Studies on fuel failure detection in Rikkyo Research Reactor

Rikkyo Research Reactor, TRIGA Mark II, the maximum power output of which is 100 kW, has been operated since December 1961 and has experienced the integral output of 2,028.469 kWh (85.5 MWD) as of the end of January 1988. The cylindrical-shaped fuel elements are made of a UZrH alloy using 8 percent by weight (wt-%) uranium (20% enriched), 91 wt-% zirconium, and 1 wt-% hydrogen, and they are cladded with aluminum of 0.76 mm thickness. The total inventory of 235U in the core is ca. 2.3 kg, and the number of fuel elements inserted in the core at present is 67. Almost all of these fuel elements have been in the pure water as the primary coolant (the volume is ca. 20 tons) in the reactor tank for these 26 years, and none of them have been taken out as spent fuel from the reactor tank. Thus the total burn-up degree is about 3.7%, in other words, the average burn-up degree per one fuel element is about 1.3 MWD. Considering the fact that the fuel failure in TRIGA reactors occurred almost only for pulsing reactors while our reactor has experienced no pulse operation and the average burn-up degree per fuel element is still low, we do not have so much fear that a serious trouble for our fuel elements might happen in very near future. However, in order to keep the wholesomeness of this reactor as long as possible, it seems quite pertinent to have a good supervision on the status of the possible leakage of FP from our fuel elements. In one sense, our aluminum-clad fuel elements which have been used in water as long as more than 26 years may be a useful tool for establishing an effective method of detecting the leakage of FP and rapidly finding the location of the defective fuel. Thus we have recently begun to undertake this studies in various approaches. In this paper is stated mainly the recent data since a preliminary report was made in November 1986. (author)