Study for nickel deposition on the fuel cladding by DH transition under simulated PWR conditions

The optimization of dissolved hydrogen (DH) concentration in the primary coolant has been studied for mitigation of PWSCC and dose rate reduction. The Influence on PWSCC by the DH concentration has been reported on in several theses. On the other hand, little research has been reported on the Influence on dose rate by the DH concentration. It is considered that DH concentration effects the chemical form of Nickel, and especially the metallic-Nickel changes into Nickel oxide as NiO under the lower DH concentration at normal PWR operating temperature. Due to the solubility of metallic-Nickel being higher under lower DH concentration, Nickel is relatively easy to dissolve on the fuel cladding surface. Therefore, it is estimated that the generation of Co-58 and radiation source decreases by optimization of DH concentration. In this study, the deposit experiments were conducted on the Zry-4 specimen in different DH concentrations. DH concentrations were varied within a controllable value with the normal pressure range of Volume Control Tank (VCT) at actual PWR. The Zry-4 specimen was mounted on the heater pin in the loop testing apparatus to simulate the fuel cladding surface in PWR. After the deposit experiments, the deposits on the fuel cladding surface were observed with a SEM/EDS, and the chemical forms of the deposits were analyzed with a XRD. Elemental composition of deposits was measured by a chemical analysis method. As for the test results, it may be concluded that the chemical forms of Nickel deposits on the fuel cladding vary with DH concentration. Therefore, it is considered that radiation source reduction on the fuel cladding is expected by optimizing DH concentration. (author)