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Illustrative Assessment ofImpacts on Non-HumanBiota from Operational

Radioactive Releases from aGeneric Geological DisposalFacility

INPUT TO THE OPERATIONALENVIRONMENTAL SAFETY ASSESSMENT

Version 5

26 January 2011



Illustrative Assessment ofImpacts on Non-Human Biotafrom Operational RadioactiveReleases from a GenericGeological Disposal Facility

INPUT TO THE OPERATIONAL ENVIRONMENTAL

SAFETY ASSESSMENT

Version 5

26 January 2011

SKM EnvirosD5 Culham Science Centre,

Abingdon,Oxon,OX14 3DBTel: +44 (0) 1865 408281Fax: +44 (0) 1865 407582Web: www.skmenviros.com

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Illustrative Assessment of Impacts on Non-Human Biota from Operational Radioactive Releases from a GenericGeological Disposal Facility

SKM Enviros

PAGE i

Executive Summary

This document constitutes an input to the Operational Environmental Safety Assessment

(OESA), which forms part of the Environmental Safety Case (ESC) for a generic

Geological Disposal Facility (GDF). It presents an illustrative assessment of the

radiological impacts to non-human biota (NHB) associated with operational releases from

emplaced waste in a GDF.

In line with other documents contributing to the ESC, this document is based on generic

rather than site-specific assumptions. It is anticipated to evolve along with the OESA as

the MRWS site selection process progresses.

For the purpose of this report, and at this stage in preparing a generic assessment of

potential impacts to the environment from emplaced wastes during the operational phase,

routes for exposure are associated solely with gaseous emissions.

A conceptual model and illustrative assessment methodology for the exposure of non-

human biota is described. Atmospheric dispersion of the release has been modelled using

Plume (part of the PC CREAM assessment code). Environmental transfer, occupancy

and dosimetric modelling for non-human biota have been based on the ERICA approach,

where possible. An additional approach for assessing dose rates from radon and itsdaughters has also been used.

A generic illustrative assessment of the potential doses to non-human biota has been

undertaken. The total calculated dose rates for all of the organisms considered are

insignificant in consideration of reviews of the effects of ionising radiation on organisms

reported by the International Atomic Energy Agency (IAEA) and United Nations Scientific

Committee on the Effects of Atomic Radiation (UNSCEAR), which give a broad conclusion

that exposure of terrestrial organisms to dose rates of 40 μGy per hour, and exposure of

aquatic organisms to dose rates of 400 μGy per hour, would be unlikely to lead to

observable effects in these populations.

These results suggest that significant future effort on refining the assessment of impacts

from gaseous operational releases on non-human species would not be warranted. For

example, it may be appropriate to take account of additional information, as the MRWS

process progresses. Any actual radiological dose from off site discharges from a GDF will

be determined by site-specific factors, and will be a function of actual gaseous discharge

rates during each year of GDF operation in combination with local environmental factors.

As the understanding of discharges (both radioactive and non-radioactive) from a facility

develops, the assessment of impact on non-human biota may need to be considered

further. This may be achieved by commissioning research and development to address




areas of uncertainty in a manner that takes account of their potential significance, and thestatus and progress of our programme to implement a GDF.

SKM Enviros

PAGE ii




Contents

1. Introduction 1

1.1.

Background 1

1.2. The Waste Inventory 2

1.3. Purpose and Scope 3

1.4.

Structure of the Report 5

2. Operational Source Term and Discharge Assumptions 6

2.1. Gaseous Emissions from Emplaced Waste 6

2.2.

Radioactive Gaseous Emissions 7

2.2.1. Carbon-14 8

2.2.2. Radon-222 9

2.2.2.1 Radon emanating from the host rock 10

2.2.3. Tritium 10

2.3. Derivation of Gas Generation Rates for Use in Assessment of Dosesfrom Off Site Radioactive Aerial Discharges 11

2.3.1. Carbon-14 bearing gases 11

2.3.2. Radon-222 12

2.3.3. Tritium 12

2.4.

Summary - Radioactive Gas Release Rates for Use in Off Site DoseAssessment Calculations 13

3. Approach to Assessment 14

3.1.

Conceptual Model 14

3.2. Methodology for Assessment of Dose Rates to Non-Human Biota 14

3.2.1. Reference Organisms 14

3.2.2. Routes of Entry to the Environment 15

3.3. Quantitative Assessment Models 16

3.3.1. PC CREAM 17

3.3.2. ERICA Assessment Tool 17

3.3.3. Quantitative Assessment of Radon Discharges 19

3.3.4. Limitations in the Assessment Methodology for Non-Human Biota 20

4. Illustrative Quantitative Assessment Methodology 22

4.1. Generic Assumptions and Parameters 22

4.1.1. Stack Height and Roughness Length 22

4.1.2. Meteorological Data 23

4.1.3. Deposition Velocity and Washout Coefficient 23

4.1.4.

Receptor Distances 23

4.1.5.

Uptake of Radioactivity by Biota and Dispersion in the Physical Environment 23

4.1.6.

Dosimetric Assumptions 24

SKM Enviros

PAGE iii




SKM Enviros

PAGE iv

4.2.

Assessment of Impact 26

5. Illustrative Assessment Results 28

6. Conclusions 30

7. References 31




1. Introduction

1.1. Background

Following the adoption of geological disposal as the preferred option for dealing with the

UK’s higher activity waste, the Nuclear Decommissioning Authority (NDA) established the

Radioactive Waste Management Directorate (RWMD) to manage the delivery of

geological disposal for higher activity wastes, as required under Government policy

published in the Managing Radioactive Waste Safely (MRWS)[1], White Paper. RWMD is

developing a work programme to achieve a safe, secure, sustainable and publicly

acceptable outcome. The “Geological Disposal: Steps towards implementation” report [2]

describes the work programme that is planned to implement geological disposal.As part of

this work programme RWMD adopted a staged approach to develop safety cases to carry

out assessments and make decisions in line with the stages of the MRWS programme. At

this stage there is no identified potential site for a GDF, therefore there is no information

either about the geology and hydrogeology at a site, or about the detailed design of a

GDF that might be constructed there. The information presented in this report is at a

generic level independent of geology or a specific design.

A generic Disposal System Safety Case (DSSC) [3] was established to provide evidence

to demonstrate that a geological disposal system meets all applicable regulatoryrequirements. The DSSC describes the approach to the assessment of safety for a

geological disposal facility (GDF) in three generic geological settings. It does this by

setting out the methods and types of data that will be used to assess the safety of

transport [4], construction and operation of a GDF [5], and the environmental safety of a

GDF [6] both before and after it has been backfilled, sealed and closed.

The generic DSSC is made up of a hierarchy of documents which collectively provide

evidence to demonstrate that a GDF will be safe. Further information is available relating

to the full suite of DSSC documents including the illustrative GDF layouts for the three

illustrative geological disposal concept examples [7] and the disposal system technical

specification [8].

The present report is an input into the generic Operational Environmental Safety

Assessment (OESA), which contributes to the generic Environmental Safety Case.

The OESA will include an assessment of the radiological and chemotoxic hazards

associated with operation of the disposal facility – for both people and the environment

(including non-human biota) located off site from a GDF. This information will be fed into

the Strategic Environmental Assessment (SEA) and Environmental Impact Assessment

(EIA) work, and integrated with the assessment of non-radiological effects, to provide an

SKM Enviros

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overall understanding of potential effects on sensitive environmental resources andbiological receptors.

