NUCLEAR DECOMMISSIONING IN THE UK2017.radioactivewastemanagement.org/images/slide/Session... ·...
Transcript of NUCLEAR DECOMMISSIONING IN THE UK2017.radioactivewastemanagement.org/images/slide/Session... ·...
NUCLEAR DECOMMISSIONING
IN THE UK
Pete Burgess
Radiation Metrology Ltd 1
Radiation Metrology Ltd
Where • Government nuclear research sites, e.g. Harwell, Winfrith
and Dounreay
• Materials test reactors – high neutron flux reactors for
determining the consequences of neutron irradiation such
as embrittlement
• Plutonium production reactors at Sellafield – see first slide
• Research reactors at universities etc.
• Reactor designs that were not taken forward into series
production – the SGHWR at Winfrith for example
• The earlier power reactors – power reactors have been
operating since 1956
• Weapons sites – mainly AWE at Aldermaston
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Where (2)
• Nuclear submarine dockyards
• Uranium production facilities – a surprising number in the
early days
• Enrichment plants
• Fuel manufacture sites
• Fuel reprocessing sites
• Waste disposal sites now deemed inadequate
• Minor university and commercially operated research
facilities
• Radiopharmaceutical manufacturing plants at Amersham
and Cardiff
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Waste in the UK
•“By 2030 Britain will have
generated approximately 1.4 million
cubic metres of LLW, 260 thousand
cubic metres of ILW and 3 thousand
cubic metres of HLW”
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Clean
•Only by history. NOT by
measurement • No reasonable possibility of contamination or activation
• Difficult on old sites
• All measurement does is to give you, at best, a Less Than value
• Measuring for longer or using better equipment can reduce this
• But not eliminate it.
• Think on Limit of Detection or, I think more usefully, Maximum
Missable Activity
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Out of Scope • Can’t be clean by history but levels are trivial, and
possibly undetectable – e.g. the desks in an
administrative building – or detectable (just) but very
low
• Most of the mass – building concrete, building cladding,
generators, soil, “clean side” plant, service piping etc.
• Uses values derived (currently) from EU RP122 but
may move to IAEA RS-G-1.7
• We have a very useful guidance document
• http://www.nuclearinst.com/write/MediaUploads/SDF%2
0documents/CEWG/Clearance_and_Exemption_GPG_
2.01.pdf
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Higher categories
• Surface contaminated but can be cleaned
• Metal, in particular, with surface contamination
• Process pipework, ventilation ducting
• Strip, clean by grit blasting mainly
• Residue (pipe scale, surface paint, rust and used grit) goes as LLW
• The product goes as Out of Scope
• Studsvik in Workington are the main contractors
• Ship to Studsvik in Sweden
• Magnox heat exchangers
• Significant surface activity – activation products mainly
• https://www.youtube.com/watch?v=oysSDAZpEg8
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Further categories • VLLW
• Most of the rest of the mass
• Definitely radioactive – concrete from pond walls, soil contaminated
by pipe leaks, low level activated concrete from close to reactors
• Nuclide specific but limit is typically 200 Bq/g
• NORM scale from oil production - http://www.nsnorm.com/services
• Goes to specified hazardous waste disposal sites
http://www.augeanplc.com/FileDepository/locations/enrmf/Permit%
20-
%20receipt%20and%20disposal%20of%20radioactive%20waste.p
df-
• Most protests were about the number of trucks, not the radioactivity
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LLW • Building rubble, active pipework
• Sludge from cooling circuits and ponds
• Glove boxes and soft waste
• Main site is the Low Level Waste Repository (LLWR) at
Drigg in Cumbria
• Nuclide specific limits for LLWR but overall 12 GBq/tonne
beta and 4 GBq/tonne alpha
• Been in use since the 1950s. Well managed to begin with
• Then a (very) casual period – see photo
• Now properly managed again
• Very keen on grouted half-height ISOs – no voids,
basically a solid block
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Map and pictures
• The old way – tumble tipping
• And now
• But Very Close to the sea
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ILW
• Reactor components
• Debris from fuel reprocessing
• Steel components cropped off fuel elements
• Redundant sources
• Plutonium contaminated material from fuel and weapons
manufacture
• No disposal route as yet – sites are building their own
storage. Winfrith shown above
• And not likely to be for a long time
• Probably will end up at Sellafield (somewhere). Most is
already there
• Local council volunteered to host a site but Cumbria
Council blocked it
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Treatment • Compacted and grouted in 500 litre stainless steel drums
• Or packed into Mosaik boxes – hugely expensive
• http://www.gns.de/language=en/23371/gns-yellow-box
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High level waste
• The really scary stuff – Sv/h at metres from the cannisters
• Mainly from fuel reprocessing at Sellafield and Dounreay
• Requires cooling as it generates kW/litre
• Was mainly in liquid form – not a good idea!
