engleski

Influence of NPP Krško Core Nuclear Characteristics on RCCA Ejection Accident

The mechanical failure of control rod mechanism pressure housing may result in the ejection of a Rod Cluster Control Assembly (RCCA) due to the pressure difference. The consequences include a rapid reactivity insertion together with an adverse core power distribution, possibly leading to localized fuel rod damage. Worth of the ejected control rod and power distribution in reactor core obtained with remaining control rods will determine quantity of damaged fuel rods. The reactivity transient is terminated by the Doppler reactivity effect due to increased fuel temperature, and by subsequent reactor trip activated by high neutron flux signals, before conditions are reached that can result in significant fuel melt. Single control rod ejection calculation has been performed for NPP Krsko HFP and HZP core conditions using best-estimate calculation tools and conservative but realistic assumptions. Control rod ejection accident was analyzed with 3D neutron kinetics computer code PARCS v2.5 in order to assess behaviour of the reactor core during and after the accident. Special attention was paid to thermal-hydraulic model of the core which was realised using modified COBRA code. More positive reactivity inserted means higher fuel enthalpy and fuel temperature in local rods, so additional sensitivity runs were performed to explore influence of uncertainty in RCCAs reactivity worth and in delayed neutron fractions on limiting core conditions. After performing core wide calculation, for most adverse conditions, analysis was made for hot fuel rods with FRAPTRAN computer code, to determine mechanical and thermal performance and possible damage of the rods.

NPP Krško; Core Nuclear Characteristics; RCCA Ejection Accident

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