[en] The pressure tube type nuclear reactor of the present invention is suitable to injection of poisons into a reactor in a short period of time for emergency reactor scram. That is, the reactor of the present invention comprises a poison injection port disposed to an annular channel disposed between a pressure tube having gases filled therein and a calandria tube. With such a constitution, since an existent pipeline for carbonic acid gas can be used as a channel for injecting poisons, flow pressure loss can be reduced. As a result, poisons can be injected to the reactor core in a short period of time. A performance of emergency reactor scram is improved. (I.S.)