Exposure of a temporary worker in excess of annual dose constraint during transfer operation of irradiated neutron detector.

At TAPS-4, irradiated Cobalt Self Powered Neutron Detectors (SPND) enclosed in a canister are stored in Interim Storage Facility (ISF). On May 17,2014 job of transfer of a canister from ISF to Solid Waste Management Facility (SWMF) was taken up.

This job involves retrieving canister containing SPND from ISF to transportation flask for shipment to SWMF. An assembly consisting of electromagnet (EM) and intermediate shield fixed on hoist was used to transfer the canister from ISF to Transportation flask. The EM was used to hold & lift the canister from ISF and to subsequently lower it in the transportation flask. After placing the canister in Transportation Flask, the EM was being retrieved during which high radiation field was detected in the area by local radiation monitors. In view of this the remaining activities of transfer operation were suspended and the area was immediately evacuated.

Investigation of the event revealed that during the transfer of the canister to the Transportation Flask, the screws holding the cover plate with the canister failed. As a result, the cover plate remained attached with the EM while the remaining portion of the canister got transferred to the Transportation Flask. The worker responsible for de-energizing the EM misinterpreted the dropping of canister as seen from camera monitors as de-energization of the EM though actually he missed the step of de-energization of the magnet. The canister cover plate had high radiation field due to loose contamination from irradiated Cobalt SPND. As a result when the EM was retracted from Intermediate Shield, the cover plate attached to EM got unshielded and resulted in high radiation field in the area. Subsequently as recovery action, canister cover plate was transferred to the Transportation Flask remotely with adequate protection and radiological shielding.

A temporary worker (contract worker) involved in the job received radiation dose of 15.7 mSv exceeding the annual dose constraint of 15 mSv for a temporary worker.

Location: TARAPUR-4
Event date: Sat, 17-05-2014
Nuclear event report
Legenda & explanation