Working Party on Pu-MOX fuel physics andinnovative fuel cycles (WPPR)

Hideki TakanoJapan Atomic Energy Research Institute, Japan

R&D Needs in Nuclear Science

6-8th November, 2002 OECD/NEA,Paris

Introduction(1) NSC has been addressing issues related to plutonium utilization in

LWRs and FBRs.

Former days than 1992 For FBRs, HCLWR, FRs, much works of benchmark calculations has

been conducted to validate the data and methods. As a result, nuclearcodes and data is comparable to standard LWR benchmarkcalculations.

Especially, for HCLWR application, computational codes based onultra-fine group method which calculates accurately resonanceshielding including intermediate neutron spectrum region weredeveloped, and furthermore continuous energy Monte Carlocalculations were proposed as a reference solution. After this, themethodology is used as standard calculation for variousbenchmark calculations.

Introduction(2) In June 1992, the NSC set up a Working Party on Physics of

Plutonium Recycling and in 1997 has started Working Party onPhysics of Plutonium Recycling and Innovative Fuel Cycles (WPPR).

In the Working Party, benchmark studies cover issues and perspectives,Pu-recycling in PWR, void reactivity effect in PWR, fast Pu-burner reactors, Pu-recycling in fast reactor, multiple Pu-recycling in standard and advanced PWR, MOX in BWR, andbenchmark calculations of MOX fuel lattice experiments in LWR.

Plutonium-recycling in PWR(1)

Three benchmarks were devised for MOX in PWRs.

Two simple infinite pin cell problems investigated issues related toMOX utilisation in PWRs with plutonium, both of typical and poorisotopic quality.

The typical type is a plutonium isotopic vector with higher fissilefraction using commercial PWR MOX.

The latter type is a plutonium of low isotopic vector expected for aself-generation recycling mode in PWR MOX.

The benchmarks were to investigate whether nuclear data and latticecodes require further development and validation to calculate theplutonium fuel core physics performance.

Plutonium-recycling in PWR(2)

WPPR expected that good agreement between the various solutionswould be obtained.

In both benchmarks, the spread in k infinity was 3- 5 % includingthe solutions of commercial established codes verified for uraniumfuels, and if the solutions are not included, the spread decrease, but notless than 1%. Furthermore, this spread in k infinity increase with theplutonium contents.

This means a need for improvement in both methods and basic datafor higher plutonium isotopes and minor actinides.

The calculational methods have to take into account resonance self-and mutual shielding treatment, especially including self-shieldingtreatment of Pu-242.

Plutonium-recycling in PWR(3)

The third benchmark was a void reactivity effect in PWR. A supercell configuration of a 30x30 array of PWR UO2 and MOX

fuel cells was carried out in the VENUS experimental reactor at MOL. The calculated results shows that the infinite lattice calculations gives

a non-negligible spread of the results, but the void effect is positivefor high content of MOX in the fuel .

A similar tendency is derived from the results of the two-dimensional calculations for an uranium assembly with a centralmoderated or voided region fuelled with MOX pins of differentplutonium content.

Plutonium-recycling in PWR(4) As a result, the design of MOX fuel assemblies needs the use of the

newest nuclear data bases and very detailed and sophisticatedspectra and assembly codes.

The use of codes only verified for UO2 fuel should be avoided.

In the void reactivity effect benchmark, experimental validation wouldalso be needed, in particular for reactivity coefficients, in the case ofdegraded plutonium isotopic composition.

In recycling of plutonium in PWRs the important related physics issueswould be the plutonium content limitation to avoid positive voideffects and the minimisation of minor actinide production.

Plutonium-recycling in PWR(5)

The summary conclusions from the PWR benchmark studies are asfollows:

• The nuclear calculational methods have to take into accountrigorous resonance self shielding, mutual shielding over thewhole energy region for the fuel and cladding nuclides, andappropriate and well tested calculation methods should beused.

• Sufficient quality of basic nuclear data is needed for U-238and the Pu isotopes, but also for higher acitinides and themajor fission products, and modern nuclear data libraries suchas ENDF/B-VI, JEF-2 and JENDL-3 are essential.

• Experimental verification related to maximum plutoniumcontent in the case of degraded plutonium isotopiccomposition is needed in clean lattice configurations withdifferent moderator-to-fuel ratios.

Pu-recycling in Fast Reactor (1)

Two topics of benchmarks examined for fast reactor systems are thephysics of plutonium recycle in both oxide and metal fuelled fastreactors to determine the potential of the fast reactors for consumingplutonium and fissioning minor actinides.

In this benchmarks, the physics related to in particular consumingplutonium rather than establishing a breeding cycle and reducing thesource of potential radio-toxicity has been widely investigated.

