Radiological aspects in hot-spot removal from spare LPRM location

TAPS 1 and 2 comprises of twin units of boiling water reactors with vertical core. Cruciform shaped control blades are driven from the bottom through the core using hydraulic Control Rod Drives (CRD). Neutron flux monitoring is carried out by in-core devices such as Source Range Monitors (SRM) and Intermediate Range Monitors (IRM) during the start up and later using Power Range Monitors located in the biological shield. In addition, Local Power Range Monitor (LPRM) assemblies located symmetrically in the core gives information about axial and radial neutron flux. There are 18 LPRM locations out of which 13 are used for mounting the LPRMs and rest are spare locations. The lower head of the reactor vessel has 120 nozzles for CRD housings, SRM, IRM, LPRMs, vessel drain etc. Maintenance work on CRDs, removal and installation of in-core monitoring devices etc are carried out from under vessel area in the primary containment known as CRD housing area. While conducting radiation survey of CRD housing area after shut down of unit no. 1 for the 21st refueling outage, hotspot was observed at spare LPRM 40-21 location showing radiation level of 10 Gy/h and general back ground of 30-50 mGy/h. An attempt is made to bring out the ALARA efforts taken during hot spot removal. Options like hotspot shielding were not feasible. After core unloading, efforts were made to remove the hotspot. To assess the change in radiation levels, a high range silicon semiconductor based gamma monitor's detector was mounted on a pole and fixed near hotspot. Radiation level at the detector was 6 Gy/h. In addition detector of another low range area monitor was fixed at the trolley in the CRD housing area and radiation level was 7 mGy/h. Monitors of both the detectors were kept outside the primary containment in low back ground area at reactor building 103' El. Hotspot removal was attempted initially by vacuuming from the top of reactor vessel using 1 inch dia tube from spare LPRM location. Vacuum receiver was provided with adequate shielding anticipating accumulation of high active material. As it was not successful, high pressure waterjet was attempted. Increase in radiation levels in the CRD housing area was observed. This was probably due to change in the orientation of the hot object. Later further attempts of vacuuming using 0.5 inch dia tube, use of magnet, flower shape tube etc, were made and were not helpful. As a last resort, high pressure waterjet using 0.5 inch dia tube was attempted. It dislodged the hotspot and probably pushed it above bottom grid. Radiation level at spare LPRM reduced to 3 mGy/h confirmed the removal of hotspot. Underwater hotspot removal attempts were made from RB 200'El and the monitoring was done at RB 103'El with continuous communication. Collective dose consumed for hotspot removal was 23 person-mSv. Use of high range monitor, remote operations, dynamic planning, proper communication and above all management's commitment were helpful for executing the work with minimum dose consumption and without any violation. (author)