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

Summary MOX (Mixed Oxide) Fuel Recycling in PWRs 1.French Context 2.Physics of Pu Recycling in PWRs 3.Void Effect in PWR cores with Plutonium 3.Codes and methods

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: Pu Rec. With FBR Open UOX Cycle

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 : MWd/ton heavy metal Pu Recycling in France : a Year-Lasting Experience

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

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

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). 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. Physics of MOX Recycling in PWR

MOX fuel in PWRs 3/4 : 1.A smaller Delayed-neutron Fraction (  eff), 2.An almost absent Xenon poisoning, 3.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

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 Physics of MOX Recycling in PWR Original assembly design

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 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 * Lower than 0.5

Xenon-poisoning Effect at equilibrium1500 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

Sensitivity of PWR core to the Plutonium content:Sensitivity of PWR core to the Plutonium content: a.ReactivityQuite Low ( 600 pcm / % Pu)* b.Void EffectVery High (5 000 pcm / % Pu)* c.Control Rod WorthMedium d.Soluble Boron WorthMedium e.Burnable Poison WorthMedium f.Power and Temperature EffectsLow *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

Design constraintsDesign 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. Physics of MOX Recycling in PWR

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

Reactivity swing in a Voided core: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

Moderator vs. Void Effect in UOX & MOX Fuel MOX UOX Reactivity Void Fraction 0100 Void effect in MOX fueled cores Full Void Reactivity depending on Plutonium content Moderator Effect Void Effect

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

X.S. Behavior vs. Energy 0.2 Log E Résonances Zone 1/v 1.0 Fission à seuil E56 Pu240 U238, Pu240, … U238 U235, Pu239 Void effect in MOX fueled cores

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

Thermal Fission X.S Void effect in MOX fueled cores

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

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

Studies on Heterogeneous Void OCDE Benchmark sample UO2 MOX

Études de Vidange Hétérogène 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.HMOX14.40 b.MMOX 9.70 c.LMOX 5.40 d.(UO2 3.35)

Studies on Heterogeneous Void 1.In the MMOX sample with water, typical parameter values are respectively: 2.ZoneKinf*Imp*. 3.UO MOX Sample a.*Rounded-off values

Studies on Heterogeneous Void 1.In the central-void MMOX sample, typical parameter values are respectively: 2.ZoneKinf*Imp*. 3.UO MOX Sample *Rounded-off values

Studies on Heterogeneous Void K Inf with water Void 1.UO2 M. Inf1.3697* 0* 2.MOX M. Inf *0.7738* * 3.Sample1.3427*1.3458*+ 170* a.* Rounded-off values

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

Main calculation challenges: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: -Pu39, Pu41 0,3 eV, -Pu40 1,0 eV, -Pu eV; d. Mutual resonance self-shielding. Void effect in MOX fueled cores

Qualification basis. Quite rich, including:Qualification basis. Quite rich, including: a.GODIVAU35, b.JEZEBELPu39, Pu40, c.EOLE -ERASME S, R, (L) Pu hard spectrum -EPICUREU38, Pu, d.VENUS -VIPOseriesU38, Pu e.[SUPR seriesWG Pu] Void effect in 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

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

Voided Zone 3.7% UOX EPICURE mock-up Experiment

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

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

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

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

CONCLUSIONCONCLUSION 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 history, now …. Void effect in MOX fueled cores