Year
1974
Abstract
A new Leached Hull Monitor (LHM) featuring high sensitivities, wide applicability to various kinds of reactor fuels, and excellent suppression of interference, was built and tested at Rad Tech. The purpose of a LHM is to detect the presence of undissolved fuel in chopped hulls of irradiated fuel elements after a proper leaching time. The monitor detects the presence and measures the amount of the fission product, -*-^Ce. This isotope has a high cumulative yield of 6%, and a half life of 285 days. Its daughter product, 144prj emits several distinctive gamma-ray lines, the highest of which has an energy of 2186 keV, and a rather low yield of 0.74% per disintegration. The LHM detects this passive radiation which is the most penetrating and least sensitive to interference from other radiations. The interference comes mainly from activities induced in the fuel cladding by neutron captures. The most important induced activity in Zircaloy cladding of light-water reactor (LWR) and stainless-steel cladding ef fast-breeder reactor (FBR) fuel rods is 60co. Other interferences of less importance are ^Co, ^^Zr, ^^Nb, and some fission-related products such as 10°Ru and ^-^Cs. The new LHM reduces substantially this interference by using a fast-slow electronics system, a proper gamma-ray filter, and a rotation-collimation plus vertical-scanning system. The main components of the system are: a gamma-ray spectrometer [NaI(T£) or Ge(Li)], a fast-slow electronic signal processing system, analog and digital outputs for gross and net cerium response, and a remotely positioned source for calibration and normalization. Performance of the system in terms of the suppression of interference and sensitivity was demonstrated in an extensive series of measurements simulating an actual LWR irradiated fuel. Based on the tests, the minimum detectable amount of undissolved uranium for typical high and medium burnup LWR fuel (>25000 MWD/T) under ideal conditions (i.e., all the detected Ce is related to residual fuel), has been estimated. Very high sensitivity to Ce, and hence to residual fuel, has been demonstrated. Some inherent problems with the passive LHM are discussed, along with methods to diminish their effect.