Gas Cooled Fast Reactor for Gen IV. Service

Transcription

1 The First COE-INES International Symposium, INES-1 Keio Plaza Hotel, Tokyo, Japan, October 31-November 4, 2004 Gas Cooled Fast Reactor for Gen IV. Service P. Hejzlar, M. J. Pope M.J. Driscoll, and W.C. Williams Massachusetts Institute of Technology CANES Center for Advanced Nuclear Energy Systems DOE support through INEEL LDRD and I-NERI program is acknowledged CANES MIT 5/2003 1

2 Outline Generation IV goals Sustainability considerations affecting design choices Safety Key challenges Approaches to address post-loca cooling Selection of the most promising decay heat removal strategy Economic considerations affecting design choices Summary of the main design features of a GFR concept that can meet Gen IV goals and deserves further development

3 Generation IV Goals Nuclear Energy Enterprise with Gen IV Reactors Sustainability Safety Economy

4 Sustainability Traditionally high utilization of resources (motivated early development of fast reactors with high breeding ratio - blankets) New emphases in Gen IV Waste minimization Proliferation resistance To reduce waste long-term radiotoxicity to that of natural U in <1000yrs full recycling of TRU with losses <0.1% needed Enhanced proliferation resistance favors avoidance of Pu separation and maintenance of dirty plutonium isotopics throughout the cycle

5 Sustainability driven design choices Use accumulated TRU from spent LWR for 1 st GFR core Design GFR with BR=1, no blankets to avoid clean Pu Recycle TRU without Pu separation, Depleted U feed If enough GFRs deployed, LWR TRU inventory eliminated After full transition to GFR, enrichment could be eliminated Today Nat - U Enriched U UO 2 U +TRU + FP (1 st core) U +TRU + FP Conversion & Enrichment Plant LWR Fuel Fabrication Plant LWR Reprocessing Plant U +TRU + FP GFR Depleted U Storage of LWR spent fuel 0.1% TRU loss + FPs Storage of Depleted U CANES MIT 5/2003 High -Level Waste Storage

6 Challenges-sustainability driven The outlined strategy can meet all 3 sustainability goals, but brings several challenges: TRU core has smaller delayed effective neutron fraction TRU core has more positive coolant void reactivity Core with BR=1 without blankets is more difficult to design GFR cost of electricity must be lower than from LWRs to attract large deployment by utilities and gradual transition to GFR park

7 Safety Safetywas the key issue in the GCFR development in the 70ties and is today as well reactivity increase from coolant depressurization post LOCA decay heat removal

8 Safety coolant void worth Wide-spread belief gas cooled reactors do not pose coolant void reactivity problem (coolant neutronically transparent) Does not hold for gas-cooled GFRs with hard spectrum CO2core-average density =0.137g/cc (1/5th of water) significant moderation and increase of fission to capture ratio in HMs upon LOCA, moreover large scattering xs How about He (density=0.01g/cc) less challenging Can be decreased through leakage enhancement as in LMRs?

9 Safety coolant void worth Coolant void reactivity for (U-TRU)C pin fuel with Ti cladding and Ti reflector Coolant Void Reactivity [$] MPa MPa Height to Diameter Ratio Leakage effect negligible for He cooled reactors, but works for S-CO2 cooled cores Most promising (U-TRU)C/SiC or (U-TRU)C/Ti matrix cores with Ti or Zr3Si2 reflector

10 Safety post LOCA cooling Approaches investigated Solid matrix core with decay heat removal by conduction and radiation Solid matrix core with heat pipes Low pressure drop core cooled by natural circulation loop with emergency cooling heat exchangers + containment et elevated pressure

11 Approach 1: Long life, low power density design Low power density design possible candidate? Addresses LOCA Coolant holes CERMET or METMET fuel in a metallic matrix Replaceable lowdensity reflector or void Replaceable outer reflector Synergistic twin to thermal MHR-GT Same power density as MHR-GT - 8kW/l Long core life 50 years Permanent side reflector Active core Decay heat after depressurization removed by conduction and radiation through vessel wall CANES MIT 5/2003

12 Approach 1: Very long life core k eff Long-life nuclear battery fhm=20wt% fhm=25wt% fhm=30wt% fhm=35wt% fhm=40wt% EFPY fhm=weight fraction of HM in matrix CANES MIT 5/2003 Enrichment=13.8wt% Pu Zirconium matrix Core OD=3m, Height=8m Pu from spent LWR fuel 50year core life feasible with MET- MET fuel Within allowable fluence and burnup limits Pu composition and mass at discharge same as at BOL - provides fuel for next reactor

13 Approach 1: Decay heat removal Temperature (C) 1400 Niobium-alloy matrix Inref Core-max Core-out Ref-out Vess-in Vess-out Panel Time (hrs) Decay heat from 300MWe-core can be dissipated at Tmax=1200 C

14 But very high fuel cycle cost!!! Fuel Cycle cost (mills/kwhr) FCC-PWR(4%) FCC-GCRF(13%) FCC-GCFR FCC = Bd=50MWd/kg Bd=180MWd/kg C plη T T-PWR T-GCFR Specific power (kw/kghm) xt 1 e xt Core residence time for fixed burnup GFR For U235 enriched fuel η=45%, L=0.90 Bd=180MWd/kgHM discount rate x=10%/yr C=3936 $/kg for e=13% PWR η=33%, L=0.90 Bd=50MWd/kgHM discount rate x=10%/yr C=1200 $/kg for e=4.5% Fabrication 200$/kg SP=38kW/kgHM Twin to MHR-GT not economically feasible Specific power should not be much below 20MWd.kg, Shoot for 25kW/kgHM (BWR) CANES MIT 5/2003

