Analysis of the dose-rate time development after pulse generation in a TRIGA-type reactor

The TRIGA Mark-II reactor of the 'Jozef Stefan' Institute can be operated in the pulse mode. During a pulse, radioactive gases (Ar-41, N-16 and O-19) are produced in the reactor coolant by neutron-induced nuclear reactions. These gases propagate through the reactor water column towards the reactor platform. An analysis of different contributions to the gamma radiation on the platform after the pulse generation was performed. The time development of the absorbed dose-rate was measured with a high-pressure ionization chamber and the gamma-ray spectra were recorded with a high-resolution n-type HPGe spectrometer and a low-resolution NaI(Tl) spectrometer. The spectra were recorded in several consecutive short-time intervals after the pulse in order to determine the time dependence of individual contributions of different radionuclides to the observed dose-rate. The results of the measurements are presented and a plausible explanation of the mechanisms behind the observed dose-rate time development is offered. (author)