GAS-COOLED FAST REACTOR

Lehrstuhl für Nukleartechnik - Technische Universität München

Boltzmannstr. 15 85747 Garching

www.ntech.mw.tum.de

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1 GAS-COOLED FAST REACTOR

1.1 CONCEPT DEVELOPMENT

The Generation IV Roadmap selected the Gas-cooled Fast Reactor (GFR) concept as

one of the six technologies for further development under Generation IV. The System

Arrangement for the international research and development of the Gas-cooled Fast

Reactor nuclear energy system was signed in November 2006 by Euratom, France,

Japan and Switzerland.

Two projects have been identified departing from two basic needs: the development

of an innovative fuel and its associated fuel cycle technologies and the design and

safety analysis of the GFR system:

the fast neutron Fuel, other Core Materials, and specific Fuel Cycle process

(FCMFC)project

the Design and Safety Management (D&SM) project

The GFR really occupies a particular position as one of the four GEN IV fast neutron

systems together with Sodium-cooled Fast Reactor (SFR), Lead cooled Fast Reactor

(LFR) and Supercritical Water Cooled Reactor (SCWR) in its fast spectrum version,

as well as one of the two Gas Cooled system with the Very High Temperature

Reactor (VHTR).

During the past ten years, the start-up of two experimental reactors (HTTR and HTR-

10), and the launching of several industrial projects (PBMR, GT-MHR), have testified

of the significant interest renewal for the High Temperature Reactors (HTR). This

renewed interest for the HTR technology is not limited to thermal reactors and this

explains the GEN IV GFR selection. With the GFR, one seeks to combine the

advantages of the HTR:

refractory core with particle fuel having high confinement properties

inert He coolant allowing high temperatures

high thermodynamic efficiency

possibility of energy conversion by a direct cycle with a gas turbine

to those of the fast neutrons (GEN IV fuel cycle).

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The GFR study objectives are to propose:

an attractive possible alternative to liquid metal cooling for fast neutron

reactors

a “sustainable version” of the VHTR with the integral recycling of actinides

and the search for a self-sustainable fuel cycle

There are several reasons for this :

the potential benefits of medium size modular and compact power stations,

which could enhance nuclear economic competitiveness, and could allow a

fully passive safety approach for the management of the most serious

accidents.

the progress made on the gas turbines (recent developments of combined

cycles), opening prospects of high thermodynamic efficiencies by

considering a direct Brayton cycle (about 47% with 850°C turbine inlet

temperature).

the attractive safety characteristics of the HTRs due to the particle fuel

(excellent fission products confinement), the refractory character of the core

materials, and the huge thermal inertia brought by the large graphite

inventory used as moderator, reflector and structural material.

a renewed interest for high temperatures (> 850°C) for electricity generation

with high efficiency, but also for cogeneration purposes like thermochemical

cycles for an hydrogen massive production.

In the framework of the FP6 of the European Union the GCFR project started in 2005

for 48 month until the February 2009. This project involved several European

institutions that are listed below:

National Nuclear Corporation Limited Reactor Services, Nnc Ltd - United

Kingdom

Commission Of The European Communities - Directorate General Joint

Research Centre -Belgium

Empresarios Agrupados Internacional, S.A. - Spain

Framatome Anp Sas - France

Technische Universiteit Delft - Netherlands

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British Nuclear Fuels Plc - United Kingdom

Consorzio Interuniversitario Per La Ricerca Tecnologica Nucleare - Italy

Paul Scherrer Institut – Switzerland

Nuclear Research And Consultancy Group - Netherlands

Commissariat L'energie Atomique - France

The expected results of this project are:

establish the preliminary viability by 2007

confirmation of the viability by 2012

to complete a conceptual design by 2019 followed by the construction

an experimental GCFR of limited power, called Experimental Technology

Demonstration Reactor (ETDR), is foreseen in this period (2015) to qualify key

technologies as a precursor to a prototype reactor

The intention is to exploit the favourable characteristics, attributable in part to the

gas coolant, to have a selfgenerating core (converting the natural uranium fuel in the

core to fissile material), whist avoiding a breeder blanket around the core, which

meets the sustainability requirements and brings advantages for proliferation

resistance and the economics.

