The GRR-1 is a 5-MW pool-type, light-water-moderated and-cooled reactor fueled with MTR-type fuel elements. Recently received Be reflector blocks will soon be added to the core to add additional reactivity until fresh LEU fuel arrives. REBUS-3 xy fuel cycle analyses, using burnup dependent cross sections, were performed to assist in fuel management decisions for the water- and Be-reflected HEU nonequilibrium cores. Cross sections generated by EPRI-CELL have been benchmarked to identical VIM Monte Carlo models. The size of the Be-reflected LEU core has been reduced to 30 elements compared to 35 for the HEU water-reflected core, and an equilibrium cycle calculation has been performed

Deen, J. R.

Papastergiou, C.

Snelgrove, J. L.

UNT Digital Library

GREEK RESEARCH REACTOR PERFORMANCE CHARACTERISTICS AFTERADDITION OF BERYLLIUM REFLECTOR AND LEU FUEL*J. R. Deen and James L. SnelgroveANL/EP/CP---77 803Argonne National Laboratory DE93 002930Argonne, Illinois, USAand8" ' " _ _ -' -" " C. Papastergiou"_ .- .- "__, _ _ s National Center for Scientific Research= _ _,=._ _ __e_ t_ _ ... t,.d -F--a_. _,_ '= ,_ _ o-" Athens, GreeceO_ = E o=._o_oS=E =o _ o -, = _ _ _,,,, To be Presented at the ......, _. _ _ O. _'."t - - _ _ _ ) -_ 1992 International Meeting on=_, = 8 .. ,_ _ Reduced Enrichment for Research and Test Reactors= _ _ _ ,_ g-_ _ __,_ September 27 - October 1, 1992._ _ ,, ._8,.. r, o _ _ Roskilde, Denmarkt: = ._ _.E _ .." o e*r_ o ,o i_, o _ [= ¢1 _) The submitted manuscript has been authoredby a contractor of the U.S. Governmentunder contract No. W-31-109-ENG-38.Accordingly, the U. S. Government retains anonexclusive, royalty-free license to publishor reproduce the published form of thiscontribution, or allow others to do so, forU. S. Government purposes. iASiER*Work supported by the U.S. Department of Energy,Office of Arms Control and Nonproliferationunder Contract No. W31-109-ENG-38 _.to.(.;BISTRIBUTIONFTHISOOCUMENTIS UNLtMll._.... .._......_-.... ..... . .. ....... . ..._.._,....._._.-_-...*'_'_'*''"_•• • ......... 0.GREEK RESEARCH REACTOR PERFORMANCE CHARACTERISTICS AFTERADDITION OF BERYLLIUM REFLECTOR AND LEU FUELbyJ. R. Deen and James L. SnelgroveArgonne National LaboratoryArgonne, Illinois, USAandC. PapastergiouNational Center for Scientific ResearchAthens, GreeceABSTRACTThe GRR-1 is a 5-MW pool-type, light-water-moderated and-cooled reactorfueled with MTR-type fuel elements. Recently received Be reflector blockswill soon be added to the core to add additional reactivity until fresh LEU fuelarrives. REBUS-3 xy fuel cycle analyses, using burnup dependent crosssections, were performed to assist in fuel management decisions for the water-and Be-reflected HEU nonequilibrium cores. Cross sections generated byEPRI-CELL have been benchmarked to identical VIM Monte Carlo models.The size of the Be-reflected LEU core has been reduced to 30 elementscompared to 35 for the HEU water-reflected core, and an equilibrium cyclecalculation has been performed.INTRODUCTIONThe Reduced Enrichment Research and Test Reactor (RERTR) Program and the NationalCenter for Scientific Research "Demokritos" initiated a new joint study program in 1991 toinvestigate the conversion of the Greek Research Reactor (GRR-1) from HEU to LEU fuel.This study is a continuation of a previous joint study _ completed in 1981. This paper presentsthe results of modeling the current HEU core operations and the core changes expected uponadding a new Be reflector on two core faces. Since LEU fuel is not expected before August1993 and fresh lIEU fuel supplies are very low, some attention is being devoted to fuel cyclestrategies to increase the lifetime of the Be-reflected HEU core. One equilibrium Be-reflectedLEU core design will be presented that has fuel cycle characteristics similar to the currentHEU core. In addition to providing guidance on fuel management strategies, thesepreliminary calculations will provide fuel element loadings for use in analyzing the transitionto an LEU core.GRR-I CORE AND FUEL CHARACTERISTICSThe GRR-1 is a 5-MW