On-Line Noble Gas Fission Product Monitoring in Experimental Breeder Reactor II

As part of the defective-cladding detection program at Experimental Breeder Reactor II, the Reactor Cover Gas Monitor was installed to provide on-line monitoring for noble gas fission products in the argon cover gas which fills the reactor vessel above the sodium coolant. With a 2- × 2-in. sodium iodide scintillator as the detector, it has been possible to identify ¹³³Xe, ^85mKr, and ¹³⁵Xe, which are always present in low concentrations due to an irreducible background fission source in the reactor. This apparently straightforward monitoring project turned out to be much complicated by the presence of relatively large concentrations of ²³Ne [from the reaction ²³Na(n, p)²³Ne on the sodium coolant]. The ²³Ne diffuses from the sodium to the cover gas at a rate which is exceedingly sensitive to the bulk sodium temperature. Small temperature changes can produce large changes in the ²³Ne concentration. The Compton continuum from ²³Ne is counted in the channels adjusted for the fission product gases, causing large and alarming surges in the indicated concentrations. While this effect is not yet understood, it has been alleviated by “aging” the sample stream in a delay line to permit additional decay of the 38-sec ²³Ne. After the delay line was installed, it was possible to monitor ¹³⁵Xe very effectively; ¹³³Xe and ^85mKr still could not be monitored clearly because of the interference from the Compton continua of higher energy gamma lines, principally that of ¹³⁵Xe.