Experiences of Ar-41 monitoring at the proximity of emission source

As a safety index in the air breathed at control area by radiation operators and for the public at a nuclear facility, a mean concentration limit at each three months out of peripheral monitoring area is decided by a regulation rule (reactor room:1x102 Bq/L, out of peripheral monitoring area: 5x10-1 Bq/L. And, a CsI plastic scintillator for concentration control in reactor room during operation of a KUR, and an ionization box type stationary monitor at each vent are prepared, and each alarming concentration (momentary value) is decided for its controlling reference value (reactor room: 3.7x102 Bq/L, vent: 6x101 Bq/L). In recent years, air concentration in the reaction room during 5 MW operation was 10 to 20 Bq/L, and that at vent was about 30 to 50 Bq/L. The reason why concentration at vent was higher than that in the reactor room air was due to emission (2.5 m3/min) of high concentration air such as air in experiment pores near the reactor and so forth as sweep gas into duct. Recently, on setting in and out a sample of pneumatic system, concentration in a stack exhaust sometimes showed an momentary increase to alarm. Therefore, here were described on some investigations and researches aiming at cause elucidation on variances and emission reduction of the reactor room air and vent concentration accompanied with additional installation of such some sweep gas systems. (G.K.)