Precursor analysis: harbingers of damage in the reactor core
If something in a nuclear power plant does not work as planned, this is referred to as an "event". On average, there are several such events in German nuclear power plants each year, as the records of the competent supervisory authorities, who are notified in these cases as required by the German Nuclear Safety Officer and Reporting Ordinance, show. However, the obligation to report an event alone does not suffice to determine its safety-relevance.
GRS has been evaluating these reportable events on behalf of the competent federal ministries since 1975. If it turns out that such an event has real or potential safety significance and can be applied to other nuclear power plants, GRS prepares a so-called Information Notice . To judge the safety significance of an event, GRS in some cases also determines the probability of core damage. To do so, a method is applied that is called "precursor analysis".
What is a precursor analysis?
Precursors are all those events in nuclear power plants that increase the probability of core damage, e.g. through an impairment of safety equipment, operational disturbances, or an incident.
The purpose of the precursor analysis is to calculate this core damage probability. If e.g. part of the emergency cooling system has failed during a reportable event, the precursor analysis will look at all disturbances and incidents in which the failed system will be challenged. Here, one object of the studies is also the probability with which other safety equipment intended for fuel cooling could have failed. The result thus obtained is a core hazard probability.
If fuel cooling is at risk, this does not necessarily lead to damage to the reactor core. The former will only be the case if the emergency measures provided are unsuccessful and the core overheats. However, even if the core has already suffered some damage, the design still provides safety barriers to prevent or at least limit a release of radioactive materials.
The result of the precursor analysis puts a number on the safety significance of an incident. Common worldwide practice is to consider only those events as precursors in which the core damage probability is higher than 1 in one million, or in short 10-6.
What are the uses of precursor analyses?
The method of the precursor analysis allows a differentiated view of reportable events, going beyond the rather rough classification of the German Nuclear Safety Officer and Reporting Ordinance and the International Nuclear Event Scale (INES) . The aim is to quantify and render objective the safety significance of an event on the basis of a standardised methodology. This way, possibly existing weak points can be detected and assessed, which in turn makes it possible to derive ideas for optimising safety.
A precursor as such does, however, not yet allow a conclusion regarding the overall safety level of a particular plant. The result of the precursor analysis as well as the occurrence of the precursor is a stochastic snapshot and is subject to statistical variation. Yet precursor analyses carried out over longer periods of time contribute to obtaining an objective picture of the safety level of a nuclear power plant.
Precursor analyses and the probabilistic safety analysis
The results of the precursor analyses are also used for optimising the method of the probabilistic safety analysis (PSA) . Beside the precursor analysis, the probabilistic safety analysis is a further method that allows a numerical representation of a core damage risk on the basis of probabilistic calculations. Contrary to the precursor analysis, though, its starting point is not a concrete event but the broad spectrum of events that can lead to a deviation from normal specified operation.
The method behind the PSA is to assume that technical failure is only a matter of time. Hence there is always a certain probability of an incident. In so-called event-trees, the different development scenarios of such events are played through and their frequency is calculated. The precursor analysis draws on the results of the plant-specific PSA of the nuclear power plant where the event occurred. In doing so, this PSA is adapted such that it models the event that has occurred.
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