Preparation and Testing ORIGEN-ARP Library for VVER Fuel Design

14 Preparation and Testing ORIGEN-ARP Library for VVER Fuel Design Maksym YEREMENKO, Yuriy KOVBASENKO, Yevgen BILODID State Scientific and Technical Centre on Nuclear and Radiation Safety (SSTC NRS), Radgospna st. 35-37, 03142 Kyiv, Ukraine The main results of neutron-physical constant libraries preparing and testing for assessment VVER reactors spent fuel isotope composition with the SCALE control module ORIGEN-ARP are presented in this report. The libraries were tested on the basis of the comparison between the calculation results with the experimental data, and with the calculation benchmarks. It should be mentioned that the particular attention was paid to important in the view of criticality analysis isotopes that are contained in the spent nuclear fuel. KEYWORDS: Nuclear Power Plant (NPP), reactor VVER, spent fuel, burnup credit, SCALE, ORIGEN-ARP, benchmark 1. Introduction Improvement of NPP capabilities in terms of spent nuclear fuel (SNF) management and storage, upgrading near-reactor cooling ponds and shipping casks, construction of SNF storage facilities require the relevant nuclear safety substantiation. According to the existing normative base, nuclear safety as to SNF management and storage system shall be analyzed without fuel burnup taking into account (i.e. fuel shall be treated as fresh one). This is too much conservative, and does not meet state-of-the-art science and technology. Presently, there are researches undertaken both in Ukraine and in many other countries, aimed at substantiation, and development, of such normative requirements allowing to use burnup credit while analyzing criticality of SNF management and storage systems. One of these activity main components is to identify the isotopic content of fuel spent in VVER-440 and VVER-1000 type reactors constituting the basis of nuclear power in Ukraine. To determine isotopic content of VVER-440 and VVER-1000 SNF, we use the package of computer codes SCALE 1) and, in particular, code ORIGEN-ARP 4). This program is among the most advanced ones with which help the isotopic content of SNF can be calculated. However, this program (its neutronic constant libraries) is oriented, firstly, at using for PWR, BWR, and CANDU reactors. That is why, while using this code to calculate isotopic content of VVER SNF, one shall, first of all, prepare the libraries of neutronic constants for VVER fuel, test them, and substantiate their applicability. Here, the main results are presented as to testing ORIGEN-ARP libraries of neutronic constants prepared by the authors taking into account geometrical structure, and material composition of fuel assemblies for VVER-440 and VVER-1000. This work is the continuation of our comprehensive testing of code package SCALE for VVER reactors fuel. For the previous stage results as regards testing SCALE criticality sequence calculation, see, for instance, references 1) and 2). Now, the testing was carried out by comparison of the results calculated by ORIGEN-ARP with the results of experiments, and calculations made by other programs. As the experimental data, the results of studying isotopic content of VVER-440, and VVER-1000 SNF completed in different years were used. As the calculated data used for testing ORIGEN-ARP with libraries for fuel assemblies of VVER-440, and VVER-1000, the special calculated benchmarks were used. While testing libraries, the special attention was paid to those isotopes, which are suggested to be used while undertaking the work on substantiation of nuclear safety taking the spent fuel burnup into account: - first of all, these are fission isotopes, i.e. those participating in chain reaction: 235U, 236U, 238U, and 239Pu, 240Pu, 241Pu; - secondly, actinides: 234U, 238Pu, 242Pu, 241Am, 243Am, 237Np; - as well as 18 main products of fuel isotopes fission caused by chain reaction: 95Mo, 99Tc, 101Ru, 103Rh, 105Pd, 108Pd, 109Ag, 133Cs, 135Cs, 143Nd, 145Nd, 147Sm, 149Sm, 150Sm, 151Sm, 152Sm, 153Eu, 155Gd. Here, the results of completed testing are presented. Upon which base the conclusion is made concerning possibility of using the libraries prepared for code

