Development of a Device for Detecting Fuel Failure in a Pool-Type Reactor Effective Extraction of Fission Rare Gases by Introducing Gas Bubbles

Studies have been made for developing a sensitive device for monitoring possible release of fission products from fuel elements of a pool-type research reactor. Gas bubbles were introduced into the reactor-coolant water to extract dissolved fission rare gases, ⁸⁹Kr and ¹³⁸Xe, and their daughter nuclides, ⁸⁹Rb and ¹³⁸Cs, were counted with high efficiency. Various causes that influence the efficiency of extracting the rare gases and of collecting the daughter nuclides were studied by two methods: (1) a “filter method”, in which the air covering water was sucked and the daughter nuclides were caught onto a filter paper, and (2) a “bottle method”, in which the bubbled gas was sampled into a bottle, and the gas was later washed with water, and the activities on the filter or of the water were counted. The items affecting the efficiency of extraction include the flow rate of gas introduced, the position (the depth and the radial direction) of the gas exit in the reactor water, and whether the coolant was circulated or not. Analysis was made of the effect of the depth on the extraction efficiency of dissolved gas into bubbles. On the basis of these results, a fuel-monitoring device for routine use was tentatively designed, both in the filter and in the bottle methods, and tests were made. The filter method was found to be unsatisfactory for practical use. The bottle method enabled an automatic system which functions well enough to be used as a fuel-failure monitoring device.