pool-type, light-water-moderated and -cooled reactor fueled withMTR-type HEU fuel elements. The core consists of 29 or 30 standard fuel elements and sixcontrol fuel elements. Each standard element consists of 18 flat fuel plates, and the controlelements have ten fuel plates. A Ag-In-Cd control rod is inserted into the center of eachcontrol element. In the future the core will be used for various isotope production projectsand Si-doping. The dimensions of the fuel plate and coolant volume of the proposed LEUfuel element are identical to those of the existing HEU design. A comparison of HEU andLEU fuel element characteristics is presented in Table 1.TABLE 1STANDARD AND CONTROL FUEL ELEMENT CHARACTERISTICSDESIGN PARAMETER STANDARD CONTROLHEU LEU HEU LEUEnrichment, wt. % 93 19.75 93 19.75Fuel Meat Composition UAI U3Si2 UAI U3Si223sU/Element, g 180.8 222.2 100.3 123.4Number of Plates/Element 18 18 10 10Plate Thickness, cm 0.152 0.152 0.152 0.152Fuel Meat Thickness, cm 0.050 0.076 0.050 0.076Clad Thickness, cm 0.051 0.038 0.051 0.038Water Channel Thickness, cm 0.290 0.290 0.290 0.290,,,THE FUEL CYCLE ANALYSIS MODELThe fuel cycle analyses were performed using REBUS-3 with five neutron groups in xygeometry) The increased accuracy that would be gained by the use of three-dimensionalanalyses would be small for the GRR-1 because of its low discharge fuel burnup. Theburnup-dependent microscopic cross sections were obtained from a slab geometryrepresentation of important core materials using EPRI-CELL) Based on previous studies, theside plate was separated from the fuel-clad-moderator zone to provide more accuratereactivity and power peaking predictions from the diffusion theory model. The fuel zonecross sections were found to be in excellent agreement with those obtained from a VIM4Monte Carlo calculation of the fuel cells using ENDF/B-IV data.Modeling of ex-core materials _onsisted of representing the graphite thermal column and thePb gamma shield located between the core face and the thermal column. Other ex-corefeatures, including experiments, beam tubes, and irradiation figs, were omitted from thismodel because of insufficient information. We believe this simplified model is sufficientlyaccurate for this stage of the study.An estimate of the reactivity bias of the fuel cycle model was obtained from an xyz diffusiontheory model of an Ml-fresh GRR-1 startup core configuration 4B-2. With control rods attheir critical positions, l%trwas calculated to be 1.010. This overprediction in reactivity isprobably caused by inaccuracies in radial and axial ex-core material descriptions as well as bythe diffusion model itself. This reactivity bias has been adopted for all fuel cycle analyses.The extrapolation distances for the planar fuel cycle model were obtained from this fresh fuelstartup core calculation.ANALYSES OF THE HEU WATER-REFLECTED CORE23SUloadings for each element in core and in the storage pools were provided by the GRR-1staff and used to model the actual HEU core fuel cycle beginning on October 26, 1990. Thecore consisted of 29 lIEU standard elements and six control elements arranged as shown inFig. 1. The GRR-1 235U loadings were used in conjunction wi_ an EPRI-CELL depletionmodel of the standard and control elements to obtain a complete set of fuel isotopics for eachelement in the core.The operation of the October 26, 1990 core was followed for 6.66 fpd, after which a freshfuel element was added and the highest burnup fuel element was discharged. This refueledcore was depleted for an additional 22.61 fpd and shut down on May 31, 1991. At this pointa spent fuel element was added to the core in position D-6 without discharging any fuel. Thisincreased the core size to 30 standard elements and six control elements and the _35Ucoreloading by 126 g. This core configuration has remained unchanged to date. The currentoperation schedule has been significantly reduced due to the shortage of additional HEU fuelsupplies.The typical operation schedule for a week at the GRR-1 consists of full-power operation forfive