control-plot analysis of the operational safety of power-generating units in a nuclear power plant

Abstract

Formulation of the Problem. There are several approaches to estimating the operational safety of operating power-generating units of a nuclear power plant on the basis of information about failures. One such approach, which is descriptive, consists of a direct statistical analysis of failures (initial events) neglecting the sequence of failures and the probability that a failure will develop into a serious accident [1]. The second approach, recommended by IAEA [2], makes it possible to take into account the seriousness of failures in the operation of the power-generating unit by means of an expert analysis of the initial events taking into account qualitative criteria. If the initial event is an equipment failure, then this approach is equivalent to the FMECA method (failure mode, effects, and criticality analysis), standardized in the international standard Mt~K 1085 and the national standard GOST 27.310 "Analysis of modes, consequences, and criticality of failures. Basic principles." A common drawback of both approaches is their static nature, as a result of which it is impossible to take into account preceding failures and there is no predictive element.The method proposed in [3, 4] for assessing the operational safety level of the power-generating unit consists of investigating its reliability rating, which is a random quantity, proportional to the expected (probable) occurrence frequency of a serious accident associated with melting of the active zone. Thus the rating takes into account both the degree of reliability of the systems, equipment, and personnel as well as the seriousness of possible consequences of incidents. It can therefore be regarded as an integral indicator of the operational safety of the power-generating unit. Failure analysis based on the rating makes it possible to investigate these events on a scale reflecting the probabilistic measure of the evolution of the failure into a serious accident within a fixed time interval (for example, within a month); this distinguishes this approach advantangeously from the approaches described above. In addition, the safety rating makes it possible to take into account the entire history of the failure, beginning with the startup of the power-generating unit; this expands the possibilities of safety analysis. The objective of the present paper is to improve further the methodology of operational safety assessment using a rating based on the construction of safety control plots.Safety Control Plots. Analysis of the dynamics of the changes in the safety rating of the power-generating units of the Balakovo, South Ukraine, and other nuclear power plants, performed on the basis of failure data for the observation period up to 1992, showed that the safety rating r(t) of an individual power-generating unit is a time series (realization of a random process) r(t), characterizing the expected occurrence frequency of a serious accident, which is due to events which have happened in the unit over the period t. Figures 1 and 2 display plots of the change in the saf...