Loss of coolant accident

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As coolant accident ( English loss-of-coolant accident , LOCA ) is in the core technology a fault refers to the leakage of coolant from the cooling circuit of the nuclear reactor leads.

Design basis accident

An assumed major loss of coolant accident is usually the basis for the design of the residual heat removal and emergency cooling systems as well as the containment of a nuclear reactor. The breakage of a main coolant line is assumed, in the most dangerous conceivable manner, namely so that both break ends are completely open and twice the line cross-section is available for the coolant to exit.

The design basis accident is generally referred to as the Greatest Accident to be Assumed ( GAU ), i.e. the most serious accident, the probability of which is high enough that fixed design precautions must be taken against it.

Meltdown or of interpretation border accident called an even more severe accident, the probability of occurrence but is considered to be so small that configuration-measures appear as not necessary; nevertheless, precautions are also taken against it - some improvised - such as (as can be implemented in the Fukushima event) the possibility of feeding water into the reactor with tank fire engines. However, there are no specifications for this in the approval process and therefore no model assumptions. If a worst-case scenario occurs, only assessments of the system behavior and empirical values ​​from previous events are available.

Newer concept

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A newer security "philosophy" regards the previous assumptions as outdated for several reasons :

  • on the one hand as insufficient because of the higher probability of occurrence of smaller LOCAs, which - as in the Three Mile Island accident - develop into core meltdown accidents;
  • on the other hand, it is excessive compared to the concept of the so-called basic safety of main coolant lines (HKL). Due to the improved testing possibilities of the quality of the lines as well as the leak-before-break principle, these are regarded as basic safe , i.e. H. the likelihood of a main coolant line break is rated as extremely low.
    • Doubts about this new concept, which is more favorable for the builder of new and the operator of existing nuclear power plants, arose in 2008 when cracks at so-called mixed seams were discovered in the Wolf Creek nuclear power plant (USA), which according to calculations took 1.9 to 2.6 years would lead to a leak and practically at the same time to the breakdown of the main coolant line;
  • thirdly as inadequate because of the likelihood of disturbances in the energy supply,
    • like after the fire of a transformer in Krümmel on June 28, 2007 and the subsequent failure of the reserve transformer on June 30, 2007.
    • like after the flooding and total failure of the emergency power supplies in the Fukushima I nuclear power plant on March 11, 2011.

Control of loss of coolant accidents

Nuclear power plants have various structural properties and facilities to control loss of coolant accidents. Using the example of a pressurized water reactor as it is in operation in many German nuclear power plants, the sequence of countermeasures in the event of such an incident can be described in the following four phases. These are initiated one after the other if the coolant has not already stopped leaking by the respective point in time. The water escaping from the leak collects at the bottom of the containment and forms the so-called sump under the pressure vessel in the containment.

  • Pressure relief: The coolant flows through the leak as a water-steam mixture into the containment . The pressure in the primary circuit drops. The reactor is automatically shut down by the safety system.
  • Accumulator feed: The pressure in the primary circuit has dropped sufficiently. Water storage tanks under pre-pressure then automatically flood the reactor core . The hot fuel elements (see decay heat ) continue to be cooled.
  • Core flooding: Before the pressure accumulators are empty, further cooling of the reactor core from the flood tanks begins. If the temperature of the fuel assemblies has been successfully reduced, high water pressure is no longer necessary. However, because of evaporation, not only does the coolant have to be circulated, but also supplemented.
  • Sump circuit: When the flood tanks are emptied, the system automatically switches over to circulation mode. After-cooling pumps continuously convey the water from the sump back into the primary circuit via reheat coolers.

Cooling pool

In addition to the fuel elements in the nuclear reactor itself, there is also a decay pool with water as a coolant to dissipate the decay heat from spent fuel elements in the reactor building or in an auxiliary building of a nuclear power plant . A cooling pool is not under pressure, so that if the coolant is lost, a makeshift attempt can be made to refill it from the outside (using fire brigade syringes, police water cannons, etc.).

See also

Individual evidence

  1. ↑ M. Sailer: Safety aspects of light water reactors , 1990
  2. ↑ ENSI : Experience and research report , 2008
  3. ↑ Trafobrand just escaped