IAEA Education and Training Seminar/Workshop on Fast Reactor Science and Technology

Transcription

1 IAEA Education and Training Seminar/Workshop on Fast Reactor Science and Technology October 1 5, 2012 Centro Atómico Bariloche, Argentina The Gas-Cooled Fast Reactor: History, Core design and Main Systems Dr Richard Stainsby AMEC Booths Park, Chelford Road, Knutsford, Cheshire, UK, WA16 8QZ Phone: +44 (0) , Fax +44 (0) richard.stainsby@amec.com

2 Contents 1. Why have gas cooled fast reactors? 2. Gas cooled fast reactor concept: a historical perspective 3. The present day: Generation IV gas cooled fast reactors 4. Performance requirements for the Gen IV GFR system 5. R&D requirements for the Gen IV GFR system 6. Specific challenges: Core and fuel 7. Specific challenges: Decay heat removal 8. Power conversion system 9. Plant layout 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 Historically, liquid metal reactors have a strong 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, very stable nucleus, void coefficient is small (but still positive), single phase coolant eliminates boiling and optically transparent. 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 3

4 Gas cooled fast reactor concepts: a historical perspective US, General Atomics The GCFR programme Started in the 1960 s Capitalised upon High Temperature (thermal) Reactor (HTR) experience: Peach Bottom and Fort St Vrain Funded by US DOE Collaboration with European partners Helium cooled reactor with a multi-cavity pre-stressed concrete pressure vessel. Featured a vented fuel pin fuel element design to reduce fuel clad stresses. 4

5 General Atomics GCFR concept 5

6 Germany: the Gas Breeder Memorandum Germany: the Gas Breeder Memorandum (1969) The German research centres at Karlsruhe and Jülich, together with industrial partners, Defined three concepts, all cooled by helium, Fuel assemblies extrapolated from sodium cooled fast reactors, Pre-stressed concrete pressure vessels Steam cycle, Some work was carried out on coated particle fuels and direct cycle power cycles. 6

7 Europe: the Gas Breeder Reactor Association ( ) A number of organisations joined to form the Gas Breeder Reactor Association. The first design produced by the group was GBR-1, a 1000MWe helium cooled reactor with metallic clad pin type fuel and a secondary steam cycle. GBR-2, 1000MWe reactor using coated particle fuel, slightly elevated outlet temperature, helium coolant, GBR MWe reactor using coated particle fuel, CO 2 coolant GBR-4 design was developed to overcome the complexities of the particle bed fuel elements. metallic clad fuel pins held in spacer grids. the clad surface was ribbed to maximise the core outlet temperature whilst respecting clad temparture limit. 7

8 GBR-2 (left) and GBR-3 (right) particle bed fuel assemblies 8

9 GBR-4 reactor layout 9

10 UK: ETGBR/EGCR (1970s-1990s) Based on UK Advanced Gas cooled (thermal) Reactor architecture Metallic clad fuel Carbon dioxide coolant Pre-stressed concrete pressure vessel 10

11 Japan: Prismatic Block Fuel (1960s present day Japan investigated block fuel containing coated particles and packed bed (GBR-2 type) fuel elements. 11

12 The present day: Generation IV Gas Cooled Fast Reactors Generation IV: A renewal of interest in fast reactors for sustainability, waste minimisation and non-electricity applications. Six systems are proposed, three of which are fast reactors, sodium, lead and gas cooled fast reactors (and now the molten salt reactor is being developed to be a fast reactor) 12

13 The Gen IV GFR system Now 2400 MWth 13

14 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 14

15 GFR Performance requirements Self-generation of plutonium in the core to ensure uranium resource saving. No 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 and diverse (nonelectricity) uses of high-quality heat. 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). 15

16 R&D Requirements Definition of a GFR reference conceptual design and operating parameters meeting the following requirements: Self-breeding cores with optional need for fertile blankets. Capability for multi-recycling of plutonium and minor actinides. Selection of an adequate core power density to meet requirements of economics, reactor fleet deployment, and management of safety issues. Coupling between the reactor and process heat applications. Identification and study of alternative design features (lower temperatures, indirect cycle). Definition of an appropriate safety architecture for the reference GFR system and its alternatives, considering that: Implementation of defence-in-depth is a key to achieving a robust safety architecture. Probabilistic methods will complement the deterministic approach. Definition of the ALLEGRO conceptual design and its safety architecture, in coherence with that of the GFR. Development and validation of computational tools needed to analyze performance and operating transients (design basis accidents and beyond). 16