Such fast reactor systems may have an important role in managingplutonium stocks until fast breeder reactors are needed.

Pu-recycling in Fast Reactor (2)

The benchmark excises were considered to cases with non-breedingfast system with conversion ratios in the range 0.5 to 1.0.

The fast burner reactors / LWR symbiosis was investigated. Resultsindicated a potential for significant nuclear waste toxicity reduction.

In the metal fuelled FR benchmarks, high leakage cores, highercontent of minor plutonium isotopes and higher actinide isotopes allneed further validation work, including critical experimentperformance.

Recycling of Plutonium in AdvancedConverter Reactor

As advanced converter reactors, the Japanese ATR, variant of theCANDU, the High conversion LWRs (HCLWR) were reviewed.

The advanced converter reactors have a role as an intermediate stepbetween today’s thermal reactors and future fast breeder systems.

For HCLWRs, in some lattice experiments such as LWHCR of PSI,ERASME/S (moderator ratio 0.5), ERASME/R (moderator ratio0.9), the comparison of measured and calculated lattice parametersappear to be well knowledge of the neutronics and physics of suchreactors.

Multiple Plutonium-Recycling in PWRs (1)

A shortcoming of the first Pu-recycling benchmarks was that onlyone of the cases considered corresponded to plutonium of goodisotopic quality, and to the scenario which might arise after manygenerations of MOX recycling (with extremely poor isotopic quality).Analysis of the intermediate steps was missed.

CEA suggested a benchmark in which five consecutive generationsof multiple recycling in a PWR would be followed. In thespecification of the benchmark, attempts were made to make it asrealistic scenario as possible, taking into account such details as thetime delays in pond storage, MOX fabrication etc. and the dilutioneffect when MOX and UO2 assemblies are co-reprocessed.

Multiple Plutonium-Recycling in PWRs (2)

As in the previous benchmark exercise, the benchmark wasrestricted to the level of the lattice codes. This is the logical first stepuntil the underlying nuclear data and lattice code calculations showadequate agreement.

Two cases were considered; one for a standard 17x17 PWR latticesuch as used in many of today's PWRs (designated the STD-PWR)and another lattice with an increased moderator to fuel ratio (3.5:1compared with 2:1 for the STD-PWR). The latter case, designated theHM-PWR (for highly moderated), was intended to cover a proposedPWR design for use with MOX only.

Multiple Plutonium-Recycling in PWRs(3) The burnup reactivity, reactivity coefficients, microscopic cross-

sections, isotopic evolution and isotopic toxicity evolution with timewere compared. The broad conclusions and observations are asfollows:

Since the earlier benchmarks, considerable progress has been made innuclear data libraries and methods. The discrepancies between thedifferent data libraries and lattice methods are now generallywithin reasonable bounds. The observed spread of results is nowconsistent with the uncertainties in the underlying nuclear data.Multiple recycle scenarios therefore appear to be practicable andfeasible in conventional PWRs, at least in the near term.

Questions are a possible positive void coefficient would almostcertainly preclude recycle beyond the second generation.

Multiple Plutonium-Recycling in PWRs(4)

The HM-PWR degrades the plutonium isotopic quality morerapidly than the STD-PWR, and this negates the benefit of the softerspectrum. The HM-PWR also seems to pose more difficulties forpresent nuclear data libraries and codes, as evidenced by the largernumber of discrepant results seen in the HM-PWR benchmark.Therefore, even the HM-PWR is of questionable practicability withrespect to later recycle generations.

In view of these considerations, the WPPR agreed that there is nocompelling reason to continue further benchmark studies at thelevel of the lattice codes.

Power Distribution Within MOX FueledAssemblies (1)

The main objective of the benchmark was to compare differenttechniques of fine flux assessment derived from coarse meshdiffusion calculations or transport calculations.

Ten institutions contributed and more than 15 calculation schemeswere examined including the majority of the methods used for reactordesign: collision probability, Sn transport (finite difference and nodal),diffusion(finite difference and nodal), Monte Carlo, powerreconstruction methods, etc.

Power Distribution Within MOX FueledAssemblies(2)

Larger discrepancies appear at U-MOX interfaces and it waspointed out that discrepancies may also appear in non-symmetricconfigurations.

In conclusion, fine flux reconstruction can be achieved to asatisfactory precision except for local singularities.

The second phase benchmark exercise has been undertaken for powerdistribution within MOX fueled assemblies. Concerning corecalculations, a big spread of 860 pcm from the average value of keffwas observed. This is due to cross section libraries and methodused. As for the pin-by-pin power distributions, the average ofparticipants overestimates the power in the central MOX assemblywith a spread of about 3 % for each zone.

