Uranium-235 atoms can spontaneously break-apart (fission) releasing heat energy, radiation, fission fragments and neutron particles. If the neutrons are travelling at slow enough speed (called thermal neutrons) they can strike and split other uranium-235 atoms breaking them apart too, releasing yet more heat energy and neutrons.
If there is enough uranium present beyond a certain point called the critical mass, then the nuclear fission process can escalate producing a self-sustaining chain reaction affecting all other uranium-235 atoms nearby. Some neutrons are not released instantly during fission but can take a few seconds to emerge.
These delayed neutrons allow enough time for human operators and computers to control the overall speed of the nuclear chain reaction called driving the reactor.
The core of a reactor contains uranium nuclear fuel rods, a moderator such as water to slow-down (thermalise) the neutrons produced, control rods made from boron steel or cadmium to selectively absorb neutrons for adjusting the reactor power, and a recirculating coolant to remove heat.
Most commercial nuclear power reactors are fuelled by low enriched uranium fuel rods which are carefully inserted into the core. Reactor-grade uranium fuel contains a mixture of uranium-235 (3%) and uranium-238 (97%).
The chain reaction inside the core of a nuclear power station is carefully controlled by adjusting the flow of neutron particles within the core called the neutron flux, so that they just reach equilibrium criticality.
The neutron multiplication factor k=1.00000 when the number of neutrons generated inside the core is exactly equal to the number of neutrons leaving the core.
If the core reactivity is slightly less than 1 then the core is sub-critical (k<1) and the chain reaction gradually dies out. If the reactivity is exactly 1 then the core is critical (k=1) and the chain reaction becomes self-sustaining until all of the uranium-235 fuel is used-up. If the core reactivity rises above 1 then the core becomes supercritical (k>1) and the chain reaction increases, boosting reactor power and electricity output.
But if the reactivity increases very much greater than 1 then the core risks prompt supercriticality (k>>1) where the power level may suddenly spike out of control triggering a nuclear explosion accident.
The 1986 Chernobyl nuclear disaster was caused by a prompt supercriticality event after key safety systems were deliberately disabled by the operators of the power station during an experiment.
What types of reactor are used for modern nuclear power stations?
Large commercial nuclear power stations planned for construction during this decade are mostly based on Generation III+ Pressurised Water Reactor (PWR) technology. The four leading designs are the US Westinghouse Advanced-Passive AP1000 reactor, the French Areva EPR Evolutionary Pressurised Water Reactor, the Japanese Mitsubishi Advanced Pressurized Water Reactor (APWR) and the Russian Rosatom VVER-1200.
There are also many other kinds of older nuclear power stations operating based on Generation I reactor designs built in the 1950s - 1960s, Generation II designs built in the 1970s - 1980s and Generation III designs built in the 1990s and 2000s.
In December 2009 South Korea won the largest nuclear export contract in the world worth $20 billion, to supply four Generation III KEPCO APR-1400 pressurised water reactors to the United Arab Emirates. Boiling Water Reactor (BWR) technology used in the American-Japanese General Electric Hitachi Economic Simplified Boiling Water Reactor (ESBWR) and Canada's Advanced Canada Deuterium-Uranium Reactor (CANDU ACR-1000) might also be built in some countries.
The Generation III+ reactor designs presently under construction in Western Europe and China will begin operating this decade.