ORIGEN-ARP to determine isotopic content of VVER-440 and VVER-1000 fuel. Errors of identifying concentration of particular isotopes are being determined. The report was prepared within the framework of project «Validation of ORIGEN-ARP and SCALE SAS2H for VVER and RBMK Fuel Designs». 2. Analysis of the data used for testing. Let s consider in more detail the experimental results and calculation benchmarks data used for testing. 2.1 Fuel for VVER-440 Reactor 2.1.1 Publications 6) and 7) Information presented in 6) and 7) is based upon the results of measuring isotopic composition of SNF from VVER-440. Burnup of fuel, and concentration of isotopes were determined experimentally. Error is presented to each value of fuel burnup, and concentration of isotope, and this equals to about 5 7 %. Unfortunately, isotope concentration was determined long time ago (more than 20 years ago) for obsolete types of fuel. During experiment, only concentration of fuel isotopes, and actinides, was measured. The data required for the detailed modeling burnup conditions and for the subsequent storage of fuel are presented insufficientl. Because of this, we ve used the average parameters. Fuel structure is typical for the reactors VVER-440 (Figure 1). 2.1.2 Publication 8) For measurement, four FA were used from VVER-440 of Novo-Voronezh NPP. Initial enrichment of fuel was 3.3% and 3.6%. Selected assemblies were used during 1, 3, аnd 5 campaigns. Totally, isotopic content of 21 specimens with different burnup levels was determined. Reference 9) presents the data needed to calculate loading FA during operation inside core: schemes of fueling and refueling, schedules of in-service loading, etc. Burnup of fuel, and concentration of isotopes were determined experimentally. Error is presented to each value of fuel burnup, and concentration of isotope, and this equals to about 5 7%. Concentration of isotopes was measured quite long time ago, and for obsolete fuels. During experiment, only concentration of fuel isotopes, and actinides, was measured. Conditions of burnup and subsequent storage of fuel are described enough particularly. Fuel structure is typical for the reactors VVER-440 (Figure 1). 2.1.3 Publications 10) and 11) AER Calculational benchmark ( 10) and 11) ) is developed to study possibility of using various codes to determine isotopic content of SNF, and subsequent using this information while calculating criticality of fuel storage systems (using burn-up credit for VVER fuel). It was developed in the framework of the International Scientific Team AER (Atomic Energy Research) gathered experts from many countries were VVER reactors are now under operation (Bulgaria, Hungary, Russia, Slovakia, Czechs Republic, Finland). This benchmark presents description of the used fuel assembly, operation modes, and the obtained results. Burnup of fuel, and concentration of isotopes were determined by calculation. Error is not presented. Benchmark was calculated by a number of organizations. The following codes were used: CASMO-4, WIMS 7, NUKO, HELIOS, MCU, and ORIGEN2. Fuel burnup and subsequent storage conditions and, also, fuel structure are presented enough particularly. 2.2 Fuel for VVER-1000 2.2.1 Publication 12) Publication 12) presents the results of isotopic content measurement as to the VVER-1000 spent fuel. For this measurement, one FA was used from VVER-1000 under operation at Zaporizhya NPP. Its initial enrichment was 3.92%. This was under operation during one campaign, and unloaded from the core in 1999. Isotopic composition of three speciments with different burnup levels is presented. This assembly was operated over comparatively not so long time ago, and SSTC NRS has the information necessary to calculate loading FA throughout the period of its operation inside the core (fuel pattern, and refueling, reactor power during operation of unit, etc.). Burnup of fuel, and concentration of isotopes were determined experimentally. Error is presented for each value of burnup, and isotope concentration, being equal approximately to 1 7 %. Isotope concentrations were measured quite recently, and for the fuel of new type. However, this assembly was operated for one campaign, and the achieved level of burnup is insufficient. During the experiment, only fuel isotopes, and actinides, concentration was determined. Conditions of fuel burnup, and subsequent cooling are described quite in detail. Fuel is of the known design. 