Loviisa NPP (unit 2) low pressure Main Steam Safety Valves (MSSV) were renewed during 2014 overhaul. New valves are pilot operated and qualified for two phase conditions. The old valves were spring loaded and qualified only for steam flow conditions.
During the testing of new MSSVs, unexpected behavior was detected. The opening of new MSSV included significant delay in case of repeated openings. This led to a need to investigate this phenomena more closely since such opening delay could have negative impact to plant safety.
This paper presents the aftermath of the discovered opening delay in the NPP and describes how thermal hydraulic modelling and analyses were used to gain in depth understanding of pilot operated MSSV. In addition, it is presented how the analysis work was used to solve the discovered challenges.
Modelling work was conducted using Apros Process Simulation Software  which is the main tool for deterministic safety and engineering analysis of Loviisa NPP [2–4].