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Insights into MOX Fuel Recycling in PWR Cores: Physics & Design Considerations

Explore the detailed physics and design strategies of MOX fuel recycling in PWR cores, focusing on plutonium utilization, fuel burn-up, and reactor safety in a French context.

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Insights into MOX Fuel Recycling in PWR Cores: Physics & Design Considerations

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  1. MOX Recycling in PWR Zone Vidangée 3.7% UOX Giovanni B. Bruna IRSN – DSR dir

  2. Summary • MOX (Mixed Oxide) FuelRecycling in PWRs • French Context • Physics of Pu Recycling in PWRs • Void Effect in PWR cores with Plutonium • Codes and methods

  3. Pu Recycling in France :a Year-Lasting Experience • In 1976 France adopted a « partially closed » cycle in 900MWe PWRs aiming at • Improving the fossil fuel utilization • Limit Pu build-up • Use the huge amount of depleted Uranium, • Reduce the amount of wastes (and their activity • Concentrate Pu in reactors: Open UOX Cycle Pu Rec. With FBR

  4. Pu Recycling in France : a Year-Lasting Experience • MOX loading in 900 MWe PWR cores: • Three-zoned assembly, • At equilibrium, 1/3 of the core assemblies contain MOX fuel, • Average Pu enrichment of the fuel : 7,0%, • Objective burn-up : 50000 MWd/ton heavy metal

  5. Pu Recycling in France : a Year-Lasting Experience Current MOX Assembly Gd-poisoned Assembly CYCLADES L.S. – 12 Gd2O3 pin/ass. Low-enrichment pins Intermediate-enrichment pins Water tubes eau 8 % C Gd2O3pins High-enrichment pins Water tubes

  6. Physics of MOX Recycling in PWR • MOX fuel in PWRs 1/4: • A grain-structured fuel • Pin power distribution, • Pin thermo-mechanical behavior, • Volatile F.P. release, • A lower number of fission per MWth • Fission energy release • Pu : 210 Mev / fission, vs. U : 200 Mev / fission • P.F Build-up • Short-term Residual power

  7. Physics of MOX Recycling in PWR • MOX fuel in PWRs 2/4: • A Fission efficiency (per gram) • ~ U235 for WG Pu, • < U235 for RG Pu • A roughly equivalent Doppler Coefficient, • A slightly higher Moderator Coefficient, • A reduced absorber worth (up to 60 – 70 % for the assembly): • Soluble boron, • Control clusters, • Poisons (burnable and not-burnable).

  8. Physics of MOX Recycling in PWR • MOX fuel in PWRs 3/4 : • An increased competition among fuel, structural materials and moderator, and a slightly increase of leakage. • Shorter prompt neutron lifetime, • An increased epi-thermal efficiency, • A reduced capacity to escape traps. • A lowered thermal fission, • An increased epi-thermal and fast fission, • Improved fast neutron utilization.

  9. Physics of MOX Recycling in PWR • MOX fuel in PWRs 3/4: • A smaller Delayed-neutron Fraction (b eff), • An almost absent Xenon poisoning, • A smaller reactivity swing vs. Burn-up (higher Internal Conversion ratio ~0.75 vs. 0.60) Contribution from main Isotope Families to reactivity swing vs. Fuel Burn-up

  10. Physics of MOX Recycling in PWR • Pin-wise Power Control • Compensation of physical effects through the assembly design FISSION REACTION RATES vs. LETHARGY (Infinite medium calculations)

  11. Physics of MOX Recycling in PWR • Pin-wise Power Control • Compensation of physical effects through the assembly design Original assembly design

  12. Physics of MOX Recycling in PWR • Pin-wise Power Control • Compensation of physical effects through the core loading strategy OUT-IN

  13. 242Cm 243Cm 244Cm 32 years Possible simplification  - 25 minutes Real process 163 days 16 hours 18,1 years n - 2n 241Am 242Am 243Am n   ~ 5 hours Fission products and energy production by fusion 13 years  238Pu 239Pu 240Pu 241Pu 242Pu 2,10 days 2,35 days 33 minutes 237Np 5,57 days 23,5 minutes 235U 236U 237U 238U 239U Physics of MOX Recycling in PWR • Fuel Burn-up / Breeding Process • Actinide build-up chain

