CBRN event in Spain on Thu 28th January 1999

On 1999/01/28 the licensee noticed that several penetrations of the Reactor Building (RB) were not properly sealed, so that there was some communication between the annulus (space between metallic spherical containment - housing the reactor coolant system - and the reactor building) and the environment.
The licensee had been opening up some RB penetrations since August 1998 in order to lay some cables in the framework of a design modification aimed to upgrade the electrical system.
At normal operation there is an Annulus Ventilation System that keeps the annulus pressure under atmospheric. This is monitored by a 24 hour surveillance requirement, that was always carried out.
Penetrations sealing had been beheld properly as fire barriers, however their function as radioactivity confinement was overlooked until the licensee discovered the deficiency on 1999/01/28.
Upon noticing the issue, the licensee performed a test to check out operability of the Annulus Emergency Ventilation System (TL-9), the one in charge of keeping annulus under-pressure in case of an accident. The acceptance criteria for such a test is to get in the annulus a pressure of at least 40 mm of water column below the environment pressure, the actual value found was just 5 mm below. The licensee immediately entered the action required by the Technical Specification and properly sealed those penetrations.
Rating:
On-site and off-site criteria are not applicable as no spread of contamination took place. Actually the annulus pressure kept below environment during existence of the deficiency.
It is rated on the defence in depth criterion, Table I is chosen as there was no initiator.
The safety function degraded was "confinement of radioactivity". In normal operation it was within Technical Specifications, as the requirement to keep underpressure was always fulfilled.
In case of LOCA, an Unlikely accident, the leaks of containment would enter the annulus and threaten to escape to the environment through the defective seals. The system in charge of preventing this challenge, TL-9, was outside Technical Specifications, as it was unable to get the specified under-pressure; as the TL-9 got some under-pressure, the leaks through the RB penetrations improperly sealed would be inwards, anyway, therefore the safety function is Adequate (Box 3-C)
Final: Ines Level 1