Nuclear fission is an important source of energy. Globally, around ten percent of all electricity comes from nuclear plants. In Europe, this is around a quarter. Not everyone is keen on nuclear power. However, we are not likely to survive the coming decades without nuclear power. The transition to sustainable sources is developing slowly, whilst the demand for energy in the world is growing fast. The biggest growth in nuclear power is expected outside Europe (China, India, Latin America). In the Netherlands, nuclear power is limited. Around four percent of the total consumption in the Netherlands is generated by the nuclear plant in Borssele.
Nuclear fission: the process
In nuclear fission, atomic nuclei from the isotope U-235 are split into smaller nuclei, releasing radiation, heat and neutrons. These neutrons can in turn be captured by other atomic nuclei of U-235, causing new fissions, which results in a chain reaction. A controlled chain reaction takes place in a nuclear reactor. To keep the chain reaction going, at least one of the 2 to 3 neutrons released in a fission must create the next fission. This is the case in a stable reactor. We then call the reactor 'critical'. When more than one neutron remains in each fission, you get a 'snowball effect'. 2 neutrons each produce a fission that both also produce 2 neutrons = 4 neutrons. These 4 neutrons produce 4 fissions, which results in a total 4x2=8 neutrons, etc. If this happens in a reactor, we call the reactor supercritical. This is done in a controlled way to bring a reactor up to power.
When less than 1 neutron remains per fission, the fission process stops. We now have a subcritical reactor (the reactor is now 'off'). In practice, a critical reactor is stopped (made subcritical) by introducing materials that trap neutrons, for example by inserting control rods between the fuel rods. This creates a shortage of neutrons, which stops the chain reaction.
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(Credit illustration: ©Ji Xing, Daiyong Song, Yuxiang Wu - CC BY 4.0, https://commons.wikimedia.org)