A higher temperature coolant would be less dense, and therefore a less effective moderator. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. Thermal neutrons are more likely than fast neutrons to cause fission. 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. In some reactors, the coolant also acts as a neutron moderator. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, and other points in the process interpolated in cents. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators 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. 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, and so considerable time is required to determine exactly when a reactor reaches the critical point. 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. 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.Īt 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. The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions.Ĭontrol rods that are made of a neutron poison are used to absorb neutrons. Main articles: Nuclear reactor control, Nuclear reactor physics, Passive nuclear safety, Delayed neutron, Iodine pit, SCRAM and Decay heat However, in some reactors the water for the steam turbines is boiled directly by the reactor core for example the boiling water reactor. 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. The heat is carried away from the reactor and is then used to generate steam.
CoolingĪ nuclear reactor coolant - usually water but sometimes a gas or a liquid metal (like liquid sodium) or molten salt - is circulated past the reactor core to absorb the heat that it generates. This decay heat-source will remain for some time even after the reactor is shut down.Ī 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 × 10 13 joules per kilogram of uranium-235 versus 2.4 × 10 7 joules per kilogram of coal).
Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. This is known as a nuclear chain reaction.
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.
The heavy nucleus splits into two or more lighter nuclei, (the fission products), releasing kinetic energy, gamma radiation, and free neutrons. When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.