Heavy water leak from a defuelled coolant channel in a PHWR during Inspection, with reactor in shutdown state
Monday, May 12, 2003
Narora Atomic Power Station (NAPS) is a twin unit station having 2x 220 MWe PHWR type of reactors. NAPS Unit-2 was under shutdown with its Primary Heat Transport (PHT) system in cold and depressurized state. In Service Inspection of coolant channels was in progress. For inspection of a coolant channel, the channel is defuelled and the normal seal and shield plugs of the channel are removed using the fuelling machine. An inspection probe having a special seal plug is then installed in the channel and an extension tube is connected to the plug which has a drive motor connected at the other end. The probe is then moved inside the channel to carry out inspection. Inspection of four coolant channels was completed successfully. On 25th April 2003, at 1130 hrs, during inspection of the fifth channel, when the probe was moved for inspection, PHT system water started leaking from the channel. This happened as the extension tube was not properly connected to the probe and got detached during the movement. Plant emergency was declared at 1140 hrs and access control was enforced in reactor building. The airlock doors of the containment building which were open at this time to permit passage of cables connected between the inspection probe and the inspection control station outside the reactor building, were closed in a short time. There was no interruption in core cooling as shutdown cooling pump continued to operate normal. Coolant channel was boxed up as per emergency operating procedure by putting a blind flange on the coolant channel face. The plant emergency was called off at 1225 hrs. There was no radiation overexposure of any plant person and release to the environment was also within specified limits.