Analysis of causes for high silicon content in the in-containment refueling water storage tank of a nuclear power plant

Addressing the issue of abnormally high silicon content in the In-containment Refueling Water Storage Tank (IRWST) of a Hualong One unit at a nuclear power plant during its first cycle and subsequent operation, this paper identifies two main pathways leading to the increase in silicon content through a systematic investigation of potential factors such as civil engineering materials, equipment operation, and the air environment, combined with leaching experiments and operational data coupling analysis. The first pathway is the release of initial exogenous sediment left over from the installation and commissioning stage during pump startup operation. The second is due to the "open-top" structural design of the IRWST; during power operation, under the unfiltered internal circulation mode of the EVR system, airborne silicon-containing particulates in the reactor building air continuously settle and dissolve into the water pool. The results indicate that the introduction from the air environment is the key root cause leading to the continuous deterioration of this issue. Ultimately, the plant successfully restored the water quality to within limits using a nanofiltration (SRD) device.