STEPS: source term estimation based on plant status phase 0 - the technical specifications of the containment module

In the framework of Project RODOS (Real-Time On-Line Decision Support System for Nuclear Emergencies in Europe), the European Atomic Energy Community (EAEC) of the Commission of the European Communities has commissioned the development of a unified concept, body of knowledge, models and software package meant to assist the evaluation of the source term of severe nuclear accidents in light water reactors of the types prevailing on the Continent. Code-named STEPS, for 'Source Term Estimation based on Plant Status', the project has evolved as Contract RODOS D (FI4P-CT96-0048), between EAEC and consortium of expert European centres including Commissariat a l'Energie Atomique, Institut de Protection et de Surete Nucleaire, (CEA-DPI-SEAC) as Coordinator and Forschungszentrum Karlsruhe GmbH (FZK-INR), the Finnish Centre for Radiation and Nuclear Safety, (STUK-NSD), the Technical Research Centre of Finland, Energy, Nuclear Energy (VTT-ET-NE), and Eidgenossische Technische Hochschule - ETH Zurich, Centre of Excellence (ETH-CERS) as Contractors. For the Phase 0 of the project, an IFIN-HH expert has been assigned by ETH-CERS to develop the Technical Specifications of the delivery component of the intended STEPS package, the CONTAINMENT Module. Sponsored by ETH-CERS headquarters in Zurich, the work was done on the premises and with the logistic support of CEADPI-SEAC at Fontenay-aux-Roses, with the feedback processing and computer code development subsequently performed in Bucharest. The Technical Specifications of the STEPS CONTAINMENT Module were guided by specific terms of reference, including: (i) the capability of the software to function as a source term interface between targeted nuclear power plants and the RODOS System; (ii) 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. On the technical side, the specifications had to focus on the possible location of radioactive leaks (sources) in a representative variety of LWRs, the most probable routes that leaks would follow from containments to the environment (pathways), the depletion, including safety-engineered reduction mechanisms mitigating releases (release phenomenology and control modeling) and the evaluation of the radioactivity effectively escaping from plant confinements (off-site risk potential). The main thrust and scientific challenge in developing the specifications rested on the effort to identify, critically review, adapt and articulate into a 'do-able' structure a disconcerting variety of models and constitutive equations for the complex phenomenology of radioactive releases to reactor containments, and its dynamics featuring natural and safety-engineered depletion, on the way to the environment. The emerging CONTAINMENT models and codes, developed in two versions, essentially rely on an inter twinned box-structuring of the release and paths, coupled mass-balance equations and hierarchical inputs addressing different possible levels of knowledge on the plant status during the accident. Subject to international review by the STEPS Contracting Parties, the Technical Specifications of the CONTAINMENT module and the feasibility demonstration codes developed in Bucharest were recognized (Zurich, July 1997; Helsinki, January 1998; Zurich, April 1998) as a valid platform, instrumental collection of models, algorithms and user-interfaces for the foreseen code development. (author)