in84061.txt at Page 1 of 6 2/25/00

Transcription

1 in84061.txt at Page 1 of 6 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C SSINS No.: 6835 IE INFORMATION NOTICE NO : OVEREXPOSURE OF DIVER IN PRESSURIZED WATER REACTOR (PWR) REFUELING CAVITY Addressees: All nuclear power plant facilities holding an operating license (OL) or construction permit (CP). Purpose and Summary: This information notice is provided to inform recipients of a whole-body exposure in excess of regulatory limits to a diver engaged in underwater repair work. At the Palisades Nuclear Generating Plant on March 18, 1984, a diver received a whole-body exposure totaling about 4.5 rems to his right thigh during a series of three dives while working on the fuel transfer tilt machine (upender) in the flooded reactor refueling cavity. The dose received during the first two dives totalled about 1 rem. Failure to account for the diver's change of work position relative to a known, high level radiation source during the third dive caused this whole-body overexposure. The diver kneeled in a layer of radioactive sludge on the tilt pit floor. This diving event is the second instance in which a lack of adequate management and radiological controls for planned underwater work led to a diver receiving a whole-body overexposure. After reviewing this and the previous event (described in IE IN 82-31, July 28, 1982), the NRC staff has developed further guidance to help licensees properly control and manage underwater work to prevent overexposures. It is expected that licensees will review the information provided for applicability to their facilities and consider taking actions, as appropriate, at their facility. Suggestions and guidance contained in this information notice do not constitute NRC requirements and, therefore, no specific action or written response is required.

2 in84061.txt at Page 2 of 6 Description of Circumstances: The refueling cavity side upender was found in need of repair (one leaking hydraulic hose). Underwater surveys in the upender area taken on March 16 indicated radiation dose rates of 350 to 900 mrems/hour. An RWP was written on March 17, but no ALARA review was conducted for the repair work. Underwater radiation survey instrument malfunctions delayed starting the job until March 18. Before the first dive, check surveys indicated underwater dose rates in the upender work area ranging from 1,000-7,000 mrems/hour. The difference between these survey results and the March 16 results demonstrates the large Page 2 of 4 dose rate gradients in the work area and the difficulty in accurately positioning the survey instrument. Diving operations commenced March 18 on the afternoon shift. The diver was equipped with thermoluminescent dosimeters (TLDs) and self-reading dosimeters, but no continuous readout alarming dosimeter or dose rate survey instrument was required by the RWP. After each of the first two dives, the diver's TLDs were read and these exposure results along with the planned dive durations were used to establish a conservative, allowable stay-time for the following dive. The TLD results indicated an exposure of about 3.6 rems to the diver's thigh had been received during the third dive (twice the expected dose for the third dive). When informed of the diver's exposure the duty health physicist stopped all diving operations and began an investigation of the incident. Discussion: A review of the incident by the licensee and NRC regional personnel found several key factors that contributed to the overexposure. 1. Lack of Job Planning and Controls Contrary to licensee administrative procedures, no formal ALARA review was performed and no maintenance work order was generated for the repair work. According to licensee representatives, had an ALARA review been performed, the resultant RWP would have required the diver to have a dose rate survey instrument with him at all times. The RWP did not stipulate further survey requirements. The plant radiation protection (RP) staff responsible for on-the-job supervision of the repair work received no formal, prework