15 How to increase conduction /radiationlimited power density? Separated Fuel/coolant channels Coupled

16 Two possible options of distributed heat sinks 1. Use coolant channels available for normal operation cooling Simplicity, no additional structures, better neutron economy Dictates natural convection cooling, low HT rates, hence elevated pressure Loss of cont. pressure lead to core damage (different from PWRs) 2. Introduce additional heat sink (HPs) independent of coolant channels Independent of state of coolant, no need for higher pressure Rugged and simple devices, passive initiation of operation But, Impaired neutronic performance (parasitic absorption) Exposure to high fluence, challenge for materials Introduction of additional modes of reactivity change Interference with refueling, additional vessel penetration CANES MIT 5/2003

17 1. Additional Heat Sinks Power density 50W/cc feasible with 37 heat pipes in block cores Significant neutronic penalties Issues of heat pipe failure, containment penetration Materials, which meet mechanical, thermal, neutronic and economic targets are difficult to find Detachable connector Common headers Heat pipe Reflector block Fuel block CANES MIT 5/2003

18 2. Convective cooling at elevated pressure 3x 50% cooling loops low pressure drop core after depressurization of primary system, containment pressure increases and provides elevated pressure needed for natural circulation But Relatively large pipes needed Potential for vapor ingress to core in case of HX tube failure containment must reliably maintain elevated pressure Water cooling Reactor vessel Guard containment Hexagonal blocks with coolant channels Emergency cooling Heat Exchanger reflector Core This approach selected as the most promising

19 Natural circulation performance for CO 2 and He Post LOCA core temperature profiles Temperature ( o C) Average Channel Coolant Average Channel Wall Hot Channel Coolant Hot Channel Wall Temperature ( o C) Average Channel Coolant Average Channel Wall Hot Channel Coolant Hot Channel Wall P = 5.0 bars mdot = kg/s CO 2 Core Axial Location (m) P = 13 bars mdot = kg/s Helium Core Axial Location (m) Limits peak cladding temperature=1200c, maximum core-average outlet T=850C 2% decay heat can be removed by natural circulation CO2 much better than He requires backup pressure of 5bars versus 13 bars for He

20 Challenges:Operation in Atypical Flow Regimes Decay heat removal operation Moody friction factor /D Re (-) Mixed convection regime has higher uncertainties on heat transfer Experimental loop at MIT under construction (INEEL LDRD)

21 Challenges: Instability of helium laminar flow Pressure drop (Pa) Point of instability reached when gravity head =20% of total dp dp-tot LOCA-COLA shows instability here -600 Tout=1260C CANES MIT 5/2003 Hot channel mass flow rate (kg/s) Helium flow in the core is laminar and has high temperature rise ~1000C Large temperature dependence of kinematic viscosity on temperature Small helium density, thus small gravity pressure drop Hence Ledinegg-type instability and temperature runaway in hot channel possible CO2 coolant no such problem dp/dm Derivative dp/dmdot (Pa-s/kg)

22 Design selections to achieve high safety Use combination of active and passive safety systems 3x50% emergency/shutdown cooling loops with diverse power supply (Diesels, fuel cells, microturbines) Active blowers are first line of defense Passive emergency cooling by natural circulation is a backup CO2 coolant is primary choice. It can remove 2% decay power at containment pressure of 5bars typical containment design pressure, but not compatible with carbide fuel, hence innovative tube-in-duct assembly with VIPAC (U,TRU)O 2 explored Helium coolant secondary choice needs active systems to operate for first 24 hours until decay heat is reduced to 0.5% when natural circulation can take over at reasonably low backup pressure (cost of containment is key)

23 Economy Key design choices driven by economic considerations Direct cycle Indirect cycle has large economic penalty as a result of smaller plant efficiency (~ 4% ) Additional cost of IHX and blowers, more complex system Power density 100W/cc, SP 25 kw/kghm Recompression supercritical CO2 power cycle for balance of plant achieves high efficiency at temperature compatible with more materials (thermal efficiency of 49.5% at 650C temperature of British AGRs) It is simple and very compact First analyses showed 25% lower capital cost than Rankine cycle and 10% lower cost than helium Brayton cycle But significant R&D needed Backup, nearer-term solution helium coolant, direct Brayton power cycle

24 Supercritical CO2 recompression cycle Point P T Enthalpy (MPa) ( o C) (kj/kg) PRECOOLER 1 8 MAIN COMPRESSOR 2 LOW TEMPERATURE RECUPERATOR 8 FLOW MERGE RECOMPRESSING COMPRESSOR FLOW SPLIT 5 6 TURBINE 4 HIGH TEMPERATURE RECUPERATOR REACTOR For conservative compressor (89%) and turbine (90%) efficiencies thermal/net efficiency =49.5%/ 45.3%

25 Comparison of turbine size Steam turbine: 55 stages / 250 MW Mitsubishi Heavy Industries Ltd., Japan (with casing) 1 m Helium turbine: 17 stages / 333 MW (167 MW e ) X.L.Yan, L.M. Lidsky (MIT) (without casing) Supercritical CO 2 turbine: 4 stages / 450 MW (300 MW e ) (without casing)

26 Summary of design choices to meet Gen IV goals Closed cycle with TRU recycle, TRU from LWR spent fuel feed the first core, no Pu separation Self-sustaining core, BR=1, no blankets, diluted Pu vector Low pressure core,3x50% emergency/shutdown cooling loops Active system with redundant and diverse power supply as first line of defense, passive decay heat removal by natural circulation at containment pressure 5 bars as a backup First choice Direct SCO2 cycle, tube-in-duct assembly with VIPAC (U,TRU)O 2 cycle Second choice Direct helium cycle, matrix or plate (U,TRU)C fuel, first 24 hours active cooling, passive cooling after 24 hours