Fission product retention in the ACACIA (AdvanCed Atomic Cogenerator for Industrial Applications) reactor primary system

The transport and deposition of fission products in the ACACIA (AdvanCed Atomic Cogenerator for Industrial Applications) high temperature reactor primary system are investigated. The study focuses on the behaviour calculated with the MELCOR computer code of 4 nuclides: Cs-137, Sr-90, Ag-110m, and I-131. After a ten-year operation period, the total activity of the released nuclides in the primary system is about 58 GBq. The highest activity is produced by Cs-137 (52 GBq), followed by I-131 (4 GBq), and Ag-110m (1.8 GBq). The contribution of Sr-90 is very low (1600 Bq). The highest activity is found in the precooler (56 GBq). The main reason is the condensation of the volatile species CsOH and CsI in this component. Other components with high activities are the recuperator (1.4 GBq) and the compressor (0.007 GBq). These components are contaminated by Ag-110m. The gas ducts in the energy conversion unit are contaminated by Ag-110m (0.043 GBq) and I-131 (0.011 GBq). Contamination as a consequence of a Loss Of Coolant Accident (LOCA) or a Loss of Flow Accident (LOFA) is difficult to estimate, because it depends on a number of phenomena. Under the assumption that 10 fuels elements are damaged, the activity is about 44 GBq. In this accident, Ag-110m produces most of the activity (31 GBq), followed by Cs-137 (12 GBq), and I-131 (1.4 GBq). Although the activity of Sr-90 is high (0.023 GBq), it is negligible compared to the activity of the other nuclides. Obviously, the activity is proportional to the number of damaged fuel elements. The distribution of the nuclides over the components is not considered in view of the large number of uncertainties. 9 refs