Improvement of the determination of radioactivity concentrations of short-lived radioisotopes with the time-dependent leakage model of VVER-type nuclear reactor

The leak-free operation is a fundamental necessity for the entire duration of each nuclear reactor cycle. While the physical and chemical processes resulting from fuel failures typically do not avert the safe operation of the reactor, they can have direct operational and economic consequences. The state of leak-free operation in the reactor core is monitored on-line by gamma spectra measurements of the primary coolant. Estimating the burnup and leakage parameters of defective fuel elements is achieved through calculations employing relevant leakage models. This paper presents a novel approach to assessing the time-dependent sample delay time, which is used to calculate the time-dependent radioactivity concentrations of the fission products in the primary coolant from their number of nuclei calculated by the time-dependent leakage model. The performance of three models with different basic assumptions was individually and collectively investigated. The new sample delay time model allows a more accurate estimation of the time-dependent radioactivity concentration, especially of the short-lived ¹³⁷Xe, ¹³⁸Xe, ¹³⁸Cs, and ¹³⁴I isotopes in the primary coolant.