Development of JOYO MK-III core management code system 'HESTIA'

As part of the JOYO upgrading program (MK-III program), the JOYO MK-III core management code system 'HESTIA' was developed in order to improve the calculation accuracy concerning the core and fuel management and irradiation condition evaluation in the MK-III core. The neutronic calculation of HESTIA was modified to improve the power and neutron flux distribution. The calculation geometry was changed to Tri-Z geometry from Hex-Z geometry which was used in the MK-II core management code system 'MAGI' . The number of calculation mesh per subassembly was increased to 24 meshes in the radial direction, and the fuel region was divided into 20 meshes in the axial direction. The number of neutron energy group was increased from 7 to 18, and that of gamma energy group was increased from 3 to 7 groups respectively. As a result, HESTIA can accurately calculate the local neutron flux distribution and spectrum change within the fuel subassembly at the boundary between the fuel and reflector regions, which can not be fully simulated by MAGI. It was also confirmed that HESTIA can improve the power distribution in the driver fuel subassembly adjacent to radial reflector. As to thermo-hydraulic calculation, the porous body model was adopted to improve the calculation accuracy of coolant temperature. This model can take into account the detailed power distribution and the turbulent heat transfer in a fuel subassembly. It was found that the calculated value by HESTIA agreed well with that of the subchannel model. In order to verify the calculation accuracy of HESTIA, JOYO MK-II core characteristics calculation was conducted using HESTIA, and calculation results were compared with those of MAGI. The MAGI calculation results were already confirmed by the core performance test and post irradiation examination data. The comparison of both calculation code systems showed that the excess reactivity agreed within 0.01% Δk/kk', the maximum neutron flux agreed within 3%, the core average burn-up agreed within 3%, the coolant temperature difference between reactor inlet and outlet agreed within 7degC. HESTIA will be used for the purpose of the core and fuel management in the MK-III core. The calculation accuracy of HESTIA will be further confirmed by comparing with the MK-III initial core performance test results and the subsequent power and neutron flux distribution measurement by means of multiple foil activation method. (author)