Control rod

Control rods are used in nuclear reactors to control the fission rate of uranium and/or plutonium. Their compositions includes chemical elements such as boron, cadmium, silver, and/or indium, that are capable of absorbing many neutrons (without themselves fissioning). These elements have different neutron capture cross sections for neutrons of various energies. Boiling water reactors (BWR), pressurized water reactors (PWR), and heavy-water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons. Each reactor design can use different control rod materials based on the energy spectrum of its neutrons. Control rods are used in nuclear reactors to control the fission rate of uranium and/or plutonium. Their compositions includes chemical elements such as boron, cadmium, silver, and/or indium, that are capable of absorbing many neutrons (without themselves fissioning). These elements have different neutron capture cross sections for neutrons of various energies. Boiling water reactors (BWR), pressurized water reactors (PWR), and heavy-water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons. Each reactor design can use different control rod materials based on the energy spectrum of its neutrons. Control rods are inserted into the core of a nuclear reactor and adjusted in order to control the rate of the nuclear chain reaction and, thereby, the thermal power output of the reactor, the rate of steam production, and the electrical power output of the power station. The number of control rods inserted, and the distance to which they are inserted, strongly influence the reactivity of the reactor. When reactivity (as effective neutron multiplication factor) is above 1, the rate of the nuclear chain reaction increases exponentially over time. When reactivity is below 1, the rate of the reaction decreases exponentially over time. When all control rods are fully inserted, they keep reactivity barely above 0, which quickly slows a running reactor to a stop and keeps it stopped (in shutdown). If all control rods are fully removed, reactivity is significantly above 1, and the reactor quickly runs hotter and hotter, until some other factor slows the reaction rate. Maintaining a constant power output requires keeping the long-term average neutron multiplication factor close to 1. A new reactor is assembled with its control rods fully inserted. Control rods are partially removed from the core to allow the nuclear chain reaction to start up and increase to the desired power level. Neutron flux can be measured, and is roughly proportional to reaction rate and power level. To increase power output, some control rods are pulled out a small distance for a while. To decrease power output, some control rods are pushed in a small distance for a while. Several other factors affect the reactivity; to compensate for them, an automatic control system adjusts the control rods small amounts in or out, as-needed. Each control rod influences some part of the reactor more than others; complex adjustments can be made to maintain similar reaction rates and temperatures in different parts of the core. Typical shutdown time for modern reactors such as the European Pressurized Reactor or Advanced CANDU reactor is 2 seconds for 90% reduction, limited by decay heat. Control rods are usually used in control rod assemblies (typically 20 rods for a commercial PWR assembly) and inserted into guide tubes within the fuel elements. Control rods often stand vertically within the core. In PWRs they are inserted from above, with the control rod drive mechanisms mounted on the reactor pressure vessel head. In BWRs, due to the necessity of a steam dryer above the core, this design requires insertion of the control rods from beneath. Chemical elements with usefully high neutron capture cross-sections include silver, indium, and cadmium. Other candidate elements include boron, cobalt, hafnium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Alloys or compounds may also be used, such as high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium nitrate, gadolinium titanate, dysprosium titanate, and boron carbide–europium hexaboride composite. The material choice is influenced by the neutron energy in the reactor, their resistance to neutron-induced swelling, and the required mechanical and lifespan properties. The rods may have the form of tubes filled with neutron-absorbing pellets or powder. The tubes can be made of stainless steel or other 'neutron window' materials such as zirconium, chromium, silicon carbide, or cubic 11B15N (cubic boron nitride). The burnup of 'burnable poison' isotopes also limits lifespan of a control rod. They may be reduced by using an element such as hafnium, a 'non-burnable poison' which captures multiple neutrons before losing effectiveness, or by not using neutron absorbers for trimming. For example, in pebble bed reactors or in possible new type 7-moderated and -cooled reactors that use fuel and absorber pebbles.