Inadvertent Blowdown Event in a Nuclear Boiling Water Reactor

Abstract

On March 6th 2007, the Leibstadt Nuclear Power Plant in Switzerland experienced an automatic blowdown of eight safety/relief valves installed on the main steam lines caused by a faulty electrical manipulation while performing planned maintenance during full power operation. Due to the temperature measurements inside the reactor recirculation system and the reactor pressure vessel this event, at a first glance, appeared to be Event No. 23 (Automatic Blowdown event) as an Emergency (Service Level C) Condition in accordance with the relevant reactor pressure vessel Thermal Cycle Diagram. According to the ASME Code Section III, Service Level C limits permit large deformations in areas of structural discontinuity which may necessitate the removal of a component from service for inspection or repair. This paper presents a summary of thermal-hydraulic, stress, fatigue, and fracture mechanical evaluations as well as plant inspections performed to demonstrate the impact of the event on the reactor pressure vessel and associated components and to fulfill the requirements of the Swiss Federal Nuclear Safety Inspectorate. It is shown that the primary circuit of the plant was not inadmissibly stressed by the event and that it was acceptable from a safety-related point of view to return the plant to service. Corresponding to the 7-level International Nuclear and Radiological Event Scale this event was rated afterwards as level 1 (anomaly) by the Swiss Federal Nuclear Safety Inspectorate.

Towards a Consolidated Approach for the Assessment of Evaluation Models of Nuclear Power Reactors

“…The depressurization of the RPV caused flashing and consequently a level swell in the downcomer. As a result of the swell in the upper part of the down-comer, a steam/water mixture reached the steam lines and from there extended to the SRVs 26 .…”

“…During the initial phase of the event the RPV experienced a fast depressurization. The pressure gradient in the primary system leads to significant stresses on the RPV wall and the associated internal structures 26 . To quantify these stresses the chosen QoIs are the dome pressure time derivative, i.e.…”

“…To quantify these stresses the chosen QoIs are the dome pressure time derivative, i.e. dp/dt and the pressure drops over the sensitive locations in the core as specified by Engel 26 . It has been found that these pressure drops behave all very similar and therefore, only two are shown, i.e.…”