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| Nuclear Reactor |
Just as conventional power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the thermal energy released from nuclear fission.
Fission
When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons; collectively known as fission products. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.
The reaction can be controlled by using neutron poisons, which absorb excess neutrons, and neutron moderators, which reduce the velocity of fast neutrons, thereby turning them into thermal neutrons, which are more likely to be absorbed by other nuclei. Increasing or decreasing the rate of fission has a corresponding effect on the energy output of the reactor.
Commonly used moderators include regular (light) water (75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.
Heat generation
The reactor core generates heat in a number of ways:
The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms. Some of the gamma rays produced during fission are absorbed by the reactor, their energy being converted to heat. Heat produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shut down.
A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal).[3][4]
Cooling
A nuclear reactor coolant — usually water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. But in some reactors the water for the steam turbines is boiled directly by the reactor core, for example the boiling water reactor.
Reactivity control
Main articles: Nuclear reactor control, Passive nuclear safety, Delayed neutron, Iodine pit, SCRAM, and Decay heat. The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions. Control rods that are made of a neutron poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it. At the first level of control in all nuclear reactors, a process of delayed neutron emission by a number of neutron-rich fission isotopes is an important physical process. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "prompt neutrons") released immediately upon fission. The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state, allows time for mechanical devices or human operatures to have time to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention.
In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so more neutron moderation means more power output from the reactors. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.
In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors power output can be increased by heating the coolant, which makes it a less dense poison.[citation needed] Nuclear reactors generally have automatic and manual systems to Scram the reactor in an emergency shut down. These systems insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.
Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit. Xenon-135 is normally produced in the fission process, and acts as a neutron absorbing "neutron poison", which acts to shut the reactor down, but can be controlled in turn within the reactor by keeping neutron and power levels high enough to destroy it as fast as it is produced. The normal fission process also produces iodine-135, which in turn decays with a half life of under seven hours, to new xenon-135. Thus, if the reactor is shut down, iodine-135 in the reactor continues to decay to xenon-135, to the point that the new xenon-135 from this source ("xenon poisoning") makes re-starting the reactor more difficult, for a day or two, than when first shut down (this temporary state is the "iodine pit.") If the reactor has sufficient extra capacity, it can still be re-started before the iodine-135 and xenon-135 decay, but as the extra xenon-135 is "burned off" by transmuting it to xenon-136 (not a neutron poison), within a few hours the reactor may become unstable as a result of such a "xenon burnoff (power) transient," and then rapidly become overheated, unless control rods are reinserted in order to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure, was a key step in the Chernobyl disaster.
Reactors used in nuclear marine propulsion (especially nuclear submarines) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without refueling. For this reason many designs use highly enriched uranium but incorporate burnable neutron poison directly into the fuel rods. This allows the reactor to be construced with a high excess of fissionable material, which is nevertheless made relatively more safe early in the reactor's fuel burn-cycle by the presense of the neutron-absorbing material which is later replaced by naturally prodused long-lived neutron poisons (far longer-lived than xenon-135) which gradually accumulate over the fuel load's operating life.
