The Generation IV Gas Cooled Fast Reactor

Dr Richard Stainsby

AMEC

Booths Park, Chelford Road, Knutsford,

Cheshire, UK, WA16 8QZ

Phone: +44 (0)1565 684903, Fax +44 (0)1565 684876

e-mail: Richard.stainsby@amec.com

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Contents

1. Why have fast gas cooled fast reactors ?

2. The GFR system

3. Performance requirements for the Gen IV GFR system

4. Specific challenges

5. Decay heat removal

6. GFR Safety systems

7. Risk minimisation

8. Conclusions

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Why have gas cooled fast reactors ?

Fast reactors are important for the sustainability of nuclear power:

More efficient use of fuel

Reduced volumes and radiotoxicity of high level waste

Sodium cooled fast reactors are the shortest route to FR deployment, but:

The sodium coolant has some undesirable features:

– Chemical incompatibility with air and water

– The strongly positive void coefficient of reactivity

– Avoiding sodium boiling places a restriction on achievable core outlet temperature.

Gas cooled fast reactors do not suffer from any of the above:

Chemically inert, void coefficient is small (but still positive), single phase coolant eliminates

boiling.

But …

Gaseous coolants have little thermal inertia – rapid heat-up of the core following loss of

forced cooling;

– Compounded by the lack of thermal inertia of the core structure + very high power

density

Motivation is two-fold: enhanced safety and improved performance (c.f. SFR)

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The Gen IV GFR system (as per the Gen IV Roadmap)

Now 2400 MWth

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Current concept - Gas~Steam turbine combined cycle

Indirect steam cycleIndirect SC CO2

cycle

Indirect

combined cycle

Direct He Brayton

cycle

He

H2O hp

He-N2

He

He CO2s

H2O He

Indirect steam cycleIndirect SC CO2

cycle

Indirect

combined cycle

Direct He Brayton

cycleIndirect steam cycle

Indirect SC CO2

cycle

Indirect

combined cycle

Direct He Brayton

cycle

He

H2O hp

He-N2

He

H2O hp

He-N2

He

He CO2s

H2O He

reactor

primary

circulator

main heat

exchanger

He-N2 turbine

Heat recovery steam

generator

He-N2

compressor

feed pump

steam turbine

condenser

The indirect combined cycle is proposed for the ANTARES HTR and is the reference cycle for

the GenIV GFR system

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Cut-away view of a proposed 2400 MWth

indirect-cycle GFR

re-fuelling

equipment

core control and shutdown

rod drives

steel reactor pressure

vessel

core barrel

main heat exchanger

(indirect cycle)

Decay heat

removal heat

exchanger

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GFR Performance requirements

Self-generation of plutonium in the core to ensure uranium resource

saving.

Optional fertile blankets to reduce the proliferation risk.

Limited mass of plutonium in the core to facilitate the industrial

deployment of a fleet of GFRs.

Ability to transmute long-lived nuclear waste resulting from spent fuel

recycling, without lowering the overall performance of the system.

Favourable economics owing to a high thermal efficiency.

The proposed safety architecture fits with the objectives considering

the following elements:

Control of reactivity/heat generation by limiting the reactivity swing over the

operating cycle; the coolant void reactivity effect is minor.

Capacity of the system to cool the core in all postulated situations,

provision of different systems (redundancy and diversification).

A “refractory” fuel element capable of withstanding very high temperatures

(robustness of the first barrier and confinement of radioactive materials).

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Specific Challenges (1): Fuel

The greatest challenge facing the GFR is the development of robust

high temperature refractory fuels and core structural materials,

Must be capable of withstanding the in-core thermal, mechanical and

radiation environment.

Safety (and economic) considerations demand a low core pressure drop,

which favours high coolant volume fractions.

Minimising the plutonium inventory leads to a demand for high fissile

material volume fractions.

Candidate compositions for the fissile compound include carbides,

nitrides, as well as oxides.

Favoured cladding materials include:

refractory metals and SiC for pin formats

refractory metals and ceramic matrices (e.g. SiC, ZrC, TiN) for dispersion

fuels in a plate format

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Potential GenIV GFR fuel forms

Ceramic plate

Ceramic pin

e

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Specific challenges (2): Decay heat removal (DHR)

HTR “conduction cool-down” will not work in a GFR

High power density, low thermal inertia, poor conduction path and small

surface area of the core conspire to prevent conduction cooling.

A convective flow is required through the core at all times;

A natural convection flow is preferred following shutdown

–This is possible when the circuit is pressurised

A forced flow is required immediately after during when depressurised:

–Gas density is too low to achieve enough natural convection

–Power requirements for the blower are very large at low pressure

The primary circuit must be reconfigured to allow DHR

Main loop(s) must be isolated

DHR loop(s) must be connected across the core

Conclusion: the reliability of the DHR function is dependent on the

reliability of the primary circuit valves.

