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Preliminary radiation doses assessment for NPP Krško SBO sequence

Accidents at nuclear power plants may require urgent measures to prevent deterministic effects and reduce stochastic consequences. The need for fast decision making is restricted with great uncertainty of what is known about the situation in the early phase of an accident. In particular, magnitude, beginning and duration of a release are subject to considerable uncertainty and meteorological information might be unavailable or vague. This calls for simplistic and robust decision criteria which is defined in emergency preparedness zones. Recent developments on international level, especially after Fukushima accident emphasized need to have harmonized emergency preparedness zones in case of close proximity of nuclear power plants to the national borders. Example of such a case is Nuclear Power Plant Krško (NEK) that is located just 10 km from Croatian- Slovenian border. Emergency zones in Croatia and Slovenia do not have adequate level of harmonization. Croatian State Office for Radiological and Nuclear Safety (DZRNS – Državni zavod za radiološku i nuklearnu sigurnost) initiated an activity to gather background information based on which harmonization of emergency preparedness planning zones could be done between Croatia and Slovenia. In order to achieve this, use of NEK PSA (Probabilistic Safety Assessment) Level II, LERF (Large, early release frequency) and Release Frequency (RF) results has been explored. Additionally, impact of recent modification PCFVS (Passive Containment Filtered Vent System) has been investigated. Release category corresponding to the SBO (Station BlackOut) was selected to investigate the influence of PCFVS modification that enables controlled and filtered release of containment structure. IAEA available code INTERRAS (INTERnational Radiological Assessment System) module ST-Dose (Source Term to Dose) was used to estimate projected radiation doses from a radioactive plume to the environment. With this module it is possible to make preliminary assessment of consequences to the environment in case of adverse reactor conditions or releases from radioactive materials. Its intended usage is for quick assessment of consequences of radiological release before and during initial phases of nuclear accident when radiological measurements is still not possible or is limited. This paper represent preliminary finding as the conservative boundary conditions in calculation yield doses close to intervention levels.

radiation doses; INTERRAS; Krško NPP; SBO; release rates; iodine; LERF; filtered venting

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