Reactivity control rods

During the MK-II operation, extensive data were accumulated from start-up and core characteristics tests. These core management and core characteristics data were compiled into a database [2]. The core management data includes core specifications and configurations, atomic number densities before and after irradiation, neutron and gamma flux, neutron fluence, fuel bumup, and temperature and power distributions. The core characteristics data include excess reactivities, control rod worths, and reactivity coefficients, e.g., temperature, power and bumup. These core characteristics data were recorded on CD-ROM for user convenience. 

Between the time at which positive reactivity is inserted and control rods begin to move, the reactor behavior depends primarily on the Doppler effect to limit the severity of the accident. The full-power operating conditions correspond to the lowest Doppler feedback in normal operation because the Doppler effect is approximately inversely proportional to the absolute fuel temperature. Also, energy required for core failure is normally a minimum at full core power. Thus, the core is most vulnerable to reactivity insertions at full power. The ejection of the most reactive control rod was assumed at full power. Worth of the most reactive control rod is 60. An instantaneous ejection of this rod was assumed. 

Types of reactivity control systems Two independent protection systems with mechanical drives of reactivity control rods Each system can shut down the reactor and maintain it in a subcritical state... 

According to Russian regulations, two independent and diverse protection systems shall be provided. The reactor uses two groups of reactivity control rods with mechanical drives of different types. Each of these systems can shut down the reactor and maintain it in a sub-critical state from any nominal or emergency state, provided that the most effective rod does not actuate. 

In the current design, all reactivity control rod clusters are built as active. Passive shutdown rods actuated by flow decrease beyond a certain limit (the same as proposed for the BN-800) are under investigation. 

During BMN-170 development, the design principles to provide plant safety was aimed at an optimum combination of reliance on intrinsic safety features and application of engineered (active and passive) systems. To be specific, the protection system being developed employs hydraulically suspended reactivity control rods that effectively influence the reactivity and convert the reactor to a sub critical state when flow rate is reduced through the core [XXI-9]. Use of a passive emergency cool down system allows complete removal of residual heat. 

Currently, the most severe beyond design basis accident related to NPP blackout and nonactuation of all reactivity control rods and additional failure of heat removal to the secondary circuit has been considered. 

The spherical fuel particle measuring about 1 mm in diameter consists of an inner nuclear kernel coated in successive layers of carbon and ceramics. Thousands of the particles are packed in graphite matrix into a spherical pebble of roughly tennis ball size or a cylindrical compact about the size of man s thumb. A pebble bed core contains a large number of fuel pebbles (for example, 27,000 in the HTR-10 core), and the helium coolant flows in the void volume formed in the pile of the pebbles. On the other hand, a prismatic core contains many hexagonal graphite blocks (150 in the HTTR core) in which the fuel compacts are embedded and the hehum coolant flows in the channels provided in the block. Both cores are surrounded by graphite reflector and enclosed in steel pressure vessel. Reactivity control rods (RCRs) are inserted from above the reactor pressure vessel (RPV). 

Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions which terminate polymer chain reactions are also an important part of the polymerization mechanism. Killing off the reactive intermediate that keeps the chain going is the essence of these termination reactions. Some unusual polymers can be formed without this termination these are called living polymers. 

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. 

A scram causes the control rods to drop into the core, absorb neutrons and stop the chain reaction. Some rods perform both controlling and scram functions. The control rods are raised to increase the neutron flux (and power) or lowered to reduce it by magnetic jacks (W and CE) or a magnetic "clamshell" screw (B W). The chemical volume and control system (CVCS - not siiown) controls the water quality, removes radioactivity, and varies the reactivity by controUing the amount of a boron compound that is dissolved in the water - called a "poison." Thus, a PWR coiiirols reactivity two ways by the amount of poison in the water and by moving the control rods. 

BWRs do not operate with dissolved boron like a PWR but use pure, demineralized water with a continuous water quality control system. The reactivity is controlled by the large number of control rods (>100) containing burnable neutron poisons, and by varying the flow rate through the reactor for normal, fine control. Two recirculation loops using variable speed recirculation pumps inject water into the jet pumps inside of the reactor vessel to increase the flow rate by several times over that in the recirculation loops. The steam bubble formation reduces the moderator density and... 

The low-power-density, low enrichment reactor core uses soluble boron and burnable poisons for shutdown and fuel bumup reactivity control. Low worth grey rods provide load following. A heavy uranium flywheel extends the pump coastdown to allow for emergency action during loss-of-flow transients. 

At 0119 10, the operator began to increase the rate of feedwater return to reduce the recirculation flow to increase the water level in the steam drums. At 0119 45, the reduced inlet water. stopped water from boiling in the core. The absence of the steam voids reduced the reactivity, and control rods were withdrawn, such that only 6 to 8 rods were in the reactor, rather than the required 30. Then, to avoid reactor trip from steam drum or feedwater signals, their scram circuils v ere locked out (a safety regulation violation). 

Excess reactivity Failure of automatic rod control Heat imbalance lemper- ature increase Control rod drive Operator action, interlocks HIE... 

Control Rod Drive System. Positive cure reactivity cuulrol is maintained by the use of movable control rods interspersed throughout the core. 

Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

The term eff — 1 is called excess reactivity, and /eff — l)/ eff is called reactivity. Because the fissile material is continuously used up by fission and because the fission products absorb neutrons, a certain excess reactivity is necessary to operate a nuclear reactor. This excess reactivity is compensated by control rods that absorb the excess neutrons. These control rods contain materials of high neutron absorption cross section, such as boron, cadmium or rare-earth elements. The excess reactivity can also be balanced by addition to the coolant of neutron-absorbing substances such as boric acid. 

The subsequent events led to the generation of an increasing number of steam voids in the reactor core, which enhanced the positive reactivity. The beginning of an increasingly rapid rise in power was detected, and a manual attempt was made to stop the chain reaction (the automatic trip, which the test would have triggered earlier, had been blocked). However, there was little possibility of rapidly shutting down the reactor as almost all the control rods had been completely withdrawn from the core. The continuous reactivity addition by void formation led to a prompt critical excursion. It was calculated that the first power peak reached 1(X) times the nominal power within four seconds. Energy released in the fuel by the power excursion suddenly ruptured part of the fuel into minute pieces. Small, hot fuel particles (possibly also evaporated fuel) caused a steam explosion. 

Use of europium oxide as a neutron absorber in the control rods avoids gas generation under irradiation and gives a slower loss of reactivity with neutron exposure than boron carbide (10%). The main problems are obtaining adequate critical nuclear... 

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