Patent GB-1365115-A (GB)
Title: Nuclear reactors
IPC: G21C17/12
Filed: 1971-09-07
Abstract: 1365115 Controlling reactors SIEMENS AG 7 Sept 1971 [8 Sept 1970] 41770/71 Heading G6C A control device for a pressurized-water reactor consists of a control element 2 composed of a series of grey finger control rods 4 secured to a central drive rod 6 through a common spiderlike holder 5, and a shutdown element 3 composed of a series of black finger shutdown rods 7 secured, through another spider-like holder 8, to a drive tube 9 coaxially surrounding the drive rod 6. Relative movement is possible between the two elements 2, 3 so that the control element 2 can be inserted into the reactor core to a selectively variable extent whilst the shutdown element 3 is withheld from the core, and so that the shutdown element 3 can be moved to a fully-inserted position in the core when required. The shutdown element 3 can only be withdrawn from its fully inserted position by withdrawing the control element 2 from an inserted position in the core. The drive rod 6 and drive tube 9 extend up through a tube section 11 to a drive means located above the reactor pressure vessel 10, this drive means comprising a jack 12 composed of a lift coil 13, a grip coil 14 and a hold coil 15 for raising and lowering the drive-rod 6, and below the jack 12 a hold coil 16 for the drive tube 9. On rapid shutdown, the finger shock-absorbers present in the fuel assembly immediately below the control device normally become effective, but it may happen by way of exception that the shutdown element 3 falls on to the control element 2 when it is at half-height in the core. For this reason, a widened, lower end portion 20 of the drive tube 9 is constructed to act as a shockabsorber cylinder, there being provided on an enlarged portion 19 of the drive rod 6 housed in a chamber 25 within the portion 20 a bead-like projection 23 so that only a narrow outlet gap 24 remains between the two portions 19, 20 to permit liquid present in the chamber 25 to escape. A coil spring 26 is provided coaxially with the drive rod 6 above its enlarged portion 19 for further braking the descending shutdown element 3. The lower end of the drive-rod 6 has a telescopically insertable end portion 27, which is cushioned against the actual rod 6 by a coil spring 28 and acts as a final shock absorber when the control element 2 encounters the upper closure plate of the fuel assembly. Disposed coaxially with this coil-spring 28 is a soft weighing spring 29, which remains continuously compressed only when the shutdown element 3 is fully run in so that the springs 28, 29 are loaded not only by the control element 2 but also by the shutdown element 3. Thus, when the control element 2 is fully run in the condition of the spring 29 provides an indication of the position of the shutdown element 3. Such an indication is not, however, always unambiguous because it would also indicate full insertion of the shutdown element 3 if this became jammed to the drive-rod 6 at half-height with the drive rod 6 fully run-in so that the spring 29 was loaded by both the elements 2, 3. An additional position indication system is therefore used, comprising an indicating rod 37 having two pins 38 at its lower end extending outwardly through apertures in the rod 6 and connected to a ring 40. When the shutdown element 3 falls in, the rod 37 is pulled downwards owing to the fact that the lower hold ring 42 of the spring 26 encounters the ring 40. The position of the upper head of the rod 37 can be ascertained from variations in the inductance of a magnet coil 17 above the jack 12.