to six hours per week day; the reactor is shut down at all other times. From the exactshutdown and operation intervals, the core excess reactivity was calculated for June 3-7, 1991using the REBUS fuel cycle model. The calculated core reactivities at the beginning and endof each irradiation day are compared to hypothetical equilibrium steady-state operation overthe same period in Fig. 2. On Monday morning, with t35Xe at its lowest level of the week,the core l%rf = 1.066. At the end of the day, the core reactivity has fallen to k = 1.060 due tothe buildup of _3SXe. For the remainder of the week, the core reactivity increases during theday due to the burnout of _35Xeaccumulated during the preceding night's shutdown period.During the week the startup excess reactivity decreases each day except Friday, when a slightincrease in excess reactivity has been observed at the GRR-1 and predicted using this model.From these results, shown in Fig. 3, one can also note that the GRR-1 never reachesequilibrium _35Xe. The 1'_9Sm concentration changes very little and remains betweensaturation and equilibrium values during the entire week, as shown in Fig. 4. This operationschedule, with short irradiation intervals and relatively long shutdown periods, maintainspoison concentrations low enough to provide the excess reactivity required for continued operation.HEU Water Reflected GRR-1 Reference Core 4B-13654321A B C D E FFigure 1GRR--1 REBUS Reactivity During June 3-7,1991tt.o9 : II I1I!= EQU'ILIB ,,1.07 ......................_......................L.....................J.....................,......................'........... -_ s'rARTUPSHL'TDWNOj I ..........................................I t.06 ....................... _............. ,. ...... -_..................... J"............................................._ ,II l ...................... i ......................1.05 ...................... t ..................... .L..... I!O C fIItI1/.•. ,,-., n_ ..................... t ..................... a ..................... ..................... . - ,li I I, 1 : 'i: l ' l1.03 " 6.0 -'.O,.o i.o _'.o _.o ,;.o _.oTime, daysFigure 2GRR-I REBUS Xe-135 Inventory During June 3-7, 19914,0.0 ........t.,,30.0 .................. _......... ._ .... J........ _...... _........ i ....I ................................I' 1 : ._o EQUILIB_, , o STARTUPT , sH Dw_ ' ,i !.w,4 _ ! !20.0- ......................_.......................t.....................s,.. ...................j......................j............................................_ .7 °X oA.-_ta.n....................... _...................... L............ e. ......J-..................... J...................... ,............ _........ ,.....................I II i,, ,,I, lI i| ii: o IIi io.o ..... i ................. !0,0 LO 2.0 3.0 4.0 5.0 6.0 7.0Time, daysFigure 3GRR-I REBUS Sm-14,9 Inventory During June 3-7, 1991500.0o EQUILIB400.0 ................ _.......... _ .............. _".............. ' ....... ' ........ STARTUP0_ SHUTDWNCk.O O OO _O A AA2300.0 ............................................._"....................._"....................._......................' .........................................ii: ,,I i_oo.o ..... , - -o.o _..o z.o 3.0 ,,.o _.o S.o ?.oTime, daysFigure 4ANALYSES OF THE HEU Be.REFLECTED COREThe actual depletion of the current water-reflected core has been modeled up to the currenttime and projected to January 1, 1993, assuming only 50 MWh of operation per week inorder to conserve fuel. The Be blocks were received during this past summer and areexpected to be positioned as shown in Fig. 5. The core size will be reduced from 30 standardand six control elements to 25 standard and five control elements by removing the highest-burnup elements. This Be-reflected core has an excess reactivity at startup of kefr= 1.0382,which is very close to the beginning of cycle reactivity of an HEU water-reflected equilibriumcore. This core loses about 85¢ in reactivity for each 10 fpd of operation. Therefore, the Be-reflected HEU core can be operated on the reduced operation schedule with the remainingfresh HEU fuel inventory until the summer of 1993 when the LEU fuel supplies arrive. If itshould be deemed necessary to substitute an additional control element for a standard fuelelement, approximately $2 in reactivity would be lost. Other fuel cycle options that mightprove necessary would be an increase in core size using previously discharged fuel elements.ANALYSES OF THE LEU Be-REFLECTED COREAn equilibrium fuel cycle calculation of a preliminary LEU core configuration, shown inFig. 