INTEGRATED SEVERE ACCIDENT ANALYSIS FOR A VVER-1000: ACCIDENT PROGRESSION, UNCERTAINTY QUANTIFICATION, AND RADIOLOGICAL CONSEQUENCES

Nuclear safety aims at the protection of people, society and environment from the consequences of hypothetical accidents in Nuclear Power Plants (NPPs). This is particularly true in the case of Severe Accidents (SAs) with partial or complete core meltdown and potential radioactive release to the environment. In such conditions, a rigorous assessment of accident progression, Source Term (ST), and radiological consequences is essential for the development of accident management strategies guidelines and for emergency preparedness. The present study investigates a hypothetical Large Break Loss of Coolant Accident (LBLOCA) in the hot leg during a Station Blackout (SBO) sequence for a generic VVER-1000 reactor. The analysis relies on the integral SA code ASTEC (v3.1.1) to simulate accident progression, fission products release from the core, and ST to the external environment. The KATUSA tool and the JRODOS decision support system are then used for uncertainty and sensitivity analysis and for radiological dispersion and consequence evaluation under different meteorological conditions, respectively. As a case-study, the analysis has been performed taking into consideration the Zaporizhzhia NPP area in southeastern Ukraine. The integrated use of a SA code, an Uncertainty Analysis (UA) tool, and a consequence assessment system provides a comprehensive computational framework for SAs analysis. The results from the UA reveal that the uncertainty band for the total activity released to the environment is not negligible. In a complementary way, sensitivity analysis points out containment leakage area and fuel burnup as the factors driving the ST variability. Moreover, atmospheric dispersion and consequences analyses indicate limited off-site radiological consequences, in both winter and summer seasons, under maximum release conditions.