Experimental study of fission product release from research reactor fuel and transport in the reactor coolant system

Experimental study of heat transfer processes, maximum permissible power and fission product release from fuel elements in power-cooling mismatch conditions was performed in the high flux research reactor MARIA operated at the Institute of Atomic Energy in Swierk in Poland. One multitube fuel assembly of the MARIA reactor with aluminium-uranium fuel layers in aluminium cladding was equipped with thermocouples and coolant flow and pressure sensors, and the cooling circuit was provided with neutron detectors, coolant sampling lines and a gamma dose rate measurement system. After successful heat transfer measurements, power-cooling mismatch conditions were initiated, with fuel temperatures rising to 300 deg. C. Then the temperature was abruptly lowered, which led to fuel canning rupture and fission product release. Delayed neutron flux measurements and gamma dose rate measurements were taken and the coolant activity sampled, which permitted determination of the activities of released fission products. Analysis has shown that good agreement of calculated and measured data is obtained for fission product releases equal to 1.1% for Kr and 0.625% for Xe and I, which corresponds to fission product escape from the fuel layers due to recoil energy. No solid fission products such as 137Cs were released. Measurements on the primary cooling circuit made it possible to determine the fraction of gaseous fission products released to the pressurizer gas volume. (author)