The ESC and OESA will be updated in line with development of the DSSC, at regular

intervals as appropriate to meet regulatory expectations. Over time, the design options

under consideration and the choices RWMD has to make will change the emphasis on

strategy to one of implementation. This approach is consistent with a staged development

and approval process.

1.2. The Waste Inventory

The MRWS White Paper [1] provides an estimate of the volumes and characteristics of

higher activity radioactive waste and other materials that could possibly, come to be

regarded as wastes that might need to be managed in the future through geological

disposal. This ‘Baseline Inventory’1 is based on the 2007 UK Radioactive Waste

Inventory (UK RWI) [9]. It includes materials not currently classified as waste - spent

nuclear fuel, and separated plutonium and uranium stocks. However, it excludes low level

waste (LLW) unsuitable for surface disposal that can be managed under the

Government’s “Policy for the Long Term Management of Solid Low Level Radioactive

Waste in the United Kingdom” [10].

The Government’s policy (paragraph 3.8 in the MRWS White Paper) is that, pending a

decision whether the radioactive materials included in the Baseline Inventory should be

declared as waste, their possible inclusion will be factored into the design and

development of a geological disposal facility. Therefore for planning purposes the

Baseline Inventory is used as the basis for developing a disposal system specification

[8,11] and, in turn, GDF engineering designs that meet this specification. These facility

designs [7] provide the basis for assessments of the associated safety and environmental,

social and economic impacts and for assessment of costs.

The MRWS White Paper sets out the Baseline Inventory in terms of volume and activity,

for the different waste types, as shown in Table 1. The White Paper emphasises that thefigures are only indicative and that the assumptions that support them may change. For

design and assessment purposes, more detailed information related to the characteristics

of individual waste packages is needed and therefore a ‘Derived Inventory’ has been

developed. The ‘Derived Inventory’ is based directly on the Baseline Inventory but with

information presented on a ‘per package’ basis and taking account of the effects of

radioactive decay (and build-up) as appropriate. In addition to the Baseline, the Derived

1 It should be noted that at present the Baseline Inventory is based on UK RWI figures, and as such, currently

contains waste expected to be managed under the Scottish Government’s policy of interim near-surface,near-site storage as announced on 25 June 2007.

PAGE 2




Inventory has also considered an ‘Upper Inventory’ which accounts for potential increasesin waste and material volumes that may arise through for instance, alternative scenarios

for reprocessing or the introduction of new-build nuclear power stations. The Derived

Inventory is fully described in the Disposal System Technical Specification [8].

Table 1 Basic data for the Derived Inventory Reference Case considered in the GenericDSSC

1.3. Purpose and Scope

This report is an input into the OESA as it develops through the MRWS site selection

process. It provides an overview of the methodology and results of an illustrative generic

assessment of doses to non-human biota resulting from operational releases of gases

from emplaced wastes in a GDF.

For the purposes of this generic assessment, a number of key assumptions have been

made about off site emissions from emplaced wastes during the operational phase:

PAGE 3




• As a result of appropriate design, filtration and effluent management, the only primaryemissions from the emplaced wastes will be gaseous.

• A number of the surface facilities, such as decontamination, inspection and

maintenance facilities, active laundry and laboratories, may give rise to radioactive

liquids requiring disposal. Any radioactive liquid arisings would be collected and

treated as appropriate in an Active Effluent Treatment Plant (AETP). Treated liquids

would be monitored prior to discharge to ensure that all discharges from the site are

kept within acceptable limits. Liquid discharges from the surface facilities will be

managed as described above and are not expected to be significant, and are therefore

not further assessed in this report. This will be confirmed in future assessments

during the MRWS site selection process. There are two potential sources of liquiddischarges during the operational period that require consideration:

o Liquid radioactive wastes may arise from underground activities such as

waste package inspection and maintenance – these will be pumped or

transferred in a bowser to the surface and then treated in an AETP and

discharged in the same way as liquid effluents arising in surface facilities.

o Depending on the geological setting, there may be groundwater intrusion

into the active areas during the operational period; this would be managed

and kept to a minimum level. Any liquid ingress would be diverted away

from waste packages and collected to minimise the potential for corrosion

or contamination. Any groundwater collected would be monitored and

discharged to ensure compliance with applicable limits; this is expected to

be through a system separate to that relating to any potentially radioactive

liquid wastes.

• Radioactive liquid effluent discharges from the underground facilities would be

managed as described above and are not expected to be significant, and are therefore

not assessed further in this report.

• It is currently assumed that any solid radioactive waste arising from activities taking

place in the surface facilities will be LLW and would be disposed of within the GDF.

• Underground activities include the removal of waste packages from shielded waste

transport containers in a shielded inlet cell, inspection and monitoring activities, and

transfer to and emplacement of waste packages in the appropriate disposal vault or

tunnel. As with the surface based activities, it is currently assumed that most

radioactive solid waste arising from these activities would be LLW (there could

conceivably be some ILW) and would be disposed of within the GDF. This would

require provision of appropriate waste packaging and handling facilities within the GDF

and definition of appropriate waste acceptance criteria. Any such solid waste arising

would need to be compared with on-site waste acceptance criteria and following

PAGE 4




suitable conditioning and packaging is assumed to be determined to be compliant andaccepted for disposal on site. Therefore there would be no solid radioactive waste

from the underground facilities discharged off site.

Consequently, at this stage in preparing a generic assessment of potential impacts to the

environment from emplaced wastes during the operational phase, routes for exposure are

associated solely with radioactive and non-radioactive gaseous emissions.

The exposure of human receptors and non radiological (toxicological) effects are beyond

the scope of this report and have not been discussed further.

1.4. Structure of the Report

The remaining sections of this report are summarised, below.

Section 2 addresses the source terms and describes the basis for the predicted gaseous

discharges.

Section 3 presents the conceptual and quantitative models applied and limitations of the

models used in assessing radiological impacts.

Section 4 discusses the specific approaches, assumptions and parameters used in

undertaking an illustrative dose calculation. It lists the reference organisms and dose

factors employed.

Section 5 presents the results of a prospective dose assessment based on discharges of

gases anticipated to be released from a GDF and the model assumptions and parameters

described in earlier sections of the report. It remarks on the significance of the dose rates

determined.

Section 6 concludes with recommendations for further work.

Section 7 provides references.

PAGE 5




2. Operational Source Term and DischargeAssumptions

2.1. Gaseous Emissions from Emplaced Waste

Understanding what constitutes the source term of radiological and non-radiological

emissions from emplaced wastes within the controlled environment of the vaults is central

to understanding the operational environmental hazards.

The components within a GDF that have the potential to generate gases and are of most

relevance to gas production would include:

• the wastes, including both metals (e.g. steels, Zircaloy, Magnox, aluminium and

uranium) and organic materials (e.g. cellulose and synthetic polymers such as

polyvinylchloride (PVC)) [9];

• the waste encapsulants, such as a cement grout or organic polymer;

• the container materials, including iron or steel (the most likely to be used for

ILW / LLW containers) and some of a variety of metals, including steel, copper,

titanium and nickel alloys, being considered for containers for other higher activity

wastes [12]• the buffer or backfill materials (through radiolysis of associated water); and

• structural materials, such as steel reinforcement used in underground construction.

