Patent GB-983596-A (GB)
Title: Nuclear reactor
Inventor: ASHCROFT DAVID JOHN
IPC: G21C1/08
Filed: 1962-08-30
Abstract: 983, 596. Controlling nuclear reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Aug. 8, 1963 [Aug. 30, 1962], No. 33301/62. Heading G6C. A liquid-moderated nuclear reactor has hollow nuetron-absorbing control elements which displace liquid moderator in moving with an open end foremost into the reactor core, filler elements being so positioned within the liquid moderator they are each surrounded by a control element when these are located within the core, the filler elements being without a fissile content and substantially of a moderator material which has neutron slowing-down power less than that of the liquid moderator displaced by the filler elements. A reactor of this type is shown in Fig. 1 and has a core 11 in which fuel elements are housed in fuel tubes 12 through which pressurised light water is circulated as a primary coolant and which are housed in a region defined by a baffle 13 in a pot 14 in the lower half of a reactor vessel 17. A secondary coolant, also light water, is circulated downwards through the annular space between the pot and the baffle and upwardly through the core between the fuel tubes. A thermal shield 15 is interposed between the baffle and the pot and has apertures 16 to permit downward flow of the secondary coolant between it and the baffle. Neutron moderation is effected by the primary and secondary coolants. In its upward passage the seconddary coolant is allowed to boil to form a mixture of steam and water which is separated by cyclone steam separators 33, the steam passing through scrubber units 34 to a steam outlet 19. The fuel tubes 12 are disposed below an intermediate support plate 27 while extension tubes 32, each being an extension of a fuel tube, extend between the intermediate support plate 27 and an upper support plate 28. After its passage through the fuel and extension tubes, the primary coolant is collected at a ring header 35 and circulated by pumps 23 to a helical toroid pressurizer and thence back to the fuel and extension tubes. The control elements are in the form of hollow open-ended rods 39 of neutron-absorbing material (e.g. stainless steel alloyed with 4% by wt. boron) which are insertable into the core (i.e. the region below the support plate 27) over filler element 42 by hydraulic means, there being connections between the rods 39 and individual headers 43 so that the rods can be operated independently of each other. Each rod 39 (Fig. 2) is closed by a head 60 at its upper end which is slidable on a fixed piston rod 61 suspended from a web 63 at the lower end of a support shaft 64 itself suspended from the support plate 28. Apertures 65 through each web 63 admit secondary coolant into the space above the head 60 so that the pressure above and surrounding the head is at the secondary outlet pressure. A fixed piston 66 is screwed on to the lower end of the rod 61 and carries circumferential rings 67 which make a fluid-tight, sliding seal with the interior of the rod 39. Each rod 39 is positioned over a filler element 42 in the form of a hollow Zircalloy cylinder filled with graphite 73 and having at its upper end a cap 74 and a threaded rim 70 screwed over a similar threaded rim at the lower end of the piston 66. At its lower end each filler element 42 is closed and tied to the lower ends of the clustered fuel tubes 12. As each rod 39 is introduced into the core it slides over the filler element 42 below it and is guided and steadied by the element. These elements displace water by graphite which is a poor moderator in comparison with water with the result that the thermal neutron flux in the regions vacated by the rods is substantially undistorted; this is in contrast to the flux peaking which would occur in the regions vacated by the rods were these regions to be filled with water. On the other hand, some moderation by the filler elements enhances neutron absorption in the control rods in that any high energy neutrons which penetrate into the hollow interior of a control rod on the first encounter with the absorber material thereof are slowed down by the filler element and are therefore less likely to escape absorption on the second encounter. Movement of each rod is effected hydraulically by means of secondary coolant water introduced into the rod through the hollow piston rod 61. At its lower end the piston rod 61 had apertures 75 to allow water to pass from the piston rod into the control rod. The control rods are moved by regulating the fluid pressure within the headers 43. Those control rods designated as shut-off rods are, during normal operation, maintained in their uppermost positions by having their headers 43 in communication with the outlet from the secondary coolant pumps and hence the interiors of the rods are filled with secondary coolant water at a pressure equal to that of the water at the inlet to the core, there thus being a pressure differential between the interiors and exteriors of the rods which is equivalent to the pressure drop across the core. If this pressure drop falls, for example owing to failure of the secondary coolant pumps, the rods descend into the core. This descent can be accelerated by means of coiled springs surrounding the rods and acting between the fixed support shafts 64 and shoulders at the lower ends of the rods. Those rods which are employed for long term reactivity control are retained within the core by placing their headers in communication with secondary coolant water at core outlet pressure. When it is desired to raise any of them, their headers are placed in communication with the secondary coolant pump outlets when the rods are raised to their uppermost positions and then operate as shut-off rods. The control rods of this reactor have only two stable positions, viz. their uppermost and lowermost positions and accordingly they cannot be employed for operational control of the reactor. This is effected by varying the steam content and therefore the moderating power of the secondary coolant. Specification 983,595 is referred to.