17 Specific Challenges: Core and Fuel The greatest challenge facing the GFR is the development of robust high temperature, high power density 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: oxide dispersion strengthened steel (ODS), vanadium and SiC for pin formats ceramic matrices (e.g. SiC, ZrC, TiN) for dispersion fuels in a plate format 17

18 Potential GFR fuel forms Ceramic pin e Ceramic plate 18

19 Ceramic fuel element consisting of an assembly of fuel plates 19

20 Two Types of GFR (as in 2007) ALLEGRO (1st core) GFR MWThermal 100 MW.m -3 Height : 2,06 m Diameter : 2,23 m 169 pins of (U,Pu)O 2 Operating Temperature : 880 C Cycle : 1705 EQFPdays 2400 MWThermal 100 MW.m -3 Height : 3,60 m Diameter : 6,60 m 397 plates of (U,Pu)C /SiC Operating Temperature : 1260 C Cycle : 2493 EQFPdays Slide courtesy of Gerald Rimpault (CEA) 20

21 Comparison of GT-MHR and GFR2400 cores GFR 2400 MWth 24m 3 GTMHR 600 MWth 92m 3 21

22 GFR Cores (as in 2007) ALLEGRO (50 MWth*) GFR 2400 MWth * 2007 value, now 75 MWth Slide courtesy of Gerald Rimpault (CEA) 22

23 Reactivity Feedback Effects BOL ALLEGRO EOL BOL GFR 2400 EOL At the beginning of Life: Dr HeALLEGRO 0,25 $ < Dr HeGFR $ Dr DopplerALLEGRO 1 $ < Dr DopplerGFR $ ALLEGRO: T1 = 880 o C, T2 = 180 o C GFR 2400: T1 = 1260 o C, T2 = 180 o C Slide courtesy of Gerald Rimpault (CEA) 23

24 Current Fuel Form The objectives of the plate fuel concept were to: Achieve good heat transfer to minimise fuel temperature Minimise the clad to pellet gap size by ensuring that the critical dimensional change of the pellet due to swelling is minimised (the pellet s longitudinal axis is its shortest dimension). To improve fission product retention by containing a pellet s share for this fission gas locally within its cell failure of a cell releases only a small fraction of the total fission gas within a plate. Honeycomb structure emulates the behaviour of coated particle fuel. Structure is SiC fibre-reinforced SiC. An internal refractory metal liner is required prevent diffusion of fission products through the SiC/SiCf structure or flow of fission products through micro-cracks. An external refractory metal liner may be required to prevent inward diffusion of reactor coolant (helium) from separating the internal metal liner from the SiC/SiCf structure. Bonding of all of the metal-lined SiC/SiCf components into a single leak-tight fuel plate is considered to be very difficult and thus carries a high degree of technological risk. The plate fuel concept has been relegated to be a reserve technology with metal-lined SiC/SiCf pins now being the preferred option. 24

25 GFR fuel design: Fuel pin and sub-assembly Gas plenum & internals Pellet/clad gap End plug Fuel pellets (UPuC) Inner liner Composite ceramic/metal subassembly Super-Alloy exo-skeleton CMC cladding (SiC-SiC f ) Outer liner SiC/SiCf segments Image courtesy of CEA Image courtesy of SRS 25

26 Specific challenges: 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 must be isolated DHR loop(s) must be connected across the core 26

27 Comparison of passive heat conduction paths and power densities for GT-MHR and GFR2400 cores GFR 2400 MWth 24m 3 GTMHR 600 MWth 92m 3 27

28 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 28

29 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 29

30 Power Conversion: Why is GFR different from other nuclear reactors? GFR is intended to operate at high-temperature so we need an engine that can exploit this high temperature heat source. A conventional Rankine (steam) cycle will not make best use of heat of such high quality. The architecture of GFR is based on using the gas returning from the power conversion system to cool the reactor pressure vessel (RPV). This places an upper limit on the amount of waste heat recovery (recuperation) we can employ. Like all gas-cooled reactors, GFR uses a (particularly) low density coolant that consumes a lot of power to circulate. The coolant circulation power can consume a significant fraction of the power output, It is important to minimise the core pressure drop and to minimise the primary flow rate (pumping power is proportional to Q 3 ). 30

