Nuclear Fission Reactors PWR


Nuclear fission

In nature Uranium occurs at two isotopes: U-235  0.7% and U-238 91.3%. The U-235 is fissile, meaning it splits (fissions) when it absorbs an extra neutron. When it fissions, it gives off 3 neutrons that can start other U-235 fissions. This a chain reaction. If it happens quickly, it is an atomic bomb, under control it is a power station.

U-235 is also radioactive, meaning it spontaneously decays by giving off an alpha particle and can be detected by a radio. This has nothing to do with fission.

There are many different designs, and these are looked at briefly on these 3 pages.


Nuclear Fission Reactor

Wikipedia description of fission

Nuclear power stations use Uranium 235 or plutonium 239. When slow neutrons are captured, the nuclei  become unstable and break apart forming daughter elements, these too are unstable and break down into a chain of other elements. Each stage gives off heat.

Eventually some of these absorb neutrons and slow the reaction down. At this stage the fuel is removed. The fuels rods are stored under water until all the decay products have finished their own decays. It can be disposed of or purified to remove the U325 and P239 for re-use.

Nuclear fission is very expensive. In every country in the world it has been heavily subsidised by the government. The price of uranium is 3-7% of the total cost of delivering electricity from a nuclear power plant. 

The economics only make sense if the fuel is reprocessed or used in breeder reactors.

However despite decades of research no one has developed an economic fast breeder reactor.


Radioactive waste

At present the waste is building up faster than it is being buried. The decisions are being postponed as too difficult. Most waste is cooling down at power stations prior to underground disposal.

There is talk of "burning" (fissioning) the waste in a fast reactor.


Heating water to make steam

As the nuclei break up they  move about and cause water molecules to move as well. This movement is heat.

The neutrons given off collide with water.
The gamma rays produced during fission also make water molecules move.

A kilogram of uranium-235 (U-235) releases  three million times more energy than a kilogram of coal.

Removing the heat - cooling

There are different reactor designs:

  • PWR, pressurized water reactor - high pressure water removes the heat and heats more water to become steam.. About 2/3 of reactors are PWR.
  • BWR, Boiling Water Reactor boils water in the core to produce steam directly. About 1/3 of reactors are BWR
  • PHWR pressurized heavy-water reactor uses heavy water, D2O, as moderator and coolant. Heavy water has much less neutron capture so improves neutron economy. Uranium does not need to be enriched meaning he extra capital costs of the heavy water can be recovered by the cheaper fuel.
  • GCR, Gas Cooled Reactor uses gas, CO2 or He to carry the heat to the heat exchangers.
  • Molten sodium cooled reactor has the advantage of being able to operate at atmospheric pressure. But contact with water will cause an explosion, with air, a fire.
  • Mercury and lead-bismuth have also been used.
  • Molten salt cooled reactor Again, can operate at atmos. pressure. (different to a molten salt reactor) 

Controlling the fission

The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions.
Control rods absorb neutrons, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it.

Neutron 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 (slow) neutrons. Thermal neutrons are more likely than fast neutrons to cause fission.