3 in84061.txt at Page 3 of 6 briefing. Shift turnover between the RP technicians covering the job was ineffective. No formalized RP procedure existed to detail special precautions, equipment, survey requirements, job-specific training, and other requirements conducive to effective RP coverage for this repair work. 2. Inadequate Surveys and Instrumentation Except for the prework surveys taken on March 16, none of the upender area underwater surveys performed before the incident were documented. On March 18, underwater check surveys taken before the first dive indicated dose rates up to seven times (7,000 mrem/hour) the March 16 prework survey results, but this was not documented or reported to responsible RP supervision. The Palisades RWP procedure requires that a job be stopped if unplanned changes in the working environment occur. Evidently, information regarding these higher dose rates were not turned over to the oncoming RP relief crew. A diver was not used to obtain representative work area surveys. Contrary to the Technical Specification requirement (6.12) for controlling individuals entering high radiation areas, the diver was not required to have a dose rate monitoring device in the work area to warn of unexpected changes in dose rates. The underwater survey instrument (a large-volume ion chamber with a 40 ft cable) was difficult to use effectively, hard to position precisely, and repeatedly malfunctioned. Because of its demonstrated poor performance and Page 3 of 4 fragile condition, the RP technicians reportedly tended to limit its use. The instrument's large volume (7.5 in. long and 5 in. in diameter) made it too cumbersome to reach into the cramped areas around the upender. After each dive, the diver's dosimeters were processed and the results were used for planning the next dive. As was expected by the licensee, the diver's dosimetry results for all three dives indicated an extreme dose gradient over the body. However, the licensee failed to account for the diver's different work position/posture for the third dive. The diver was much closer to a known radioactive source -- a sludge-like contamination layer on the tilt pit floor. While kneeling on the floor during the third dive, the diver's thigh area was in a rem/hour radiation field for about one-half of the dive time. Guidance:

4 in84061.txt at Page 4 of 6 The nuclear industry uses divers to perform a variety of maintenance and repair tasks. The potential for significant exposures in very short periods of time demands stringent work and radiation protection controls. Water shielding offers dose savings, but it also presents difficulties in accurately assessing the dose rates in work areas. Extreme dose rate gradients allow highly localized areas of radiation to go undetected unless extremely detailed, carefully conducted surveys are performed. Finally, another unique problem is that the diver can move through the water shield. This factor adds another dimension to the RP control problem. Since the 1982 diver overexposure, several licensees have requested guidance for improving their radiological control program to support diving operations. Review of the procedures of several commercial diving companies disclosed that they contain many of the following elements: 1. A specialized written procedure for diving operations to ensure effective radiological coverage and control. This procedure establishes minimum prerequisites in job planning, RP coverage requirements, survey technique/frequency, worker training, prework briefing, periodic RWP updating, placement of dosimeters, etc. 2. During diving operations, continuous coverage is provided by qualified RP technicians. These technicians have stop-work authority and clear management guidance on when to exercise this important control function. 3. Minimum acceptance criteria is established for pool water clarity and underwater lighting to help ensure adequate working area visibility. 4. The underwater work area is decontaminated if contamination presents a significant exposure potential. 5. When practicable, physical barriers are provided to prevent diver access to fuel/irradiated components and other high-radiation areas. Diving cages or work platforms can successfully limit a divers mobility to a well surveyed and controlled area. Special warnings (e.g., underwater colored lights) can be used to mark high-dose-rate areas/components. Each diver is equipped with a safety line and continuous voice communication with Page 4 of 4 surface personnel. Emergency procedures for diver rescue are provided

5 in84061.txt at Page 5 of 6 and understood by everyone involved in the diving operations. 6. Diver access control is planned to avoid traveling over or near high-radiation areas. 7. Before any diving operation, a radiation survey of the diving area is conducted using two independent radiation exposure monitoring devices. Survey instruments are functionally checked (response checked) daily before diving operations. TLDs can be used to help confirm dose rate instrument readings. A survey map of the area is updated to reflect current status of ongoing work. 8. Underwater confirmatory surveys of the work area are performed by the diver. Because of the difficulty in placing a survey instrument from the surface, underwater surveys by a trained diver are more effective in locating hot spots. 9. After any movement of spent fuel or other highly radioactive components, an underwater radiation survey is conducted before any diving operations resume. 10. Each diver is equipped with a calibrated, alarming dosimeter which is functionally checked each day before diving operations begin. Additionally, each diver is equipped with a remote-readout radiation detector which can be continuously monitored by health physics technicians, or each diver carries a dose rate survey instrument. Individual dose rate monitoring capability is necessary because of the dose rate gradient of the water. The divers surface and have their dosimeters checked periodically; any significant deviation from the expected dive work pattern or radiation levels are grounds for terminating the diving operations and reassessing the conditions. No written response to this information notice is required. If you require any additional information about this matter, please contact the Regional Administrator of the appropriate regional office or this office. Technical Contact: J. E. Wigginton, IE (301) R. L. Pedersen, IE (301) Edward L. Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement

6 in84061.txt at Page 6 of 6 Attachment: List of Recently Issued IE Information Notices