Information Notice No. 97-68: Loss of Control of Diver in a Spent Fuel Storage Pool
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
September 3, 1997
NRC INFORMATION NOTICE 97-68: LOSS OF CONTROL OF DIVER IN A SPENT FUEL
STORAGE POOL
Addressees
Holders of a facility license or construction permit issued for a power reactor
pursuant to 10 CFR Part 50.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to inform addressees of inadequacies in licensee control of work in a spent fuel
storage pool at a power reactor facility which resulted in a diver getting close
to very high radiation fields emanating from recently discharged spent fuel. It
is expected that recipients will review the information in this notice for
applicability to their facilities and consider actions, as appropriate, to avoid
similar problems. However, suggestions contained in this information notice are
not NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances
On April 3, 1997, the Calvert Cliffs Unit 2 facility, owned and operated by
Baltimore Gas and Electric Company (the licensee), was in Mode 6 with reactor
defueling on hold because of a malfunction of the Unit 2 fuel transfer system.
The fourth in a series of diving activities to effect repairs to the fuel
transfer system was conducted in the spent fuel pool. Previous dives had been
made in the refueling cavity to repair the system. The diver entered the spent
fuel pool at about 9:00 a.m. to commence work on an upender limit switch at the
south end of the fuel transfer area, the only surveyed and authorized work area.
The fuel transfer area runs the length of the west side of the Unit 2 spent fuel
pool. No wall or shield (other than the pool water) separates the area from the
fuel storage racks on the east side.
As with the previous dives, normal diving controls were specified by a licensee-
approved procedure and a job-specific radiation work permit. Multiple
thermoluminescence dosimeters (TLDs) were attached to the diver's wrists, head,
chest, back, and thighs and feet. Monitoring of the diver�s dose was provided
in real time with teledosimetry devices attached to his wrists, thighs (above
knee), chest, and back. The diver was also provided with two radiation detector
probes attached to a shaft approximately 76 cm (30 in) long for the purpose of
surveying his immediate vicinity. Each teledosimetry device was set to alarm at
the surface monitoring station on detecting an integrated dose of 1.0 mSv (100
mrem) or a dose rate of 8.95 mSv/hr (895 mrem/hr). Radiation protection (RP)
technicians continuously
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monitored the instrument readouts, and relayed the information through an
intercom to the diver who had no local indication of monitor readings or
alarms. Unlike previous dives into the refueling cavity which employed
underwater closed-circuit television (video) to visually monitor the diver, a
technician at the pool surface was assigned to observe the diver through a
floating window box during the fourth dive.
Following the repair of the limit switch, the diver asked for some materials
that he needed to complete the work. While he was waiting for the materials
to be lowered, the diver told the support team that he wanted to inspect a
kink in the upender cable. He was referring to a cable kink at the north end
of the pool. However, the RP technicians assumed that the diver was referring
to the cable in the authorized work area. Accordingly, the health physicist
(HP) technician approved the request, and the diver headed toward the north
end of the pool. At the north edge of the authorized work zone, the diver
inflated his diving suit, ascended and hovered above the pool floor to inspect
cabling. He then vented his suit, descended to the pool floor and continued
toward the north leaving the authorized area. The observer at the surface did
not detect the diver�s unauthorized entry into the north end of the fuel
transfer area because the vented air bubbles ascending through the pool water
obscured his view of the diver. The observer subsequently was distracted with
other duties and never regained visual contact with diver. Therefore, the
dive tender continued to provide cable and breathing air line to the diver
unchallenged.
