MOX Recycling in PWR Zone Vidange 3 7

MOX Recycling in PWR Zone Vidangée 3. 7% UOX Giovanni B. Bruna IRSN – DSR dir

Summary • MOX (Mixed Oxide) Fuel Recycling in PWRs 1. Pu Recycling in France 2. Design & safety features 2. Void Effect in PWR Plutonium fueled cores

Pu Recycling in France: Year-Lasting Experience 1. In 1976 France adopted a « partially closed » cycle in 900 MWe PWRs aiming at 2. Improving the fossil fuel utilization • Limit Pu build-up • Use the huge amount of depleted Uranium, • Reduce the amount of wastes (and their activity 3. Concentrate Pu in reactors: Open UOX Cycle Pu Rec. With FBR

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

Pu Recycling in France: a Year-Lasting Experience Current MOX Assembly Gd-poisoned Assembly Low-enrichment pins CYCLADES L. S. – 12 Gd 2 O 3 pin/ass. Intermediate-enrichment pins High-enrichment pins Water tubes eau 8 % C Gd 2 O 3 pins

Physics of MOX Recycling in PWR 1. 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

Physics of MOX Recycling in PWR 1. MOX fuel in PWRs 2/4: • A Fission efficiency (per gram) • ~ U 235 for WG Pu, • < U 235 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).

Physics of MOX Recycling in PWR 1. MOX fuel in PWRs 3/4 : - An increased competition among fuel, structural materials and moderator, and a slightly increase of leakage. § - An increased epi-thermal efficiency, § - Shorter prompt neutron lifetime, A reduced capacity to escape traps. A lowered thermal fission, An increased epi-thermal and fast fission, § Improved fast neutron utilization.

Physics of MOX Recycling in PWR 1. MOX fuel in PWRs 4/4 : 2. A smaller Delayed-neutron Fraction (b eff), 3. An almost absent Xenon poisoning, 4. 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

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)

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

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

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

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

Physics of MOX Recycling in PWR 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° Moderator Coefficient > UOX

Physics of MOX Recycling in PWR 1. Sensitivity of PWR core to the Plutonium content: a. b. c. d. e. f. 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)

Physics of MOX Recycling in PWR 1. Transient sensitiveness to Plutonium content - LOCA - RIA - Main Steam Line Break (RTV) 2. Additional Control Rods, 3. Constraints on the Loading Strategy, 4. System Modification

Physics of MOX Recycling in PWR 1. Design constraints: 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.

Void effect in MOX fueled cores 1. 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.

Void effect in MOX fueled cores 1. 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.

Void effect in MOX fueled cores 1. Moderator vs. Void Effect in UOX & MOX Fuel Void Effect 0 100 Void Fraction Moderator Effect MOX Reactivity UOX Full Void Reactivity depending on Plutonium content

Void effect in MOX fueled cores 1. X. S. Behavior vs. Energy Zone 1/v Pu 240 Fission à seuil U 235, Pu 239 Résonances U 238, Pu 240, … U 238 0. 2 0. 3 1. 0 1. 8 6 60 100 8 E 5 Log E

Studies on Heterogeneous Void 1. Homogeneous Void : Progressive et uniform void of the sample, 2. Heterogeneous Void : Non-uniform, spotted Void of the sample; some regions are privileged, 3. The void fraction is the same but the reactivity swing is far different.

Studies on Heterogeneous Void 1. Accounting for leakage effect reduces the reactivity swing significantly 2. For sake of conservatism, the design calculations are always performed in an infinite medium, no leakage modeling approximation.

Studies on Heterogeneous Void 1. Coupling Effect a. The reactivity of each region changes with the void fraction, b. The neutronics importance of the region (i. e. , the asymptotic contribution of the region to the reactivity) changes too, in the meantime. 2. The actual reactivity of the sample depends on region-wise importance (as a weighting function).

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

Studies on Heterogeneous Void OCDE Benchmark sample UO 2 MOX

Studies on Heterogeneous Void 1. OCDE Benchmark 2. 3*3 assembly sample with 10*10 pins/ass. ; (1. 26 cm pitch): Inf. Medium Calc. with a variable Pu enrichment central MOX assembly: a. HMOX b. MMOX c. LMOX d. (UO 2 14. 40 9. 70 5. 40 3. 35)

Studies on Heterogeneous Void 1. In the MMOX sample with water, typical parameter values are, respectively: 2. Zone Kinf* Imp*. 3. UO 2 1. 3697 0. 88 4. MOX 1. 1447 0. 12 5. Sample 1. 3427 a. *Rounded-off values

Studies on Heterogeneous Void 1. In the central-void MMOX sample, typical parameter values are, respectively: 2. Zone Kinf * Imp*. 3. UO 2 1. 3697 0. 96 4. MOX 0. 7738 0. 04 5. Sample 1. 3458 *Rounded-off values

Studies on Heterogeneous Void K Inf Water K Inf Void 1. UO 2 Inf. M. 1. 3697* 2. MOX Inf. M. 1. 1447* 0. 7738* -41900* 3. Sample 1. 3427* 1. 3458* + 170* a. *Rounded-off values 0*

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

Void effect in MOX fueled cores 1. Main calculation challenges: a. Space and Energy Heterogeneity; b. Streaming inn the voided regions; c. Self-shielding and dependence on the temperature of epi – thermal resonances: - Pu 39, Pu 41 0, 3 e. V, - Pu 40 1, 0 e. V, - Pu 42 1. 8 e. V; d. Mutual resonance self-shielding.

MOX 3. 7% UOX Low and High Enrich. UOX-MOX EPICURE Qualification of Void calculations: MOX fueled cores

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.

Qualification of Void calculations: MOX fueled cores 1. 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.

Qualification of Void calculations: MOX fueled cores , • Analysis of results: • Despite K Inf • 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.

Qualification of Void calculations: MOX fueled cores 1. 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).

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.

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 ….