A long-term flow-induced vibration and wear test was performed for a full-scale 17×17 PWR fuel mockup, and the test results were compared with numerical simulations. The flow-induced vibration on a fuel assembly or fuel rods may cause Grid-to-Rod Fretting (GTRF) and result in the leakage of fuel rods in PWRs. GTRF involves non-linear vibration of a fuel rod due to the excitation force induced by coolant flow around a fuel rod. So, the numerical simulation is performed by VITRAN (Vibration Transient Analysis Non-linear) and Computational Fluid Dynamics (CFD). VITRAN code was developed by Westinghouse to simulate fuel rod flow induced vibration and GTRF. In this paper, it was confirmed that the code can reproduce GTRF wear for NFI fuel assembly. CFD calculation is performed to obtain the axial and lateral flow velocity around the fuel rods, reflecting detailed geometries of fuel assembly components like bottom nozzle, spacer grids. The numerical simulation reasonably reproduced the vibration and wear test for NFI fuel assembly.

Thermohydraulic behavior in spent fuel pool is quite important in evaluating safety of a nuclear reactor under accidental conditions. When the liquid level of spent fuel pool decreases, fuel rods are exposed, heated up and finally failed. In order to avoid overheating of fuel rods, spray cooling is planned. Spay water is expected to enter rod bundle and cool down fuel rod. Therefore, quantity of spray water flowing into rod bundle has to be evaluated. A series of experiments were carried out for this phenomenon. The results showed that the water flowing into rod bundles is objected to the flooding and correlated by flooding correlations. However, the parameters of flooding correlations depended upon spray water flow rate and showed higher values compared with previous correlations.

Thermohydraulic behavior in spent fuel pool is quite important in evaluating safety of a nuclear reactor under accidental conditions. Particularly, accurate prediction of void fraction in spent fuel pool is indispensable for evaluating cooling characteristics of spent fuel. In view of these, experimental and analytical studies were carried out for void fraction in spent fuel pool. The experiment was performed to measure the heat-up and void fraction transient during the postulated SFP accident. In this experiment, a simulated 7x7 BWR rod bundle that consists of 49 heater rods, 7 spacer grids and upper tie-plate was used. The measured data was compared with the some driftflux correlations under the low pressure and the low flow rate condition related to SFP accident.