Internal exposure control measures during heat exchanger replacement in thermosyphon evaporators: a case study on radiation protection during decommissioning.

Large-scale decommissioning or partial refurbishment of process equipment in nuclear facilities can pose considerable challenges for controlling radiation exposure. While there is substantial literature on external exposure control during decommissioning activity, the works on internal exposure control are limited. Moreover, unlike external exposure, which can be planned and budgeted through extensive radiological mapping, internal exposures can escalate rapidly during decommissioning activities and can lead to severe exposures. This paper presents a case study on replacing six heat exchangers in thermosyphon evaporators at a spent nuclear fuel reprocessing facility, with radiological challenges analogous to small-scale decommissioning in both scope and complexity. By carrying out root cause analysis (RCA) of historical internal exposure events, we identified four categories of causes for internal exposure: radiological environment, respiratory equipment, monitoring & measurements, and safety culture. Within the facility's established radiation-protection programme, we implemented a set of RCA-derived measures that included localized ventilation systems for increased air exchanges, newly designed airline respirator manifolds with real-time flow alarms, validated in-cell air-monitoring protocols, personal air sampling in the breathing zone of workers, and intent-based training for personnel. The implementation of the methods delineated in the paper, resulted in an achievement of zero internal exposure cases, for a work that spanned for 3 years and involved more than 2000 man-hours spent inside active process cells.