PURPOSE: To prevent the damage of a container caused by the burning of combustible gas generated in the container at the time of accident and to suppress the migration of radioactive nuclide into gaseous phase in order to prevent the leakage of the radioactive nuclide into the surrounding environment of an nuclear reactor.

CONSTITUTION: A dry well 6 and a wet well 7 of a container 1 is connected with a medicine injecting pipe 4. A medicine storing tank 5 is provided in the halfway of the pipe. The medicines such as iodine oxidizer and carbonate are injected into suppression pool water 3. As a result the amount of hydrogen generation is suppressed, and the inflammable limit is not reached. Therefore, burning does not occur. The amount of migration of radiactive iodine into a gaseous phase is decreased. This apparatus is advantageous in cost, positively prevents the damage of the container caused by the burning of the hydrogen and can suppress the generation of the radioactive iodine.