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NPP Krško DVI LOCA Calculation using RELAP5/mod 3.3 and FRAPTRAN to Assess UFC Modification Influence

Damaged fuel assemblies have been identified at NPP Krško during 2013 outage refueling activities. As a potential root cause for fuel assemblies located at the core periphery, the baffle jetting across the baffle joints between the baffle-barrel bypass and the core was identified. In order to decrease the pressure difference across the baffle joints the Upflow Conversion (UFC) modification has been performed at NPP Krško during 2015. outage. The modification consisted in altering the reactor vessel internals in such way that the coolant downflow path in the baffle-barrel region was converted to an upflow path. Thereby, the pressure difference across the baffle joints has been minimized and the baffle jetting as a major cause of damage of peripheral fuel elements has been eliminated. As a support to the UFC modification review, thermal-hydraulic analyses of the Direct Vessel Injection (DVI) line Small Break Loss Of Coolant Accident (SB LOCA) have been analyzed using RELAP5/mod 3.3 code along with the FRAPTRAN calculation for hot fuel rod. Both the downflow configuration (pre-UFC) and the upflow configuration (UFC) were considered in the analyses to assess influence of the modification. Depending on the assumptions used in calculation small increase in Peak Cladding Temperatures (PCT) was predicted due to UFC, but safety limits are not endangered.

NE Krsko; LOCA; RELAP5; DVI; UFC modification

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