The GFR development schedule covers three main stages:

the first stage involving overcoming the main technological barriers, namely

development of an innovative, refractory fuel, and choice of safety options.

the second stage is the construction of the Experimental Technology

Demonstration Reactor (ETDR), a low-power reactor that will provide the

capability to establish the viability basis for core design, fuel technology,

neutronic control, and instrumentation,

the third stage, the construction of a medium-power, electricity generating

prototype, comparable to that contemplated for the SFR reactor line

The decision whether go ahead with construction of ETDR is to be taken in 2012, at

the outcome of feasibility and definition studies, with commissioning scheduled for

2020.

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1.2 TECHNICAL ASPECTS

In Table 1.1, the GFR design objectives are summarized.

Table 1.1: GFR design objectives

Several preliminary designs of the reactor have already been proposed in France, US

and Japan.

In Figure 1.1 the layout of a small size (300 MWe) reactor is shown. The pressure at

the turbine inlet is 7 MPa thus leading to a net efficiency of about 47%. The

proximate/guard secondary containment is designed to sustain a maximum pressure

of 2 MPa. Each auxiliary cooling loop is designed to extract a maximum power of 18

MW with a primary He natural circulation under 2 MPa. This pressure explains the

large thickness of the proximate containment.

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Figure 1.1: Layout of reactor, building and heat removal system

The reference system is high-temperature Helium cooled direct Brayton cycle reactor

(5-7 MPa and 850ºC outlet temperature). This system presents challenges. At least

as a step towards the high-temperature direct cycle, the indirect cycle with Helium

as primary coolant at lower temperatures and supercritical (SC) CO2 cycle as

secondary coolant has to be seen as a second possibility fulfilling the priority goal of

sustainability with more margins for materials and safety. This would nevertheless

allow to achieve the efficiency aimed at and to preserve synergies with the VHTR

(Helium as primary coolant).

The preliminary dimensioning of the ETDR core (Figure 1.2) will be grounded on

representativeness criteria, with respect to irradiation conditions, as compared to

those for the reference provided by the commercial 2,400 MWth GFR. The aim is to

achieve, at the same time, identical cladding and fuel temperatures, a fast flux and

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dose/burnup ratio close to those to be exhibited in the GFR, along with a core

characterized by power of 50 MW, and average power density of 100150 MW/m3.

Figure 1.2: 3D model of the ETDR reactor core and building

The vessel, comprising a cylindrical shell, and hemispherical bottom, is 3 m in

diameter, with a height of 8 m, for a thickness of 10 cm (the components are 9Cr1Mo

steel forgings). Total mass (vessel head included) stands at around 120 t. Helium

enters the vessel through a side pipe connector, descending to the bottom via an

annular space, ascends through the core to reach the hot plenum above the core,

and exits via the side connector. The primary loop layout, outside the vessel, is of the

cross-duct type, featuring two concentric ducts: the piping holding the “hot” gas

being inserted into the duct channeling the “cold” gas.

Owing to its low power, and in order to simplify the system, and minimize capital

cost, ETDR features no energy conversion system, and is not intended to produce

electricity.

It will be cooled by pressurized helium, by means of an isobaric circuit fitted with

blowers. The secondary loop is a pressurized-water circuit, the ultimate heat sink

being the atmosphere.

The fuel used in this so-called “demonstration” configuration will be the innovative

fuel selected as reference for the GFR. This comes in the form of flat plates, holding

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a (U,Pu)C carbide fuel. ETDR is primarily designed to operate with the GFR

advanced fuel, however it is due to be built before the GFR fuel can be fully qualified.

Consequently, a so called “startup” core is being contemplated, of proven design, in

which irradiation, of an experimental character, of the GFR advanced fuel will be

carried out in test assemblies. For reference purposes, the startup core will comprise

(U,Pu)O2 oxide fuel pins, featuring metallic cladding (SFR technology). These entail a

definitely lower core outlet helium temperature, compared with the GFR (560 °C, as

against 850 °C).

