Special Criticality Safety Analysis for TOPAZ-Ⅱ Space Reactor

The special criticality safety accidents refer to the unexpected supercriticality incidents that may occur when a space reactor falls to the Earth’s surface after a spacecraft launch abort or re-entry into the atmosphere, with the reactor core being water flooding and immersed in water or sand. The special criticality safety accident is the design basis accident of the space reactors, and the criticality safety issue of the space reactors falling to Earth is an important safety issue that needs to be demonstrated in detail before launch. At present, the analysis of special criticality safety problems at China and abroad only simply calculate keff, and no effective analysis and demonstration methods has been proposed. With reference to the international nuclear criticality safety analysis method for fissionable materials outside reactors, a reasonable special criticality safety analysis method for space reactors is proposed on the basis of special criticality safety experimental datum obtained on a criticality assembly that has a high degree of similarity to the prototype reactors. This method calculated the keff of the space reactors under accident scenarios and superimposes the bias β and bias uncertainty Δβ derived from special criticality safety experimental datum. Based on the results of special criticality safety experiments conducted on the Narciss-M criticality assembly, combined with the possibilities of actual occurrences, it has been determined that the worst accident scenario faced by the TOPAZ-Ⅱ space reactor after falling on Earth is: The reactor core being water flooding, immersed in wet sand, and side reflector with control drum is off; The bias β=0.000 03 and bias uncertainty Δβ=0.004 1 also obtained. According to the special criticality safety analysis method proposed for space reactors, and the cold built-in reactivity of the TOPAZ-Ⅱ space reactor is 0.005 Δk/k (i.e., keff=1.005), the ks of the TOPAZ-Ⅱ space reactor in the worst accident scenario is calculated to be 1.029 10±0.000 08. After considering the bias β and the bias uncertainty Δβ, the calculated result of ktot=1.033 33>0.98, which does not meet the criticality safety requirement (ktot≤0.98). Five modification measures for the TOPAZ-Ⅱ space reactor have been proposed; after analysis and calculation, four of them can meet the criticality safety requirement and are feasible to a certain extent. The special criticality safety analysis method for space reactors proposed in this paper, compared to simply calculating the keff of space reactors under various accident scenarios, has a high degree of rationality and feasibility. This method can provide beneficial reference for the special criticality safety analysis of other types of space reactors.