The main mechanisms by which gas could be generated in a GDF have been reviewed in

a joint EC/NEA status report [13]. These are metal corrosion, radiolysis and microbial

degradation. Thus, in a GDF, processes that could generate either large volumes of bulk

gases or significant amounts of radioactive gases are:

• corrosion of metals leading to the release of carbon-14 and tritium trapped in the

metal;

• microbial degradation of organic materials, including the prior hydrolysis of cellulose to

smaller organic compounds;

• radiolysis, in particular of water and some organic materials;

but would also include:

• diffusion, notably the release of tritium by solid-state diffusion from metals;

• radioactive decay of radium, which leads to the generation of radon-222; and

• the release of radioactive gases containing tritium or carbon-14 by leaching of

irradiated graphite.

PAGE 6




The rates at which most of the gases will be generated are sensitive to environmentalfactors, which might change with time, such as: the presence of oxygen or water; the

presence of hydrogen or chloride ions; and temperature. The process of backfilling the

GDF, as may occur during the operational period, could affect the temperature and hence

rate of gas generation.

The operational activities which may give rise to off site aerial radioactive and non

radioactive discharges are described in more detail in the Operational Environmental

Safety Assessment (OESA) for both the surface facilities and those underground [14].

2.2. Radioactive Gaseous Emissions

As indicated above, radioactive discharges, during the operational period, would be

dominated by gaseous emissions. All emissions would be managed to meet regulatory

requirements as described in the OESA [14].

The generic OESA concludes that the key off site radioactive discharges of interest from a

GDF would be aerial discharges released via the discharge stack from the total waste

inventory when emplaced in the underground facilities. The rate of gas generation and

hence aerial radioactive discharge can be conservatively calculated using gas generation

data for the total inventory of waste to be disposed of in a GDF.

HLW, spent fuel, plutonium and HEU are assumed in the generic DSSC to be packaged in

high integrity, unvented disposal canisters. Radioactive discharges of gaseous

radionuclides from these disposal canisters would not be expected under normal

operating conditions [15] and therefore do not contribute to the aerial discharge from the

GDF.

LLW, ILW and DNLEU are assumed in the DSSC to be packaged in containers that would

be vented. Aerial discharges associated with gases generated in LLW and ILW

containers are assessed in this report; the key radionuclides are identified as tritium,

carbon-14 and radon-222. DNLEU does not generate tritium or carbon-14 although will

generate radon-222 through radioactive decay. The generation of radon-222 from this

source will be small in comparison to the source-term from LLW and ILW and as

described later these stocks of uranium can be packaged in a form that will allow the

radon to decay before it is released from the package and into the ventilation circuit.

Future updates to the OESA will consider this radon source in a quantitative assessment.

We do not present a quantitative assessment relating specifically for the Upper Inventory:

the additional tritium and carbon-14 inventory associated with the ILW and LLW in the

Upper Inventory is not a significant gas generation source term during the aerobic GDF

PAGE 7




operational period. The impact of radon in the Upper Inventory is not thought to representa significant challenge to the GDF due to the availability of packaging technologies that

can provide significant hold-up of radon in the waste package. Future updates to the

OESA will consider the issue of radon in the Upper Inventory in a quantitative

assessment.

Recent calculations [20] of the generation rates of gases from ILW and LLW in a GDF in a

higher strength host rock have been made based on an update to the 2004 UK radioactive

waste inventory data [16]. The results of these calculations and their underpinning

assumptions are summarised in the gas status report [15] and form the basis of the

assessment reported in the generic OESA.

We note that there are other radionuclides, including those that are gaseous species in

their own right (e.g. krypton-85) or that can be incorporated into volatile species (e.g.

selenium-79 in hydrogen selenide or dimethyl selenide), these are expected to be of less

significance [17] and have therefore not been assessed in this report.

Further details of the source terms within the wastes that could give rise to gaseous

discharges from the packages during the operational period are provided in the OESA.

2.2.1. Carbon-14There are three main intermediate level waste types leading to generation and release of

carbon-14: irradiated metals, irradiated graphite and organic wastes. To make a

conservative assessment basis we make the pessimistic assumption that these releases

are in the form of methane rather than the more reactive form of carbon dioxide which

would be trapped by carbonation of grout in the waste packages [18].

In [15], it is noted that gas generation calculations assume carbon-14 is released from the

corrosion of irradiated metals and the congruent reaction of carbon-14 bearing carbides in

the form of methane, and the rate of release is assumed to be proportional to the

corrosion rate. The release of carbon-14 from irradiated graphite is assumed to beproportional to its degradation rate and that 1% of the carbon-14 inventory released is in

the form of methane.

Microbial degradation of organic molecules containing carbon-14 may result in the

formation of carbon dioxide and methane. Carbon-14 bearing methane is also formed

from the radiolysis of some organic molecules. Widespread methanogenesis is unlikely

during the operational period although the possibility of localised niches within some

individual waste packages exists. Carbon dioxide generated by microbial action is

assumed to be trapped by the wasteform grouts.

PAGE 8




We undertake assessment of the disposability of proposed waste packages before theyare manufactured and work with the packager to ensure that the barriers provided by

wasteform and container meet the anticipated needs for disposal in a GDF [19]. In

recognition of our disposability assessment process, the quantified assessment reported

herein assumes that release of carbon-14 from the small quantities of waste that comprise

small organic molecules is not significant since it is assumed that the limited number of

relevant wastes would be specifically conditioned and packaged such that significant

quantities of carbon-14 bearing methane would not be released during the operational

period.

The ‘effective’ radionuclide inventory for carbon-14 at 2040 was calculated in the gasgeneration calculations [20] to have an activity of 1.54x103TBq for UILW and 6.41x103TBq

for SILW/LLW. These activities were calculated using [16]. As noted in [20] these

activities were then multiplied by the fraction of carbon-14 present in each waste material

to derive a partitioning of carbon-14 activity at 2040. From this data set it is clear that the

highest activity for carbon-14 is contained within the graphite present in the SILW/LLW,

with smaller amounts of activity in the stainless and mild steel. The activity of the wastes

are contained within;

• graphite,

• stainless steel,

• mild steel,

• zircaloy,

• nimonic alloy,

• magnox,

• uranium,

• corroded magnox,

• corroded uranium, and

• non metals, various types.

2.2.2. Radon-222

Radon-222 is generated from radium-226, which is present in high concentrations in a

limited number of intermediate level waste streams. In this assessment, such radium-

bearing wastes have been assumed to be packaged in an appropriate manner to reduce

their radon emissions (e.g. encapsulation in a low permeability material). As discussed

above, the disposability assessment process facilitates the production of waste packages

that are expected to be compliant with the needs of disposal in a GDF.

PAGE 9




The ‘effective’ radionuclide inventory for radium-226 at 2040 was calculated in the gasgeneration calculations [20] to have an activity of 2.32x101TBq for UILW. There is

uilibrium with its daughter products2, and therefore

As noted above, future updates to the OESA will consider this radon source

In addition to the waste, the host rock itself may be a source of radon-222. This is a site-

sp host rock is not considered in the generic

Tritium may be present in ILW metals (e.g. fuel cladding) as hydrides or as dissolved

ould also be present in other materials (e.g. irradiated graphite) and

for UILW and

calculated to be no ‘effective’ radium-226 inventory for SILW/LLW at 2040. These

activities were calculated using [16].

DNLEU in the form of a uranium oxide will also be a source of radium-226 and hence

radon-222. DNLEU is not in secular eq

the radium-226 activity in these waste streams will continue to increase during the period

of operation. However, being in the oxide form, the releases are expected to be a small

fraction of releases from the more mixed and heterogeneous forms present in ILW.