BWR MOX Benchmark

The previous work of the WPPR addressed fast reactors and PWRsand did not consider BWRs. As BWRs were intended for MOXburning by several member countries, the WPPR felt that it wasimportant to address the imbalance in its activities.

The specified benchmark compares calculations for a modern 10X10BWR design with assemblies containing MOX rods and U/Gd rodswith a large internal water structure. The fissile content allows for anaverage discharge burn-up of > 5050 GWd GWd/t/t and the isotopiccomposition of the plutonium corresponds to Pu from burnt UO2 fuelwith discharge burn-ups of > 50 Gwd/t.

BWR MOX benchmark

The results from participants showed that a similar accuracy isachieved for BWR as for the PWR benchmarks.

For BOL (beginning of life), an agreement within 1% for k-infinityis observed, and a difference slightly more than 1 % is found whencomparing ENDF/B-VI against JEF-2.2. For burn up cases, theagreement is better than that for BOL cases.

Innovative Plutonium fuelwithout uranium(1)

For ex-weapons and civilian plutonium, the plutonium burning inreactors has been emphasized as a means of minimising the risks ofproliferation.

From this perspective, the most effective way is adopting plutoniumwithout uranium. By using inert carriers for plutonium it is possibleto avoid the production of fresh Pu-239 from U-238 captures.

A specific task group established by PSI, JAERI, CEA andPolitecnico Milano summarised the present status and major issuesassociated with R&D of plutonium fuels with non- uranic carrier,concentrating in particular on the physics.

Innovative Plutonium fuelwithout uranium (2)

Some benchmarks have been carried out, in case of partially loadedIMF core in a PWR, the agreement among the solutions provided arewell for UO2, but discrepancies are larger for MOX fuel.

Further problem was encountered with the kinetic parameters inwhich the discrepancies are observed.

Inert Matrix Fuel Work Shop is held every year since 1995, andresearch and development is under way to find potential fuelcandidates.

On-Going Works

In the 5-th WPPR meeting on November 1996, future works werediscussed and the need for experimental analysis for MOX fuelcritical experiments in LWRs was proposed in order to validatethe calculations against the experiments as essential progression.

Participants agreed and several experiments were candidated suchas TCA (JAERI), VENUS-2(SCK/CEN), EOLE (Cadarache), andso on. These data sets are not available immediately, and wouldrequire the agreement of the original sponsors to release part or all ofthe data.

VENUS-2

The VENUS-2 programme carried out by SCK/CEN. This coreconsists of 3.3% and 4.0 % enriched UO2 rods and 2.7% Pucontent MOX rods. The comparison of calculations and experimentsare conducted for keff and pin-power distributions. The firstbenchmark is 2-D configuration model, and the second phase is 3-Dmodel. Participants were adopted the blind benchmark procedurewhich the experimental values are not given to the participants.Thirteen solutions were submitted and the agreement is generallysatisfactory(<500ppm reactivity, 5 percent on power in the UO2regions and 10 percent on power in the MOX region). The higherspread of pin powers in the MOX region is partly a result of the MOXrods being positioned at the core periphery.

Benchmarks proposed presently(1)

– HTR Benchmark

The WPPR address the issue of pebble bed reactors to avoid overlapfor the IAEA prism type benchmark. The PBMR concept is moreinteresting for physics studies than the prismatic type.

Proposals for HTR plutonium physics benchmark are presented byM.Delpech (CEA) and H.I, Rutten (Framatom). Furthermore, U.Kasemeyer presents the analysis of experiment carried out on HTR atthe PROTEUS facility. The experimental data are of interest, butthe PROTEUS geometry is complicated and difficult to model. Soit is a candidate for work in a second phase study.

Benchmarks proposed presently(2)

MOX core transient benchmark

A benchmark exercise on Kinetic Parameters viaCalculation/Experiment Comparison of Reactivity Effect inCROCUS is proposed. It is a 3-D problem and involve 4 waterheights and two control rods, six cases in all. The first phase is theUO2 fuel benchmark, and a similar exercise on MOX will follow asa further step.

In fact, large discrepancies due to data libraries have been found atPSI(20% in some cases).

This benchmark was endorsed by the WPPR.

Benchmarks proposed presently(3)

Benchmarks co-operated with TFRPD (The Expert Group onreactor-based Pu Disposition)

Three benchmarks are co-operated with TFRPD, that is, 3-D VENUS-2MOX Benchmark, which specifications have been prepared as BlindBenchmark on The 3-D VENUS-2 MOX Core Measurements at May 2001,KRITZ-2 Benchmarks consist of three low enriched uranium fuel cores andone MOX fuel core, and VVER-1000 MOX Benchmark proposed by KI.

Furthermore, a study related to plutonium management in the medium term(i.e. beyond LWR MOX, but prior to fast reactors) had been addressed.