Patent Number: 
Section: claims

1. A concrete cask comprising:an inner shell made from metal;an outer shell made from metal;a shielding body composed of concrete and provided between said inner shell and said outer shell;heat transfer fins provided between said inner shell and said outer shell; andan accommodation portion formed inside said inner shell for accommodating a radioactive substance therein thereby being kept from the outside of the cask, whereinsaid concrete includes portland cement, andsaid heat transfer fins each has an inner shell-side and an outer shell-side and is configured such that said inner shell-side is in contact with the inner shell and the outer shell-side is formed with at least a portion that is not in contact with the outer shell; orsuch that said outer shell-side is in contact with the outer shell and the inner shell-side is formed with at least a portion that is not in contact with the inner shell. 2. The concrete cask according to claim 1, comprising at least a first heat transfer fin provided in contact with said outer shell and a second heat transfer fin provided in contact with said inner shell, the first heat transfer fin and the second heat transfer fin being provided so as to overlap each other and so that there is a clearance between said first and said second heat transfer fins in said overlap portion. 3. The concrete cask according to claim 2, wherein when the length of the overlap portion of said first and said second heat transfer fins is denoted by w1 and the clearance between said first and said second heat transfer fins in the overlap portion is denoted by a1, then the following relation is satisfied:a1·(2·λc·w1·Lc)/(λf·t),where λc: thermal conductivity of the concrete (W/m·K);Lc: thickness of the concrete shielding body (m);λf: thermal conductivity of the heat transfer fins (W/m·K);t: thickness of the heat transfer fins (m). 4. The concrete cask according to claim 1, wherein the side of said heat transfer fins that forms said separation portion is formed to have substantially an L-like shape so as to be provided with an opposite surface facing said inner shell or said outer shell. 5. The concrete cask according to claim 4, wherein if the separation clearance of said separation portion is denoted by a2, the following relationship is satisfied:a2·(2·λc·w2·Lc)/(λf·t),where λc: thermal conductivity of the concrete (W/m·K);Lc: thickness of the concrete shielding body (m);λf: thermal conductivity of the heat transfer fins (W/m·K);t: thickness of the heat transfer fins (m);w2: length of said opposite surface in the width direction (m). 6. The concrete cask according to claim 1, wherein said heat transfer fins are formed to have substantially an I-like shape, when viewed from the shell end. 7. The concrete cask according to claim 1, wherein said separation portion is composed so as to separate completely the heat transfer fins and the inner shell or outer shell. 8. The concrete cask according to claim 1, wherein said heat transfer fins are disposed at an angle to the radial direction of said shielding body. 9. The concrete cask according to claim 1, wherein openings are formed in said heat transfer fins. 10. The concrete cask according to claim 1, wherein said shielding body comprises a metal material in at least one shape of grains, particles, or fibers. 11. The concrete cask according to claim 1, wherein said shielding body contains 15 mass % or more of hydroxide retaining water as crystals with a melting point and decomposition temperature higher than 100° C. 12. The concrete cask according to claim 1, wherein said shielding body is sealed so as to be shielded from outside air. 13. The concrete cask according to claim 1, wherein said radioactive substance is contained in a canister which includes a body and a lid, and said canister is placed in said accommodation portion. 14. A concrete cask comprising:an inner shell made from metal;an outer shell made from metal;a shielding body composed of concrete and provided between said inner shell and said outer shell; andan accommodation portion for accommodating a radioactive substance inside said inner shell thereby being kept from the outside of the cask, whereinsaid shielding body is composed of said concrete including portland cement and a metal material that has a high thermal conductivity, wherein said cask has heat fins located between said inner shell and said outer shell. 15. The concrete cask according to claim 14, wherein the thermal conductivity of the shielding body is 4 (W/m·K) or more. 16. The concrete cask according to claim 14, wherein said shielding body comprises a metal material in at least one shape of grains, particles, or fibers. 17. The concrete cask according to claim 14, wherein said shielding body contains 15 mass % or more of hydroxide retaining water as crystals with a melting point and decomposition temperature higher than 100° C. 18. The concrete cask according to claim 17, wherein said hydroxide shows poor solubility or insolubility in water. 19. The concrete cask according to claim 14, wherein said radioactive substance is contained in a canister which includes a body and a lid, and said canister is placed in said accommodation portion.