Snap-in insert for reactivity control in spent nuclear fuel pools and casks

Abstract

A neutron absorbing apparatus, for insertion into a fuel cell storage system, includes a corner spine, a first wall and a second wall, each wall being affixed to the corner spine to form a chevron shape. Each wall includes an absorption sheet affixed to the corner spine, each absorption sheet being formed of a metal matrix composite having neutron absorbing particulate reinforcement, and a guide sheet affixed to and covering a fractional upper portion of the absorption sheet and extending over a top of the absorption sheet. The absorption sheet extends along the corner spine a greater length than the guide sheet.

What is claimed is: 
     
         1 . A method of retrofitting a spent nuclear fuel storage system, the method comprising:
 inserting a neutron absorbing apparatus into a first cell of an array of cells each configured to hold a spent nuclear fuel assembly, wherein each cell is separated from each adjacent cell by a cell wall, the neutron absorbing apparatus comprising:
 a corner spine; and 
 a first wall and a second wall, each affixed to the corner spine to form a chevron shape, wherein each wall comprises:
 an absorption sheet affixed to the corner spine, the absorption sheet comprising a metal matrix composite having neutron absorbing particulate reinforcement; and 
 a guide sheet affixed to the absorption sheet, the guide sheet extending over a top edge of the absorption sheet, and 
 
   wherein at least one of the first wall and the second wall further comprises a first locking protuberance coupled to the respective guide sheet and protruding through an opening formed in the respective absorption sheet; and   creating a second locking protuberance in a first cell wall of the first cell adjacent to the neutron absorbing apparatus, wherein the first locking protuberance and the second locking protuberance are positioned to interlock to retain the neutron absorbing apparatus in the one cell during removal of the fuel assembly from the first cell.   
     
     
         2 . The method of  claim 1 , wherein the guide sheet is affixed to and covers a fractional upper portion of the absorption sheet. 
     
     
         3 . The method of  claim 2 , wherein an extension portion of the guide sheet which extends over the top edge of the absorption sheet is obliquely angled to the absorption sheet to protect the absorption sheet from damage during a process of loading the fuel assembly into the cell. 
     
     
         4 . The method of  claim 1 , wherein each of the first and second locking protuberances comprises resiliently deflective first and second tabs, respectively. 
     
     
         5 . The method of  claim 4 , wherein the first tab is obliquely angled to its respective absorption sheet. 
     
     
         6 . The method of  claim 5 , wherein the first tab projects outwards away from its respective absorption sheet to engage the second tab of the first cell wall which projects inwards towards the first cell. 
     
     
         7 . The method of  claim 4 , wherein the first tab has a lower part affixed to its respective guide sheet and a free upper part obliquely angled to the lower part. 
     
     
         8 . The method of  claim 7 , wherein the upper part of the first tab is bent away from the guide sheet to extend through the opening of the respective absorption sheet. 
     
     
         9 . The method of  claim 8 , wherein the first tab protrudes beyond an outer surface of the absorption sheet by between about 0.125 inch to 0.254 inch. 
     
     
         10 . The method of  claim 7 , wherein the lower part of the first tab is riveted to the guide sheet. 
     
     
         11 . The method of  claim 1 , wherein the first tab is formed from 301 stainless spring steel, tempered to ¾ hard. 
     
     
         12 . The method of  claim 11 , wherein the first tab is resiliently deflective by about 0.125 inch. 
     
     
         13 . The method of  claim 1 , wherein the absorption sheet extends along the corner spine a greater length than the guide sheet. 
     
     
         14 . The method of  claim 1 , wherein the first and second walls of the neutron absorbing apparatus are oriented perpendicular to each other. 
     
     
         15 . The method of  claim 1 , wherein the step of creating the second locking protuberance further comprises cutting a half-shear tab in the first wall of the first cell, the half-shear tab extending inwards towards the first cell. 
     
     
         16 . The method of  claim 15 , wherein the half-shear tab is cut with a C-shaped tool 
     
     
         17 . The method of  claim 15 , further comprising pushing a wedge-shaped tool into the cell wall to bend the half-shear tab inwards. 
     
     
         18 . The method of  claim 1 , wherein the second locking protuberance is formed above the first locking protuberance of the neutron absorbing apparatus. 
     
     
         19 . A method of retrofitting a neutron absorbing apparatus into a spent nuclear fuel rack system, the method comprising:
 inserting a neutron absorbing apparatus into a first cell of an array of cells each configured to hold a spent nuclear fuel assembly, each cell being defined by plural cell walls;   the neutron absorbing apparatus comprising:
 a first wall and a second wall supported by a corner spine to form a chevron shape; and 
 a first locking tab protruding outwardly from the first wall towards a first cell wall of the first cell, 
   cutting a half-sheared second locking tab in the first cell wall of the first cell adjacent to and above the first locking tab of the neutron absorbing apparatus;   wherein the second locking tab is positioned to locking engage the first locking tab to retain the neutron absorbing apparatus in the first cell during removal of one of the fuel assemblies from the first cell.   
     
     
         20 . The method of  claim 19 , wherein the second locking tab comprises a lower part connected to the first wall of the first cell, and an upper part protruding laterally inwards towards the first wall of the neutron absorbing apparatus. 
     
     
         21 . The method of  claim 19 , further comprising a step of permanently deforming a free lower part of the second locking tab inwards after the cutting step such that a lower part of the second locking tab protrudes laterally inwards towards the first wall of the neutron absorbing apparatus to engage the first locking tab. 
     
     
         22 . A method of installing a neutron absorbing apparatus into a spent nuclear fuel rack, the method comprising:
 inserting a neutron absorption assembly into a first cell of the fuel rack, the first cell configured to hold a spent nuclear fuel assembly;   the neutron absorber assembly comprising a cantilevered first locking tab angled laterally outwards from the assembly towards a first cell wall of the first cell,   cutting a cantilevered second locking tab in the first cell wall of the first cell in a position above the first locking tab of the neutron absorbing apparatus, the second locking tab angled laterally inwards;   wherein the second locking tab is arranged to lockingly engage the first locking tab to retain the neutron absorbing apparatus in the first cell during removal of the fuel assembly from the first cell.