How does a nuclear reactor work?

How it works?

• 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.

Basic parts of a reactor

• Core,• A moderator, • Control rods, • A coolant, & • Shielding.

• The core of a reactor contains the uranium fuel. • For a light water reactor with an output of 1,000

megawatts, the core would contain about 75 tonnes of uranium enclosed in approximately 200 fuel assemblies.

• 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, (the fission products), – releasing kinetic energy, gamma radiation, and free neutrons.

• 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: nuclear chain reaction.

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• Fission -

Heat generationThe 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. • The reactor absorbs some of the gamma rays produced during fission and

converts their energy converted into heat.• Heat is 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.

• 1kg uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than 1kg of coal burned conventionally – (7.2 × 1013 joules per kilogram of uranium-235 vs. 2.4 × 107 joules per kilogram of

coal).

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.

Control rods

• For more precise control of the chain reaction, control rods are inserted into the core of the reactor.

• Pushed in, they absorb neutrons and slow down the reaction

• Pulled out, they allow it to speed up again.

In this way the chain reaction is controlled.

Coolant

• Fissions occurring in the reactor generate an enormous amount of heat.

• A liquid or gas coolant carries this heat away from the reactor to a boiler where steam is made.

Reactivity control

• 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.

Control rods • In a nuclear power station, the uranium is first formed into

pellets and then into long rods.• The uranium rods are kept cool by submerging them in

water/... • When they are removed from the water, a nuclear reaction

takes place causing heat. • The amount of heat required is controlled by raising and

lowering the rods. • If more heat is required, the rods are raised further out of

the water and if less is needed they lower further into it.

read• 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 operators 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.

Moderator

• The neutrons produced by fission are travelling at great speeds, and in most reactors, are deliberately slowed down by a material known as a moderator.

• Slow neutrons are much more likely, when they collide with the nuclei of U-235, to cause a fission and keep the reaction going.

• A moderator: composed of light atoms and the materials most commonly used are carbon in the form of graphite, and water.

Moderator

• 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.

Moderators

Commonly-used moderators include • regular (light) water (in 74.8% of the world's

reactors), • solid graphite (20% of reactors) and • heavy water (5% of reactors).

• Some experimental types of reactor have used beryllium, and hydrocarbons have been suggested as another possibility.

Coolant as Mod.

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.

Shielding

• Shielding, typically made of steel and concrete about two meters thick, is an outer casing that prevents radiation from escaping into the environment.

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Typical control room

Sudden shut down

• 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.

Scram

• A scram or SCRAM is an emergency shutdown of a nuclear reactor –

• In commercial reactor operations, this emergency shutdown is often referred to – – as a "SCRAM" at boiling water reactors (BWR), and – as a "reactor trip" at pressurized water

reactors (PWR). – In many cases, a SCRAM is part of the routine

shutdown procedure as well.

read• In any reactor, a SCRAM is achieved by a large insertion of negative reactivity. In light

water reactors, this is achieved by inserting neutron-absorbing control rods into the core, although the mechanism by which rods are inserted depends on the type of reactor. In PWRs, the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. Any cutting of the electric current releases the rods. Another design uses electromagnets to hold the rods suspended, with any cut to electric current resulting in an immediate and automatic control rod insertion. A SCRAM mechanism is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, in four seconds or less, thus rapidly halting the nuclear reaction by absorbing liberated neutrons. In BWRs, the control rods are inserted up from underneath the reactor vessel. In this case a hydraulic control unit with a pressurized storage tank provides the force to rapidly insert the control rods upon any interruption of the electric current, again within four seconds. A typical large BWR will have 185 of these control rods. In both the PWR and the BWR there are secondary systems (and often even tertiary systems) that will insert control rods in the event that primary rapid insertion does not promptly and fully actuate.

read• Liquid neutron absorbers are also used in rapid shutdown systems for light water reactors. Following