Furthermore, when and if uranium stocks are conditioned and packaged for disposal, the

LoC disposability assessment process will be applied to ensure that the barriers providedby the wasteform and waste package are consistent with the requirements of the disposal

safety case.

Based on the above reasoning, releases from DNLEU are not quantified further in this

assessment.

in a quantitative fashion to confirm the robustness of this approach.

2.2.2.1 Radon emanating from the host rock

ecific issue. Radon-222 emanating from theOESA but will be addressed at a later site-specific stage.

2.2.3. Tritium

hydrogen. It w

trapped as tritiated water on desiccants. The major contributor to the generation of tritium

during the operational period is the aerobic corrosion of metallic uranium.

The ‘effective’ radionuclide inventory for tritium at 2040 was calculated in the gasgeneration calculations [20] to have an activity of 2.54 x 105 TBq

2.57 x 104 TBq for SILW/LLW. These activities were calculated using [16]. From this data

set it is clear that the highest activity of tritium is contained within the stainless steel

present in the SILW/LLW with smaller amounts in mild steel. The activity of the wastes

are contained within;

• stainless steel,

• mild steel,

2DNLEU is processed uranium i.e. it is not in the state it would be naturally occurring in the environment.

PAGE 10




• zircaloy,

• aluminium,

• magnox, and

ion of Gas Generation Rates for Use in Assessment of Doses

from Off Site Radioactive Aerial Discharges

The possible generation rates of carbon-14 in methane during the operational phase are

discussed in the gas status report [15], based on the calculations and underpinning

[20]. These gas generation rates are considered

y [15]. Immediately after the

• 6 TBq per year (representative of the operational period peak gas generation rate).

• uranium.

2.3. Derivat

2.3.1. Carbon-14 bearing gases

assumptions reported in reference

herein. We assume that release of carbon-14 from wastes comprising small organic

molecules does not need to be considered.

For most of the operational period of a GDF, the rate of carbon-14 bearing methane

generation for all sources combined is estimated to be about 0.5 TBq per year.

This generation rate is calculated to increase if vaults are backfilled with a cementitiousmaterial. Such backfilling has the effect of heating the waste packages as it cures

(cement curing is exothermic) and backfilling may make available free water that can be

used in e.g. corrosion reactions.

The assumptions used in reference [20] lead to a maximum generation rate of 6 to 8 TBq

per year on backfilling3. These are currently expected to represent likely upper bounds

during the operational period prior to closure of the facilit

closure of a GDF (a period excluded from consideration in the OESA), the rate of

carbon-14 gas generation peaks at about 8 to 10 TBq per year [15, 20]. On the basis of

the calculations reported in reference [20], and acknowledging the uncertainties in the rateof gas generation, we have used a selection of values for the carbon-14 bearing gas

generation rates during the operational period:

• 0.5 TBq per year (representative of the operational period average gas generation

rate); and

3

Higher values in the range 20 to 30 TBq per year arose during the post-closure phase in some variantcalculations.

PAGE 11




We also consider a carbon-14 bearing gas generation rate of 10 TBq per year – this istaken herein to be a conservative bounding gas generation rate, noting that this does not

he

OESA.

e of radon-222, a generation rate of approximately 1,000 TBq per year has been

estimated for a radium-226 inventory of 23.2TBq at 2040, as reported in [20] on the basis

inventory [16]. However, this generation rate is conservative and not

ison with the in-package radon-222 generation rate [21]. In

bustness of this approach will be considered in a future

on rates of tritium during the operational phase are discussed in the

gas status report [15] based on the calculations reported in reference [20].

F operational history for carbon-14 bearing gases, and acknowledging the

• 1-2 TBq per year (representative of the operational period average gas generation

rate); and

imply such a generation rate is expected to occur in the timeframe considered by t

2.3.2. Radon-222

In the cas

of the updated 2004

appropriate for estimating release to the atmosphere, as it does not include any retention

and decay of radon-222 within waste packages. Retention of radon-222 by waste

packages can reduce release rate significantly as a consequence of its relatively short

half-life (3.82 days).

The retention of radon-222 within a waste package is expressed in terms of an ‘emanation

coefficient’, which corresponds to the fraction of radon-222 that is released from a waste

package in compar

determining the rate of off-site release of radon-222 for the generic OESA, we have taken

radon-222 in-package retention into account by assuming an emanation coefficient of

2x10-3 [22]. For a total radon-222 generation rate of 1,000 TBq/year, this results in an off-

site discharge release rate of 2 TBq per year, which is used for assessment purposes in

this generic OESA. As we discuss later, there is the potential to package radon-bearing

wastes in a form that provides improved ‘hold-up’ and which offers the potential to justify a

much reduced emanation rate.

As noted earlier, additional radon-222 associated with the DNLEU is not considered

significant in comparison to discharges from LLW and ILW, and is not considered in the

quantified assessment. The ro

update of the OESA.

2.3.3. Tritium

The possible generati

Assuming a GD

uncertainties in the rate of gas generation, we have used a selection of values for tritium

generation rates during the operational period:

PAGE 12




• 10 TBq per year (representative of the operational period peak gas generation rate).

2.4. Summary - Radioactive Gas Release Rates for Use in Off Site Dose

nalysis described above, Table 2 summarises the average, peak and

dose assessment calculations for off site discharges.

Assessment Calculations

Based on the a

bounding radioactive gas release rates that are used subsequently in this report in the

Table 2 Radioactive gas release rates

clide Average reRadionu lease rate Peak release rate Bounding releasefor operational period(TBq per year)

for operationalperiod (TBq peryear)

rate (TBq peryear)

Carbon-14 0.5 6-8 10

Rad 2 2on-222 2

Tritium 1-2 10 10

PAGE 13




3. Approach to Assessment

3.1. Conceptual Model

The conceptual model that is presented identifies the most important features of the

waste, potential hazards and routes (if any) by which they may impact on the environment

and the points through which impacts may be assessed.

Conceptual models for a GDF may contain different features, depending on the purpose

for which they are being developed. Features of relevance to assessing environmental

impacts arising from emplaced wastes are described below. The model and thedescriptions of the various components can be refined iteratively as more detailed site-

specific information becomes available.

Based on information presented in Section 1.3, it is assumed that discharges of relevance

would be limited to controlled emissions of gases via authorised outlets. At this stage in

demonstrating a generic assessment, it is considered reasonable that gaseous discharges

only are considered (although other routes may be considered in a more detailed, site-

specific, assessment). When constructing a quantified model, it is necessary to make

assumptions about the height of the gaseous release and about other characteristics of

the emissions (discharge velocity, temperature and so on).

Once released to atmosphere, gases disperse. The characteristics of the dispersion

would depend on local topography, including the built environment, and meteorological

conditions such as wind speed and direction, and rainfall. As a result, some of the gases

may disperse over very large areas and remain in the atmosphere for long periods. Other

fractions may be deposited to ground and be available for uptake to organisms or

accumulate in soils. Deposition may also occur over surface waters, including rivers,

lakes and the ocean. Deposition is likely to be associated with sorption, either to airborne

particulates or to water droplets in the air.

Further information on environmental transfer may be found elsewhere [14].

