The Control Rods

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. 

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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. 

The rest of the less volatile fission products along with constituents of zircalloy, stainless steel, and the control rods are assumed to be in condensed form as inert aerosols that are treated together in TRAPMELT as "other aerosols." The aerosols are modeled as agglomerating and depositing on surfaces by several mechanisms (e.g., gravitational settling). 

For the start-up of a reactor, its core is allowed to heat up by withdrawal of the control rods until the recapture ratio is slightly greater than f.O. When the optimum operating temperature is reached, the control rods are inserted until the capture ratio is exactly f, and the reaction proceeds at a steady rate. For the shutdown of a reactor, its control rods are fully inserted, reducing the recapture ratio to nearly zero. 

The main danger in the operation of a nuclear power plant is potential loss of control over the nuclear reaction. If the core overheats, it may either explode or melt down. In either event, radioactive materials escape Irom the reactor to contaminate the environment. Designers attempt to make nuclear reactors fail-safe by providing mechanisms that automatically shut the core down on overheating. One way this has been done is to design the control rods to fall into the core if their control mechanism fails. 

C22-0106. Describe the roles of the moderator and the control rods in the operation of a nuclear power plant. 

Worldwide reactors continued to be built until the accident at Chernobyl occurred. Several features made the Chernobyl accident unique to a Soviet style reactor. One was the use of graphite as a moderator, which caught fire. Another was the absence of water to contain radioactivity. But, the most important may have been an inadequate containment structure. There were also problems in controlling the stability of the reactor and the control rods had to be changed frequently in order to keep the reactor stable. 

Zirconium is used for structural parts in the core of water moderated nuclear reactors to this end Zr has several good properties and especially it has low thermal neutron cross-section. Hf, on the contrary, has a high thermal neutron absorption coefficient, so it is necessary to be able to prepare Hf-free zirconium. On the other hand, in some cases the Hf properties too may be useful in nuclear technology, in the control rods of submarine reactors. 

Hafnium has a great affinity for absorbing slow neutrons. This attribute, along with its strength and resistance to corrosion, makes it superior to cadmium, which is also used for making control rods for nuclear reactors. This use is of particular importance for the type of nuclear reactors used aboard submarines. By moving the control rods in and out of a nuclear reactor, the fission chain reaction can be controlled as the neutrons are absorbed in the metal of the rods. The drawback to hafnium control rods is their expense it costs approximately one million dollars for several dozen rods for use in a single nuclear reactor. 

Significant advances have also been made in reactor safety. Earlier reactors rely on a series of active measures, such as water pumps, that come into play to keep the reactor core cool in the event of an accident. A major drawback is that these safety devices are subject to failure, thereby requiring backups and, in some cases, backups to the backups The Generation IV reactor designs provide for what is called passive stability, in which natural processes, such as evaporation, are used to keep the reactor core cool. Furthermore, the core has a negative temperature coefficient, which means the reactor shuts itself down as its temperature rises owing to a number of physical effects, such as any swelling of the control rods. 

Nuclear Boiler Assembly. This assembly consists of the equipment and instrumentation necessary to produce, contain, and control the steam required by the turbine-generator. The principal components of the nuclear boiler are (1) reactor vessel and internals—reactor pressure vessel, jet pumps for reactor water circulation, steam separators and dryers, and core support structure (2) reactor water recirculation system—pumps, valves, and piping used in providing and controlling core flow (3) main steam lines—main steam safety and relief valves, piping, and pipe supports from reactor pressure vessel up to and including the isolation valves outside of the primary containment barrier (4) control rod drive system—control rods, control rod drive mechanisms and hydraulic system for insertion and withdrawal of the control rods and (5) nuclear fuel and in-core instrumentation,... 

Reactor Assembly. This assembly (Fig. 3) consists of the reactor vessel, its internal components of the core, shroud, top guide assembly, core plate assembly, steam separator and dryer assemblies and jet pumps. Also included in the reactor assembly are the control rods, control rod drive housings and the control rod drives. 

Each fuel assembly that makes up the core rests on an orificed fuel support mounted on top of the control rod guide tubes. Each guide tube, with its fuel support piece, bears the weight of fuur assemblies and is supported by a control rod drive penetration nozzle in the bottom head of the reactor vessel. The core plate provides lateral guidance at the top of each control rod guide tube. The top guide provides lateral support for the top of each fuel assembly. 

The channel is a square-shaped tube fabricated from Zircaloy 4. The outer dimensions arc 5,518 inches (14 centimeters) by 5,518 inches (14 centimeters) by 166,9 inches (424 centimeters) long, The reusable channel makes a sliding seal lit on the lower tie plate surface. It is attached to the upper tic plate by the channel fastener assembly, consisting of a spring and a guide, and a cap screw secured by a lock washer. The fuel channels direct the core coolant flow through each fuel bundle and also serve to guide the control rods. 

Because of the presence of sleam voids in lire upper part of [he core, there is a natural characteristic for a BWR to have the axial power peak in the lower part of the core. During the early part of an operating cycle, bottom-entry control rods permit a partial reduction of this axial peaking by locating a larger fraction of the control rods in the lower part of the... 

The control element assemblies consist of an assembly of 4. 8, or 12 fingers approximately 0.8-inch (2-centimeter) outside diameter and arranged as shown in Fig. 14. The use of cruciform control rods, as in boiling water and early pressurized water reactors, necessitates large water gaps between the fuel assemblies to ensure that the control rods will scram (prompt shutdown) satisfactorily. These gaps cause peaking of the power in fuel rods adjacent to the water channel compared to fuel rods some distance from the channel. 

In PWRs, the fuel is U02, enriched typically to 3.3% 235U while for BWRs, the fuel is U02, enriched to 2.6%. (Natural uranium is 0.72% 235U). The fuel elements are clad in Zircaloy, a zirconium alloy that includes tin, iron, chromium, and nickel that prevents fission product release and protects them against corrosion by the coolant. The control rod material in BWRs is B4C, while PWRs have Ag-In-Cd or Hf control materials. 

Uranium fuel is placed in a containment vessel surrounded by circulating coolant, and control rods are added. Made of substances such as boron and cadmium, which absorb and thus regulate the flow of neutrons, the control rods are raised and lowered as necessary to maintain the fission at a barely self-sustainable rate so that overheating is prevented. Energy from the controlled fission heats the circulating coolant, which in turn produces steam to drive a turbine and produce electricity. 

The SL-1 (Stationary Low Power No. 1) was a 3-MW (thermal) boiling water reactor operated by military personnel at the National Reactor Testing Station, Idaho. As a result of interference with the control rods, there was an explosion on 3 January 1961 in which about 5 tonne of coolant were expelled from the pressure vessel (Horan Gammil, 1963). 

Because hafnium has a high absorption cross-section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used to make the control rods of nuclear reactors. It is also applied in vacuum lines as a getter —a material that combines with and removes trace gases from vacuum tubes. Hafnium has been used as an alloying agent for iron, titanium, niobium, and other metals. Finely divided hafnium is pyrophoric and can ignite spontaneously in air. 

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