Thermodynamic behavior of nitrogen gas used in pressurized water reactor boron injection tanks

The boron injection tank (BIT) of a pressurized water reactor safety system is filled with borated water and pressurized by a nitrogen blanket. Accurate evaluation of the gas polytropic index (n) during the injection process is important to assure required flow from such tanks. An experimental setup simulating a typical BIT system was built and used to evaluate n of the gas under different test conditions. When the N2 regulator valve was left open, n at the end of the injection process was much lower than 1.0. For the conservative situation with a closed N2 regulator valve, however, n was close to 1.0 under all test conditions. Test duration, which was a strong function of initial gas properties and gas expansion rate, had a noticeable effect on n at the endpoint. Although heat transfer from tank walls had a minor effec at the endpoint, its effect on n after time t from starting injection was noticeable