Assessment of contamination distribution of "Monju", 9; Study on contamination behavior and dominant factors in the primary main cooling system

Mashimo, Ryutaro; Nanri, Tomohiro ; Hayashi, Hirokazu ; Muranaka, Makoto*; Kato, Mitsuaki*; Kishimoto, Yasufumi*

In the decommissioning of Monju, secondary contamination is being assessed through non-destructive external measurements of equipment and activation analysis of core components. This paper reports on the characteristics of contamination forms in the primary main cooling system based on measurement results obtained using a portable Ge detector. Over 25 years after the performance test, the gamma-emitter levels in the primary cooling system piping were below the clearance; Co-60 and Nb-94 dominated. The generation of Nb-94 is likely influenced by physical factors, such as surface degradation of Inconel 718 used in the flow straighteners. In the future, in addition to clarifying the migration behavior of Nb-94, evaluations of beta-emitters such as Ni-63 are also planned.