1. А Е Ц “К О З Л О Д У Й” - Е А Д N P P K O Z L O D U Y – P L C 17 th Symposium of AER Y alta, Crimea, September 24-28, 2007 WWER-1000 SPENT FUEL NUCLIDE INVENTORY AT THE KOZLODUY NPP K. Kamenov, D. Hristov NPP Kozloduy, Bulgaria K_Kamenov@npp.bg DVHristov@npp.bg

2. Subject   This paper contains a presentation and discussion of selected isotope inventory results for different types of WWER-1000 spent fuel assemblies. Codes used  The required nuclide inventory calculations are performed using the computer code system SCALE 4.4a [1]. A specific ORIGEN-S library [2,3,4], developed at the Kozloduy NPP for typical irradiation conditions is used for each different fuel assembly type.

3. Introduction   According to the procedures for spent fuel transportation across the border for final disposal, beside the residual heat release in each cask, there is a requirement to report the concentrations of selected major fuel isotopes. These are 235 U, 236 U, 238 U, 239 Pu, 240 Pu, 241 Pu, 242 Pu. As well as the total amounts of 235 U+ 236 U+ 238 U and of 239 Pu+ 240 Pu+ 241 Pu+ 242 Pu.   The burnup dependent concentrations of uranium and plutonium isotopes calculated using ORIGEN-S are compared with data submitted by the fuel supplier.   Furthermore, a comparison is made between the ORIGEN-S calculated concentrations (of all major U and Pu isotopes, as well as of 237 Np, 241 Am and 243 Am) and corresponding results obtained using the HELIOS-1.5 [5] lattice code.

4. Comparison between ORIGEN-S and fuel supplier data (1)   As presented in [3,4], in addition to the standard 17x17 ORIGEN-S library, a specific library for each different WWER fuel assembly type has been developed at the Kozloduy NPP. These libraries have been verified against the standard 17x17 library and HELIOS-1.5 calculated data.   The present estimation of the relative deviations between the ORIGEN-S calculated isotope concentrations and data submitted by the fuel supplier is made for a profiled TVSA type fuel assembly with 4.30wt% initial 235 U enrichment and 6 Gd pins. All calculations are performed at typical irradiation conditions. The reported fuel supplier data are produced using the TVS-M lattice code. The concentrations of the U and Pu isotopes in [kg/tHM] are presented in Fig.1÷7.

5. Fig. 1 Burnup dependence of the 235 U concentration (ORIGEN-S vs TVS-M)

6. Fig. 2 Burnup dependence of the 236 U concentration (ORIGEN-S vs TVS-M)

7. Fig. 3 Burnup dependence of the 238 U concentration (ORIGEN-S vs TVS-M)

8. Fig. 4 Burnup dependence of the 239 Pu concentration (ORIGEN-S vs TVS-M)

9. Fig. 5 Burnup dependence of the 240 Pu concentration (ORIGEN-S vs TVS-M)

10. Fig. 6 Burnup dependence of the 241 Pu concentration (ORIGEN-S vs TVS-M)

11. Fig. 7 Burnup dependence of the 242 Pu concentration (ORIGEN-S vs TVS-M)

12. Comparison between ORIGEN-S and fuel supplier data (2)   It can be concluded that the results are in good agreement. The relative deviation between ORIGEN-S and fuel supplier data for 235 U are 0÷2.5%, for 238 U: 0÷0.1% and for 239 Pu: 0÷3.8%.   The total amounts of the U and Pu are also evaluated, as required by the procedures for spent fuel transportation across the border. The relative deviations between the total amounts of U and Pu isotopes are respectively 0÷0.05% and 0÷3.2%.   The same calculations have been carried out for the old type nonprofiled 4.4wt% assembly. The ORIGEN-S calculated data based on the standard 17x17 library are compared with the fuel supplier data. The results are in the same range as those for the considered TVSA fuel type.

13. Comparison between ORIGEN-S and HELIOS-1.5 data (1)   HELIOS-1.5 calculated TVSA isotope concentrations ( 235 U, 238 U, 239 Pu, 237 Np, 241 Am, 243 Am) are used for verification of the ORIGEN-S calculated data. All calculations are performed at typical irradiation conditions, not accounting for the actual power history. The results are presented in Fig. 8÷13.   The relative deviations between ORIGEN-S and HELIOS-1.5 data, for 235 U are 0÷2%, for 238 U: 0÷0.12% and for 239 Pu: 2.5÷8%. The relative deviations between the total amounts of 235 U+ 236 U+ 238 U and of 239 Pu+ 240 Pu+ 241 Pu+ 242 Pu are respectively 0÷0.15% and 4÷8.2%.   The ORIGEN-S calculated concentrations for almost all selected isotopes (except 238 U) are slightly higher than the respective HELIOS-1.5 calculated data.