6, has been made. One fresh standard element was loaded at the beginning of eachcycle, and one control element was loaded at the beginning of each fourth cycle. The totalresidence time for ali fuel was 24 cycles. The cycle length was chosen to be 10.3 days witha core consisting of 24 standard fuel elements and 6 control elements. The smaller LEU corehas the same excess reactivity at the beginning and end of the cycle and the same cyclelength as the reference water-reflected HEU equilibrium core. The power peaking in the LEUcore has increased by 20%, which is inversely proportional to the core size decrease. Theequilibrium discharge burnup in the LEU core was reduced to 24%. This core represents aninitial attempt to design a replacement LEU core. If the LEU number of elements wasreduced by removing one standard element in location C-6 in order to provide an in-coreirradiation position, the core reactivity would be reduced by $2.5 for the same cycle length.Another option would be to remove one control element to provide an in-core irradiationposition, in which case the reactivity penalty would be decreased.HEU Be Reflected GRR-1 Core9 Be Be H20 H20 Be Be,,,H208 H20 H20 Be Be H20 H20H20 ,7 H20 H20 Be Be H20 H20__StandardFuel6 Assemblies,, _Control"' -'__ FuelH20 5 Assemblies.,43H20,,.I • Il Be Be Be Be Be BeA B C D E FH20Figure 5 Be Core HEU Configuration with 25 + 5 ElementsLEU Be Reflected GRR-1 Core9 Be Be H20 H20 Be Be8 H20 H20 Be ....Be H20 H20 H20H207 H20 H20 Be Be H20 H20_____.Standard6 FuelAssemblies,,,- --------Control5 ---__-FuelH20 Assemblies43 H202I Be Be Be Be Be BeA B C D E FH20Figure 6. Be Core Configuration With LEU Fuel 24 + 6 ElementsLocations of experimental facilities will significantly affect the final core configuration. Thereactivity performance could be increased without a change in core size by replacing onecontrol element with a standard element if the core can be adequately controlled using five Lcontrol elements.CONCLUSIONSThe GRR-1 has had to significantly reduce its operation schedule caused by a shortage offresh HEU fuel. Once ali the necessary preparations for installation of the Be reflector forthe HEU core have been completed, additional reactivity can be added to extend the corelifetime until LEU fuel arrives next summer. The use of the reactor for isotope productionand Si-doping will significantly affect the final decision regarding the most suitable LEU coreconfiguration. The issues of transition from HEU to equilibrium LEU cores by the additionof one element per cycle must also be addressed.REFERENCES1. C. Papastergiou and J. Deen, "Neutronic Calculations for the Conversion of the GRR-1Reactor from HEU Fuel to LEU Fuel," unpublished ANL report, (November, 1981).2. B.J. Toppel, "A User's Guide for the REBUS-3 Fuel Cycle Analysis Capability,"ANL-83-2, (March, 1983).3. B.A. Zolotar, et al., "EPRI-CELL Code Description," Advanced Recycle MethodologyProgram System Documentation,", Part II, Chapter 5, (September, 1997).4. R.N. Blomquist, "VIM - A Continuous Energy Neutronics and Photon TransportCode," pp. 222 - 224, ANS Proceedings of the Topical Meeting on Advances inReactor Computations, Salt Lake City, Utah, (March 28 - 31, 1983).v

Greek research reactor performance characteristics after addition of beryllium reflector and LEU fuel

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Greek research reactor performance characteristics after addition of beryllium reflector and LEU fuel

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