• Now converted into glass blocks and stored inside
stainless steel containers in a tube store
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UK progress -reactors
• All the Magnox reactors shut down and very nearly
completely defueled.
• All materials test reactors closed and defueled
• All research and prototype reactors closed. Many now
completely removed
• Magnox turbine halls demolished – mostly Out of Scope
• Fuelling machines removed
• Some of the heat exchangers removed –contaminated but
most not activated. Gas reactor ones are much less
contaminated than PWR heat exchangers
• Some recycled at Studsvik
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Magnox reactor
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Magnox
• Reactor pressure vessel is huge compared to PWR and
BWR designs. Typically 7m in diameter
• Reactor buildings will remain but reduced in height
• Fuel element debris vaults being emptied – high end LLW
and ILW – Co-60 dominated
• Other debris vaults being cleared – control rods
• Steel rods with boron ends
• Out of scope to ILW on one rod!
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Berkeley then and now
• Two 138 MW(E) Magnox
• Quite low power
• 1962 to 1989 – unusually short life
• On site ILW store operating
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Harwell (where I work)
• Started in 1946
• Peaked at 5000 workers in the 1960s
• 14 reactors in total
• Two significant air cooled “atomic piles”, GLEEP and
BEPO – very low power. Graphite with natural uranium in
aluminium cans. GLEEP ran from 1947 to 1990
• De-fuel, remove the concrete bioshield
• Break up the moderator
• Incinerate to remove the tritium
• Store
• Plus 2 materials test reactors
• HEU and higher power
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Then and now
• Only the
most active
buildings
remain –
reactor
vessels,
stored
waste,
plutonium
chemistry
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Chemical plant
• B351 – uranium contam
• Lots of asbestos
• Brick – recycled after crushing
• Steel structure recycled
• Pipework etc. as low level waste
• All the steel flooring plates
recycled after monitoring
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Harwell Liquid Effluent Treatment Plant
• Everything that went into any building drain
• Cs-137, Sr-90, uranium, plutonium, Co-60, C-14 etc.
• We had to cut the trees down – we tried hard not to
• Soil, brick, pipework and concrete contamination
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Liquid effluent treatment plant
• “Main active drain” – excavated as low level waste,
significant number of MBq particles trapped inside
• Trade waste drain – less active levels
• Domestic waste
• LETP provided treatment and filtration
• Big range of activity levels
• Major project success – for one area, an estimated 22
Half Height ISO containers reduced to 2
• Good monitoring, careful radiochemical analysis
• Use of Exemption Orders
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Springfields Gaseous Diffusion Plant
• Enrichment by diffusion of UF6
• Designed to concentrate the lighter fissile U-235 (0.7 % of
mass) from the heavier non-fissile U-238 (99.3 % of
mass)
• High value metals wet decontaminated
• Then smelted to reduce volume
• Ingots assayed for radioactivity
• Most released as out of Scope
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Delicensing
• Major facilities operate under a nuclear site license
• Nuclear Installations Act 1965 and subsequent
modifications
• Delicensing on the basis of “no danger”
• On the basis of existing, published guidance6, HSE
considers that an additional risk of death to an individual
of one in a million per year, is ‘broadly acceptable’ to
society.
