Calculation code for predicting releases in the event of severe NPP accidents
If a severe accident occurs in a nuclear power plant (NPP) during which radioactive substances may reach into the environment on a large scale (so-called release), disaster control authorities must take timely measures to protect the population. This can range from recommendations to shelter, take iodine tablets or for the authorities to evacuation the areas affected by the release and resettle the people living there in the long run. The exact measures to be taken will depend on when and how much radioactive material is likely to be present in which area. This depends on the one hand on the weather conditions prevailing at the time of release and on the other hand on the so-called source term.
The source term provides data on the expected amount and type of radioactive material released in a NPP accident. With this data, dispersion codes such as ARTM can predict the distribution of radioactive substances under certain weather conditions. While the weather data are continuously recorded by meteorological stations and can be determined quickly, the source term and its forecast is the result of a complicated calculation procedure. The determination of the source term and its adaptation in the course of the accident is the responsibility of the operator and is defined in the Basic Recommendations for Emergency Preparedness in the Vicinity of Nuclear Installations.
Basis: Plant data and PSA results
With the FaSTPro (Fast Source Term Prognosis) calculation code for source term prognosis developed by GRS, possible source terms can be predicted at the push of a button in the event of a nuclear power plant accident. The source terms can be transmitted very quickly to authorities or institutions such as the Federal Office for Radiation Protection (BfS) via ready-made forms or data acquisition. For example, it is possible to transfer the data directly to the RODOS decision support system of the BfS, with which the Federal Office, for its part, makes forecasts of the radiological situation.
The FaSTPro code uses the data of the Probabilistic Safety Analysis (PSA) of the respective plant to determine source terms. In German plants, a PSA is performed every 10 years as part of the periodic safety reviews. To put it simply, a source term is defined in a PSA for every possible accident sequence that will cause damage to the reactor core. The PSA indicates the probability with which the respective accident sequence will occur. In addition to the information from the PSA, the code uses plant data such as pressure or temperature for its source term calculations.
In practice, the determination of source terms then looks like this: The user of the code is asked to provide various pieces of information about the system status or to answer questions about it. These questions may be: "Time of reactor scram?" or "How high is the currently measured coolant pressure?” A scroll-down menu allows the user to click on the correct answer for his plant, using the multiple-choice method (see Figure 1). When querying data, questions can also remain unanswered or be marked "not known" if, for example, measurement displays have failed or if there is contradictory information about the values. These missing answers are replaced by the respective PSA data for the further calculation.
In a further step, the plant data are compared with the PSA data. If the plant data show, for example, that the pressure in the reactor drops below 9 bar within 200 seconds and radioactivity is measured, the system will indicate a possible loss-of-coolant accident when comparing the data with those from the PSA and will then indicate the probability of occurrence of such an event. In this way, the number of possible accident sequences and thus also the source terms in question can be limited on the basis of the existing plant data. The processing time until the user has a selection of possible source terms at his disposal takes only a few seconds. In addition, the relatively simple use in the stress situation of an accident can help to prevent wrong decisions.
The joining of the plant data determined in each case with the PSA data is carried out on the basis of a so-called Bayesian network. The special feature of such a network is that it can infer an effect not only from a cause, but also in the opposite direction. In our case, the network can therefore not only infer the effect of "core damage" from the cause "loss of coolant with simultaneous failure of safety systems" but can also in reverse infer possible causes for the malfunction from the parameters observed in the system and provide them with a probability of occurrence. Figure 3 shows the concept in a simplified manner.
What is the logic behind?
GRS already developed a first version of the FaSTPro code back in 2008 as part of a project sponsored by the Federal Environment Ministry (BMU) and the Federal Office for Radiation Protection (BfS). Since then, the GRS code has been continuously developed and, for example, adapted from the original variant for pressurised water reactors for application to boiling water reactors. Currently, five of the seven German plants still in operation use the GRS code. In its "Recommendation on the Forecast and Estimation of Source Terms in the Event of Nuclear Power Plant Accidents", the Commission on Radiological Protection generally recommends the use of computer-aided probabilistic methods for source term prognosis. GRS itself uses FaSTPro in its Emergency Centre, which also supports the Federal Radiological Situation Centre.
In addition, GRS is currently involved in a project sponsored by Federal Ministry of Economics and Energy (BMWi) in which FaSTPro is extended for releases during non-power operation in nuclear power plants, e.g. during an overall maintenance and refuelling outage or in the post-operational phase.
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