Bitumen is present in viscodampers that have been installed on Olkiluoto 3 surge line to prevent pipeline vibrations. The viscous mass in the dampers is significant. Viscofluid response to LOCA loads or effects on emergency core cooling system (ECCS) sumps/strainers has not been studied in the past at all. Release of the viscofluid cannot be excluded in case of a surge line break, and an (excessively) conservative assumption that all viscomass would be transported on operating strainers would lead to an unacceptable conclusion that ECCS recirculation cannot guaranteed. Therefore, information was needed on how the viscofluid behaves under LOCA loads, and how it influences coolant recirculation.
LUT carried out experiments to understand behaviour of bitumen viscomass under Loss-of-Coolant Accident conditions, and to develop an “additional trap” to prevent bitumen ingression into the In-containment Refuelling Water Storage Tank in case of a surge line guillotine break. Tests were performed on bitumen material properties, bitumen disintegration under highpressure steam jets, bitumen transport with hot borated water, and bitumen behaviour on filter structures. Based on test insights, an additional trap design was proposed for Olkiluoto 3 that can trap released bitumen with high confidence.
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