Control rod

From Free net encyclopedia

A control rod is a rod made of a chemical element capable of absorbing many neutrons without decaying themselves. They are used in nuclear reactors to control the rate of decay of uranium and plutonium.

A control rod is removed from or inserted into the central core of a nuclear reactor in order to control the neutron flux - increase or decrease the number of neutrons which will split further uranium atoms. This in turn affects the temperature of the reactor, the amount of steam generated, and hence the electricity produced.

Control rods often stand vertically within holes through the pile, and are lifted partially out to allow a chain reaction to occur. The number of control rods lifted and the distance by which they are lifted can be varied to control the rate of the reaction.

In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the reaction. Quickly shutting down a reactor in this way is called scramming the reactor.

Originally the control rods hung above the reactor, suspended by a rope. In an emergency a person assigned to the job would take a fire axe and cut the rope, allowing the rods in fall into the reactor and stop the fission. At some point the title of the person assigned this duty was given as SCRAM, or Safety Control Rod Ax Man (although this may be a backronym). This term continues in use today as the phrase "scram" to describe a shutting down of a reactor by dropping the control rods.

Mismanagement or failure of control rods was a cause or an aggravating factor of many nuclear accidents, including the SL-1 explosion.

Control rods are commonly composed of silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, gadolinium, and europium.

In many water moderated reactors it is common to add a small amount of boric acid to the collant to act as a chemical shim this has the effect of lowering the reactivity of the core in the same way as a solid control could. This chemical shim along with the use of burnable neutron poisons are used to assist regulate the long term reactivity of the core,[1] while the control rods are used for rapid changes to the reactor power (eg shutdown and start up).

Homogenous neutron absorpers have oftein been used to manage criticality accidents which involve aqueous solutions of fissile metals, in several such accidents either borax (sodium borate) or a cadmium compound has been added to the system. The cadium can be added as a metal to nitric acid solutions of fissile material, the corrosion of the cadmium in the acid will then generate cadmium nitrate in situ.

In carbon dioxide cooled reactors such as the AGR if the solid control rods were to fail to arrest the nuclear reaction, nitrogen gas can be injected into the primary coolant cycle. This is because nitrogen has a larger absorption cross section for neutrons than carbon or oxygen, hence the core would then become less reactive.

Image:Neutroncrosssectionboron.jpg

As the neutron energy increases the neutron cross section of most isotopes decreases, it is interesting to note that ^{10B is the boron isotope which is responsible for the majority of the neutron absorption. Boron containing materials can be used as neutron shields to reduce the activation of objects close to a reactor core.ja:制御棒}