2.2.2 Publication 13) Here, the calculation benchmark specification, is presented which was developed for study of different codes possibilities for the modeling of modern VVER-1000 reactor fuel types (fuel with gadolinium burnable absorber, and МОХ fuel). The main attention was paid to K inf calculation and to determination of spent fuel isotope composition. Benchmark was

prepared in the framework of NUCLEAR ENERGY AGENCY ORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENT (NEA/OECD). This benchmark presents spent assemblies description, their operation conditions and result of calculations for fuel burnup and isotopes concentration. Uncertainty isn t indicated. Several organizations had carried out the benchmark calculation. The following codes were used: MCU, TVS-M, WIMS8A, HELIOS аnd MULTICELL. Burnup conditions and fuel structure are described enough particularly. Below in this report only the data were used on determination of isotope composition of fuel with gadolinium burnable absorber. INPLEVEL = 1 is used. If we need to model the deviations from this regular structure, then the commands INPLEVEL=2 or 3 should be used. SAS2H capacities of modeling fuel elements lattice irregularity was used. SAS2H control module calculational model for the VVER-440 reactor assembly is presented in the Figure 2. Provided the material quantity is kept constant for the corresponding calculational zones, the following equivalent dimensions (cm.): r 1 =0.440; r 2 =0.515; r 3 =0.64054; r 4 =7.21858; r 5 =7.40303; r 6 =7.56054; r 7 =7.71805- were calculated. 3. Preparing of neutron-physical constant libraries for ORIGEN-ARP. It should be mentioned that ORIGEN-ARP is the method of isotope content in spent fuel calculation based on the neutron-physical constant libraries extrapolation. These libraries were received with the help of control module SAS2H 5). Taking into account this fact, the ORIGEN-ARP calculations were done after the required ORIGEN-ARP libraries were calculated with the help of control module SAS2H 5). After receiving the necessary libraries, the spent fuel isotope composition was calculated for the comparison with the data of experiments and benchmarks. While ORIGEN-ARP libraries calculating, SCALE standard library 44GROUPNDF5 was used. The choice of this library is conditioned, firstly, by proposals of SCALE codes package manuals and, secondly, by precence of the particular gadolinium isotopes in the library. 1- central tube; 2- fuel rod; 3- fuel assembly shroud Fig. 1 Scheme of Fuel Rod Lattice of VVER-440 Fuel Assembly 3.1 Neutron-physical constant libraries for VVER-440 fuel The SAS2H control module is based on the ORIGEN-S code modeling fuel in one-dimensional approximation. A fuel assembly is described as infinite lattice of fuel rods. For VVER, this lattice is hexagonal. However, a number of fuel rods disturb the regularity of this lattice. Central tube, inter-assembly water gap and assembly shell are the elements that disturb regularity of gexagonal fuel pins lattice (Figure 1) for the VVER-440 fuel. These components essentially impact on the neutron spectrum and fuel irradiation conditions, therefore, the possibility of their modeling should be provided in calculations. The SAS2H capabilities allow consideration of some differences between the actual fuel assembly design and the standard one. This is done through the INPLEVEL command permitting different options in modeling a fuel assembly. For example, if a completely regular fuel lattice is modeled, the command 1 2 3 4 5 6 7 - Uranium-water mixture (4) - Zirconium, central tube (2) or assembly shroud (6) - Water (1,3,5,7) Fig. 2 Model of VVER-440 fuel for control modulesas2h