  14. Physics of MOX Recycling in PWR • Fuel Burn-up / Breeding Process •Contribution of Actinide families to the reactivity swing vs. Fuel burn-up [MOX] *Lower than 0.5 *Lower than 0.5

  15. Xenon-poisoning Effect at equilibrium 1500 pcm Soluble Boron Worth ( per ppm) 7 pcm Black Control Rod Worth (per Rod) 600 pcm Gray Control Rod Worth (per Rod) 450 pcm Doppler Coefficient 3 pcm/K° Physics of MOX Recycling in PWR Moderator Coefficient > UOX

  16. Physics of MOX Recycling in PWR • Sensitivity of PWR core to the Plutonium content: • Reactivity Quite Low ( 600 pcm / % Pu)* • Void Effect Very High (5 000 pcm / % Pu)* • Control Rod Worth Medium • Soluble Boron Worth Medium • Burnable Poison Worth Medium • Power and Temperature Effects Low *1% increase of Plutonium content (RG Pu)

  17. Physics of MOX Recycling in PWR • Transient sensitiveness to Plutonium content • LOCA • RIA • Main Steam Line Break (RTV) • Additional Control Rods, • Constraints on the Loading Strategy, • System Modification

  18. Physics of MOX Recycling in PWR • Design constraints: Limit the Plutonium enrichment in the fuel and its core content to guarantee the safe operation against: • The Soluble Boron and Control Rod Worth decrease, • The Modified et more sensitive Operating conditions, • The Increased Uncertainty.

  19. Void effect in MOX fueled cores • Neutronics behavior of PWR cores in case of LOCA is sensitive to the Plutonium content because: -The MOX Moderator Coefficient is slightly different compared to UOX - The Void Effect depends on the core ◊ Overall Plutonium content, ◊ Plutonium isotope composition, ◊ Heterogeneity.

  20. Void effect in MOX fueled cores • Reactivity swing in a Voided core: The reactivity swing in a Voided core results from compensations among a large number of huge individual isotope and reaction-rate contributions having opposite sign: • Every isotope contributes through several rates (absorption, fission, slowing-down …) • Every individual component worth can be far bigger than the whole Void Worth, • Big Uncertainty • Very large Sensitiveness of Void Worth to the base data and the computation methodology.

  21. Void effect in MOX fueled cores • Moderator vs. Void Effect in UOX & MOX Fuel Void Effect 0 100 Void Fraction Moderator Effect Full Void Reactivity depending on Plutonium content MOX UOX Reactivity

  22. Void effect in MOX fueled cores Fission Rates vs. Lethargy (MOX fueled Assembly in Infinite Medium, no leakage) Unités arbitraires O Elastic Scattering U238Resonance Traps Pu239 Fission U238 Inelastic Scattering Thermal Capture Fission Spectrum Region Epi-thermal Region Pu240 Capture Léthargie

  23. Void effect in MOX fueled cores • X.S. Behavior vs. Energy Zone 1/v Pu240 Fission à seuil U235, Pu239 Résonances U238, Pu240, … U238 Log E 0.2 6 8E5 0.3 1.0 1.8 60 100

  24. Void effect in MOX fueled cores • Thermal Absorption X.S.

  25. Void effect in MOX fueled cores • Thermal Fission X.S

  26. Studies on Heterogeneous Void Infinite Medium Assembly Calculation Homogeneous Void Heterogeneous Void

  27. Studies on Heterogeneous Void • Homogeneous Void : Progressive et uniform void of the sample, • Heterogeneous Void : Non-uniform, spotted Void of the sample; some regions are privileged, • The void fraction is the same but the reactivity swing is far different.

  28. Studies on Heterogeneous Void • Accounting for leakage effect reduces the reactivity swing significantly • For sake of conservatism, the design calculations are always performed in an infinite medium, no leakage modeling approximation.

