Twenty years of experience in monitoring 41Ar in a research reactor and decrease of its discharge into the...

The radioactive gas 41Ar has been produced at high concentration by neutron activation near the reactor core in the Kyoto University Research Reactor. A pipe line for an exhaust stream, so-called sweep gas, was fabricated at the construction of the reactor in 1964 in order to exhale 41Ar from the facilities above to the environment. Other exhaust lines with decay tanks were established separately from the sweep line for both the cold neutron source in 1986 and the heavy-water tank in 1996, respectively, because a higher amount of 41Ar was thought to be produced from these facilities due to the improvement. As a result, a slight change in the flow rate of the exhaust was found to have a great deal of influence on both the 41Ar concentration in the reactor room and the rate of emission from the stack. By monitoring the exhaust air from the decay tanks, the mechanism for decreasing the emission was clarified together with identifying an obstacle, i.e., the condensate against the steady state flow, formed in the exhaust pipe. By setting the flow rate suitably in the exhaust line, the rate of 41Ar emission from the biological shielding into both the work place in the reactor room and the environment has been controlled as low as reasonably achievable.