Shim rods

Reactor of NS 910 was frozen , all absorber rods being completely inserted. In such condition the alloy in shrouds of shim rods and automatic control rods was frozen . Cables were dismantled from all CPS actuating mechanisms. Operational time of the reactor was about 80% of its lifetime. 

Nuclear safety of reactors of the damaged NSs is ensured through the application of emgineering and organizational measmes excluding extraction from (displacement in) the core of control rods and shim rods (no way for power supply to actuating mechanisms of the control and protection system, application of mechanical stops, impossibility of using hand drives). 

Position Indicators. The regulating- and shim-rod posit ion indicators have been designed to indicate the exact position of the rods at any time. Rod positions may be read by means of direct-ruading dial indicators at the. platform of the top plug and by means of remote selsyn-driven (dial) indicators at the console in the control room. 

The shim-rod primary indicators at the top plug platform consist of standard counters with five digits driven through suitable reduction gearing attached to a flexible shaft, which in turn is driven directly from the... 

The shim-rod secondary position indicator at the console consists of a dial pointer which- rotates. 9 for each inch of. rod travel , thus for a total travel ofi the rod of 31 in., the dial will rotate 279 . . v... 

The secondary indicator in both cases is operated by means of a receiver selsyn. The regulating-rod-driver sclsyns are located at the., regulating-rod driver, while those for the, shim rods are located under the - top, plug platform. All are driven by means of suitable gearing,. and in all cases either the primary or secondary indicators may be removed for servicing without affecting the other. 

The motor coupring functions.as the connection between the speed-reducer shaft-and the motor, shaft-.,. 11--consists of a splined-collet which-fits the spline-shaft of tlie motor and. is taper7pinned to the speed-reducer drive shaft. The taper pin acts as a shear pin in case an excessive load is placed upon the shim rod. The collet is.threaded and tapered for an adjustable nut. When the nut is tightened, it clamps the collet to the splined motor shaft, thus eliminating any lost motion between motor and speed-reduqer shaft. 

Bottom Absorber, Shim-safety Rod. The bottom shock absorbers for the shim-safety, rods are located on a plate mounted on the bottom plug. They consist merely of dash-pots into which plungers mounted on the bottom of the. shim rods fit. Since they are on the bottom plug they are always filled with water, which acts as the damping fluid. 

Fuel assemblies (23) Beryllinm assemblies (14) Interstices Beryllium reflector Lower grid support Loner assembly grid Shim, rods-. -. ... 

Hot Drive of Shim Rods. The motor drives for the shim rods have been described in Section 2.4.5. Here it is necessary only to recapitulate those features directly relevant to control of rod motions. 

The shim-rod motors are three-phase 220-volt 1/6-hp squirrel-cage motors, of 1800 rpm rating. Start, stop, and reverse are the only control actions called for. Reversal is accomplished. by phase, interchange. This simple control picture is, however, slightly complicated by two considerations the desire to have a finer speed control than is obtained by simple on-off switching, and concern as to the possible safety implications of transition from three-phase to single-phase operation by loss (open circuit) of one power wire. 

Following down from the "traise and scram"< position of S-5 is seen to lead to the individual withdrawal circle, provided S-.21 is on "raise" and provided the rod seat interlock (second precaution to ensure that rod is not being raised) is actuating relay R-51. The notation S-21 is used to represent any one of the eight individual shim rod switches, S-21 to S-28. Similar notations apply to the insert interlock MI-1 and the limit switch relay R-21. These conditions on withdrawal simply imply that one is not at the same time trying to run the motors in the opposite direction, and that the rod is not already withdrawn as far as it can go. 

Preferred shim rods are. also inserted automatically by regulating-rod limit switches., If. the regulating rod is. withdrawn less than 7 in. (,R9)., preferred rods are inserted, and,. if the withdrawal, is less.than 5 in. (RIO), all rods are inserted. This simultaneous insertion of. all rods is called "reverse." It may at,all times.be initiated manually on S-1 or.by a level of 20iX. above Nf, Reverse, may also. be. caused by. certain period or level signals. If, the run auxiliary r,elay, R-3,.-is not picked-up, a., level greater than... 

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