This section explains how a typical light water reactor is constructed; H. a reactor cooled with ordinary (light) water. This type is by far the most common in the world. Other types of reactors are discussed below.
Every nuclear fission reactor must contain enough fissile material to achieve the critical mass for nuclear fission. In most cases, uranium 235 (235 U) is used, sometimes also plutonium 239 or uranium 233, which can be bred from thorium 232, or a mixture of different fissile isotopes. The fissile material (nuclear fuel) is contained in fuel rods, which in turn are bundled into fuel assemblies. These fuel elements are installed in the reactor core with a relatively small spacing and can be replaced individually.
The fuel elements should safely enclose the fissile material and, above all, the highly radioactive fission products – if possible, even in the event of severe accidents. However, their resilience is z. B. limited for cases with a sharp rise in temperature. Even in normal operation, fuel element damage with leaks occasionally occurs, which can lead to severe contamination of the entire primary cooling circuit.
Control of performance
Between the fuel elements, there are several neutron-absorbing control rods (also control rods or control rods) which are used to regulate the output (see above) and to switch off the reactor.
Removal of heat
The heat generated is removed from ordinary (light) water in the light water reactor. This is chemically prepared and additional substances (e.g., boron salts) can be added.
The cooling water is pumped through the reactor by powerful coolant pumps at high pressure in a closed cooling circuit and flows through between the fuel rods. Depending on the type of reactor, the water in the reactor is evaporated and then enters a steam turbine, or it remains liquid due to the high pressure and reaches a steam generator as a heat exchanger (see below).
Basically, the heat transport through the cooling water has two important functions: on the one hand, the heat is used in this way, and on the other hand, the reactor is protected from overheating. The cooling water has another function: it moderates the neutrons (i.e., it slows them down) and thus enables criticality to be reached with a significantly lower amount of fuel or a lower degree of enrichment than without a moderator. This also has a beneficial safety-related side effect: in the event of a sharp increase in temperature, which leads to the evaporation of the water, its moderating function is greatly reduced, which can weaken or even stop the nuclear chain reaction.
Even after the reactor has been switched off (ie after the chain reaction has stopped), effective cooling must be guaranteed since the so-called post-decay heat of the radioactive fission products must be dissipated. Otherwise, a failure of the cooling system can lead to serious reactor accidents, up to and including a core meltdown.
Most reactors contain an additional reactor containment around the reactor pressure vessel, which is also known as containment. It should prevent radioactive substances from escaping if the reactor pressure vessel fails. In its lower part, cooling water can also be collected, which can escape in the event of leaks.
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