• http://www.onr.org.uk/delicensing.pdf
• Interpreted as less than 10 µSv/annum to a future
occupier
• ALARP still applies. If you can reduce doses even further
without huge expenditure, then do so.
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Approach
• Post operational clean-out (POCO)
• Assess what is left
• Strip buildings and dispose of the waste appropriately
• Demolish and dispose of unwanted buildings
• Monitor and remediate drains
• Remove any contaminated soil generated by spills
• Produce a detailed site gamma map
• Pull together all the evidence that the site has been
cleaned up well enough
• Activity limits based on IAEA RS-G-1.7 sum of quotients
• Submit to the Office for Nuclear Regulation
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Fingerprints • Waste characterisation often refers to a
fingerprint, rather than quantifying individual nuclides
• Any detection mechanism is radiation type and energy dependent
• Only gammas are easily detectable on a metre by metre basis
• Many assessments rely on one or two gamma emitting easily detectable nuclides
• Which are not always the ones that dominate radiologically
• Cs-137 for fission (662 keV at 85 %), Co-60 for activation (1.17 and 1.31 MeV at 100 % each, Bi-214 for radium (609 keV)
• So getting the fingerprint right is vital
Gamma map
The limiting activity – nuclide specific
• Express each nuclide as a fraction of the total activity (e.g. 15% Co-60, 85 % low toxicity at 100 Bq/g)
• Divide each fraction by the limiting activity (e.g. 15% Co-60/0.1 + 85% others/100)
• Sum the results (in the example above this will be 1.509)
• Invert that to get the limiting fingerprint activity in Bq/g
• Divide the fraction of the nuclide to be measured by this sum to get a limiting Bq g-1 value for that nuclide
• Taking Co-60 as the most likely nuclide to be measured this is 15%/1.509 = 0.099 Bq g-1
• Sounds complicated but isn’t
• Easy with a spreadsheet
Example - Fuel cooling pond concrete
• Total activity limit =
0.16 Bq g-1
• Two useful gamma
emitters
• Easy gamma
monitoring target –
lots of Cs-137 and
Co-60
Nuclide Major
emission
Fraction
(%)
RS-G-
1.7
limit
(Bq/g)
Cs-137
Gamma +
medium E
Beta
46 0.1
Co-60 Gamma +
low E Beta 17 0.1
H-3 Very low E
beta 23 100
Fe-55 Very low E
X-ray 4 100
Ni-63 Low E beta 7 100
C-14 Low E beta 2 1
Sr-90
(+Y-90) High E beta 1 1
And then? • They appoint a contractor to review the submission,
particularly the monitoring aspects.
• PHE is currently the main one.
• They do not duplicate measurements.
• They think about what was measured and where.
• What could be missed?
• Example: sloping flask loading area.
• It had been monitored in strips from the high end to the
low end.
• PHE concentrated on walking along the low end.
• Contaminated paint flakes in the gutter!
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The difficult bits 1
• Fingerprints with no useful detectable gammas and bulk
contamination
• Options include gross beta measurement if there is a high
energy beta
• Sampling followed by radiochemistry – slow, expensive
and very, very easy to miss localised spills
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Contamination by the naturals • Uranium and thorium are everywhere, in the mg/kg
region.
• Equivalent to several times 0.01 Bq/g, the Out of Scope
level
• The way out is to take lots of samples around the site
• Then look at on site samples
• If they fall within the same distribution, claim for zero
contamination
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Essential reading
• The DQO process – how to put together a credible
monitoring plan
• https://www.epa.gov/sites/production/files/documents/guid
ance_systematic_planning_dqo_process.pdf
• -----------------------------------------------------------------------------
• An excellent guide to the statistics of detection
• Lloyd A. Currie, Limits for Qualitative Detection and
Quantitative Determination: Application to Radiochemistry,
Anal. Chem. 40, 586-593 (1968).
• Unfortunately, not freely available on the web.
• Worth Paying For
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Very useful
• But very hard work!
• Marssim
• https://www.epa.gov/radiation/multi-agency-radiation-
survey-and-site-investigation-manual-marssim
• Needs lots of translation from ancient units into SI
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Success!
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