3.2 Neutron-physical VVER-440 fuel constant libraries for account actual location of the fuel rod selected for the further analysis. The ORIGEN-ARP codes libraries for the fuel VVER-1000 were calculated with the help of control module SAS2H according to the same scheme as for the fuel VVER-440. Fuel assembly sketching is presented in the Figure 3. Position of fuel pins with gadolinium refer only to fuel assembly from the 13) benchmark NEA/OECD. In the other fuel, the usual fuel pins are situated at these places. 1 2 3 4 5 - Fuel with gadolinium (NEA/OECD benchmark ) or water (1) - Uranium-water mixture (4) - Zirconium, fuel pin cladding or central tube (2) - Water (3,5) Fig. 4 Model of VVER-1000 fuel for control module SAS2H 1 fuel pin; 2 guide tube for control rod or burnable absorber; 3 central tube; 4 fuel pin with or without gadolinium. Fig. 3 Scheme of Fuel Rod Lattice of VVER-1000 Fuel Assembly For the modeling of fuel pins lattice irregularity of VVER-1000 reactors assembly, the calculational model (presented in the Figure 4) SAS2H control module capacities were used as in the previous case. Under the condition that the material quantity is kept constant, the following equivalent dimensions for the calculational zones (cm) were calculated: - fuel with gadolinium (benchmark NEA/OECD) r1=0.55; r2=0.65; r3=0.66942; r4=2.866; r5=2.92056; - fuel without gadolinium r1=0.55; r2=0.65; r3=0.66942; r4=2.866; r5=2.92056. All the above-mentioned in sections 3.1 and 3.2 is related to the conditions of modeling a fuel assembly as a whole, i.e. calculations of isotopic content averaged by FA are concerned. Experimental data of isotopic content were obtained for individual fuel rods; in so doing, location of these fuel rods in the FA lattice impacts on their isotopic composition. In the above regard, the experiments should be modeled taking into It s impossible to do this, in calculations with the help of ORIGEN-ARP sequence because of the sotopes concentration for the fuel assembly is determined in this calculations altogether. It is possible to take into account the real fuel pins placement while isotope concentration calculations with the help of SAS2H sequence (these results are presented in the report 14)). 4. Results of calculations Tables 1 and 2 present the mean uncertainties of the particular isotopes concentration calculation for the spent fuel VVER-440 and VVER-1000, which were received while calculating of experiments and benchmarks. In accordance with the experiments and benchmarks data, the fuel was modeled, which burnup and storage time are presented in the Table 3. Generally, it can be mentioned that according to the calculations carried out by ORIGEN-ARP, the correctness of the used models and libraries was demonstrated. Results have negligible deviation from the benchmark data. Coincidence with the experiment data is worse, but one should remember that it s impossible to take into account the real fuel pins position in the assembly while experiment condition modeling with the help of ORIGEN-ARP. It can involve the additional deviations.

Тable 1 Deviation of calculation results from experiment and benchmarks data for VVER-440 fuel experiment benchmarks 234 U -10.2% - 235 U 4.5% -0.8% 236 U -2.3% 4.0% 238 U -0.2% -1.6% 237 Np - 3.2% 238 Pu -12.7% -0.9% 239 Pu 3.2% -3.2% 240 Pu -1.6% -1.4% 241 Pu 18.9% 0.5% 242 Pu 17.5% 7.2% 241 Am - 4.1% 243 Am 79.4% 11.5% 95 Mo - 0.4% 99 Tc - 0.9% 101 Ru - -0.8% 103 Rh - 3.2% 109 Ag - 12.5% 133 Cs - 2.5% 143 Nd - 0.2% 145 Nd - 0.9% 147 Sm - 3.0% 149 Sm - -5.3% 150 Sm - 13.6% 151 Sm - 15.1% 152 Sm - 14.1% 153 Eu - 0.3% 155 Gd - -35.8% Тable 2 Deviation of calculation results from experiment and benchmarks data for VVER-1000 fuel experiment benchmarks 234 U -17.0-235 U -2.6-0.9 236 U -19.5 17.9 238 U 0.2 0.0 238 Pu -31.3-239 Pu -11.9-3.9 240 Pu -12.6-0.1 241 Pu -15.1 4.0 242 Pu -15.1 12.7 241 Am -81.6-243 Am 40.8-149 Sm - 1.0 155 Gd - 0.0 Тable 3 Experiments and benchmarks data in referance to the burnup and the storage time of fuel. Burnup, VVER MwtDay/kgU Cooling time, years Exp. Bench. Exp. Bench. 