Patent GB-1012433-A (GB)
Title: Improvements in nuclear reactors
Inventor: DEIGHTON THOMAS
IPC: G21C7/20
Filed: 1961-03-10
Abstract: 1,012,433. Nuclear reactors. BABCOCK & WILCOX Ltd. March 7, 1962 [March 10, 1961], No. 8933/61. Heading G6C. [Also in Division F2] The fuel elements in a nuclear reactor are positioned at their upper ends by end locating means including abutments adapted slowly to yield when subjected to pressure exerted by the adjacent ends of the fuel elements but quickly to follow retraction of these ends. Each fuel element may therefore expand slowly substantially without impediment on heating up since the pressure due to the expansion is exerted over a considerable period of time. Strong shock forces, however, which act for short periods of time, such as those resulting from accelerations exceeding 1g to which mobile nuclear power plants are liable, are resisted by the abutment which limits longitudinal movement of the fuel element under the shock forces to a small value. Suitably, each abutment is in the form of an hydraulic damping unit 18 (Fig. 2) secured to a central hub 52 at the top of a riser chimney 15 which has its lower bevelled end 44 resting on a transition piece 43. Each fuel element 19 comprises fuel rods 34 connected at their upper ends to the transition piece 43 by a support member 35, and at their lower ends to a guide 37 by a support member 36. The fuel element is housed in a box 13 connected to a hollow pedestal 23 at its lower end, a screw-threaded sleeve 28 securing the pedestal 23 to a bottom support plate 12 which extends across the core of the reactor. When the fuel element 19 is lowered into the box 13, the support member 36 rests on an upper guide surface 20 on the pedestal 23 and the guide 37 serves to centre the fuel element 19 by engaging a second annular guide surface 31 on the sleeve 28. A lifting frame 38 fixed inside the support member 36 has a central boss 41 which may be gripped by a grab (not shown) to lift the fuel element 19 out of the box 13. A flange 45 projecting from the riser chimney 15 rests on an annular seating 46 formed in an aperture in an upper support plate 14 and serves as a stop for one end of a compression spring 48 which is coiled around the chimney 15, stressed to yield only to a shock loading in excess of lg, and housed within the aperture into the top of which is screwthreaded a spider 54. Radial arms 56 of the spider 54 converge to a central ferrule 57 which partially houses the damping unit 18 which is held in place by a plug 58 screwed into the top of the ferrule 57. The underside of the plug 58 is formed with a punch 62 which, should the unit 18 fail to operate, pierces a rupture disc 63 to enter a cavity 61 formed in the top of the unit 18. The damping unit 18 (Fig. 3) comprises a hollow piston 64 lapped into a cup-shaped cylinder 65 formed in two parts 65A, 65B screwed together. The hollow piston 64 has upper gas ports 68 and lower inclined liquid ports 71, the latter leading from an annular chamber 72 formed around the piston to a liquid reservoir 73 within the piston. A non- return piston 74 which enables liquid to flow out of the reservoir into the lower part 65B of the cylinder 65 but not in the reverse direction comprises a disc 75 having off-centre openings 76 and pressed by a spring 77 against a flange 79 disposed around a central opening 78 leading from the bottom of the reservoir. Downward relative movement of the piston 64 causes the openings 76 to be blanked off by the flange 79, but if the piston 64 moves upward, the differential pressure across the disc 75 biases it downward against the spring 77 so that it moves away from the flange 79 and liquid flows through the openings 76 from the reservoir 73 to the cylinder 65. An end plug 84 seals the top of the piston 64 and a corrugated stainless steel bellows 96 is arranged to permit lengthwise movement of the piston 64 within the cylinder 65 without leakage of fluid from the damping unit. The liquid used in the damping unit may be terphenyl or water, the space above the liquid being filled with helium. During slow expansion of the fuel elements, the annular clearance between the lapped portions of the piston 64 and the cylinder 65 is sufficient to allow the liquid displaced by the movement of the cylinder 65 to leak slowly through into the annular chamber 72, this liquid flowing into the reservoir 73 when it reaches the inclined liquid ports 71. If the reactor is subjected to a sudden vertical shock tending to cause relative movement of a fuel element in an upward direction, this tendency is resisted by the attached damping unit because of the relatively slow leakage of liquid between the lapped portions of the piston 64 and the cylinder 65. Should the reactor be subjected to cyclic vibrations in the vertical direction, the repeated rapid upward movement of a fuel element will be opposed by the attached damping unit while the downward movement will be accompanied by a downward movement of the cylinder 65. The vertical oscillations of the fuel element are thus rapidly damped out. The invention is described with particular reference to a watercooled and -moderated reactor.