14. Fig. 8 Burnup dependence of the 235 U concentration (ORIGEN-S vs HELIOS-1.5)

15. Fig. 9 Burnup dependence of the 238 U concentration (ORIGEN-S vs HELIOS-1.5)

16. Fig. 10 Burnup dependence of the 239 Pu concentration (ORIGEN-S vs HELIOS-1.5)

17. Fig. 11 Burnup dependence of the 237 Np concentration (ORIGEN-S vs HELIOS-1.5)

18. Fig. 12 Burnup dependence of the 241 Am concentration (ORIGEN-S vs HELIOS-1.5)

19. Fig. 13 Burnup dependence of the 243 Am concentration (ORIGEN-S vs HELIOS-1.5)

20. Comparison between ORIGEN-S and HELIOS-1.5 data (2)   As mentioned above, the ORIGEN-S calculations were performed using a specific library generated at averaged burnup conditions (power, boron concentration, temperature) and with geometry shown in Fig. 14. This geometry model is obviously quite simplified and can lead to higher uncertainties and deviations in the calculated isotope concentrations. Unlike ORIGEN-S, HELIOS-1.5 is able to make the exact geometry and material description of the assembly (Fig. 15). Thus, the HELIOS-1.5 calculations should be considered as more reliable or at least appropriate for the verification of ORIGEN-S calculated data.   Concerning the overestimation of the 239Pu content by about 8 % (Fig. 10), it can be explained in the following way.

21. Comparison between ORIGEN-S and HELIOS-1.5 data (3)   An essential inaccuracy is introduced at the stage of the ORIGEN-S library generation – as shown in Fig. 14, the material in the central region is Gd 2 O 3. Thus, since the huge thermal neutrons absorbers 155 Gd and 157 Gd are surrounded by water, an excessive flux spectrum hardening takes place in the central part of the model. With a more realistic representation of the Gd pin as a mixture of fuel and Gd 2 O 3 this spectrum distortion will be much smaller. This is demonstrated in Fig. 16 wich shows a comparison between 239 Pu contents obtained using a KNPP generated ORIGEN-S library and another one, prepared by Dr. Christoskov from the Faculty of Physics of the University of Sofia.   However, even with the currently used KNPP ORIGEN-S libraries the results for WWER-1000 fuel are still sufficiently good – apparently because the Gd content in this fuel is much lower than in PWR fuel types.

22. zon e Radiusmaterial 10.05644 cmGd 2 O 3 -density 7.41 g/cm 3, T=951 К 20.563 cmWater-density 0.723 g/cm 3, 0.6g/kg B, T=575.15 К 30.6508 cm 98.97% Zr+1% Nb+0.03 Hf,  =6.55 g/cm 3, T =575.15 К 42.8620 cmNuclear fuel (МХ500) 52.8792 cm 98.47% Zr+1% Nb+0.5% Fe+0.03Hf,  =6.55 g/cm 3, T=575.15 К 62.9206 cmWater-density 0.723 g/cm 3, 0.6g/kg B, T=575.15 К Fig. 14 Simplified TVSA geometry for ORIGEN-S library generation module SAS 2H

23. Fig. 15 Real 1/6 TVSA geometry for HELIOS-1.5 calculations

24. Fig. 16 Burnup dependence of the 239 Pu concentration (ORIGEN-S vs HELIOS-1.5)

25. Conclusions   Concentration data about some major fuel isotopes, calculated with ORIGEN-S and HELIOS-1.5, are presented and discussed in this paper. The results are compared with relevant fuel supplier data.   It is concluded, that ORIGEN-S, if used with a specific library for each different fuel type, can provide reliable isotope concentration estimations.

26. References   [1] SCALE-4.4a, Modular Code System for Performing Criticality and Shielding Analyses for Licensing Evaluation, RSICC Code Package C005445.   [2] I. D. Christoskov, “ORIGEN-S Library Generation for different WWER fuel types - Report for stage IV of implementation of contract №2300372/14.05.2003 Kozloduy NPP”, July 2004.[3]   [3] D. V. Hristov, “Preparation and verification of libraries for ORIGEN-S module in SCALE4.4a, with cross-sections for WWER-1000 TVSM fuel”, 15th Symposium of AER, Znojmo, Czech Republic, 3-7 October 2005.   [4] D. V. Hristov, “Preparation of libraries for ORIGEN-S module in SCALE4.4a, with cross-sections for WWER-1000 TVSA fuel”, 11th Meeting of AER Working Group E, Hrotovice, Czech Republic, 11-12 April 2006   [5] Studsvik Scandpower “HELIOS Documentation”, 11 September 1998.