The NEPTUN test facility at Würenlingen, Switzerland, has been modified to enable light water high conversion reactor (LWHCR) representative reflooding and boiloff experiments to be carried out. Results from a first series of forced feed reflooding tests, simulating cold-leg injection, are presented for a range of values of the flooding rate, rod power, and initial rod temperature parameters. Rewetting of the LWHCR fuel bundle simulator was found to be possible in each case. Analysis of the NEPTUN-III reflooding experiments with RELAP5/MOD2 yield discrepant results, and it has been shown, in the context of calculations of the boiloff experiments, that some LWHCR specific models and correlations need to be developed.