The Generation IV Gas Cooled Fast Reactor

Transcription

1 The Generation IV Gas Cooled Fast Reactor Dr Richard Stainsby AMEC Booths Park, Chelford Road, Knutsford, Cheshire, UK, WA16 8QZ Phone: +44 (0) , Fax +44 (0)

2 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 2

3 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) 3

4 The Gen IV GFR system (as per the Gen IV Roadmap) Now 2400 MWth 4

5 He Current concept - Gas~Steam turbine combined cycle He main heat exchanger He-N 2 turbine steam turbine He He-N2 H2O hp reactor primary circulator He-N 2 compressor Heat recovery steam generator feed pump condenser The indirect combined cycle is proposed for the ANTARES HTR and is the reference cycle for the GenIV GFR system 5

6 Cut-away view of a proposed 2400 MWth indirect-cycle GFR main heat exchanger (indirect cycle) core barrel Decay heat removal heat exchanger steel reactor pressure vessel re-fuelling equipment control and shutdown rod drives core 6

7 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). 7

8 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 8

9 Potential GenIV GFR fuel forms Ceramic pin e Ceramic plate 9

10 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. 10

11 Schematic diagram of the DHR system in natural convection mode Exchanger #2 pool H2 Exchanger #1 Secondary loop H1 dedicated DHR loops core guard containment 11

12 Primary circuit components configured in DHR mode DHR circulator (or natural circulation). open DHR check valve DHR heat exchanger reactor 1200 C main heat exchanger D P core closed check valve main circulator 12

13 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. 13

14 2400 MWth indirect-cycle GFR inside a spherical guard vessel 14

15 Reactor building 15

16 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) 16

17 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. 17

18 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? 18

19 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. 19

20 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. 20