A FAST RESPONDING TRITIUM IN AIR MONITOR FOR USE IN OPERATIONAL AREASOF HEAVY-WATER POWER-REACTORS

A fast responding and sensitive tritium-in-air monitor, suitable for m onitoring in heavy water-based nuclear power reactors has been describ ed. The detector is a flowcell of 6.3 ml volume, made of stainless ste el body and Teflon window and packed with 600 mg of 5 mu m thick dry p lastic scintillator films. It is coupled to a matched pair of photomul tipliers and measurement is made in the coincidence mode of counting. Tritium counting efficiency for direct passage of sample air through t he detector is 65%. The sampling rate is 1 to 10 I/min. For an unconta minated detector, in the absence of any background activity, the instr ument records only 2 counts/min as background which is equivalent to t he passage of 8000 Bq/m(3) (0.01 DAC) of tritium in air. Microprocesso r based electronic compensation has been provided for gaseous radionuc lide and gamma background interference. Trial runs of the monitor in o ur nuclear power plants have shown that the monitor can function on a continuous basis in presence of large levels of gamma background and g aseous radionuclide interference.