In Table 1.2 it is possible to compare the specifications of the 2400 MWth concept

versus the demonstration and startup configuration of ETDR reactor.

Table 1.2: Comparison of the main characteristics for a 2,400-MWth GFR core, and

for the two cores planned for ETDR

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1.3 TECHNICAL PROBLEMS

The GFR will use the HTR technologies and will benefit from progress made for the

VHTR technology, the R&D issues that are basilar for the development of a GFR

reactor are:

HTR reactors synergy

o Particle fuel

o High temperature materials

o Helium technology

o Calculation tools

o Fuel cycle

VHTR reactors synergy

o very high temperature materials

o Intermediate IHX

o Advanced particle fuel

o I-S cycle for H2 production

Fuel for fast spectrum

Integral actinide recycling

Cycle processes

Safety systems

It will rely on experimental means, shared at the international level and structured in

the GFR GEN IV R&D plan. The objective is to build a GFR prototype (approximately

600 MW thermal, producing electricity), making it possible to acquire operation

experience. This prototype will provide the elements for the deployment of the GFR

in the longer term.

An essential intermediate stage towards the development of this prototype is the

realization of the Experimental and Technology Demonstrator Reactor (ETDR) that

will be the first GFR ever built. The fast-neutron variant of the gas-cooled reactor line

stands as a radical breakthrough, compared to thermal-neutron reactors involving

the same coolant, particularly with respect to the fuel, and safety systems.

The ETDR mainly aims at meeting the needs for qualification on:

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the behavior of the various sub-assembly types (fuel, absorber, reflector)

under representative conditions of irradiation and flow

the core operation (monitoring systems, regulation and protection)

the safety options for the GFR

the handling of fuel

The reference fuel matrix for the Generation IV GFR is a “cercer” dispersion fuel,

based on a balance between conductivity and high temperature capability. Current

fuel designs are based on dispersion fuels (either as fibers or as particles) in an inert

plate/block type matrix, or solid solution fuel clad in a refractory ceramic (e.g.,

SiC/SiC composites). The reference fuels chosen for the GFR are UN and UC for

their

high heavy metal density

high conductivity

minimal impact on neutron spectrum (although limited irradiation data exists).

The matrix materials are dependent on the coolant and operating temperatures, and

they can be classified into three categories:

ceramic (for high temperatures)

refractory metal (for modest to high temperatures)

metal (for modest temperatures).

As the fuels are of ceramic composition, the resulting fuel forms can be classified

into two categories: cercer and cermet. The fuel would be extruded into the matrix,

where the matrix would have a “honeycomb” appearance. The particles may be

coated, but unlike the thermal spectrum gas reactor fuel, they will most likely have

one coating to maximize the heavy metal content within the matrix.

In Figure 1.3 is a graphical representation of the dispersion fuel types being

considered.

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Figure 1.3: Dispersion fuel concepts

The sub-assembly designs associated to the three possible fuel forms are illustrated

in Figure 1.4:

Plate-type sub-assembly for the dispersed fuel which is preferred because it

allows low in-core pressure drops and could be designed to fulfill

thermomechanical criteria (block-type sub-assembly has also been

considered but could not fulfill these latter criteria).

Pin-type sub-assembly for pelletized fuel which is a well known concept and

allows acceptable core pressure drops.

The 10% core volume fraction provision assumed for in-core structural

materials would need quite likely to be slightly increased for the pin-type sub-

assembly.

In the case of the particle fuel, those 10% are already used by the particle

coatings and one need additional space for the sub-assembly. It could be a

particle bed type subassembly. In that situation, even with a double size

packaging (to reduce the void), it is very difficult to reach a significant heavy

atom content in the core. Those concepts must be associated with relatively

high levels of core pressure drops and low core power density. Particle fuel

could be also imbedded into a matrix to form block or plate-type sub-

assembly but this solution tends to further reduce the in-core heavy atom

content.

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At the CEA reference option is the dispersed fuel in a platetype sub-assembly; the

carbide (nitride still remains a possible option) is the actinide compound and SiC is

used for both the matrix and structural material. The alternative is the pin-type sub-

assembly with the same choice of materials (the use SiC cladding). In Figure 1.4 the

different fuel assembly options are shown.