3.2. Methodology for Assessment of Dose Rates to Non-Human Biota

3.2.1. Reference Organisms

Assessments of the potential impact of radiation on non-human biota are generally

undertaken by identifying the nature of the ecosystem and a range of characteristic

‘reference organisms’ within it. A range of such organisms is typically chosen to

encompass different trophic levels and variations in lifestyle, geometry and uptake

characteristics that determine exposure. Generic assessment approaches have been

PAGE 14




developed, which allow assessment receptors to be specified in the form of ‘referenceorganisms’4.

The range of reference organisms for the terrestrial ecosystem that are relevant to an

assessment of atmospheric releases are summarised in Table 3.

Table 3 Generic reference organisms for the terrestrial ecosystem

Generic Organisms

Amphibian

Bird

Bird egg

Detritivorous invertebrate

Flying insect

Gastropod

Grasses & herbs

Lichen & bryophyte

Mammal (deer)

Mammal (rat)

Mammal (fox)

Reptile

Shrub

Soil invertebrate (worm)

Tree

Extensive databases of the assessment parameters (occupancy, geometry, concentration

factor and dose conversion coefficients) are needed to assess potential dose rates to

such organisms. Such databases have been established over the last decade, notably aspart of the EC 6th Framework project ERICA (Environmental Risk from Ionising

Contaminants: Assessment and Management), which was completed in February 2007

(see more details below).

It should be noted, however, that impacts on sensitive species, indicator species,

endangered or locally important species may not be fully represented by the assessments

undertaken for reference organisms. Additional assessments and alternative dose rate

criteria may be necessary in such cases.

3.2.2. Routes of Entry to the Environment

Potential pathways from an initial gaseous discharge can be summarised as follows:

• Non-human biota (fauna) – inhalation, ingestion (including water uptake), aerosol skin

contact.

4 Reference organisms have been defined as: A series of imaginary entities that provide a basis for theestimation of radiation dose rate to a range of organisms which are typical, or representative, of acontaminated environment. These estimates, in turn, would provide a basis for assessing the likelihood and

degree of radiation effects. It is important that they are not a direct representation of any identifiable animalor plant species [FASSET, 2003].

PAGE 15




3.3.1. PC CREAM

This software modelling tool comprises a suite of models and data which can be used to

undertake radiological dose assessments for members of the public of routine and

continuous discharges from virtually any type of installation [26]. PC CREAM was

developed for the European Union but parts of the system have been used throughout the

world.

PC CREAM is comprised of a main radiological dose assessment module, ‘Assessor’,

supported by 5 modules that model the transfer of radionuclides through the environment

and provide estimates of activity concentrations in various environmental media following

a continuous release. The Plume module is the package for modelling the atmosphericdispersion and was used to determine the atmospheric concentrations of the gaseous

discharges assessed for this project. It takes into account prevailing meteorological

conditions during the release, the roughness of the land surface and the physical

characteristics of the radionuclides being released. Plume estimates atmospheric

dispersion using a Gaussian plume model, dry deposition using a source depletion model

and wet deposition using a washout coefficient approach [26].

3.3.2. ERICA Assessment Tool

The ERICA approach and assessment tool were developed as a consequence of two EC

funded research projects: FASSET and ERICA (ERICA was effectively a continuation and

consolidation of the FASSET project), as outlined in more detail in a previous report for

RWMD [25].

The ERICA assessment tool [23] is a software programme that allows dose rates to

selected biota to be assessed. The tool contains data for three ecosystems (terrestrial,

freshwater and marine). A number of reference organisms are included that are

representative of the types of organisms that are likely to be present in each type of

ecosystem.

A dose calculations for a reference organisms is conducted on the basis of a simplified

(e.g. ellipsoid) geometry representative of the dimensions of the main body of the

organism (i.e. extremities such as legs, wings etc are not included). Dose conversion

coefficients (DCCs) for both internal and external exposure to radionuclides are specific to

these organisms and geometries.

External DCCs allow the absorbed dose rate (hereafter referred to as simply dose rate) to

a reference organism to be estimated from the average concentration of a radionuclide in

an environmental compartment (air, soil, sediment, water) of a reference ecosystem

(expressed in for example µGy per hour per Bq/kg). External dose rates are calculated

PAGE 17




using the DCCs and taking into account the proportion of time that an organism spends indifferent compartments of the reference ecosystem (occupancy factors).

In the case of internally incorporated radionuclides, the internal concentration of

radionuclides is required. Internal activity concentrations for reference organisms are

calculated through the application of concentration ratios, which assume equilibrium

conditions and a uniform distribution of the radionuclide within the organism (i.e. with no

accumulation of radionuclides within individual tissues). Internal DCCs are then applied

that relate the average concentration of a radionuclide in a reference organism to the dose

rate.

Radiation weighting factors5 are applied to take account of the differing biological

effectiveness6 of different types of ionising radiation. The following default ERICA

weighting factors were applied: 10 for alpha radiation, 3 for low energy beta, and 1 to

gamma/high energy beta.

Total doses to the reference organisms are calculated as the sum of both internal and

external dose.

The tool interacts with a number of databases and other functions to estimate

environmental media activity concentrations, activity concentrations in biota, and dose

rates to biota. It also interfaces with the FREDERICA radiation effects database (which isa compilation of the scientific literature on radiation effects data for different wildlife groups

and, for most data, broadly categorized according to four effect umbrella endpoints:

morbidity, mortality, reproduction, and mutation). FREDERICA is a merger of two

European projects FASSET and EPIC – (FASSET Radiation Effect Database plus

ERICA).

The ERICA assessment tool is organised in three tiers, as outlined below.

• Tier 1 is designed to be simple and conservative. It requires a minimum of input data

and enables the user to exit the process if the assessment meets a predefined

screening criterion. The default screening criterion is an incremental dose rate of 10

μGyh-1. Within the Tool, Environmental Media Concentration Limits (EMCLs) 7 are

5Radiation weighting factor (defined in ERICA as follows): The value of a radiation weighting factor represents

the relative biological effectiveness of the different radiation types, relative to X- or gamma-rays, in producingendpoints of ecological significance.

6 Relative Biological Effectiveness: the ration between the dose of the given radiation needed to produce a

given biological effect to that of the reference radiation.

7 The Environmental Media Concentration Limit is defined in the ERICA Project Glossary as follows: ‘the

Predicted no effects dose rate or screening dose-rate (μGy h-1

) divided by the value F which is the dose rate

that an organism will receive for the case of a unit concentration in environmental media (μGy h

-1

per Bq l

-1

orkg of medium)’.

PAGE 18




calculated on the basis of this dose rate for all reference organism/radionuclidecombinations. The Tool compares the input media concentrations with the most

restrictive EMCL for each radionuclide to determine a risk quotient (RQ) 8. This

process also includes application of an assessment factor (default value of 3).

• Tier 2 allows the user to undertake a more interactive assessment, and to change the

default parameters and to select and specify reference organisms. The evaluation is

performed directly against the screening dose rate. Dose rates and RQs are

generated for each reference organism selected for assessment. Decisions regarding

Tier 2 results may be informed by ‘look-up effects tables’ from FREDERICA for

different wildlife groups.

• Tier 3 allows probabilistic risk assessment and access to the FREDERICA database.

It allows up-to-date scientific literature to be applied (which may not be available at

Tier 2) on the biological effects of exposure to ionising radiation in a number of

different species. Together, these allow the user to estimate the probability (or

incidence) and magnitude (or severity) of the environmental effects likely to occur.

Tiers 1 and 2 have been applied in this illustrative assessment.

