Estimation of activity concentrations of radioactive gases inside containment building of fast breeder reactor FBTR: Leakage of active cover gas

Leakage of active reactor vessel cover gas through liquid metal seal or various injection and rejection points of the reactor vessel cover gas system, increases radiation levels in the reactor containment building (RCB) of a liquid metal cooled fast reactor (LMFR). Detection of the leakage passage is often difficult. In this article, dispersion of fission product noble gases (FPNGs) and Ar-41, inside the RCB of fast breeder test reactor (FBTR) at Kalpakkam, has been studied using turbulent diffusion equation for the case of release of active cover gas from a location inside RCB. The resulting estimate of the time-evolution of the gross gamma activity concentration at a detector location, after a controlled release of active cover gas in small quantities from a known point, has been compared against the recorded detector signal. This showed substantial match, with coefficient of determination, R-2 at 0.84. If there is a steep increase in the activity release rate driven by either an increase in the activity concentration of reactor vessel cover gas (RVCG) due to a clad failure incident or by an increase in the leakage rate of the RVCG due to minor loss of effective isolation of the cover gas region, there will be well discriminated response to it over the prevailing background, in the time series data of gross gamma activity concentration recorded by the gas activity monitors in RCB. This response will be different for different locations of release of active cover gas, making detection of the release site possible, if we know possible release locations, and if the increase in activity release rate is steep enough to allow significant discrimination in response, for different release locations.