Comparison of heat capacity and thermal time constant between BWR fuel and simulated heater rod

It is important to know the thermal characteristics of BWR fuel, i.e. heat capacity and thermal time constant, in order to evaluate the thermal hydraulics at BWR accidents and the events under thermal-hydraulic and neutronic coupling condition. Further, since the heater rod simulating BWR fuel is used in the tests for BWR accidents and for BWR thermal hydraulics coupled with neutronics, it is important to know the thermal characteristics of the heater rod. Therefore, the author investigated the thermal characteristics of BWR fuel and the heater rod by performing experiments and analyzing with J-TRAC code capable to analyze 2-dimensional heat conduction problem. The heat capacity per unit length of BWR fuel cpρA (kJ/mK) was estimated to be 0.34 kJ/mK - 0.36 kJ/mK in 300 deg. C - 800 deg. C. The heat capacity of the heater rod was almost identical with each other regardless of the differences in rods and positions. It was higher with higher temperature. The heat capacity of the heater rod used in the test for BWR accidents was about 0.38 kJ/mK at 600 deg. C, which was about 9% higher than the average (0.35 kJ/mK) of BWR fuel. On the other hand, the heat capacity used in the test for BWR thermal hydraulics coupled with neutronics was about 0.42 kJ/mK at 600 deg. C, which was about 20% higher than the average of BWR fuel. Thermal time constant was affected by surface heat transfer coefficient, thermal diffusivity, and gap conductance. When the surface heat transfer coefficient is small, it controls the heat transfer and thermal time constant depends mainly on the surface heat transfer coefficient. When the surface heat transfer coefficient is large, the heat conduction controls the heat transfer and thermal time constant depends mainly on the thermal diffusivity. In the former case, one point heat transfer model is applicable and the thermal time constant is proportional to the inverse of the surface heat transfer coefficient. In this case, the thermal time constant was estimated at the surface heat transfer coefficient of 1 kW/m2K to be about 10 s and about 13 s for BWR fuel and the heater rod, respectively. On the other hand in the latter case, the thermal time constant converges to the constant value dependent on thermal diffusivity rather than on surface heat transfer coefficient. In this case, the thermal time constant were estimated to be about 5 s for BWR fuel and less than 1 s for heater rod. When gap conductance is small, it affects on thermal time constant rather than thermal diffusivity does. (author)