Shim-safety control rods

Pertinent data on the shim-safety rods are summarized in Table 2.3.A. 

Weight of roH Weight of magnetic dutch Weight of drive shaft Total weight of rod, shaft, and clutch Water pressure load on rod, assuming approximately 40 lb/in. Drive motor power. Drive motor speed Water flow area (upper section)  

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Reactor criticality is attained and maintained with cadmium-sheet safety, shim, and control rods that move vertically along the outer side of the 36-in.-diameter tank. Primary features for reactor safety are the quick dump system for the water moderator, and the availability for sudden insertion of 3 cocked safety rods (similar to the control rods), plus the control rods. Further details of the reactor design and safety aspects are discussed in Appendix B. About 24 reactors basically similar to the Argonaut have been or are being constructed in various parts of the world. 

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

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). 

The MTR control rods are of two main classes, the shim-safety rods and the regulating rods. All of them are designed to lower the reactivity of the reactor when they are inserted, and when fully in, i.e., resting in their shock absorbers, they will Overcome ah excess reactivity of greater than 40X. It will be shown in Chap. 6 that it is necessary to have approximately 19% excess reactivity built into the reactor hence the control rods provide an adequate margin of safety. 

Lower to move in downward direction a shim-safety rod holder not holding a rod (.i. e., empty). Note "Lower" is also used to mean change of seryo control point by running motor-operated rheostat..) - ... 

A4.2.2 Comtrol Sjrstea. Except for the control system, which includes both shim-safety and regulating rods, the reactor has practically no moving parts. Because this system is roughly the brain and nervous system of the reactor and should operate for its lifetime, it was important - that the control mechanics be tested very thoroughly.. .. 

For these reasons the Mock-Up was provided with two complete shim-safety rod channels and one regulating rod channel. The control, room contained the operating console which served as the study model for the MTR desk and a relay panel, both equipped with the necessary electrical equipment for testing system performance. 

Control rods are used primarily for power distribution shaping and for shim control of long-term reactivity changes, which occur as a result of fuel irradiation. The flow control function, which is used to follow rapid load changes, reduces requirements on speed of control rod response and thus improves plant safety. Every 2-3 months, the control rod patterns are altered to provide more uniform fuel and control rod burnup. In normal daily operation, little control rod movement is required for depletion of reactivity. The resulting low frequency of control rod changes reduces the possibility of operator error. 

In case of design basis accidents, the IMR detects abnormal condition and trips the control rods. Since the IMR has no soluble boron system as a chemical shim, control rod worth is enough to maintain cold shutdown conditions. Additionally, in case of a trip failure, stand-by shutdown systems inject borated water to shutdown the reactor. Residual heat is removed by a passive stand-alone direct heat removal system (SDKS). The SDKS works without operator action and external supports and keeps core conditions within the safety criteria. 

What considerations dictate the design of control rods Of safety rods Shim rods ... 

Control rods are usually designated as safety, shim, or control (regulating) rods. The safety rods shut down the reactor and are normally interlocked in a given position until they are needed to scram the reactor. Shim rods are used for coarse adjustment to criticality. Control (regulating) rods are used to control the reactor at a steady power level and to safely increase or decrease the power level. 

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. 

Probably 9 rods will be required, 4 of them safety rods, to be used for stopping the reaction completely in emergencies, one of them, centrally located, the operating control, to be automatically moved by changes in the neutron density, and 4 of them shimming rods to be manually adjustable and to serve to control the effective size of the pile. 

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