Near the north end of the transfer system, the diver stopped to survey a pipe
on the west wall of the pool that he did not recognize. During the survey,
the monitors on the diver's right and left wrists alarmed and increased to 90
mGy/hr (9 rad/hr) and 23 mGy/hr (2.3 rad/hr) respectively. The RP technicians
instructed the diver to retreat to a lower dose area. The RP technician was
not aware that the diver had actually encountered the radiation field from
recently off-loaded spent fuel located in the racks on the east side of the
transfer area. Still believing that the diver was at the south end of the
pool, the RP technician instructed the diver to survey the area to locate the
source of the unexpected radiation. When the survey meter readout increased
to 30 mSv/hr (3 rem/hr), the dive was suspended. Only after the diver
surfaced, did the RP personnel realize that the diver had actually been in the
north end of the pool. The subsequent assessment of the event revealed that
the diver crossed about 4.6 meters (15 feet) of unsurveyed fuel transfer area
floor and came within a few feet of radiation dose rates ranging from 120 to
200 Gy/hr (12,000 to 20,000 rad/hr).
The diver's TLDs were subsequently processed, but not before he was allowed to
re-enter the radiation control area (RCA) to support another diving operation
as a standby safety diver. The licensee allowed the re-entry to the RCA prior
to dosimetry processing based on a preliminary assessment that the teledosime-
try readings indicated that the diver received no significant radiation
exposure.
Following TLD processing, the licensee calculated a maximum dose to the
extremities (right knuckles) of 8.85 mSv (885 mrem) based on a wrist TLD badge
shallow dose equivalent result of 4.24 mSv (424 mrem). The licensee also
calculated a dose of 2.7 mSv (270 mrem) to the highest exposed portion of the
whole body (arm above the elbow) as compared to a . IN 97-68
September 3, 1997
Page 3 of 5
maximum TLD reading on the head of 1.37 mSv (137 mrem). The maximum dose to
the lower extremity (ankle) was 0.021 mSv (21 mrem) shallow dose equivalent.
Discussion
The NRC has noted several deficiencies in the preplanning and controls
implemented to support the April 3, 1997, diving operations at Calvert Cliffs.
These include:
1. The scope of work was not clearly understood by all parties involved.
During the formal pre-job briefing, on the morning of the dive, it
was noted that the scope of work included an inspection of the
cable kink at the north end of the transfer mechanism �if radio-
logical conditions permitted.� The RP personnel in attendance
were not sure if the radiation survey made to support the dive
covered the north end of the pool. After the briefing, the RP
supervisor determined that the survey was limited to the south end
of the fuel transfer area and informed the dive engineering
support personnel that work in the north of the pool was not
authorized. No one gave this information to the diver or the dive
tender.
2. The diver was given inadequate instructions about the location and
magnitude of the radiation sources accessible to him.
A second communications failure took place at the dive site when
the RP technician briefed the diver on the radiation levels in the
work area. A map indicating the results of the radiation survey
was shown to the diver. However, this map was an enlarged view of
the south end of the transfer area. Due to a lack of perspective,
the diver believed he was being shown the radiation levels in the
entire pool. This mis-communication reinforced the diver's
incorrect understanding of the scope of the authorized work.
3. Positive control over the diver in the pool was inadequate.
Guidance on effective access control over divers in the spent fuel
pool is given in Regulatory Guide (RG) 8.38, "Control of Access to
High and Very High Radiation Areas in Nuclear Power Plants," and
in such industry standards as the Electric Power Research Insti-
tute's (EPRI's) "Underwater Maintenance Guide" (EPRI NP-7088-R2).
Appendix A to RG 8.38 discusses six areas of concern where control
needs to be exercised over diving operations. This list is a
compilation of the result of lessons learned from previous diving
events at nuclear power plants. The licensee failed to implement
effective controls in five of these six areas.
At the time visual contact with the diver was lost, the licensee
had, in effect, lost control of the dive. As stated in Section
1.5.4 of the EPRI guide, visual contact should be maintained
throughout the entire dive to be sure of the. IN 97-68
September 3, 1997
Page 4 of 5
diver's location and proximity to all known underwater radiation
sources. The licensee failed to recognize the significance of
maintaining visual contact with the diver. The inattentiveness
and lack of a questioning attitude by the dive support personnel
contributed significantly to the loss of control.
