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2.1 Fuel
The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full power days in a reactor's operating cycle (between refueling outage times) is related to the amount of fissile uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full power days.
At the end of the operating cycle, the fuel in some of the assemblies is "spent," and it is discharged and replaced with new (fresh) fuel assemblies. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor.
The amount of energy extracted from nuclear fuel is called its "burn up," which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.
3 Types of reactors
A number of reactor technologies have been developed. Fission reactors can be dvided roughly into two classes, depending on the energy of the neutrons that are used to sustain the fission chain reaction.
- Thermal (slow) reactors use slow or thermal neutrons. These are characterised by having moderating materials which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles, that is they are thermalised. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238, than do the fast neutrons that result from fission. As well as the moderator, thermal reactors have fuel (fissionable material), containments, pressure vessels, shielding, and instrumentation to monitor and control the reactor's systems. Most power reactors are this type, and the first plutonium production reactors were thermal reactors using graphite as the moderator. Some thermal power reactors are more thermalised than others; Graphite and heavy water moderated plants tend to be more thoroughly thermalised than PWRs and BWRs.
- Fast reactors use fast neutrons to sustain the fission chain reaction, and are characterised by the lack of moderating material. They require highly enriched fuel (sometimes weapons-grade) in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some early power stations were fast reactors, as are some Russian naval propulsion units, and construction of prototypes is continuing, see fast breeder, but overall the class has not achieved the success of thermal reactors in any application.
Thermal power reactors can again be divided into three types, depending on whether they use pressurised fuel channels, a large pressure vessel or gas cooling.
- Pressure vessels holding steam heated by the reactor are used by most commercial and naval reactors. This serves as a layer of shielding and containment.
- Pressurised channels are used by the RBMK and CANDU reactors. Channel-type reactors can be refuelled under load, which has advantages and disadvantages discussed under CANDU reactor.
- Gas-cooled reactors are cooled by a circulating inert gas, usually Helium, but Nitrogen and Carbon Dioxide have also been used. Plans to utilize the heat vary. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make steam for a steam turbine. The pebble bed reactor uses a gas-cooled design.
Most designs for fast power reactors have been cooled by liquid metal, usually molten Sodium. They have also been of two types, called pool and loop reactors.
3.1 Current families of reactors
3.2 Obsolescent types still in service
