When restarting Oskarshamn Unit 3 after the re-fuelling outage 2016 a turbine trip occurred at low reactor power. Inspections of the feed-water system in the turbine building revealed a damaged support. A water-hammer caused by an open valve that should have been closed was suspected to be the cause of the damage. Measurement data from the event showed that the pressure in the piping near the damaged support had decreased slowly to be followed by rapid pressure peaks with high amplitudes. The damaged support was reinforced to regain its function and the plant was restarted but an investigation was initiated to analyse the event.

A study of measurement data from the plant showed that this was not a single event. Similar events had occurred earlier. The event turned out to be independent of the valve being open or closed. It was also observed that main pumps that were not in operation at the time in the feed-water system had rotated in the reverse direction during the pressure decrease (normally two out of three pumps are in operation). This event could explain a lot of the support damage in the feed-water system that had occurred since the plant was licensed and that was considered to be caused by operational displacements and vibrations.

A fruitful and intense collaboration started between Engineering, Operations and Maintenance to find the root cause of the event. This resulted in a number of observations that led to a general understanding of the nature of the event. The event was caused by depressurisation in the feed-water tank caused by a turbine trip at low power. The gradual pressure drop caused the water in the piping lower down to evaporate (flash), causing a mass-flow upwards. When the evaporation stopped the flow changed direction and fell downwards causing condensation and large pressure peaks.

The event could be simulated (by RElAP5) with results that qualitatively compared well to measurements. Simulations were then used to show how the event could be mitigated and ultimately avoided.

Hopefully future events will be avoided by changing operation procedures. This has been verified when the plant had to shut-down in November 2017 due to a fuel failure. During the shut-down process no flashing event was detected.

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