Patent Number: 
Section: claims

1. A spent fuel housing square pipe for housing a spent fuel aggregate therein, the square pipe comprising: four walls and four corners arranged so as to form a square cross section, the four walls including neutron-absorbing material and having a thickness capable of preventing the spent fuel inserted therein from reaching criticality and capable of ensuring sufficient strength at a time of falling down; and  a connecting section formed on each of the four corners, at which diagonally adjacent square pipes are contacted with each other,  wherein the connecting section is formed into a terrace shape having a plurality of steps, and when the steps of the terrace shape are formed to butt against the steps of adjacent square pipes when the square pipes are assembled in a staggered arrangement. 2. The spent fuel housing square pipe according to  claim 1 , wherein an engaging portion which prevents an offset in a direction perpendicular to an axis of the square pipe is formed on a wall of each step. claim 1 3. The spent fuel housing square pipe according to  claim 1 , wherein a flux trap structure is obtained by providing a space between an inner and an outer surfaces of the wall of the square pipe. claim 1 4. The spent fuel housing square pipe according to  claim 3 , wherein a space is provided between an inner and an outer surfaces of the wall of the square pipe at the corners of the square pipe. claim 3 5. The spent fuel housing square pipe according to  claim 4 , wherein a wall between the flux trap placed inside the terrace portion and the flux trap placed inside the side face is allowed to become thicker toward the butted side. claim 4 6. The spent fuel housing square pipe according to  claim 1 , wherein a corner of the step has a sharp edge with the radius of the sharp edge being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 1 7. The spent fuel housing square pipe according to  claim 1 , wherein a corner of the step is molded so as to have a sharper edge than the inside of the corner. claim 1 8. The spent fuel housing square pipe according to  claim 6 , wherein the radius of the sharp edge is set to not more than 0.5 mm. claim 6 9. The spent fuel housing square pipe according to  claim 7 , wherein the radius of the sharp edge is set to riot more than 0.5 mm. claim 7 10. The spent fuel housing square pipe according to  claim 6 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 6 11. The spent fuel housing square pipe according to  claim 7 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 7 12. The spent fuel housing square pipe according to  claim 7 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to not more than 0.5 mm. claim 7 13. The spent fuel housing square pipe according to  claim 7 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to not more than 0.5 mm. claim 7 14. The spent fuel housing square pipe according to  claim 1 , wherein the square pipe is made of aluminum alloy to which at least enriched boron has been added. claim 1 15. A basket comprising: a plurality of square pipes according to  claim 1  assembled in a staggered arrangement, wherein a cell for housing a spent fuel aggregate is formed in a space defined by walls of the square pipes.  claim 1 16. The spent fuel housing square pipe according to  claim 15 , wherein at least two adjacent connecting sections of the connecting sections formed on the four corners of the square pipe have engaging directions having a difference of 90 degrees. claim 15 17. The spent fuel housing square pipe according to  claim 15 , wherein a flux trap structure is obtained by providing a space between an inner and an outer surfaces of the wall of the square pipe. claim 15 18. The spent fuel housing square pipe according to  claim 17 , wherein a space is provided between an inner and an outer surfaces of the wall of the square pipe at the corners of the square pipe. claim 17 19. The spent fuel housing square pipe according to  claim 18 , wherein a wall between the flux trap placed inside the terrace portion and the flux trap placed inside the side face is allowed to become thicker toward the butted side. claim 18 20. The spent fuel housing square pipe according to  claim 15 , wherein a corner of the step has a sharp edge with the radius of the sharp edge being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 15 21. The spent fuel housing square pipe according to  claim 15 , wherein a corner of the step is molded so as to have a sharper edge than the inside of the corner. claim 15 22. The spent fuel housing square pipe according to  claim 20 , wherein the radius of the sharp edge is set to not more than 0.5 mm. claim 20 23. The spent fuel housing square pipe according to  claim 21 , wherein the radius of the sharp edge is set to not more than 0.5 mm. claim 21 24. The spent fuel housing square pipe according to  claim 20 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 20 25. The spent fuel housing square pipe according to  claim 21 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 21 26. The spent fuel housing square pipe according to  claim 20 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to not more than 0.5 mm. claim 20 27. The spent fuel housing square pipe according to  claim 21 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to not more than 0.5 mm. claim 21 28. The spent fuel housing square pipe according to  claim 15 , wherein the square pipe is made of aluminum alloy to which at least enriched boron has been added. claim 15 29. A spent fuel housing square pipe comprising: a plurality of square pipes assembled in a staggered arrangement, wherein a spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes,  wherein corners of walls of each square pipe is formed into a terrace shape having a plurality of steps and when assembling the square pipes the steps of the terrace shape of adjacent square pipes are butted against each other, and  wherein a flux trap structure, which fits to the shape of the terrace portion, is formed inside of the square pipe is at least the wall or the terrace portion of the square pipe. 30. The spent fuel housing square pipe according to  claim 29 , wherein a corner of the step has a sharp edge with the radius of the sharp edge being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 29 31. The spent fuel housing square pipe according to  claim 29 , wherein a corner of the step is molded so as to have a sharper edge than the inside of the corner. claim 29 32. The spent fuel housing square pipe according to  claim 30 , wherein the radius of the sharp edge is set to not more than 0.5 mm. claim 30 33. The spent fuel housing square pipe according to  claim 31 , wherein the radius of the sharp edge is set to not more than 0.5 mm. claim 31 34. The spent fuel housing square pipe according to  claim 30 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 30 35. The spent fuel housing square pipe according to  claim 31 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to at least not more than one-half of thickness of a plate with which the square pipe is formed. claim 31 36. The spent fuel housing square pipe according to  claim 30 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to not more than 0.5 mm. claim 30 37. The spent fuel housing square pipe according to  claim 31 , wherein the sharp edge has a chamfered shape, with the chamfered dimension being set to not more than 0.5 mm. claim 31 38. The spent fuel housing square pipe according to  claim 29 , wherein the square pipe is made of aluminum alloy to which at least enriched boron has been added. claim 29 39. A spent fuel housing square pipe comprising: four walls and four corners arranged so as to form a square cross section, the four walls including neutron-absorbing material and having a thickness capable of preventing the spent fuel inserted therein from reaching criticality and capable of ensuring sufficient strength at a time of falling down; and  combining section formed on each of the four corners at which diagonally adjacent square pipes are contacted with each other so that a plurality of square pipes are assembled in a staggered arrangement, for housing a spent fuel aggregate inside the square pipe and in a space defined by walls of the square pipes,  wherein the combining section is formed into a terrace shape having a plurality of steps, and the steps of the terrace share are formed to butt against the steps of adjacent square pipes when the square pipes are assembled in a staggered arrangement.