On 31 October 1995, LaSalle Unit 1 was operating at 94 percent power. A transversing incore probe (TIP) was being withdrawn from the reactor core and moved into its shielded storage location. Because of the failure of a drive chain, the TIP was withdrwan through the storage area and into the unshielded drive unit. The problem was identified when an area radiation monitor alarm showed a higher than normal radiation level. As a precaution, all personnel were evacuated from both the Unit 1 and Unit 2 reactor buildings. No abnormal dose to personnel was reported.
The dose rate in an unrestricted corridor below the TIP drive unit was measured to be 70 mSv/hr on contact at the ceiling, 30 to 50 mSv/hr at head level, and approximately 5 to 10 mSv/hr in the general area. The dose rate at the TIP drive unit (an unrestriceted area) was calculated (based on dose rates in the corridor below) to be 230 mSv/hr at 1 meter.
This event resulted in significantly elevated radiological dose rates in both an unrestriced corridor and in the area of TIP drive units. The event is rated "level 1" in accordance with the INES User's Manual Part III, Defence in depth Criteria for Reactors, paragrapgh III-3.5, Dose control. Since there was full safety system availability and the initiator was judged to have a medium (possible) frequency, Table II of Part III indicates a level of 1. The On-site Impact Criteria of Part II are not applicable to this event. The manual states in Paragraph I-2.2, on-site criteria, "worker dose is defined at level 2 to 4. The threshold for worker dose is a dose exceeding an annual statutory limit. If none of these thresholds are exceeded, on-site impact is not relevant for rating the event."
There was no off-site release, and this event had no effect on safety-related equipment.
Event date: Tue, 31-10-1995