The licensee's investigation of this event determined that several
of the people involved did not clearly understand the scope of
their responsibilities for the planning and conduct of the diving
operation. One of the root causes identified was the practice at
Calvert Cliffs of providing management expectations on how a task
is to be performed in documents other than the formal job proce-
dure. For example, the licensee found that the individual
assigned to observe the diver did not understand that he was to
continuously observe the diver and thought that this task was
optional since it was not stated in the procedure. The require-
ment to maintain visual contact with the diver was in an RP Job
Coverage Standard instead of in a formal procedure. The licensee
is revising plant procedures so that they will contain all criti-
cal steps needed to exercise adequate controls over on-site work.
4. Licensee failed to adequately evaluate the diver�s exposure status
before authorizing additional work in the RCA.
Given the complexity of the diving environment, the licensee's
assessment of the diver�s dose based on the teledosimetry readings
was not sufficiently comprehensive. Teledosimetry located on the
diver�s thigh is not adequate to determine whether an overexposure
to the diver's extremity occurred during this event. The diver
could have received a high dose to his feet while walking across
the unsurveyed section of the pool floor without exceeding the
alarm setpoint on the thigh monitors because of the shielding
provided by the pool water. In addition, the estimate of the
whole body dose did not consider the possibility of exposure to
neutron radiation since the detectors were not sensitive to
neutrons. Subcritical spent fuel is a significant neutron source
due to alpha-n reactions and spontaneous fission of curium in the
fuel. In response to the NRC inspector�s questions, the licensee
subsequently determined that the diver would have to be within 0.6
meters (2 feet) of the fuel for neutrons to be a factor. The TLD
readings verified that the diver received no measurable neutron
dose.
Although it appears that the radiation doses received by the diver did not
exceed the dose limits given in 10 CFR Part 20, the breakdowns noted above
resulted in the diver being able to gain access to a very high radiation area
contrary to the requirements of 10 CFR 20.1602. During normal operations
spent fuel pools are neither high nor very high radiation areas since the
radioactive sources in them are usually covered by at least 3.3 meters (10
feet) of water and are thus considered inaccessible to personnel (see Regula-
tory Position 4.2 in RG 8.38). However, consistent with Regulatory Position
1.5 in RG 8.38, once an inaccessible area is made accessible, in this case by
conducting diving operations, the applicable controls for a high or very high
radiation area must be provided. This includes the access control . IN 97-68
September 3, 1997
Page 5 of 5
requirements of 10 CFR 20.1601 and 20.1602 as well as appropriate posting at
the entrance to the area consistent with the requirements of 10 CFR 20.1902.
The combination of an extremely intense radiation source and the very steep
dose gradients that can be encountered as a diver moves through his shielding
(water), make diving in areas where irradiated fuel can be accessed a uniquely
hazardous operation. Had the circumstances of this event been only slightly
altered, the diver could have been exposed to much higher dose rates. Even
with continuous teledosimetry monitoring, it is possible for a diver to
inadvertently enter a radiation field and receive a serious radiation dose, in
a matter of seconds. Establishing and maintaining proper effective controls
is critical to worker safety.
Related NRC Communications and Correspondence
The following related communications and correspondence are noted:
- NRC Information Notice 82-31, "Overexposure of Diver During Work in Fuel
Storage Pool," July 28, 1982
- NRC Information Notice 84-61, "Overexposure of Diver in Pressurized
Water Reactor (PWR) Refueling Cavity," August 8, 1984
- NRC Regulatory Guide 8.38, "Control of Access to High and Very High
Radiation Areas in Nuclear Power Plants," June 1993, Appendix A,
"Procedure for Diving Operations in High and Very High Radiation Areas"
- NRC Inspection Reports 50-317/97-02 and 50-318/97-02, May 29, 1997
- NRC Enforcement Action EA97-192 dated August 11, 1997
This information notice does not require any specific action or written
response. If you have any questions about the information in this notice,
please contact one of the technical contacts listed below.
signed by
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Ronald L. Nimitz, RI John R. White, RI
(610) 337-5267 (610) 337-5114
E-mail: rln@nrc.gov Email: jrw@brc.gov
Roger L. Pedersen, NRR
301-415-3162
E-mail: rlp@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
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