Deposition Behavior of Cesium Molybdate on Type 304 Austenite Stainless Steel in Severe Accident

The chemical behavior of cesium molybdate (Cs₂MoO₄) in light water reactors during severe nuclear accidents remains unexplored. This study demonstrated the deposition behavior of Cs₂MoO₄ on Type 304 stainless steel (SUS304) at 1530 to 530 K under dry (Ar) and humid (Ar + H₂O) conditions. Cesium molybdate was partially decomposed on the SUS304 surface, thereby inducing the oxidation of iron (Fe) and chromium (Cr) under the dry condition. Molybdenum (Mo) metal and molybdenum dioxide (MoO₂) were detected on the surface, while Cs coexisted with chromium in the oxide layer at 1500 K. Both Cs₂MoO₄ and Mo metal were identified on the SUS304 surface at 1230 K. Under the humid condition, the oxidation of the SUS304 was affected by Cs₂MoO₄ vapor. Molybdenum was detected in the form of spots in the iron oxide layer, while cesium was not detected above 1500 K. Molybdenum metal was detected on the surface of SUS304 oxide at 1230 K. Cesium molybdate was deposited on the SUS304 at 730 to 530 K under both the dry and humid conditions. The results are discussed in relation with the thermodynamic model of the Cs-Fe-Cr-Mo-O system. Thus, the chemical behavior of Cs₂MoO₄ at the interior of the reactor cooling system is elucidated.