Statistical evaluation of the online core monitoring effectiveness for limiting the consequences of the fuel assembly misloading event

In VVER type reactors, online core monitoring is used for the early indication of such abnormal events like fuel assembly misloading, inadvertent misalignment of Control Assemblies, blockage of coolant channels. The paper is focusing on the assembly misloading, which cannot be indicated by other measurements. A Monte Carlo method was developed and applied to evaluate the online core monitoring effectiveness for the indication of this abnormal event during the power increase in due time, when the consequences are still acceptable. The investigations proved the satisfactory effectiveness of the online core monitoring down to 55% power even in case when 75% of the temperature measurements was only available.