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Schematic diagram of the DHR system in natural

convection mode

Exchanger #2

pool

Exchanger #1

core

Secondary loop

dedicated DHR loops

H1

H2

guard

containment

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Primary circuit components configured in DHR mode

.

D P core

main

circulator

closed

check valve

open

DHR

check

valve

DHR heat

exchanger

DHR circulator

(or natural circulation)

D P core

reactor

main heat

exchanger

1200 °C

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Depressurised DHR

For depressurised conditions, it would always be possible to generate

enough flow through the core using a large enough fan.

If the primary circuit has depressurised to atmospheric pressure, the

power consumption is very large and the duration could be very long

Proposed solution is to surround all the primary circuit components by

another pressure vessel (known as the guard containment).

Pressure within the guard containment would be controlled such that;

After the LOCA, a minimum pressure of 10 bar remains within the primary

circuit.

This back pressure allows the power of the DHR fan to be low enough to

be supplied by batteries for the first 24 hours, afterwards, the decay heat

power is low enough for natural convection to cope.

The back pressure is a compromise between the performance and

power requirements of the DHR fan and the structural complexity of

the guard vessel.

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2400 MWth indirect-cycle GFR inside a spherical

“guard vessel”

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Reactor building

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GFR Safety systems – Shutdown

Requirement for rapid and reliable shutdown systems

The system cannot tolerate an unprotected loss of flow – even with ceramic

clad fuel.

Two levels of shutdown are incorporated in the current concept, both based

on absorber rods (CSD + DSD)

A rapid, diverse, and preferably passive, third level shutdown system is

being considered required (e.g. injection of a liquid or particulate absorber

into dedicated core elements)

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GFR Safety systems – Decay Heat Removal

A convective flow to cool the core is required at all times.

Natural convection is adequate in pressurised conditions with reasonable

height differences between the core and DHXs

Natural convection will work even if the main loops cannot be isolated –

–Driven by density (temperature) differences, not pressure differences.

Electrically driven blowers are provided for defence-in-depth in pressurised

conditions and to provide an adequate flow in depressurised conditions.

Additional and diverse electrically driven loops are provided for

depressurised DHR.

A guard containment is provided to limit depressurisation to reduce the

power requirements of the DHR blowers –

–The aim is to power the DHR blowers by batteries for the first 24 hours

and then use natural convection thereafter.

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Safety issues associated with the DHR systems

A DHR valve fails open in normal operation:

Provides a large core bypass flow – requires a check valve to prevent reverse flow in the DHR loops

A DHR valve fails to open on demand

Reduced decay heat removal capacity – easily compensated for by redundancy in other loops

Main loop isolation valve fails open during DHR

Natural convection mode – likely no consequence.

Forced flow mode – provides large core bypass – check valve required ?

DHR blower fails during DHR in forced flow mode

Provides core bypass – requires a check valve in the DHR loop.

Reliability of valves (actuated valves & check valves) in a hot helium environment.

Likelihood and consequences of guard vessel failure.

Provision of a DHR system that will be effective at atmospheric pressure

Provision of a core catcher ?

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Risk Minimisation

Improvement of passive reactivity control and introduction of a tertiary shutdown system

Exploitation of control rod driveline expansion –

–current concepts have control rods pushed out of the core from below – simple driveline expansion may give positive feedback on reactivity

–Methods of reducing driveline length on heat-up could be devised.

Passive introduction of absorber as a tertiary shutdown system

Reduction in number or elimination of valves in the DHR system

Problem is that the DHR systems (and main loops) are connected in parallel, so adding more loops will degrade the reliability if failed loops cannot be isolated.

The natural convection mode is more reliable in this respect.

Introduction of fluidic valves (with no moving parts) is a possibility.

Heavy gas injection as a decay heat removal mechanism

Provides motive force through momentum and negative bouyancy

Replacement of helium following a LOCA will allow natural convection cooling to be established more quickly.

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Conclusions

Motivation behind GFR is to develop a fast reactor system that is free

of the worst problems associated with sodium:

Coolant void coefficient

Chemical reactivity

Opacity

The trade-off is a system that has negligible thermal inertia, so

shutdown and decay heat removal are the main safety issues

GFR has traded the a reduced likelihood of a whole core disruptive

accident for an increased likelihood of core melt.

The safety systems must therefore be able to guarantee shutdown and to

provide a positive flow of coolant through the core.

Tertiary shutdown and reliable depressurised DHR are considered to

be the main safety challenges.