To provide a transportation storage cask for a radioactive material which can secure safety during normal transportation or normal storage.

The transportation storage cask for the radioactive material is provided with a relief valve 15 provided on an outer tube 4 of the cask 1 and connected with a space 3a which is an expansion allowance of a neutron shielding member 3. Relief valves 9 and 16 connected with spaces 8a and 14a which are expansion allowances for neutron shielding members 8 and 14 are provided on an outer structural member 7b of a secondary lid 7 and a bottom range cover 15, respectively. Relief pressures of the relief valves 5, 9 and 16 are set to pressures of 0.5 MPa or lower in a range wherein the maximum stresses induced in the outer tube 4, the structural member 7b of the secondary lid 7 and the bottom range cover 15 by the pressures within the spaces 3a, 8a and 14a do not exceed an allowance stress, preferably to a pressure of 0.3 MPa or lower.

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