MOX Recycling in PWR

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

2. Summary • MOX (Mixed Oxide) FuelRecycling in PWRs • Pu Recycling in France • Design & safety features • Void Effect in PWR Plutonium fueled cores

3. Pu Recycling in France: 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 4/4: • 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 • 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

23. 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.

24. 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.

25. 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).

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

27. Studies on Heterogeneous Void OCDE Benchmark sample UO2 MOX

28. Studies on Heterogeneous Void • 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)

29. 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

30. 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

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

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

33. 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.

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

35. 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.

36. 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.

37. Qualification of Void calculations: MOX fueled cores, • Analysis of results: • Despite • The same experimental techniques are used a for all measurements • The same schemes and options are adopted for computations, • The discrepancies C/ E increase significantly with the sample Pu enrichment. K Inf

38. Qualification of Void calculations: MOX fueled cores • Possible explanation 1/2: • Differences in the C/ E results can be explained by the effect of : • Measurement uncertainties • Computation precision, • Which both are sensitive to the spectrum hardiness (Pu enrichment).

39. Qualification of Void calculations: MOX fueled cores • Possible explanation 2/2 : • Measurement are less precise with increasing enrichment, because: • R.R. decrease, • Yield uncertainty increases; • Computation precision is reduced with increasing enrichment because: • The worth of the non-resolved resonance region increases; • This region is generally far less well described in the libraries; • Improvements to be made both in measurement techniques and computation.

40. Void effect in MOX fueled cores • CONCLUSION The complexity of physical problems and the difficulty in the modeling increase with MOX fueling, which demands: • A huge effort to improve the base-data and the computation tools, • New qualification needs, • A conservative approach at the design stage, • Several modification in the design and operation • A wide integration of the operational experience feed-back: • That’s current practice, now ….