Patent GB-1317589-A (GB)
Title: Venting system for a gas cooled nuclear reactor
IPC: G21C3/38
Filed: 1970-09-15
Abstract: 1317589 Reactors GULF OIL CORP 15 Sept 1970 [3 Oct 1969] 43998/70 Heading G6C In a gas-cooled nuclear reactor, a venting system comprises means defining a vent passage system 11 for conducting fission products from otherwise sealed fuel rods 12, a fission product receiving system 13 communicating with the passage system 11 and including means for maintaining the pressure therein and in the passage system 11 substantially less than the coolant gas pressure. The receiving system 13 has a lower pressure than the passage system 11 so that fission products will pass from the fuel rods 12 to the receiving system 13. The fuel rods 12 are assembled in fuel elements 18 and at least one bleed passage 14 per fuel element is provided to bleed coolant gas into the passage system 11 to facilitate the transportation of fission products to the receiving system 13. The fuel rods 12 are held in position within a housing 41 by a network of transverse beams 42 secured to plugs 43 at the upper ends of the fuel rods 12, the interiors of the fuel rods 12 communicating with passages 45 in the beams 42 through passages 43a in the plugs 43. The passages 43a and 45 form part of the vent passage system 11. The passages 45 are connected by a tube 44 and by a passage 47 in the housing wall to a hollow section 46 of the housing wall which contains a fission product trap 54 of activated charcoal adsorber between regions 51, 56 of porous material. An L-shaped passage 57 (Fig. 3) leads from the fission product trap 54 to a bevelled shoulder 40 extending round the outside of the fuel element 18 and which abuts a corresponding bevelled surface 20 on the lower part of a supporting grid structure 16 when the upper wall 19 of the fuel element is secured in position in the grid structure 16. The mouth of the passage 57 is surrounded by a boss 61 projecting outwardly from a recess 62 in the shoulder 40. The boss 61 does not extend far enough to engage the bevelled surface 20, leaving a clearance which forms the bleed passage 14. A passage 64 in the grid structure 16 is aligned with the mouth of the passage 57 to conduct gas therefrom to the receiving system 13. Any upstream inleak of coolant resulting from a leak in the cladding of a fuel rod 12 will result in a measurable activity increase in the extraction line from the affected fuel element, which increase may be readily detected and its source located to the nearest fuel element. The passages 64 from all the fuel elements are interconnected and conduct all fission product gases to a single fission product trap, the purified coolant from which is returned to a low pressure region of the primary coolant loop. The fuel element 18 is supported by means of three equi-spaced webs 21 at the upper end of its housing, which webs terminate in a central collar 22 releasably attached to the lower end of a support sleeve 23 by a bayonet-type coupling. The support sleeve 23 may be used to lower the fuel element from a receptacle in the grid structure 16 into a refuelling machine. At its upper end outside the reactor vessel (not shown), the the support sleeve 23 is threaded and carries a nut 27 which when tightened applies a load to a washer 28 and this load is transmitted through a thrust tube 29 coaxial with the sleeve 23. The lower end of the tube 29 forms a collet section 31 terminating in a bevel 32 engaging a bevelled surface 39 in the central collar 37 of a grid insert comprising three equi-spaced webs 34 extending inwardly from a cylindrical outer wall 36 seated in a recess 33 in the grid structure 16. Coolant flow, which is downward after passing the grid insert, is controlled by means of an axially movable plug 87 forming an annular orifice with a portion 86 of the upper end wall 19 of the fuel element housing, which portion 86 is of varying cross-section. The plug 87 is moved between its open and closed positions by means of a rotary actuator rod 88 which extends to the exterior of the reactor vessel through the same penetration as the support sleeve 23. The actuator rod 88 carries a threaded drive screw 92 at its lower end, which screw 92 is supported by a thrust bearing 97 seated in a cavity 98 in a support member 99 and is surrounded by a threaded sleeve 102. The sleeve 102 is connected to the plug 87 by a series of arms 83 so that when the rod 88, and hence the screw 92, are turned the plug moves axially to vary the opening of the annular orifice. In its open position, the plug 87 mates with the upper end of the support member 99 at an interface 105.