9 Guest Post Reactor Safety
The operation of nuclear reactors, especially large reactors within nuclear power plants, harbors massive dangers. Therefore, considerations and measures for the highest possible reactor safety belong to the essential aspects of the use of nuclear energy. This article discusses a number of technical and non-technical issues that play a role here.
Since nuclear fusion reactors will not be technically feasible in the foreseeable future, this article refers exclusively to nuclear fission reactors.
Containment of radioactive substances
A crucial aspect of nuclear safety is the safe containment of radioactive substances – especially the highly radioactive fission products, but also of hatched plutonium and other transuranic elements. Note that the radioactive inventory of the reactor of a typical nuclear power plant that has been in operation for some time is much higher than, for example, that released by the Hiroshima atomic bomb. Therefore, even the release of a small percentage of the radioactive inventory of a nuclear reactor means a nuclear disaster. In contrast, the direct emission of radiation is to the environment during normal operation, and the number of radioactive substances released into the environment during normal operation is quite small – less, for example, than in uranium mining and the reprocessing of nuclear fuels, and often even compared to emissions from coal-fired power plants, since coal is radioactive trace elements contains.
A number of barriers are implemented for the containment of radioactive substances:
- The first barrier is the metallic shell of the fuel rods. The cladding tubes usually consist of a very resistant zirconium alloy. During normal operation, fuel rods can be damaged to a certain extent, but only a very limited escape of radioactive substances into the cooling water.
- The cooling water becomes more or less radioactive in normal operation and must not get into the environment. Safe containment is made more difficult by the fact that the cooling water is under high pressure (hundreds of bar). It is therefore necessary to make the reactor pressure vessel and all coolant lines extremely stable. Also damage z. B. corrosion or aging caused by pressure and temperature fluctuations must be avoided or discovered and remedied in good time. In many pressure vessels z. For example, numerous cracks have been found with the help of ultrasound examinations; however, often this has not resulted in shutdown when the impression was given that the cracks were growing too slowly to pose a serious hazard.
- The next barrier is the reactor containment, also known as containment. This encloses the entire reactor and parts of the coolant circuit. Leaked coolant is also collected at the bottom of the containment, from where it can be pumped out again. In the event of very serious accidents, however, the containment can also be destroyed, for example by an explosion or by melted radioactive material.
The building structures outside the containment can on the one hand protect the reactor from external influences (in the case of a particularly strong design, even against aircraft crashes) and on the other hand form a further barrier against the escape of radioactive substances. For example, filter systems can make it possible to reduce excess pressure in a targeted and metered manner without causing significant amounts of radioactive substances to escape.
Unfortunately, all safety barriers can be damaged in serious accidents and become ineffective. In particular in the event of a core meltdown (see below), there is a considerable risk of all barriers failing.
Regulation of performance
A dangerous situation occurs when the nuclear chain reaction leads to a rapidly increasing output of the reactor – for example if the control rods do not prevent this quickly enough. The output of a reactor can then increase extremely in a very short time. Such a criticality accident leads to the destruction or damage of various components of the reactor. For example, the pressure rises rapidly in the cooling system and burst the coolant lines. The overheating of fuel rods (especially after the cooling water has been lost) leads to the release of highly radioactive substances initially inside the reactor and subsequently possibly also to the outside. In addition, if overheated fuel elements (from approx. 900 ° C) come into contact with water vapor, hydrogen can also be released which can subsequently explode, causing further severe damage. (Both in the Chernobyl reactor disaster in 1986 and in Fukushima in 2011, hydrogen explosions resulted in severe destruction and the release of large amounts of radioactive substances; an uncontrolled chain reaction only occurred in Chernobyl.) It can also happen that structures of the reactor core are similar are deformed so that the control rods can no longer be retracted into the reactor.
As an emergency measure, boron-containing substances (for example boric acid) can be added to the cooling water. Since boron is highly neutron absorbing, it can interrupt the chain reaction, even if the control rods are no longer available. However, this assumes that the boron-containing water reaches the fuel rods quickly enough.
Light water reactors (unlike graphite-moderated reactors and fast breeders) have the beneficial property of inherent stability: If the cooling water is expelled when overheating, its function as moderator is no longer applicable, and the reactor is automatically subcritical, i.e., H. the chain reaction ends. However, hydrogen explosions (see above) can still occur, as can a core meltdown (see below).
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