SCRAM, if the reactor (or section(s) thereof) are not below the shutdown margin (that is, they are still critical), the operators can inject solutions containing neutron poisons directly into the reactor coolant. Neutron poisons are water-based solutions that contain chemicals that absorb neutrons, such as common household borax, sodium polyborate, boric acid, or gadolinium nitrate, causing a decrease in neutron multiplication, and thus shutting down the reactor without use of the control rods. In the PWR, these neutron absorbing solutions are stored in pressurized tanks (called accumulators) that are attached to the primary coolant system via valves; a varying level of neutron absorbent is kept within the primary coolant at all times, and is increased using the accumulators in the event of a failure of all of the control rods to insert, which will promptly bring the reactor below the shutdown margin. In the BWR, soluble neutron absorbers are found within the Standby Liquid Control System, which uses redundant battery-operated injection pumps, or, in the latest models, high pressure nitrogen gas to inject the neutron absorber solution into the reactor vessel against any pressure within. Because they may delay the restart of a reactor, these systems are only used to shut down the reactor if control rod insertion fails. This concern is especially significant in a BWR, where injection of liquid boron would cause precipitation of solid boron compounds on fuel cladding, which would prevent the reactor from restarting until the boron deposits were removed. In most reactor designs, the routine shutdown procedure also uses a SCRAM to insert the control rods, as it is the most reliable method of completely inserting the control rods, and prevents the possibility of accidentally withdrawing them during or after the shutdown.

read

• In most reactor designs, the routine shutdown procedure also uses a SCRAM to insert the control rods, as it is the most reliable method of completely inserting the control rods, and prevents the possibility of accidentally withdrawing them during or after the shutdown.

Electricity generation

• The energy released in the fission process generates heat, some of which can be converted into usable energy.

• A common method of harnessing this thermal energy is – to use it to boil water – to produce pressurized steam, – which will then drive a steam turbine – that generates electricity!

Power Plant: Heat Steam Generator Electricity

• Power plants use heat supplied by a fuel to boil water and make steam, which drives a generator to make electricity.

• A generating plant's fuel: coal, gas, oil, uranium• Fuel heats water & turns it into steam. • The pressure of the steam spins the blades of a giant

rotating metal fan called a turbine. • That turbine turns the shaft of a huge generator. • Inside the generator, coils of wire and magnetic fields

interact - and electricity is produced.

DISADVANTAGES• Nuclear power is a controversial method of producing electricity.

Many people and environmental organizations are very concerned about the radioactive fuel it needs.

• There have been serious accidents with a small number of nuclear power stations. The accident at Chernobyl (Ukraine) in 1986 -- killed and over 100,000 people being evacuated. Radiation was even detected over a thousand miles away in the UK as a result of the Chernobyl accident. It has been suggested that over time 2500 people died as a result of the accident.

• There are serious questions to be answered regarding the storage of radioactive waste produced through the use of nuclear power. Some of the waste remains radioactive (dangerous) for thousands of years and is currently stored in places such as deep caves and mines.

• Storing and monitoring the radioactive waste material for thousands of years has a high cost.

• Nuclear powered ships and submarines pose a danger to marine life and the environment. Old vessels may leak radiation if they are not maintained properly, or if they are dismantled carelessly at the end of their working lives.

• Many people living near to nuclear power stations or waste storage depots are concerned about nuclear accidents and radioactive leaks. Some fear that living in these areas can damage their health, especially the health of young children.

• Many Governments fear that unstable countries that develop nuclear power may also develop nuclear weapons and even use them.

Advantage• Energy: The amount of electricity produced in a nuclear

power station is equivalent to that produced by a fossil fuelled power station.

• Env.-friendly: Nuclear power stations do not burn fossil fuels to produce electricity and consequently they do not produce damaging, polluting gases.

• Many supporters of nuclear power production say that this type of power is environmentally friendly and clean. In a world that faces global warming, they suggest that increasing the use of nuclear power is the only way of protecting the environment and preventing catastrophic climate change.

• Many developed countries, such as the USA and the UK no longer want to rely on oil and gas imported from the Middle East.

• Countries such as France produce ~90% of their electricity from nuclear power and lead the world in nuclear power generating technology - proving that nuclear power is an economic alternative to fossil fuel power stations.

• Nuclear reactors can be manufactured small enough to power ships and submarines. If this was extended beyond military vessels, the number of oil burning vessels would be reduced and consequently pollution.