The two Loviisa VVER-440 type reactors were commissioned in 1977 and 1980. The original designed life time of the reactors was 30 years. In 2003 Fortum, the owner and the operator of the Loviisa plant, launched an extensive safety study to prove the authorities that there was not any major safety issue why operating license could not be extended for another 20 years. In 2007 the Ministry of Employment and the Economy of Finland granted 20 and 23 years extension to the operating license for units 1 and 2, respectively. One issue, which needed further investigation, was the core cooling capability during sump circulation; i.e. were the present sump strainers good enough to prevent insulation fiber from not clogging the core coolant flow? Back in the 1990’s the original steel wire type sump strainers were replaced with stronger steel pipe type strainers. Some time later experiments were carried out to find out if insulation fiber could penetrate through the strainer holes and reduce the coolant mass flow rate through the core. The experiments indicated that the insulation fiber mixed with coolant partly penetrates through the strainer and gathers to the fuel assembly spacer grids increasing pressure loss across the core. The experiments were carried out in a rather simple test facility and also under forced single phase circulation. In those loss-of-coolant accidents (LOCA) where sump circulation takes place, circumstances are completely different. Therefore, it was decided that the APROS (Advanced PROcess Simulation) simulation software would be used to study the insulation fiber effect on core coolability during the accident. A large LOCA was chosen for the case to be analyzed. The reason for this was that during a large LOCA sump circulation begins in the early phase of the accident and a lot of emergency core cooling (ECC) water is injected into the primary circuit during sump circulation. The paper will first shortly discuss APROS simulation software. Then the test facility and the experimental results will be presented. The main issue is the analyses results. Several analyses were carried out to be able to determine the maximum amount fiber gathered in the spacer grids which the core can tolerate without overheating.

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