[en] As a part of the safety evaluation of the upgraded JRR-3, two design basis events, the channel flow blockage and the loss of heavy water flow, were analysed with a heat conduction computer program HEATING5. The results indicate that in the severest case of the channel flow blockage, where a cooling channel is completely blocked, the maximum fuel meat temperature is 150 ⁰C and the minimum DNBR is above unity. Also in the loss of heavy water flow, the maximum heavy water temperature is no higher than 94 ⁰C and remains subcooled even in the case of instantaneous flow loss. (author)