Nuclear reactor

A nuclear reactor sustains and controls a nuclear chain reaction to produce heat or a useful neutron field. Nuclear fission supplies fresh neutrons; absorption and neutron leakage remove them. A steady neutron population requires a balance between production and losses, the condition of nuclear criticality.

The functions of its components are different. Fuel supplies fissioning atomic nuclei; control rods change neutron absorption; coolant transports heat. In a thermal reactor, a neutron moderator reduces neutron energies before subsequent reactions. Slowing neutrons and cooling the fuel are separate physical functions even when the same material performs both.

Chicago Pile-1 demonstrated a controlled self-sustaining chain reaction in 1942 using a uranium–graphite assembly and neutron-absorbing controls. The decisive observation was the neutron population's response to changing absorption, rather than generation of useful electrical power.^[ref] A power reactor adds heat-transfer and conversion equipment; decay heat persists after the chain reaction is stopped.

A boiling water reactor is a nuclear reactor in which water boils inside the reactor vessel and the resulting steam directly drives a turbine. Water serves as both coolant and neutron moderator. Moisture separators and dryers remove entrained liquid before steam reaches the turbine; a condenser and feedwater system return it to the vessel.^[ref]

The heat balance makes its purpose concrete: in a simplified steady model, $\dot{Q}\simeq \dot{m}(h_{\mathrm{steam}}-h_{\mathrm{feedwater}})$, where $\dot{m}$ is the water mass flow and $h$ is specific enthalpy. With an illustrative enthalpy increase of $2MJ/kg$, transporting $2\mathrm{GW}$ of heat requires approximately $1000kg/s$. This is a thermal balance, not the electrical output, because the turbine cycle rejects heat to the condenser.

A boiling water reactor differs from a pressurized water reactor, which keeps its primary coolant liquid and transfers heat to a separate steam-producing circuit. Shutting down the chain reaction does not remove decay heat, so heat removal remains necessary after shutdown.

Chicago Pile-1 was the experimental nuclear reactor that achieved a controlled self-sustaining fission chain reaction on 2 December 1942, in a structure beneath the University of Chicago's Stagg Field stands. The project was led by Enrico Fermi.

Uranium supplied fissile material, graphite moderated neutrons, and neutron-absorbing control rods adjusted the chain reaction. Gradually changing the absorber configuration let the team test approach to criticality. The scientific milestone was controlled neutron multiplication sufficient to sustain the reaction; the pile was an experimental assembly, not a power-producing station.^[ref]

A self-sustaining neutron balance in which production offsets absorption and leakage. In a steady one-group neutron diffusion equation, criticality is a compatibility condition between material parameters and the allowed spatial mode.

Escape of neutrons from a specified region before they participate in further reactions there. A leakage fraction depends on the region's geometry, surrounding materials and energy-dependent transport and absorption, rather than its dimensions alone.

A nuclear reactor on the side of a specified moderation optimum where adding moderator increases neutron multiplication. Removing water then tends to reduce reactor reactivity, under the comparison's other fixed conditions. The optimum and the derivative depend on composition, geometry and operating state.^[ref]

A constituent that absorbs neutrons and thereby reduces their availability for a nuclear fission chain reaction. Effectiveness depends on isotope, neutron spectrum and spatial distribution, not just one thermal cross section.

The reduction of reactor reactivity caused by neutron absorption by xenon-135. In a simplified model its abundance is fed by radioactive iodine decay, while its removal is by radioactive decay and neutron capture. Changes in these pathways can delay its response to a change of neutron flux.

Energy generated in a period divided by rated power times elapsed time. At fixed efficiency, it scales annual thermal energy and fuel consumption.

The ratio of the rate of formation of fissile nuclei to the rate at which the specified fissile inventory is consumed, under stated reaction and accounting conventions. A conversion ratio below one indicates a replacement shortfall in that inventory. It is distinct from neutron multiplication: a reactor's instantaneous neutron balance and its long-term fuel-inventory balance are different questions.

A reduced time-dependent model for a neutron population amplitude coupled to delayed neutron precursor populations. Spatial and spectral shape changes are neglected or represented through prescribed effective coefficients. Such a model illustrates timescale separation without establishing the spatial behaviour or operating limits of an actual reactor.

The effective time associated with successive prompt fission generations in a specified point kinetics model. It is distinct from a macroscopic population growth period, which also depends on reactor reactivity and delayed neutrons.

A timescale-separation approximation that eliminates the rapidly adjusting neutron amplitude in favour of the slower delayed neutron precursor reservoir. It describes behaviour after the short prompt transient and need not resolve the initial adjustment.

The characteristic relation between imposed reactor reactivity and exponential growth or decay rates in a point kinetics model. Several kinetic modes can coexist; the late-time behaviour is determined by the mode with greatest real growth rate that is actually excited.

A dimensionless measure of deviation from critical neutron multiplication. Positive reactivity indicates a supercritical neutron balance; zero gives criticality. The population's actual time evolution also depends on delayed-neutron and feedback dynamics.

The change of reactor reactivity with a specified material temperature, with the other variables held as stated. Several component coefficients need not have the same sign.

The change of reactor reactivity with thermal power along a specified family of states. In a quasistatic process, the chain rule weights each temperature coefficient of reactivity by that component's temperature response to power.

The fuel-temperature derivative of reactor reactivity associated with thermal broadening of nuclear resonances, with the other state variables specified. In ordinary uranium-fueled water reactors it is negative: heating the fuel increases effective resonance capture. Its magnitude depends on spectrum, fuel state and resonance self-shielding, so its sign does not determine how its magnitude changes under a lattice redesign.^[ref]

A movable neutron-absorbing component used to change reactor reactivity. Insertion can make the fission chain reaction subcritical; residual radioactive-decay heat still requires cooling.

A water-cooled and water-moderated nuclear reactor whose primary coolant is pressurized to suppress bulk boiling in the core. Heat is transferred to a secondary circuit through steam generators.