Bubelis, E.

Hering, W.

Perez-Martin, S.

Pfrang, W.

Schikorr, M.

English

KITopen

KIT – The Research University in the Helmholtz Association www.kit.eduON THE PURSUING OF SAFETY ENHANCEMENTS IN SODIUM FAST REACTORSS. Perez-Martin, E. Bubelis, W. Pfrang, M. Schikorr, W. HeringSafety enhancements in Sodium Fast ReactorsOutcome of long lasting international collaboration in SFR safety (400+ years of reactor operation):Improved core designs, structural components and passive safety systems.Development of numerical simulation computer codes  for design as well as licensing process.Assessment of enhanced SFR core designs under ULOF transientIntro & Outline1960 1970 1980 1990 2000 2010 2022FBR National ProgramsFBR National ProgramsInternational collaborations (e.g. CABRI , …  )  EU funded Projects 2Safety enhancements in Sodium Fast ReactorsLicensing: demonstration of safety functions under normal operation and accident conditions:i) reactor shut-down maintaining safe conditionii) adequate core heat removal after shut-downiii) retention capabilities for radioactive and hazardous material to minimize its release to the environment.CP-ESFR project milestone: identification of initiating events in DBC that may challenge the safety functions.Category DBC4: initiating events not expected to occur during plant lifetime (freq.<10−4 per reactor-year):Unprotected Transients (shutdown system is assumed to fail)Runaway of control rods: Unprotected Transient Over-PowerLoss of the active core cooling capability: Unprotected Loss of FlowBreak of LIPOSO (pipelines joining the primary pump with the core diagrid). ULOF importance:Potential to progress into the coolant boiling phase (and eventually into partial or even total core destruction).Detailed consideration of the particular effects of various specific design characteristics (e.g. upper sodium plenum, absorber layers, discharge tubes, etc.).SFR Safety Analysis3Safety enhancements in Sodium Fast ReactorsSFR reactor designs in EU projectsCP-ESFR ESNII+ ESFR-SMARTTime 2009-2012 2013-2017 2018-2022Target SFR Gen-IV (SFR, LFR, GFR… ) SFRReactor power (MWth) 3600 1500 3600Total number of SA 453 291 504Number of pins per SA 271 217 271Core inlet temp. (°C) 395 400 395Core outlet temp. (°C) 545 550 545Av. core structure temp. (°C) 470 475 470Reactor performance Minor Actinides transmutation ASTRID Improved CP-ESFR reactorSafety measures decrease sodium void worth negative sodium void worthcorium discharge tubes passive SR (Curie-point triggered)4Safety enhancements in Sodium Fast ReactorsSodium void (density) reactivity: • Less neutron capture (positive effect)• Neutron spectrum hardening (positive effect)• Larger mean free path: neutron leakage increase (negative effect)Measures:• a large sodium plenum at the top of the core (where neutron leakages are increased)• axially heterogeneous fuel pins with a central fertile layer in IC (increasing neutron flux in the upper fissile layer)• shortening of the fissile zone in the IC• absorbing zone in upper shielding (reducing neutron reflection back to the fissile core during voiding)CP-ESFR Reference CP-ESFR Optimized ESNII+ ESFR-SMARTSFR reactor designs in EU projects5Safety enhancements in Sodium Fast ReactorsUnintentional simultaneous coast-down of all primary pumps + failure of the reactor shut-down system. The primary mass flow rate:ሶ𝑚 𝑡ሶ𝑚0=11 +𝑡 (halving time): 10 s, except for ESNII+ case which is 24 s.No pony motors or other devices maintaining coolantEvaluation at EOEC core conditions (except for the CP-ESFR Opt. at BOL)EOEC represents the worst-case scenario where the fuel/pin configuration is in degraded thermal and mechanical conditions and all control rods are essentially in the withdrawn position. Reactivity feedbacks: Doppler, fuel cladding, sodium reactivity, control rods driveline thermal expansion. 6ULOF transientSafety enhancements in Sodium Fast ReactorsSAS-SFR and SIM-SFR: reference codes in SFR safety evaluationsDeveloped and supported by the Karlsruhe Institute of Technology.Validated against experimental data from large-scale experiments (i.e. CABRI, SCARABEE, KNS-37, TREAT, Phenix, SPX, etc.).7SIM-SFR and SAS-SFR codesCode representation with SA channelsSFR coreFuel pin Thermal EffectsCore Neutronic EffectsFuel pin Mechanical EffectsCore Hydraulic EffectsSAS-SFRSIM-SFRSafety enhancements in Sodium Fast ReactorsReactor representation:SIM-SFR: core, the primary, secondary and tertiary cooling circuits, including IHX and SGSAS-SFR: core and primary cooling circuits including IHXSA representation:SIM-SFR: average power and a peak power fuel representation for each core zoneSAS-SFR: SA grouping based on similar TH conditions undergoing same irradiation (ESFR-SMART 34 SA groups)Reactivity feedbacks:SIM-SFR: Doppler, fuel&cladding axial exp. , diagrid radial expansion, coolant expansion and CRDL axial exp.SAS-SFR: Doppler, coolant density, fuel&cladding axial exp., fuel and clad relocation, diagrid radial exp., CRDL ax. exp. Gap model: special, high fidelity