USE OF THE AXIAL BURNUP PROFILE AT THE NUCLEAR SAFETY ANALYSIS OF THE VVER-1000 SPENT FUEL STORAGE FACILITY IN UKRAINE Olena Dudka, Yevgen Bilodid, Iurii Kovbasenko, Vladimir Khalimonchuk State Scientific and Technical Centre on Nuclear and Radiation Safety (SSTC N&RS) Stusa St., Kyiv, Ukraine 17th SYMPOSIUM of AER on VVER Reactor Physics and Reactor Safety September 24-29, 2007, Yalta, Crimea, Ukraine

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine2 Nuclear safety of fresh and spent fuel is assessed in compliance with current technical regulations, among which the following documents should be singled out «Safety Rules for Storage and Transportation of Nuclear Fuel at Nuclear Power Facilities, PNAEG ». «Basic Rules for Spent Nuclear Fuel Intermediate Dry Storage Facilities Safety Evaluation, NP ». According to this documents, the effective neutron multiplication factor K eff must remain below 0.95 in normal operation and design-basis accidents.

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine3 Fig. 1 – burnup profile over FA length

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine4 Table 1. Fuel assembly number in ZNPP storage pools till BASIC DATA FOR GENERALIZED COEFFICIENTS OF CONSERVATIVE AXIAL BURNUP PROFILE

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine5 Fuel assembly burnup was calculated by simulating a fuel campaign accounting the following experimental data  power unit load curve  control rod positions  core coolant temperature at the core input  core coolant rate  boric acid concentration

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine6 Burnup irregularity coefficient for each fuel assembly layer was calculated by the following equation: (1) GENERALIZED COEFFICIENTS OBTAINING FOR CONSERVATIVE AXIAL BURNUP PROFILE burnup at point z from the core bottom through the fuel assembly fuel burnup average value through the fuel assembly

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine7 Fig. 2 – Relative burnup profiles for arbitrary FA

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine8 Figure 1 demonstrates, fuel assembly burnup profiles have only a weak dependence on a type of fuel assembly, of initial enrichment and average fuel burnup value, what allow their generalization to all types of the spent fuel assembly independently on enrichment and burnup of FA. During profile calculation the burnup in each layer of fuel assembly is normalized to the average burnup value over the fuel assembly. Sum of values obtained through 10 layers for each fuel assembly in this case is equal to 10.

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine9 Conservative axial profile for burnup distribution for the all studied fuel assemblies was obtained on the base of the selected minimal burnup irregularity coefficients for each of 10 layers through all the spent fuel assemblies in ZNPP storage pools. Then minimal burnup irregularity coefficients for each of 10 layers through all the spent fuel assemblies in storage pools were selected.

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine10 Fig. 3 Axial conservative burnup profile

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine11 Such formation of the conservative axial profile makes it different from the obtained one by the equation (1) and sum of its values is less 10 through ten layers. conservative axial burnup profile coefficients of 10 layers SFA Table 2

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine12 The sum of irregularity coefficients for burnup values for conservative axial burnup profile through the ten layers makes 8.8. So, transfer from the actual distribution burnup profile to a conservative one results in underrating of the fuel assembly burnup average value to 12% as to its real value (for comparison, average burnup value underrating makes 50-60% at the uniform burnup profile).

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine13 At performance of estimation of nuclear safety the absolute conservative fuel burnup value in each considered fuel assembly layer should be calculated in the following way: is an average fuel burnup value in a fuel assembly (2) where APPLICATION OF GENERALIZED CONSERVATIVE AXIAL BURNUP PROFILE COEFFICIENTS

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine14 Fig. 4 – burnup profiles over FA length

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine15 As the result of equation (2) application to the average burnup of any spent fuel assembly, the most conservative burnup distribution profile should be made for ten layers. Such distribution burnup profile results in the maximum neutron multiplication factor K eff for all the fuel assemblies' types and all the points of burnup. Taking into account the axial fuel burnup distribution according to the given methodology the reserve of 12% compensates possible errors in determination of burnup, which according to the software specifications for NPPs make 7-10%.

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine16 Accounting the fuel burnup for ventilated storage casks of the dry storage facility for VVER-1000 spent fuel at ZNPP critically calculations only 5 fissionable isotopes (U-235, U-238, Pu-239, Pu-240, Pu-241) are taken into account. This introduces additional conservatism to the calculation results which makes 14% in magnitude Keff. Summing up these data the conservative reserve, which assumed for spent nuclear fuel storage safety, makes up not less than 26% in magnitude К эфф in connection with possible burnup calculation errors and U and Pu isotope concentrations.

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine17 CONCLUSIONS Present-day approach to estimation of SFA burnup for ZNPP Interim Dry Storage System for Spent Nuclear Fuel, where each fuel assembly burnup is assumed uniform over assembly length and equal to average burnup of the end parts is conservative. It results in times decrease of fuel assembly burnup value comparing to the average value as far as the fuel assembly is burned more significantly near the center as to its ends. This in its turn increases the number of the spent control rods loaded into containers required for maintenance nuclear safety. The results of the analysis of the spent fuel assembly energy- producing placed in the units’ storage pools, which are presented in the report, allow reducing soundly of conservatism to the accepted level. To avoid excessive conservatism an axial conservative burnup profile determined with coefficients, shown in Table 2, should be used for analysis of nuclear safety of spent fuel dry storage system.

State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine September , th Symposium of AER, Yalta, Crimea, Ukraine18 Application of conservative profile provides underrating of fuel assembly burnup average value to 12% as to its real value, additional conservatism in the result of only fissionable isotope accounting will cause the design factor Keff increase higher than 26%. Calculations based on the examples of two casks of ZNPP Interim Dry Storage System prove that fuel burnup axial profile which has been taken into account for substantiation of spent fuel dry storage nuclear safety, allows to reduce the number of the control rods loaded into casks at least on two without violation of the Requirements for Nuclear Safety..