3.3.3. Quantitative Assessment of Radon Discharges

The ERICA approach does not include the necessary parameters for an assessment of

impacts from radon. An alternative approach was therefore used.

The approach used in this report to assess dose rates from radon-222 and its daughters

was developed by Vives et al [24]. The approach is based on allometrically derived

respiration rates and target tissue masses, and has been designed to calculate dose rates

from radon-222 and its daughters to sensitive tissues and the whole body of terrestrial

animals. The main exposure pathway is assumed to be exposure of the target tissues of

the respiratory system to radiation arising from radon-222 daughters. The approach

assumes that there is a constant in-flow of particles and that the tissue is 100 per cent

efficient at trapping the material; radioactive decay is assumed to be the only mechanism

of removal of particulates.

For plants, a simplified approach based on using CO2 as tracer gas for radon is used to

estimate respiration rate and to calculate dose rates to sensitive tissues within plants.

8 A risk quotient is defined in ERICA to be ‘a measure of the risk caused by each contaminant to an organism.

For radioactive substances it is defined by the activity concentration of a given radionuclide in soil, water or air

divided by the environmental media concentration limit for that radionuclide’

PAGE 19




This approach was used to derive a range of internal and external DPUC (Dose per UnitConcentration) factors for a variety of reference organisms, which are presented in

Section 4.

The approach developed by Vives et al [24] applies a default radiation weighting factor of

20, based on radon exposure studies.

3.3.4. Limitations in the Assessment Methodology for Non-Human Biota

The limitations associated with the reference organism approach and the application of

the ERICA Tool are discussed in more detail in the assessment for post-closure releases

[25]. However, some key issues are outlined below for the sake of completeness.

The reference organism approach, implicit in the application of the ERICA method and

associated tool, is based on the assessment of doses to individuals rather than higher

levels of ecological organisation. While it is true that the effects information is focused on

those that are likely to have impacts on higher organisational levels, the approach does

not allow for interactions between organism types or between different types of

environmental stressor. There are limited possibilities available to take account of such

interactions, particularly in prospective assessments (where biological monitoring is not

possible). It is important to keep this limitation in mind in interpreting the results.

ERICA and other methodologies also include a range of input parameters with significant

associated uncertainties. For example, such parameters include concentration ratios and

dose conversion factors for internal and external exposure for a number of reference

organisms of different geometries. The assessment of dose rates from post-closure

releases suggests [25] that concentration ratios are the most sensitive parameters in

determining absorbed dose rates; this is consistent with the findings of other studies [27,

28].

The reliability of environmental transfer (and concentration ratio) data is dependent on the

availability of information for the particular organism, ecosystem and radionuclide

involved. The transfer of tritium and carbon-14 between the atmosphere and theterrestrial environment is particularly complex due to the role of hydrogen and carbon in

biological systems. As a consequence, concentration ratios for the carbon-14 and tritium

are based on a specific activity model, in which the ratio of the carbon-14 to total carbon

(by mass) in the organism is assumed to be equivalent to that in air. These factors

assume an equilibrium state and do not represent dynamic environmental processes.

There are significant uncertainties associated with the use of these values.

The ERICA methodology and associated parameters are based on the implicit assumption

that all radionuclides are uniformly distributed throughout the organism.

PAGE 20




With regard to the model applied for calculating dose rates from radon, Vives et al (2008)[24] compared their simplified respiratory model with models described by MacDonald and

Laverock (1988) [29], Hofmann et al(2006) [30] and Harley (1988) [31], and concluded

that their model has built-in conservatism [24]. They noted that their approach:

• Calculates doses to the sensitive tissues of the lungs and uses the highest component

to dose, corresponding to an active animal in a burrow. In comparison, the model

described by MacDonald and Laverock (1988) [29], gives doses to the whole lung, and

considers the dose to have three components: hibernation, in burrow and out of

burrow; and

• Adopts a simplified tracheobronchial model which assumes complete retention of

inhaled particulates, and considers radioactive decay to be the only mechanism for

removal of activity. In contrast, Hofmann et al.(2006) [30] and Harley (1988) [31] used

a full model of the tracheobronchial tree, with the following consequences:

• The full model predicts that a significant fraction of the radon daughters is removed by

the nasal passages.

• Such models include lung clearance processes, resulting in transport and

redistribution of the radon daughters from the alveolar region to the bronchial part of

the airways, with associated decay included in transit.

• The models consider atmospheres with various assumptions of equilibrium resulting in

varying particle size.

In addition, they noted that application of the allometric formulae (scaling down of

breathing rates of higher organisms, etc) for simpler organisms (eggs, insects and plants)

further increases the conservatism of the radon-222 approach.

PAGE 21




4. Illustrative Quantitative AssessmentMethodology

4.1. Generic Assumptions and Parameters

The principles and approaches presented here are applied in many current authorisation

submissions (although the specific models used would vary). Nonetheless, it is

anticipated that models, and the understanding of process descriptions and parameter

values, will continue to develop. The following is therefore indicative of the approach to

be undertaken and identifies the type of information which would be required in a site-specific context.

In this generic development of the quantitative model, the bounding release rates of

tritium, carbon-14 and radon-222 presented in Table 2 were applied, primarily to allow a

demonstration of the assessment technique.

In the absence of site-specific information, a number of assumptions have been required

to model the system. These are summarised in the following sub-section.

In theory, materials entering the environment may accumulate over time. In practice,

where deposition velocities and washout coefficients are zero, accumulation would belimited.

4.1.1. Stack Height and Roughness Length

For routine releases from a GDF it is assumed that the discharge stack is 15 metres high.

The effective release height depends on a combination of the height of the release point,

the characteristics of the release (velocity, buoyancy) and the influence any local buildings

or landscape features have on airflow around the release point. For simplicity, two cases

have been assessed. In the first, the effective stack height is the same as the actual stack

height (implying a neutral release); alternatively, an effective ground level release isassumed (which is taken to represent a conservative case).

The roughness length is related to the roughness characteristics of the terrain. For

general application, since typical landscapes almost always contain occasional

obstructions, it is advisable to estimate an effective roughness length, noting that an

estimated effective surface roughness is rarely accurate to more than one significant

figure. In this study, a roughness length of 0.3 metres was applied. This is considered

typical for rural locations and will give conservative results in urban and semi-urban

locations.

PAGE 22




4.1.2. Meteorological Data

Typical meteorological conditions for the UK are assumed. This is identified as 60%

neutral (Pasquill Category D) conditions, with uniform windrose. Pasquill category D is

best described as overcast conditions, and allowance for rainfall can be introduced,

varying both with duration (expressed as a % probability) and intensity (rainfall rate).

4.1.3. Deposition Velocity and Washout Coefficient

For the radionuclides and physical forms considered, the deposition velocities (the rate of

settling from the plume to ground in dry conditions) and washout coefficients (the rate of

depletion from the plume due to precipitation) are uniformly set to zero. This does notpreclude interaction with soil, since the plume would spread both horizontally and laterally

from the release point and would reach ground level.

4.1.4. Receptor Distances

Activity concentrations in air and deposition rates are determined at a distance of

300 metres from the stack, for both the location of the receptor habitats and associated

food sources.

More conservative assumptions could be made, especially for the ground level release

scenario. However, it is assumed that the site boundary is likely to be located some

distance from the stack and, without prejudice to site-specific design proposals, a distance

of 300 metres is assumed to be both representative and reasonably conservative.

