Stichting Laka

Event Report Forms (ERFs) are filed to the IAEA. Sometimes there are multiple event reports, updating the rating, the event description. Here you only see the latest event report

ERF send date: Tue, 04-12-2001
ERF description: At 20:57 on November 11, 2001, while Darlington Unit 3 was at 98% FP, a Heat Transport (HT) pressurizer bleed valve (633333-CV8) failed open. A unit automatic power reduction occurred on high bleed condenser pressure, followed by a Shutdown System #1(SDS1) trip on HT low pressure. Heat Transport Pressure dropped to approximately 5 to 5.5 MPa (g) and remained there for 15 minutes. Emergency Coolant Injection channel logic signals were initiated due to HT low flow, that sealed on 1 of 3 channels (functioned as per design), and no injection occurred.
The failed valve was closed with handswitch controls in the Unit Secondary Control Area. The turbine was manually tripped and the HT pressurizer isolated to assist in restoring HT pressure to normal (9.8 MPa (g)).
The apparent cause of steam bleed valve failing open is failure of the air supply Pressure Regulating Valve (PRV), due to over torqueing of the spring retaining nut, combined with radiation damage (note: this was a non-nuclear grade component – difference in the non-nuclear version is that the elastomer is of a less radiation resistant material, and spring retaining nut is made of plastic and not brass). Investigation is continuing.
The fuel experienced no mechanical damage or overheating, as was confirmed by fuel inspections. Assessment of stresses on PHT piping and pressure tubes identified stresses within acceptable parameters.

ERF rating date: 30-11-2001
ERF contact person: Mr. Helmy Ragheb
ERF address: P.O. Box 1046, Station B, Ottawa, Ontario, Canada K1P 5S9
ERF affiliation: Canadian Nuclear Safety Commission

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