Chemical shim control

Chemical shim control is effected by adjusting the concentration of boric acid dissolved ia the coolant water to compensate for slowly changing reactivity caused by slow temperature changes and fuel depletion. Eixed burnable poison rods are placed ia the core to compensate for fuel depletion. 

Chemical shim control, in nuclear power facilities, 17 544 Chemical shippers, 25 324 Chemical sludge, 25 912 Chemical solution(s) deposition from, 23 13 thin films from, 24 747—750 Chemical solution deposition, 23 13 as fabrication method for inorganic materials, 7 415t Chemical space, 6 16... 

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

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. 

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. 

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. 

These are made of boron carbide ia a matrix of aluminum oxide clad with Zircaloy. As the uranium is depleted, ie, burned up, the boron is also burned up to maintain the chain reaction. This is another intrinsic control feature. The chemical shim and burnable poison controls reduce the number of control rods needed and provide more uniform power distributions. 

As, however, PWRs adopt chemical shim, that is the control of reactivity through dissolution of boric acid in the reactor water, the presence of this neutron absorber decreases the safety effectiveness of the moderator temperature coefficient in fact, if the temperature increases, the amount of boron... 

The reactor core is an open PWR type core made up of 213 fuel assemblies with standard PWR fuel rod diameter and a reduced height. The 2000 MWt core is located near the bottom of the reactor pool, which is a high-boron content water mass enclosed by a prestressed concrete vessel. The PIUS reactor does not use control rods, neither for reactor shutdown nor for power shaping. Reactivity control is accomplished by means of reactor coolant boron concentration control (chemical shim) and by coolant (moderator) temperature control. 

The SPWR has no control rod and reactivity is controlled by the core inherent characteristics and chemical shim. Natural boron content in the primary coolant at the rated operating condition is 1,000 ppm in the BOEC (Beginning of Equilibrium Cycle) and 50 ppm in the EOEC (End of Equilibrium Cycle). 

Reactivity control by chemical shim is a well-established technology proved in conventional PWRs. Preliminary analyses show excellent controllability of the SPWR including reactor start-up even though it has no control-rod. 

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. 

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... 

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. 

If a rupture of an RCCA drive mechanism housing is postulated, the operation using chemical shim is such that the severity of an ejected RCCA is inherently limited. In general, the reactor is operated with the power control (or mechanical shim) RCCAs inserted only far enough to permit... 

In a PWR, with chemical shim, it is possible to change the control element bank position at which the reactor will reach critical. It is important that the critical position is correctly calculated to so that the reactor meets requirements on shutdown margin and controllability before it actually reaches critical. Some PWR operating procedures require 1/M plots during startups to assure that the reactor will reach critical at the proper control element bank height. An alternative method used to predict critical control element bank position at other... 

Second, although only a small amount of tritium is produced from the pure water, there are two other sources of tritium that will produce substantial amounts in a PWR. When boron is used as a chemical shim, fast neutron reactions with boron will produce tritium. Also, tritium is produced as a fission product, and it can escape through clad leaks into the water. Finally, use of Li OH for pH control can contribute tritium. The boron reactions are responsible for most of the tritium, however. 

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. 

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

Shim, S.C., Kim, D.S., Yoo, D.J., Wada, T., and Inoue, Y., Diastereoselectivity control on photosensitized addition of methanol to ( R)-(-t)-limonene,/. Org. Chem., 67, 5718, 2002. The trans isomer of ether 49 was formed with high diastereoselectivity (>96%) using 0.5 M methanol in ether solution at -75°C and either methyl benzoate or dimethyl phthalate as sensitizer. However, chemical yields were low. 

D. Arvanitopoulos Labros, P. Greuel Michael, M. King Brian, K. Shim Anne and H. J. Harwood, in Controlled Radical Polymerization, American Chemical Society, 1998, vol. 685, pp. 316-331. 

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