4.1.5. Uptake of Radioactivity by Biota and Dispersion in the Physical Environment

For the purposes of illustration, key reference organisms were chosen to represent the

range of trophic levels and exposure geometries likely to be present in the terrestrial

environment and to arise from an atmospheric release. For scoping purposes, these

organisms were assumed to be co-habited with the human receptors at 300 metres from

the point of release.

The transfer of radionuclides from the air to particular reference organisms is generally

modelled on the basis of equilibrium concentration ratios for carbon-14 and tritium. Default

factors for tritium and carbon-14 were used from the ERICA Assessment Tool to

determine internal activity concentrations for each reference organism. Dose rates from

these radionuclides have then been calculated on the basis of default dose conversion

coefficients for internal exposure.

To assess external exposures to reference organisms, it is necessary to make some

assumptions about the time spent in different parts of the environment (occupancy above,

PAGE 23




on and within soil); default occupancy factors from the ERICA Assessment Tool havebeen applied (Table 4). Dose rates have then been calculated on the basis of default

dose conversion coefficients for external exposure.

Table 4 Reference organisms and key parameters for carbon-14 and tritium

Organism

Occupancy (Unitless)Concentration ratios

(Bq kg-1

(fw) per Bq m-3

)

On soil In soil carbon-14 tritium

Amphibian 1 0 1.30E+03 1.50E+02

Bird 1 0 1.30E+03 1.50E+02

Bird egg 1 0 8.90E+02 1.50E+02

Detritivorous Invertebrate 1 0 4.30E+02 1.50E+02

Flying insects 1 0 4.30E+02 1.50E+02

Gastropods 1 0 4.30E+02 1.50E+02

Grasses 1 0 8.90E+02 1.50E+02

Lichens and Bryophytes 1 0 8.90E+02 1.50E+02

Mammal (Deer) 1 0 1.30E+03 1.50E+02

Mammal (Rat) 0.6 0.4 1.30E+03 1.50E+02

Reptile 1 0 1.30E+03 1.50E+02

Shrub 1 0 8.90E+02 1.50E+02

Soil Invertebrates (Earthworm) 0 1 4.30E+02 1.50E+02

Tree 1 0 1.30E+03 1.50E+02

Mammal (Fox) 0.6 0.4 6.90E+02 1.40E+02

4.1.6. Dosimetric Assumptions

The ERICA approach includes DCCs for internal and external exposure for reference

organisms, based on simplified ellipsoid geometries. The appropriate values for tritium

and carbon-14 are presented in Table 5. As indicated above, total doses from radon-222

have been calculated according to the approach outlined earlier [24]. Internal dose rates

from radon-222 and its daughters were calculated using DPUC factors for alpha andbeta/gamma decay. External doses for low energy beta and the beta/gamma components

were calculated for the defined on and in-soil occupancy.

DPUC factors for alpha radiation were provided for both for the sensitive (respiratory)

tissues and the whole body. Tables 6 and 7 summarise the reference organisms

assessed and key input parameters used.

PAGE 24




Table 5 Table of DCCs for carbon-14 and tritium

Organism

Internal low β (µGy h-

per Bq kg

-1)

Internal βγ (µGy h-

perBq

kg

-1)

External radiation(µGy h

-1 per Bq

m

-3)

carbon-14 tritium carbon-14 tritium carbon-14 tritium

Amphibian 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Bird 2.90E-07 2.48E-06 2.87E-05 8.25E-07 0 0

Bird egg 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Detritivorous Invertebrate 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Flying. insects 2.80E-07 2.21E-06 2.77E-05 1.09E-06 0 0

Gastropods 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Grasses 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Lichens and Bryophytes 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Mammal (Deer) 2.90E-07 2.48E-06 2.87E-05 8.25E-07 0 0

Mammal (Rat) 2.90E-07 2.48E-06 2.87E-05 8.25E-07 0 0

Reptile 2.90E-07 2.48E-06 2.87E-05 8.25E-07 0 0

Shrub 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Soil Invertebrates(Earthworm) 2.80E-07 2.48E-06 2.77E-05 8.25E-07 0 0

Tree 2.90E-07 2.48E-06 2.87E-05 8.25E-07 0 0

Mammal (Fox) 3.73E-07 2.47E-06 2.81E-05 8.10E-07 0 0

Table 6 Reference organisms and key parameters for radon-222 (internal α)

Organism

Occupancy(Unitless) DPUC (Internal α) (µGy Bq

-1h

-1m

3)

On soil In soil

Bronchialepitheliumcells

Tracheo-bronchialcells

Respiratoryorgan

Wholebody

Amphibian 1 0 - - - -

Bird 1 0 3.87E+01 4.17E+00 2.42E-01 3.13E-03

Bird egg 1 0 2.44E+01 2.63E+00 8.28E-01 9.72E-03

DetritivorousInvertebrate 1 0 1.58E+01 1.70E+00 2.65E+00 2.84E-02

Flying insects (Bee) 1 0 1.99E+01 2.14E+00 1.43E+00 1.61E-02

Gastropods 1 0 - - - -

Grasses 1 0 N/A N/A 5.77E-01 7.75E-02

Lichens andBryophytes 1 0 N/A N/A 1.44E+00 7.75E-02

Mammal (Deer) 1 0 3.94E+01 4.24E+00 2.31E-01 2.99E-03

Mammal (Rat) 0.6 0.4 2.55E+01 2.75E+00 7.34E-01 8.69E-03

Reptile 1 0 4.01E+01 4.32E+00 2.20E-01 2.87E-03

Shrub 1 0 N/A N/A 5.77E-01 7.75E-02

Soil Invertebrates(Earthworm) 0 1 2.10E+01 2.27E+00 1.23E+00 1.40E-02

Tree 1 0 N/A N/A 5.77E-01 7.75E-02

Mammal (Fox) 0.6 0.4 4.41E+01 4.75E+00 1.71E-01 2.26E-03

PAGE 25




Table 7 Reference organisms and key parameters for radon-222 (β & γ)

Organism

Occupancy(Unitless)

Internal DPUC (µGy Bq-

h-1

m3)

External DPUC (µGy Bq-

h-1

m3)

On soil In soil low β Β + γ low β Β + γ

Amphibian 1 0 - - - -

Bird 1 0 3.40E-06 5.30E-04 3.90E-11 7.40E-04

Bird egg 1 0 3.40E-06 4.30E-04 2.70E-10 8.40E-04

DetritivorousInvertebrate 1 0 3.40E-06 2.90E-04 2.30E-09 9.80E-04

Flying insects (Bee) 1 0 3.40E-06 3.80E-04 4.40E-10 8.90E-04

Gastropods 1 0 - - - -

Grasses 1 0 3.40E-06 2.00E-04 3.60E-09 1.10E-03

Lichens andBryophytes 1 0 3.40E-06 3.00E-04 1.60E-09 9.70E-04

Mammal (Deer) 1 0 3.40E-06 5.60E-04 3.50E-11 7.10E-04

Mammal (Rat) 0.6 0.4 3.40E-06 4.20E-04 3.00E-10 8.50E-04

Reptile 1 0 3.40E-06 5.10E-04 9.60E-11 7.60E-04

Shrub 1 0 3.40E-06 2.00E-04 3.60E-09 1.10E-03

Soil Invertebrates(Earthworm) 0 1 3.40E-06 3.40E-04 8.70E-10 9.30E-04

Tree 1 0 3.40E-06 2.00E-04 3.60E-09 1.10E-03

Mammal (Fox) 0.6 0.4 3.40E-06 5.90E-04 5.60E-11 6.90E-04

4.2. Assessment of Impact

Total absorbed dose rates to the reference organisms are generally calculated as the sum

of (appropriately weighted) internal and external dose rates.

