Time delay of thermal neutron detection during power burst in TRACY

Time delay of thermal neutron detection at a micro fission chamber has been experimentally confirmed during an initial power burst caused by reactivit y addition in the Transient Experiment Critical Facility (TRACY). In the ex periment, the power was measured by using a fast response gamma -ray ioniza tion chamber, which was newly designed exclusively for the TRACY experiment , in addition to the fission chamber. As results of the experiment, the tra nsient power measured by the fission chamber was observed about 4 ms later than that by the gamma -ray chamber during the power increasing, and then t he delay time expanded after the power peak. The time delay must be taken i nto account to evaluate accurately the transient power. To comprehend the o bserved time delay in detail, time-dependent analyses were performed using the MCNP4B code. In the analyses, time response of neutron detection at the fission chamber was calculated and used for the simulation of the transien t power measured by the fission chamber. From results of the analyses, it w as confirmed that the simulated power well agreed with the measured power a nd the observed time delay was caused from the thermalization of neutrons e mitted at the TRACY core and the following flight to the fission chamber.