A feasibility study of coolant void detection in a lead-cooled fast reactor using fission chambers

Sammanfattning: One of the future reactor technologies defined by the Generation-IV International Forum (GIF) is the Lead-Cooled Fast Reactor (LFR). An advantage with this reactor technology is that steam production is accomplished by means of heat exchangers located within the primary reactor vessel, which decreases costs and increases operational safety. However, a crack in a heat exchanger tube may create steam (void) into the coolant and this process has the potential to introduce reactivity changes, which may cause criticality issues. This fact motivates the development of a methodology to detect such voids. This thesis comprises theoretical investigations on a possible route to detect voids by studying changes of the neutron spectrum in a small LFR as a function of various types of in-core voids .The methodology includes a combination of fission chambers loaded with U-235 and Pu-242 operating in various positions. It is shown that such a combination results in information that can be made independent on reactor power, a feasible property in order to detect the relatively small spectral changes due to void. A sensitivity analysis of various combinations of detectors, fuel burnup and void has also been included in the investigation. The results show that the proposed methodology yields a reasonably large sensitivity to voids down to (1-2) % of the coolant volume. The results obtained so far point in the direction that the proposed methodology is an interesting subject for further studies.