Patent Number: 052456439
Section: claims

1. In a fuel bundle having a matrix of upstanding vertical fuel rods, a lower tie plate for supporting said fuel rods and permitting the entry of water into the bottom single phase region of said fuel bundle, an upper tie plate for permitting at least some of said fuel rods to extend to said upper tie plate and to permit the exit of water and generated steam from the upper two phase region of said fuel bundle; and, an enclosing channel having said lower tie plate at the bottom end, said upper tie plate at the upper end, and surrounding said fuel rod matrix between said tie plates to permit the flow of water and vapor moderator between said tie plates; the improvement to said fuel bundle comprising:  at least some of said matrix of upstanding vertical fuel rods being part length fuel rods commencing at said lower tie plate and extending into the two phase region of said fuel bundle below said upper tie plate so as to define between the end of said part length fuel rods and said upper tie plate a spatial interval; and,  means for defining at least one water region occupying at least one interval between one of said part length fuel rods and said upper tie plate, said means for defining at least one water region having sides and a closing bottom for maintaining a volume of liquid water within said region;  means at the top of said water region for permitting the exit and entry of water to and exit of steam from said water region whereby said water region accumulates water moderator from said passing two phase water and steam mixture adjacent said water region.  a matrix of upstanding vertical fuel rods,  a lower tie plate for supporting said fuel rods and permitting the entry of water into the bottom of said fuel bundle,  an upper tie plate for permitting at least some of said fuel rods to extend to said upper tie plate and to permit the exit of water and generated steam from the upper two phase region of said fuel bundle;  an enclosing channel having said lower tie plate at the bottom end, said upper tie plate at the upper end, and surrounding said fuel rod matrix between said tie plates to permit the flow of water and vapor moderator between said tie plates;  at least some of said matrix of upstanding vertical fuel rods being part length fuel rods commencing at said lower tie plate and extending into the two phase region of said fuel bundle below said upper tie plate so as to define between the end of said part length fuel rods and said upper tie plate a spatial interval;  means for defining at least one water region occupying the interval between said part length fuel rod and said upper tie plate; said means for defining at least one water region having sides and a closing bottom;  means at the top of said water region for permitting the exit and entry of water and steam to said water region whereby said water region accumulates water moderator from said passing two phase water and steam mixture adjacent said water region.  a discrete top filled water region overlies a discrete part length fuel rod.  a discrete top filled water region overlies more than one part length fuel rods. 2. The invention of claim 1 and wherein said means for defining said at least one water region includes a single chamber overlying a plurality of side-by-side part length fuel rods. 3. The invention of claim 1 and wherein said means for defining said at least one water region defines a cruciform shaped cross sectional water region transversely across a region of said fuel bundle, said cruciform cross sectional water region overlying part length fuel rods placed in a corresponding cruciform cross sectional region underlying said part length water region. 4. The invention of claim 1 and wherein said means at the top of said water region for permitting the entry and exit of water and steam includes means extending into the two phase flow region of said fuel bundle for diverting water into the top of said water region. 5. The invention of claim 4 and wherein said means extending into the two phase flow region of said fuel bundle for diverting water into the top of said water region overlies a surface along which water flows during normal operation of said fuel bundle. 6. The invention of claim 4 and wherein said means extending into the two phase flow region of said fuel bundle for diverting water into the top of said water region is recessed with respect to the cross sectional profile of said water region. 7. In a fuel bundle for a boiling water nuclear reactor comprising: 8. The invention of claim 7 and wherein: 9. The invention of claim 7 and wherein: 10. The invention of claim 7 and wherein said means at the top of said water region for permitting the entry of water and exit of steam includes means extending into the two phase flow region of said fuel bundle for diverting water into the top of said water region.