Abstract
The objective of this paper is to analyze the ability of a VVER-1000 core and its control system to cope with a hypothetical main steam line break (MSLB) accident in case of multiple equipment failures. The study involves the use of advanced 3D core calculation models benchmarked and validated for reactivity accidents in preceding studies. A MSLB core boundary condition problem is solved on a coarse (nodal) mesh with the coupled COBAYA/CTF neutronic/thermal hydraulic codes. The core thermal-hydraulic boundary conditions are obtained from a preceding full-plant MSLB simulation. The assessment of the core safety parameters is supplemented by a fine-mesh (sub-channel) thermal-hydraulic analysis of the hottest assemblies with the CTF code using information from the 3D nodal COBAYA/CTF calculations. Thirteen variants of a pessimistic MSLB scenario are considered, each of them assuming a number of equipment failures aggravated by eight control rods stuck out of the core after scram at different locations in the overcooled sector. The results (within the limitations of the adopted modeling assumptions) show that the core safety parameters do not exceed the safety limits in the simulated aggravated reactivity accidents.
Highlights
A major concern in a main steam line break (MSLB) reactivity accident is the risk of core overheating
This paper presents results from the analysis of thirteen variants of a hypothetical MSLB scenario, each of them assuming a number of equipment failures plus eight control rods stuck out of the core after scram at different radial locations in the overcooled sector
A specific objective is to make a step in the analysis of the ability of a VVER-1000 core and its control system to cope with such an accident without exceeding the safety limits
Summary
A major concern in a main steam line break (MSLB) reactivity accident is the risk of core overheating. In the computational analysis of such accidents the safety parameters of particular interest are the fuel centerline temperature, the departure from nucleate boiling ratio (DNBR) and the fuel rod cladding temperature. As a VVER-1000 core can contain once, twice, three or four times burnt fuel assemblies, it is of practical interest to analyze the consequences of such an asymmetric reactivity accident at the end of core life and for various combinations of equipment failure. The main objective is to demonstrate the use of state-of-the-art 3D core calculation models which have been benchmarked and partly validated for VVER-1000 reactivity accidents in preceding studies. A specific objective is to make a step in the analysis of the ability of a VVER-1000 core and its control system to cope with such an accident without exceeding the safety limits.
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