LAUSR

Abstract High-quality boiling crisis, i.e. dry out of the liquid film is an important operational safety issue for boiling water reactors. The phenomena involved in dry out are complicated and difficult to capture in full details in experiments. We follow a step-wise approach to the problem and utilize innovative, high-resolution measurement techniques. First extensive testing have been carried out on partial nuclear fuel bundle models containing typically a couple of neighboring subchannels. These first series featured adiabatic, air-water annular flows with and without the presence of prototypical functional spacers. Cold neutron imaging has been used to map the distribution of the liquid film thickness on the virtual fuel rods and on the spacer structures. As a next step a dedicated loop using a special working fluid has been designed enabling examining convective boiling two-phase flow in similar geometries as the adiabatic tests. Cold neutron and X-ray imaging can be used for the heated set-up to try to capture actual dry out phenomena. An overview of the result of the adiabatic test series and the first result with the heated experiments are given below.