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Thermal-neutron reactor
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<p>A thermal-neutron reactor is a <a href="/facts/Nuclear_reactor/8xqjIbge">nuclear reactor</a> that uses slow or <a href="/facts/Thermal_neutron/UMKR4XgJ">thermal neutrons</a>. ("Thermal" does not mean hot in an absolute sense, but means in <a href="/facts/Thermal_equilibrium/1D7nGm0d">thermal equilibrium</a> with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than the <a href="/facts/Fast_neutron/UMKR4XgJ">fast neutrons</a> initially produced by fission.) </p><p>Most <a href="/facts/Nuclear_power_plant/IJj5pJUn">nuclear power plant</a> reactors are thermal reactors and use a <a href="/facts/Neutron_moderator/1qa6YvDD">neutron moderator</a> to slow <a href="/facts/Neutron/oRCM1geb">neutrons</a> until they approach the average <a href="/facts/Kinetic_energy/aJRxUd4p">kinetic energy</a> of the surrounding particles, that is, to reduce the speed of the neutrons to low-velocity, thermal neutrons. Neutrons are uncharged, this allows them to penetrate deep in the target and close to the nuclei, thus scattering neutrons by nuclear forces, some nuclides are scattered large. </p><p>The <a href="/facts/Nuclear_cross_section/AtghNytD">nuclear cross section</a> of <a href="/facts/Uranium-235/pe0RznDM">uranium-235</a> for slow thermal neutrons is about 1000 <a href="/facts/Barn_(unit)/0yCEtKJ9">barns</a>, while for fast neutrons it is in the order of 1 barn. Therefore, thermal neutrons are more likely to cause uranium-235 to <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a> than to be captured by <a href="/facts/Uranium-238/QhcHm6q8">uranium-238</a>. If at least one neutron from the U-235 fission strikes another nucleus and causes it to fission, then the <a href="/facts/Chain_reaction/5UwHTKXS">chain reaction</a> will continue. If the reaction will sustain itself, it is said to be <a href="/facts/Critical_reaction/5UwHTKXS">critical</a>, and the mass of U-235 required to produce the critical condition is said to be a <a href="/facts/Critical_mass_(nuclear)/wRAnE21M">critical mass</a>. </p><p>Thermal reactors consist of the following: </p> <ul><li><a href="/facts/Neutron_moderator/1qa6YvDD">Neutron moderator</a> to slow down the <a href="/facts/Neutron/oRCM1geb">neutrons</a>. In <a href="/facts/Light_water_reactor/c330w1Wt">light water reactors</a> and <a href="/facts/Heavy_water_reactor/YfjI0tqR">heavy water reactors</a> it doubles as the <a href="/facts/Nuclear_reactor_coolant/7m7FX3Sn">nuclear reactor coolant</a>.</li> <li><a href="/facts/Nuclear_fuel/8RhAomlJ">Nuclear fuel</a>, which is a <a href="/facts/Fissile_material/XJR9qoxW">fissile material</a>, usually <a href="/facts/Uranium/n2sGWBzU">uranium</a>.</li> <li><a href="/facts/Reactor_vessel/9wFc9XIV">Reactor vessel</a> that is a <a href="/facts/Pressure_vessel/5mGeFbpQ">pressure vessel</a> containing the <a href="/facts/Coolant/531hqfUQ">coolant</a> and <a href="/facts/Reactor_core/RJv4geBq">reactor core</a>.</li> <li><a href="/facts/Radiation_shield/5LaNNgXe">Radiation shielding</a> to protect people and the environment from the harmful effects of <a href="/facts/Ionizing_radiation/vw0d9GaT">ionizing radiation</a>.</li> <li><a href="/facts/Containment_building/kIlXYEEK">Containment buildings</a> which are designed to contain the escape of <a href="/facts/Radiation/RwMocyeq">radiation</a> in an emergency.</li> <li>Instrumentation to monitor and control the reactor's systems.</li></ul>