Abstract
(English)
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In the event of an accident in a nuclear power reactor, the authorities should be in position to implement the counter-measures necessary to protect the surrounding population from radiological and the environment consequences of any releases. The means may be complex and require a few hours to set in place. However, the rate of development of such accidents in Light Water Reactors (LWR) is relatively slow, with in general no significant releases occurring before a several hours into the accident. It is therefore necessary to monitor the progression of the accident as soon as it is detected by the plant operator in order to forecast the future behaviour of the reactor so as to be able to recommend to the government authorities the implementation of counter-measures within a time compatible with the control of the risk to the population. For that purpose, it is necessary to estimate the nature, amount and kinetics of the radioactive products likely to be released out of the installation. Then, it would be possible to utilise RODOS system to decide the adapted counter-measures.
The present work developed a methodology, designed and implemented a computer package for source term estimation in accidental cases LWR. The computer package aims at monitoring the progression of an accident, to forecast the future behaviour of the reactor and to estimate the on-going and the potential releases, on the basis of the data regularly transmitted by the operator of the nuclear power plant concerning the state of the installation. Thus, the consequences of these releases for the environment and population may be calculated with RODOS system. It will be possible to evaluate the source term by taking into account a possible failure or recovery of a safety system and also to compare out site measurements to release be predicted and to refine the source term. The software system is constructed to be applied to every type of LWR operating in the European Union. The software is composed by simplified models for emergency preparedness situations. The operator will be guided in his analysis of the situation and will keep his responsibility for the evaluation. The approach includes a number of distinct modules which are integrated in order to constitute a whole package. Each module is designed to be independent, but will interact with other modules. Within the STEP project, the following elements are of interest:
- to develop a common methodology to estimate a source term for LWR;
- to constitute common simplified models and tools adapted to the methodology;
- to give a source term to RODOS system before measurable releases.
The ETH Zürich specific work deals with the technical specifications for the containment module and the development of a complex methodology and associated software package. The objective is the estimation of a nuclear accident source term based on the power plant technical condition. The work to achieve STEPS objectives is guided by specific terms of reference:
- the capability of the software to function as a source term interface between targeted nuclear power plants and the Real -Time On -Line Decision Support System for Off-Site Emergency Management in Europe (RODOS), and
n the comparable capability of the system to be operated as a stand-alone assessment and decision support tool for a comprehensive variety of plants, nuclear emergency classes, and potentially interested parties.
For the containment-related aspects, the technical specifications had to focus on the possible location of radioactive sources; the most probable routes that leaks would follow from containment, or otherwise accident localisation volume out to the environment (pathways): the depletion, including safety engineering reduction mechanisms mitigating releases (release phenomenology and control modelling); and the effective evaluation of the radioactivity effectively escaping from plant, confinements (off-site risk potential). A specific guideline was established, to the effect of using to the utmost extent feasible the current experience, methods and software dedicated to the subject e.g. IPSN, France.
To accommodate and harmonise the terms above, the technical specifications have adopted:
n classification criteria for types of plants/reactors/containment/ structures, as well as for accident sequences, that generally are consistent with the generic classification practices referenced in the topical literature based on the PWR/BWR dichotomy, with the purpose of providing, in principle, for an acceptable balance between reactor-specific models, algorithms and codes on the one hand, and generic models, algorithms and codes, on the other hand.
n an operational criterion of prioritising the approach to be taken up by STEPS in covering the variety of targeted nuclear units, recommending the use of IPSN SESAME methodology, and a core software structure derived from the latter, as a main stream.
To provide for an interim coverage of the units not accounted for, at different stages, by the emergency methodology as described, reference is made to alternative generic methodologies designed in order to meet norms recommended by IAEA.
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