Figure 1.4: Fuel assembly options

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1.4 ECONOMIC ASPECTS

The motivation to look at the GFR technology should be the same presented for the

VHTR technology. Essentially industry should look at a VHTR/GFR like a future

competitor of the Gas-turbine. For sure the capital cost of the investment will be

higher but the limited incidence of the cost of the nuclear fuel on the energy

production process will influence the choice in the direction of the nuclear reactor.

In comparison to the VHTR the GFR will be a fast reactor so that the reactor will also

be able to produce the fuel that it consumes. Furthermore, the GFR will be a reactor

with a breeding ratio very close to one. Thus, this reactor is not able to produce

higher quantity of fissile/fissionable material that it consumes. In order to do this the

choice should be to consider other reactors like, for example, the Sodium cooled fast

reactor or the Lead cooled Fast reactor.

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1.5 ENVIRONMENTAL AND SOCIAL ASPECTS

Fast neutron reactors are well known for their capability to fully use the uranium

natural resources, to transmute minor actinides and therefore to minimize waste

production. Since He is almost neutronically inert, the GFR neutron spectrum is

harder than the one of SFRs when the same in-core materials are used, and this

could enhance the GFR capabilities. To get full benefit of those features, the reactor

has to be associated to a coherent fuel cycle (Figure 1.5) like the GEN IV fuel cycle is.

Figure 1.5: The Gen IV closed fuel cycle

The GFR reactor is clearly designed to be compatible with the GEN IV fuel cycle.

This reactor is a “sustainable version” of the VHTR with the integral recycling of

actinides and the search for a self-sustainable fuel cycle.

Gas cooled fast reactors offer a number of advantages over liquid metal cooled core

designs. There are significant safety, economic and technical benefits to be gained

from using a benign, readily available gaseous coolant which is compatible with both

air and water, compared to sodium which reacts vigorously with water and requires

special handling and disposal. The negligible coolant void coefficient in gas cooled

cores, compared to sodium cooled systems, allows the loading of a far greater

quantity of degraded plutonium and minor actinide fuels. An additional feature of a

gaseous coolant, in terms of minor actinide incineration, is the hard neutron

spectrum that exists in gas cooled cores.

Using GCFR could be possible to change from high rates of plutonium consumption,

to moderate plutonium breeding, or to sustain existing plutonium stocks, with only

minor modifications to the basic reactor design. The recycling of minor actinides,

either homogeneously or heterogeneously can be feasibly undertaken to achieve

equilibrium minor actinide consumption.

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It means that the gas cooled fast reactor satisfies, theoretically, a wide range of fuel

cycle scenarios that may occur in the near and longer term future.

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1.6 BIBLIOGRAPHY

2007 Annual Report, GEN IV International Forum

Gas cooled Fast Reactors: Concepts and Challenges Jacques Rouault CEA, France,

The 2004 Frédéric JOLIOT & Otto HAHN Summer School AUGUST 25 –

SEPTEMBER 3, 2004 CADARACHE, France

GCFR - Gas Cooled Fast Reactor - Project Presentation (PP) - http://www.gcfr.org/

ETDR, a precursor for GFRS - Experimental instruments for concept validation-

CLEFS CEA - No.55 - SUMMER 2007

Gas Cooled Fast Reactor GCFR - Martin McDermott,Colin Mitchell,National Nuclear

Corporation Ltd, FISA 13-15 March 2006 Luxembourg

Gas-Cooled Fast Reactors – Evaluation – US. AEC, June 1972

INL FY2005 Report, Appendix 3.0 Gas-Cooled Fast Reactor, Idaho National

Laboratory, USA, 2005

INL Gas-Cooled Fast Reactor (GFR) FY04 Annual Report Submitted September 30,

2004, Idaho National Laboratory, USA,2005

Flexibility of the Gas Cooled Fast Reactor to Meet the Requirements of the 21st

Century, T D Newton, P J Smith Serco Assurance (Sponsored by BNFL)

http://www.sercoassurance.com/answers/resource/pdfs/gcfr_flexibility.pdf