Nuclear power reactor

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A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity. In a research reactor the main purpose is to utilize the actual neutrons produced in the core. In most naval reactors, steam drives a turbine directly for propulsion for a ship or submarine.

The principles for using nuclear power to produce electricity are the same for most types of reactors. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity (as in most fossil fuel plants).

There are several components common to most types of reactors:

  • Fuel: Usually pellets of uranium oxide (UO2) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.
  • Moderator: This is material which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.
  • Control rods: These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.)
  • Coolant: A liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the moderator functions also as primary coolant. Except in boiling water reactorss, there is secondary coolant circuit where the steam is made.  (see also later section on primary coolant characteristics.)
  • Pressure vessel or pressure tubes: Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.
  • Steam generator: Part of the cooling system where the heat from the reactor is used to make steam for the turbine.
  • Containment: The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. It is typically a meter-thick concrete and steel structure.

There are several different types of reactors as indicated in the following table.

Most reactors need to be shut down for refuelling, so that the pressure vessel can be opened up. In this case, refuelling is at intervals of 1-2 years, when a quarter to a third of the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be refuelled under load by disconnecting individual pressure tubes.

If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium. Natural uranium has the same elemental composition as when it was mined (0.7% uranium-235 (235U), over 99.2% uranium-238 (238238U)), whereas enriched uranium has the proportion of the fissile isotope 235U increased by a process called enrichment, commonly to 3.5 - 5.0%. In this case, the moderator can be ordinary water, and such reactors are collectively called light water reactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.

Practically all fuel is ceramic uranium oxide (UO2 with a melting point of 2800°C) and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and permeable to neutrons . Numerous rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. In the most common reactors these are about 3.5 to 4 meters long.

In a new reactor with new fuel a neutron source is needed to get the reaction going.  Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12.  Restarting a reactor with some used fuel may not require this, as there may be enough neutrons to achieve criticality when control rods are removed.

Burnable poisons are often used, especially in boiling water reactors (BWRs), in fuel or coolant to even out the performance of the reactor over the time from when fresh fuel is loaded to the time of refuelling. These are neutron absorbers which decay under neutron exposure, compensating for the progressive build-up of neutron absorbers in the fuel as it is burned. The best known is gadolinium, a vital ingredient of fuel in naval reactors that are very inconvenient to refuel, compelling the design of reactors capable of running more than a decade between refuellings.

In fission, most of the neutrons are released promptly, but some are delayed. These are crucial in enabling a chain reacting system (or reactor) to be controllable and to be able to be held precisely critical.

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