Loss-of-coolant accident

As a LOCA ( loss-of- coolant English accident, LOCA ) is the core technology referred to an incident in which escapes through a leak coolant from the cooling circuit of the nuclear reactor.

Design basis accident

An assumed large loss of coolant accident is usually the basis for the interpretation of the residual heat removal and emergency core cooling systems as well as the containment vessel of a nuclear reactor. Where the breach of a main coolant line is assumed, in the most dangerous possible way, namely so that both the broken ends are fully open and the coolant so that twice the cable cross -section is the outlet available.

The design basis accident is in common usage as Biggest Assumable accident (GAU ) denotes, ie as the worst accident, the probability is high enough that against him fixed interpretation precautions must be taken.

Super - GAU or beyond design basis accident means an even heavier accident, the probability of occurrence but is considered to be so small that interpretation arrangements appear to be not necessary; partly improvised - - made ​​arrangements about (as with the Fukushima event feasible ) the possibility to feed with water tender vehicles water into the reactor still be against it. There are, however, in the approval process no requirements and therefore no model assumptions. If an meltdown, are only estimates of the system behavior and experience of past events.

Recent concept

A recent safety "philosophy" considers the previous assumptions for several reasons as obsolete:

  • On the one hand to be inadequate because of the greater probability of smaller Locas, which - like the Three Mile Iceland accident - evolve to core melt accidents;
  • On the other over-priced compared to the concept of so-called basic security of main coolant lines ( HKL). These are considered safe base due to improved testing capabilities of the quality status of the lines, due to the leak - before-break principle, that is, the probability of a main coolant line rupture is considered extremely low. Doubts about this new, for the installer and the new operators of existing nuclear power plants cheaper, concept, however, came in 2008, when the Wolf Creek nuclear power plant (USA) cracks on so-called mixed seams were discovered, according to calculations within 1.9 to 2.6 years would a leak and carry virtually the same time the demolition of the main coolant pipe;
  • Such as after a fire in a transformer crumbs on 28 June 2007 and the subsequent failure of the transformer Reserve on 30 June 2007.
  • Such as after flooding and total failure of the emergency power supplies in nuclear power plant in Fukushima I on March 11, 2011.

Mastery of loss of coolant accidents

To manage loss of coolant accidents nuclear power plants have different structural properties and facilities. Using the example of a pressurized water reactor, as it is in many German nuclear power plants in operation, can observe the progression of countermeasures by such a disruption through the following four phases describe. These are sequentially introduced, if the leakage of refrigerant is not yet finished up to the respective point in time. Emerging from the water leakage collects at the bottom of the containment vessel and forms with the pressure vessel in the so-called containment sump.

  • Pressure release: Due to the leak, the refrigerant flows as a water steam mixture into the containment. The pressure in the primary circuit drops. The reactor is shut down automatically by the security system.
  • Accumulator supply: The pressure in the primary circuit is sufficiently decreased. Under form standing water storage automatically flood then the reactor core. The hot fuel (see decay heat ) are thereby further cooled.
  • Core flooding: Before the accumulators are empty, the further cooling of the reactor core from the flood begins containers. High water pressure is no longer necessary for successfully reduced temperature of the fuel elements. Because entering evaporation but not only, the coolant must be circulated, but are also supplemented.
  • Swamp circuit: When the tide container are emptied, the system automatically switches on the circulation mode. RHR pumps deliver the water continually back out of the swamp on residual-heat cooler in the primary circuit.

Cooling ponds

In addition to the fuel elements in the reactor core itself is located in the reactor building or in an adjacent building a nuclear power plant, a cooling pond with water as a coolant for the removal of decay heat from spent fuel. A cooling pond is not under pressure, so that loss of coolant if need be improvised from the outside (using fire hoses, fire water transport helicopters and police water cannons, etc.), refilling can be attempted.

Containment building Nuclear meltdown Wolf Creek Generating Station Nuclear reactor core Spent fuel pool Void coefficient Emergency management Triage
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