Present status and program of research on tritium measurement

The National Institute of Radiological Sciences (NIRS) of Japan has worked for 13 years on tritium measurement, and the recommendation of the Tritium Subcommittee of the Nuclear Fusion Council under the Atomic Energy Commission, Japan, was made in 1977. Since 1967, tritium surveillance programs have been in effect near nuclear power stations and facilities in Japan. The sources of samples water are rivers, reservoirs or ponds and wells near nuclear sites in the case of land water, the inlet or outlet of secondary cooling sea water; tap water sources inside the facilities; and land, sea and tap water sources far from nuclear sites for the baseline level. The tritium concentration in land water decreased exponentially from 600 pCi/l in 1967 to 150 pCi/l in 1972, and then the tritium level remained almost constant in the range of 100 to 300 pCi/l. In the case of tap water, the tritium level was lower or the same as the above. This tendency seems to reflect the level of tritium fallout. While the tritium level in coastal sea water in recent years has ranged from 20 to 60 pCi/l, it is not regarded as being influenced by the effluent from nuclear reactors and facilities. For the measurement of low tritium concentration, a low background scintillation counter was devised, and an instruction manual for tritium measurement was published to normalize the analytical technique. (Wakatsuki, Y.)