fuel pin mechanics models. SIM-SFR: dynamic model (burn-up dependence of both fuel and cladding material)SAS-SFR: URGAP model (gap size, fuel-clad contact pressure, gas content and composition and surface roughness)8ULOF Code modelling Safety enhancements in Sodium Fast ReactorsCP-ESFR:The optimized core improved the safety response by reducing peak temperatures and enlarging grace times. Not sufficient to avoid a power excursion once sodium boiling commenced. ESNII+:Upper sodium plenum provided only a small delay in boiling onset (void effect dominated by the fissile core voiding)  sodium plenum does not play a decisive role in improving total void effectTransient progression beyond pin and hexcan failure, driven by cladding failure and relocation from the fissile core zone. Optimization of the core neutron physics alone was not sufficient to avoid a power excursion during the ULOFtransient.This type of core geometrical modifications established a direction to progress for achieving higher safety levels and risk prevention of core meltdown accident.ULOF code results in CP-ESFR and ESNII+9Safety enhancements in Sodium Fast ReactorsSodium boiling starts in ch. 33 (OC) at 46.1 s, nominal power is 0.61, net reactivity is -0.12 $. SAS-SFR results for ESFR-SMART ULOF10Safety enhancements in Sodium Fast ReactorsFuel pin break-up failure mechanism: clad integrity compromised due to the high clad temperature and fuel pellet heat-up exceeds the melting limit and built-up cavity pressures.11SAS-SFR results for ESFR-SMART ULOFSafety enhancements in Sodium Fast Reactors12ULOF transient period 90-100 sSafety enhancements in Sodium Fast ReactorsNo hexcan damage: transient does not progress further and following accidental sequences (i.e. recriticality) are no longer expected.13SAS-SFR results for ESFR-SMART ULOFSafety enhancements in Sodium Fast Reactors14SAS-SFR results for ESFR-SMART ULOF504 SAs with 271 pins per SAComputational running time36 representative pins (6 ch. inner core, 30 ch. outer core)Fuel pin irradiation:Real irradiation time: 1800 dComputing time: 38 min. total  ~1 min. per SAULOF transient:Real transient time: 125 s Computing time: ~17 h total  0.5 h per SAVideoSafety enhancements in Sodium Fast ReactorsDuring ULOF sodium flowrate decreases down to natural circulation flowrate and even below depending on sodium voiding.Boiling onset sequence (outlet fissile region): i) PP* OC, ii) Av.P** OC, iii) PP IC and iv) Av.P IC.Flow blockage due to boiling not known precisely: sensitivity study to investigate the natural circulation flow reduction.SIM-SFR results for ESFR-SMART ULOF15* Peak Power, ** Average Power, IC: Inner Core, OC: Outer CoreSafety enhancements in Sodium Fast ReactorsSIM-SFR results for ESFR-SMART ULOF16Safety enhancements in Sodium Fast ReactorsThe 15% and 5% flow choking cases basically replicate the SAS-SFR ESFR ULOF results.SIM-SFR results for ESFR-SMART ULOF17Safety enhancements in Sodium Fast ReactorsAssessment of SFR performance +  verification & validation of computational toolsMeasures considered to reduce the void reactivity effect in large SFR: Sodium and boron layers (CP-ESFR Optimized core)Central fertile layer in the inner core region (ESNII+ core)Reducing inner core fissile fuel region (ESFR-SMART core).Optimizations not sufficient to avoid a power excursions, though increase grace time, not melting/rupture of the SA hexcan.Future R&D:Sodium two-phase flow: large scale code validation comparisons based on experimental data sets (KNS-37 test)New two-phase flow experimental tests reflecting the current trends in core designs.Large scale clad relocation prevention: core and SA-design measures or min. flow rate (active/passive means, pony motor)Assessment of SMRs based on SFR technology, where high benefits are expected in terms of safety and flexibility.New code strategies to find the right compromise between:1. computing capabilities (new programming languages, parallelization, etc.)2. phenomena description (neutronics, thermal-hydraulics, pin thermal-mechanics, corium relocation, etc.)3. details of reactor description (pin-by pin level, core level or up to whole plant level). Advantages for the safety analysis of advanced systems (Machine Learning, Digital Twins) compared to current Fortran-based codesAttractive research to future nuclear engineers/scientist  costly person-intense requiring support of public and private stakeholders.18CONCLUSIONS Safety enhancements in Sodium Fast ReactorsThe work presented in this paper was supported by EURATOM under the projects CP-ESFR, ESNII+ and ESFR-SMART.CP-ESFR project: 7th Framework Programme FP7-232658ESNII+ project: : 7th Framework Programme FP7-605172ESFR-SMART project: H2020 program 2014-2018 grant agreement 754501Acknowledgement19

On the pursuing of safety enhancements in sodium fast reactors

https://publikationen.bibliothek.kit.edu/1000147679/148863107

On the pursuing of safety enhancements in sodium fast reactors

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