Decay Heat from Products of 235U Thermal Fission by Fast-Response Boil-Off Calorimetry

摘要

A cryogenic boil-off calorimeter was used to measure the decay heat from the products of thermal-neutron-induced fission of236U. Data are presented for cooling times between 10 and 10s s following a 2 x 10 s irradiation at constant thermal-neutron flux. The experimental uncertainty (1 a) in these measurements was = 2%, except at the shortest cooling times where it rose to = 4%. The beta and gamma energy from an irradiated 236U sample was absorbed in a thermally isolated 52-kg copper block that was held at 4 K by an internal liquid helium reservoir. The absorbed energy evaporated liquid helium from the reservoir and a hot-film anemometer flowmeter recorded the evolution rate of the boil-off gas. The decay heat was calculated from the gas-flow rate using the heat of vaporization of helium. The calorimeter had a thermal time constant of 0.85 s. The energy loss caused by gamma leakage from the absorber was < 3%;a correction was made by Monte Carlo calculations based on experimentally determined gamma spectra.