440 8.7-54.8 30-40 0 0-5 1000 16.1-17.2 0-40 0 0 It should be underlined that the available results of experimental assessment of VVER spent fuel isotope composition and calculational benchmarks data don t allow to provide the full ORIGEN-ARP libraries testing for all the isotopes chosen for the burnup credit while analysis of storage and transportation systems criticality. So, by the moment, the criticality assessment for VVER spent fuel management systems with burn-up credit approach can be fulfilled only taking the fissionable isotope concentration into account. 5. Conclusion Analysis of the results permits the following conclusions: 1. ORIGEN-ARP sequence permits sufficiently accurate determination of isotopic composition of VVER-440 spent nuclear fuel. 2. The most comprehensive testing based on independent sources including both experimental results and calculational benchmarks has been conducted for 235U, 236U, 238U, 239Pu, 240Pu, 241Pu, 242Pu fuel elements. 3. While experiment condition modeling with the help of ORIGEN-ARP, it is impossible to take into account the real fuel pins position in the assembly. That creates additional inaccuracy. SAS2H control module usage is more reasonable for this purpose. 4. The usage of control module SAS2H altogether with the 44GROUPNDF5 library can be recommended for the ORIGEN-ARP library preparing for the further VVER-440 and VVER-1000 spent fuel composition calculation. Acknowledgements The authors are grateful to the US Government for financial support in realization of this work. References 1) Y.Kovbasenko, V.Khalimonchuk, A.Kuchin, Y.Bilodid, M.Yeremenko, O.Dudka Validation of SCALE sequence CSAS26 for criticality safety

analysis of VVER and RBMK fuel designs NUREG/CR-6736, PNNL-13694, 2002. 2) Y.Kovbasenko, Y.Bilodid Study of Local Power Distribution in Fuel of VVER-1000 Reactors with SCALE-4.3, Sixth International Conference on Nuclear Criticality Safety (ICNC 99), Proceeding v.iii, p.1104-1113, September 20-24, 1999, France. 3) SCALE: A Modular Code System for Performing Standardized Computer Analysis for Licensing Evaluation, NUREG/CR-0200, Rev. 6 (ORNL/NUREG/CSD-2R6), Vols. I, II, and III, May 2000. Available from Radiation Safety Information Computational Center at Oak Ridge National Laboratory as CCC-545. 4) S. M. Bowman, L. C. Leal. ORIGEN-ARP: AUTOMATIC RAPID PROCESS FOR SPENT FUEL DEPLETION, DECAY, AND SOURCE TERM ANALYSIS. Oak Ridge Natl. Lab., 2000. 5) O. W. Hermann, C. V. Parks. SAS2H: A COUPLED ONE-DIMENSIONAL DEPLETION AND SHIELDING ANALYSIS MODULE. Oak Ridge Natl. Lab., 2000. 6) A. Herman, W. Möller Bestimmung der Abhängigkeit von Nuklidgehalten in VVER-440 Brennstoff nach rechnerischen und experimentellen Methoden, Kernenergie 27 (1984), p. 255. 7) B.Bibichev, V. Mayorov, Yu. Protasenko, P. Fedotov. Measuring fuel burnup, and U and Pu content in FA for VVER-440 as regards activity of 134Cs, and 137Cs - Atomic Energy, 1988, v. 64, issue 2, p. 147 (in Russian). 8) A.Stepanov, T. Makarova, B. Bibichev, et al. Determination of burnup, and isotopic composition of SNF from VVER-440. Аtomic Energy, v. 55, issue 3, September, 1983 (in Russian). 9) Kravchenko Yu.N., Polyakov Yu.N., Aleshin S.S. BENCHMARK CALCULATION OF FUEL BURNUP AND ISOTOPE COMPOSITION OF VVER-440 SPENT FUEL. 8th AER Symposium on VVER Reactor Physics and Reactor 10) L.Markova. Calculation Burnup Credit Benchmark No.2 (CB2). 7th AER SYMPOSIUM on VVER Reactor Physics and Reactor Safety Hornitz near Zittau, Germany, Sept. 23-26,1997 11) L.Markova. CB2 Result Evaluation (VVER-440 Burnup Credit Benchmark). 7th AER Symposium on VVER Reactor Physics and Reactor, Slovakia, October 4-8, 1999 12) A. Tataurov, V. Kvator. R&D Report. Calculated-experimental studying nuclide composition of SNF from VVER-1000, and RBMK-1000. RNC КI, 2002 (in Russian). 13) A VVER-1000 LEU and MOX Assembly Computational Benchmark. Specification and Results. OECD 2002. 14) Y.Bilodid, M.Yeremenko, Y.Kovbasenko. TESTING THE SAS2H SCALE CONTROL MODULE ON VVER TYPE FUEL. Abstract No12. ICNC2003