  29. Studies on Heterogeneous Void • Coupling Effect • The reactivity of each region changes with the void fraction, • The neutronics importance of the region (i.e., the asymptotic contribution of the region to the reactivity) changes too, in the meantime. • The actual reactivity of the sample depends on region-wise importance (as a weighting function).

  30. Studies on Heterogeneous Void Computation sample : the central region can contain a MOX assembly Homogeneous Void Heterogeneous Void

  31. Studies on Heterogeneous Void OCDE Benchmark sample UO2 MOX

  32. Études de Vidange Hétérogène • OCDE Benchmark • 3*3 assembly sample with 10*10 pins/ass.; (1.26 cm pitch): Inf. Medium Calc. with a variable Pu enrichment central MOX assembly: • HMOX 14.40 • MMOX 9.70 • LMOX 5.40 • (UO2 3.35)

  33. Studies on Heterogeneous Void • In the MMOX sample with water, typical parameter values are respectively: • Zone Kinf* Imp*. • UO21.3697 0.88 • MOX 1.1447 0.12 • Sample 1.3427 • *Rounded-off values

  34. Studies on Heterogeneous Void • In the central-void MMOX sample, typical parameter values are respectively: • Zone Kinf * Imp*. • UO21.3697 0.96 • MOX0.7738 0.04 • Sample1.3458 *Rounded-off values

  35. Studies on Heterogeneous Void K Inf with water Void • UO2 M. Inf 1.3697* 0* • MOX M. Inf. 1.1447* 0.7738* -41900* • Sample 1.3427* 1.3458* + 170* • *Rounded-off values

  36. « Envelop » Heterogeneous Void Homogenous Void Void effect in MOX fueled cores

  37. Void effect in MOX fueled cores • Main calculation challenges: • Space and Energy Heterogeneity; • Streaming inn the voided regions; • Self-shielding and dependence on the temperature of epi – thermal resonances: • Pu39, Pu41 0,3 eV, • Pu40 1,0 eV, • Pu 42 1.8 eV; • Mutual resonance self-shielding.

  38. Void effect in MOX fueled cores • Qualification basis. Quite rich, including: • GODIVA U35, • JEZEBEL Pu39, Pu40, • EOLE • ERASME S, R, (L) Pu hard spectrum • EPICURE U38, Pu, • VENUS • VIPO seriesU38, Pu • [SUPR series WG Pu]

  39. Qualification of Void calculations: MOX fueled cores • Pin-power distribution measurement technique 1/2: • A very careful characterization of the fuel is to be performed (to avoid effect of fabrication uncertainties); • Activity is measured pin by pin through gamma spectrometry (relative values); • But U and Pu R.R. are different (due to X.S. ); • Thus gamma-scanning activities in U and Pu regions are inhomogeneous: absolute values are necessary • Activities of some F.P. the Yields of which (both U and Pu) are very well known (with equivalent uncertainty level) are measured independently as tracers, • Y-scanning activity distribution are re-normalized to obtain absolute distributions; • To obtain the power distribution from the activity, a suitable normalization is performed via a “ P/A ” conversion factor experimentally measured in reference mock-ups.

  40. Qualification of Void calculations: MOX fueled cores • Pin-power distribution measurement technique 2/2: • The process of measurement is very hazardous and complex, • It is not fully independent from data and computation, • The quality of the pin-wise experimental distribution depends on: • The fuel fabrication process (homogeneity of composition and density), • The representativeness of the experimental mock-ups The experimental techniques, • The base-data used (Yields); • The robustness of the overall reconstruction process.

  41. Qualification of Void calculations: MOX fueled cores Voided Zone 3.7% UOX EPICURE mock-up Experiment

  42. Qualification of Void calculations: MOX fueled cores MOX 3.7% UOX Low and High Enrich. UOX-MOX EPICURE

  43. Qualification of Void calculations: MOX fueled cores( EPICURE LE (Low-Enrich) UOX-UOX)

  44. Qualification of Void calculations: MOX fueled cores( EPICURE LE (Low-Enrich) UOX-UOX)

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