“…In fact, this segment fuel rod was irradiated at a relatively high linear heat rate, and the maximum linear heat rates during the first, second, third and fourth cycles were estimated as about 35, 33, 30, and 26 kW/m, respectively. 4,15) From the calculation results obtained using a fuel performance code, 16) it was considered that the maximum temperature at the pellet center reaches 1280, 1240, 1140, and 1060 C during each base irradiation cycle, taking the thermal conductivity degradation of the irradiated fuel pellet 17,18) into account. The temperature levels during the first and second cycles are sufficient to cause the migration of fission gas atoms, and it is also probable that part of the accumulated fission gas was released near the central region of the fuel pellet during base irradiation.…”