31 Direct or Indirect Power Cycles? With GFR we have the option to use the primary circuit helium to drive a gas turbine (Brayton) cycle directly: Direct cycle No primary-secondary temperature drops No high-temperature heat exchanger Activation of turbomachine Oil-free bearings required Cobalt-free alloys required Gas turbine must work with pure helium 31

32 Direct or indirect power cycles (continued) Indirect cycle Primary-secondary temperature drops High-temperature (high-risk) heat exchanger No activation of turbomachine Gas turbine can be standard aerospace technology Simple (and standard) alloys required GT can work with compressor-friendly gas 32

33 Power conversion system options PC Direct Recuperated Helium GT PC IC Compressors IC Compressors G F R Recuperator G F R GC Turbine ~ Turbine ~ PC pre-cooler IC Intercooler FP Feed water pump GC Gas circulators GC GC G F R G F R Steam Turbine Indirect Pure Steam Cycle Gas Turbine FP FP Steam Turbine Recuperator Indirect Recuperated Helium GT Images courtesy of Chris Neeson, Rolls-Royce plc 33

34 Why use a combined cycle gas turbine system? Cost of turbines Steam turbines are cheap (agricultural engineering!) Gas turbines are expensive (aerospace engineering) Low-risk technology Track record of use in many fossil-fired CCGT power plants. Solution: We only need the GT to use the highest-temperature fraction of the heat the waste heat from the turbine exhaust is hot enough to generate high-quality steam. Cycle is biased so that the bulk of the power is generated by the steam turbine to minimise the size (and cost) of the gas turbine. 34

35 GFR Reference Combined Cycle 1.Direct cycle, Tin = 480 C: η ~ 47.5 % 2. Indirect cycle, Tin = 480 C: η ~ [ ] % 3. Direct cycle, Tin = 400 C: η ~ 44.8 % 4. Indirect combined cycle, Tin = 400 C: η ~ [ ]% 5. Indirect cycle, Tin = 400 C: η ~ [ ]% 35

36 Power conversion system (indirect combined gas~steam cycle) heat recovery steam generator (x 3) He-N 2 gas turbine (x 3) decay heat removal pool (x 3) decay heat removal loop (x 3) alternator steam turbine reactor main heat exchanger condenser 36

37 Supercritical CO2 - an interesting fall-back option If the materials specialists fail to develop a suitable cladding material for the fuel, a supercritical CO2 recompression cycle can deliver similar performance for a core outlet temperature of 680 o C η = 46% Water in Water out C, 75 bar PRECOOLER 30 C 68.1 C, 75 bar 366.3C, 75 bar 75 bar MAIN AUXILIARY LOW GRADE HIGH GRADE GENERATOR COMPRESSOR COMPRESSOR TURBINE TURBINE 58.6 MW 94.2 MW MW MW MW 61.6 C 183 C 508 C 652 C 680 C kg/s 1225 kg/s 1591 kg/s 1812 kg/s kg/s 250 bar 250 bar 250 bar 250 bar 70 bar ~ HIGH GRADE IHX C MW REACTOR C 3 loops of 800 MWth LOW GRADE IHX MW LOW TEMP. RECUPERATOR LOW GRADE C HIGH TEMP. HIGH GRADE RECUPERATOR HIGH TEMP. 375 C 380 C RECUPERATOR 13.6 MW 37

38 Plant layout MWth indirect-cycle GFR inside a spherical guard vessel 38

39 Plant Layout: Reactor building 39

40 Plant Layouts: Whole plant Gas Turbine Conversion System (secondary, x 3) Diesels Gas tanks storage Nuclear Steam Supply System (tertiary) Spent fuel storage Reactor plant Control command 40

41 Conclusions The GFR concept is attractive as it avoids the coolant handling issues associated with liquid metal-cooled fast reactors: Chemical inertness of helium Excellent nuclear stability avoids activation of the coolant Transparent coolant permits simple inspection and repair GFR offers a high temperature heat source for high efficiency electricity generation and high-quality process heat. The main technical challenges lie in the development of a hightemperature, high-power density fuel and in the development of robust decay heat removal systems. An indirect combined gas/steam cycle has been chose to be the reference power conversion system as this returns good efficiency with low technological risk and good economics. 41