Tiers 1 and 2 of the ERICA assessment tool were applied to assess releases of tritium

and carbon-14. Tier 1 includes Environmental Media Concentration Limits (EMCLs),

which have been derived on the basis of the screening dose rate of 10 µGy per hour 9. The

ERICA assessment tool compares the input media concentrations (in this case activity

concentration in air) with the most restrictive EMCL for each radionuclide to determine a

risk quotient (RQ). This process also includes application of an assessment factor

(default value of 3). If the calculated sum of RQ values from Tier 1 is less than one, it is

possible to have some confidence that the screening level is not exceeded. At such levels

radiation-induced effects on non-human biota are unlikely. At Tier 2, the dose rates to

reference organisms are calculated directly and may be compared with the ERICA

9The absorbed dose, which is an expression of energy (joules per kilogram) with units of gray (Gy), is used to

express dose to biota. The unit for expressing radiological exposure to people is the sievert (Sv). The Sv is arelatively complex unit. It reflects the biological impact to people arising from radiation energy. It includes anumber of assumptions relating to the radiobiological effectiveness of different types of radioactivity, which

may not apply to other species.

PAGE 26




proposed screening level of 10 μGy/h, or any other benchmark which may be consideredrelevant.

The total dose rates associated with exposures to radon and its daughters were estimated

using another approach, [24]. These results (appropriately weighted) may be combined

with those from the other radionuclides for comparison with screening levels and/or other

benchmarks discussed in more detail elsewhere [32].

PAGE 27




5. Illustrative Assessment Results

An illustrative assessment of the potential dose to organisms as a consequence of

gaseous discharges from the GDF during the operational phase on terrestrial biota has

been undertaken using Tier 1 and 2 of the ERICA Assessment Tool [23].

Activity concentrations in the air at 300 metres from the stack (from PC CREAM) were

applied within ERICA Tier 1. The RQ values for the illustrative assessment for the

bounding releases (from Table 2) from ground level and from a stack height of 15 m are

presented in Table 8.

Table 8 RQ values from Tier 1 of ERICA Assessment Tool for bounding releases fromground level and 15 m stack heights

Radionuclide

ERICA EMCLAir

(Bq m-3

)

Risk Quotient(ground levelrelease)

Risk Quotient(15 m stack height)

LimitingReferenceOrganism

DetritivorousinvertebrateTritium 2.60E+03 1.19E-03 3.57E-04

Carbon-14 8.33E+01 3.72E-02 1.12E-02 Mammal (Deer)

Radon-222 Not available in Tier 1 assessment tool

Sum of Risk Quotients 3.84E-02 1.15E-02

The RQ values are significantly lower than one, thus indicating that the predicted activity

concentrations in air associated with the bounding releases of tritium and carbon-14 are

significantly below the screening levels and consequently below those at which any

adverse effects on non-human biota would be anticipated.

In Tier 2 of the ERICA Assessment Tool, dose rates to a range of terrestrial reference

organisms may be assessed. Default parameters were used throughout. Doses from

radon and its daughters were calculated using the approach proposed by Vives et al [ 24].

The results are presented below for bounding releases from the ground level and from a

15 metre stack height in Table 9.

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Table 9 Dose rates to terrestrial reference organisms from bounding release rates forground level and 15 m stack heights

Organism

Ground release 15m stack height

Dose rate (μGy h-1

) Dose rate (μGy h-1

)

carbon-14 tritium radon-222 Total carbon-14 tritium radon-222 Total

Amphibian 1.19E-01 3.84E-03 0.00E+00 1.22E-01 3.56E-02 1.15E-03 3.67E-02

Bird 1.23E-01 3.84E-03 2.54E-03 1.29E-01 3.69E-02 1.15E-03 7.45E-04 3.88E-02

Bird egg 7.88E-02 3.84E-03 6.69E-03 8.93E-02 2.36E-02 1.15E-03 1.95E-03 2.67E-02

DetritivorousInvertebrate 3.81E-02 3.84E-03 1.82E-02 6.01E-02 1.14E-02 1.15E-03 5.34E-03 1.79E-02

Flying insects 3.81E-02 3.59E-03 1.07E-02 5.24E-02 1.14E-02 1.08E-03 3.13E-03 1.56E-02

Gastropods 3.81E-02 3.84E-03 0.00E+00 4.19E-02 1.14E-02 1.15E-03 1.26E-02

Grasses 7.88E-02 3.84E-03 4.93E-02 1.32E-01 2.36E-02 1.15E-03 1.45E-02 3.92E-02

Lichens andBryophytes 7.88E-02 3.84E-03 4.93E-02 1.32E-01 2.36E-02 1.15E-03 1.45E-02 3.92E-02

Mammal (Deer) 1.23E-01 3.84E-03 2.46E-03 1.29E-01 3.69E-02 1.15E-03 7.20E-04 3.87E-02

Mammal (Rat) 1.23E-01 3.84E-03 5.90E-03 1.33E-01 3.69E-02 1.15E-03 1.73E-03 3.97E-02

Reptile 1.23E-01 3.84E-03 2.38E-03 1.29E-01 3.69E-02 1.15E-03 6.94E-04 3.87E-02

Shrub 7.88E-02 3.84E-03 4.93E-02 1.32E-01 2.36E-02 1.15E-03 1.45E-02 3.92E-02

Soil

Invertebrates(Worm) 3.81E-02 3.84E-03 9.04E-03 5.09E-02 1.14E-02 1.15E-03 2.65E-03 1.52E-02

Tree 1.19E-01 3.84E-03 4.93E-02 1.72E-01 3.58E-02 1.15E-03 1.45E-02 5.14E-02

Mammal (Fox) 6.26E-02 3.52E-03 1.88E-03 6.80E-02 1.88E-02 1.06E-03 5.53E-04 2.04E-02

The highest dose rates were predicted to be in the range of around 0.03 – 0.2 µGy/h,

depending on stack height and organism. These values are significantly lower than the

ERICA screening level of 10 µGy/h, at which more detailed assessment would be

recommended. As such, these values are also significantly below previous benchmark

values, included in UNSCEAR and IAEA publications [33,34], which imply dose rates of

40 and 400 µGy/h for terrestrial animals and terrestrial plants and aquatic animals,

respectively. It is therefore possible to conclude that it is extremely unlikely that these

releases would give rise to any detrimental effects to non-human species.

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6. Conclusions

This report constitutes an input to the OESA report and addresses the doses to non-

human biota located off site associated with discharges from emplaced waste during the

operational phase of a GDF. This should be considered within the context of an overall

Environmental Safety Case. At this stage in the MWRS site selection process, the

assessment is generic in nature, and does not consider specific sites or associated

geology or ecology.

The potential radiological impacts to non-human biota have been estimated using

bounding release rates. It has been assumed that emissions of gases to air woulddominate exposures of reference organisms during the operational phase. Based on the

waste forms anticipated, and on control features likely to be installed in any design of GDF

(such as filtration of discharge streams), the most likely emissions are tritium, carbon-14

and radon-222.

An illustrative assessment of dose rates to non-human biota in the terrestrial environment

from releases to atmosphere of tritium, carbon-14 and radon-222 has been undertaken.

The highest dose rates were predicted to be i

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