The hydrogen moderator of 1MW pulse spallation neutron source (JSNS) in J-PARC must be designed to get effective coolability with liquid hydrogen under such sever conditions as compact vessel size and high heat load deposited with high neutron flux irradiation. To solve this problem and to clarify the heat transfer characteristics of the impinging jet in a limited narrow space, heat transfer experiments have been carried out using an actual-scale simulation test vessel under water flowing condition. And, an evaluation method of the heat transfer coefficient from a heat conduction equation was established. From the experimental results and the evaluated heat transfer coefficients, the heat transfer characteristics of the bottom surface of moderator vessel were clarified. The operating flowing condition of the hydrogen moderator was determined from the predicted heat transfer curve for hydrogen based on those results.
