[en] The stability of MC precipitates in the Japanese PCA is investigated by the following two methods. First, recoil resolution of the precipitates under heavy-ion irradiation was investigated using techniques for in-situ observation. The size and morphology of the precipitates (>20 nm) did not change for irradiation up to a dose of 100 dpa. Secondly, the distribution of alloying elements during irradiation at high temperature was measured by X-ray EDS Energy Dispersive Spectroscopy. Irradiation caused the enrichment of Ni and Si atoms around the precipitates, while in the case of simultaneous injection of helium, Si atoms were found to be absorbed into the precipitates. (orig.)