Chemical shim system

The chemical shim system uses the soluble neutron absorber boron (in the form of boric acid), which is inserted in the reactor coolant during cold shutdown, partially removed at startup, and adjusted in concentration during core lifetime to compensate for such effects as fuel consumption and accumulation of fission products which tend to slow the nuclear chain reaction. The control system allows the plant to accept step... 

Control rods perform the reactivity control a soluble boron chemical shim system is not used in the IMR except for the backup shutdown system. 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

In conjunction with the boron thermal regeneration system (BTRS), adjust the boric acid concentration of the reactor coolant for chemical shim control solution. 

Natural circulation cooling leads to low cost power generation. The core outlet temperature of 345°C and the pressure of 15.5 MPa have been selected for the core cooling system to obtain higher thermal efficiency. The coolant system does not use chemical shim for reactivity control. To compensate for bum-up reactivity swing, a perfectly passive reactivity control system is under study, based on burnable poisons in fuel and using moderator void feedback. 

During operation, the reactor coolant pumps circulate pressurised waterthrough the reactor vessel then the steam generators. The water, which serves as coolant, moderator, and solvent for boric acid (chemical shim control), is heated as it passes through the core. It is transported to the steam generators where the heat is transferred to the steam system. The water is then returned to the reactor vessel by the pumps to repeat the process. 

As xenon-135 and samarium-149 are formed in a reactor, they reduce the multiplication factor by decreasing the thermal utilization factor, f, Since the formation of fission product poisons is a direct function of the fission rate, as power level changes the amount of poison present in the reactor also changes. Control system reactivity insertions such as rod motion and chemical shim must be made to compensate for fission product reactivity. 

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. 

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