05000133/LER-2010-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 06-24-2010 |
|---|---|
| Report date: | 07-14-2011 |
| 1332010001R01 - NRC Website | |
I. Plant Conditions
Humboldt Bay Power Plant (HBPP) Unit 3 was shut down in 1976. Spent fuel transfer to the Independent Spent Fuel Storage Installation (ISFSI) was completed in December, 2008. Unit 3 is currently undergoing decommissioning, with decontamination and dismantlement in progress.
II. Description of Problem
A. Background
On June 24, 2010, during the quarterly inventory of radioactive sources by HBPP radiation protection (RP) personnel, it was discovered that source number HBS-498 was missing. HBS-498 is a glass fiber filter paper with a mixed radionuclide gamma emitting composition. This source is used for specific radiation instrument calibrations. It was previously accounted for in the April 8, 2010, inventory. It is not definitively known that the source was not in use between the April and June inventories; however, to the best knowledge of RP personnel, it had not been used for any purpose between these inventories. An exhaustive search for the missing source was commenced following the discovery and was continued for approximately one month without success; however, the missing source was located in Unit 3 on February 1, 2011.
B. .Event Description
(i) A description of the licensed material involved, including kind, quantity, and chemical and physical form:
Source number IIBS-498 is a glass fiber filter paper, mixed radionuclide gamma source used for specific radiation instrument calibrations. The source activity was 0.35 micro-curies as of August 9, 2010.
The radionuclide composition of the source is as follows:
Nuclide� Activity (Curies) Am-241� 5.27E-08 Cd-109� 2.25E-07 Co-57� 2.27E-09 Ce-139� 4.96E-10 Hg-203� 6.08E-13 Sn-113� 4.08E-10 Cs-137� 1.90E-08 Y-88� 4.71E-10 Co-60� 2.41E-08 (ii) A description of the circumstances under which the loss or theft occurred:
On June 24, 2010, during the quarterly inventory of radioactive sources by HBPP RP personnel, it was discovered that source HBS-498 was missing. The source was previously accounted for in the April 8, 2010, inventory. It is not definitively known that the source was not in use between the April and June inventories, but to the best knowledge of RP personnel, it had not been used for any purpose between these inventories.
(iii) A statement of disposition, or probable disposition, of the licensed material involved:
Several scenarios were considered. Based upon both the circumstances surrounding the loss and the preliminary results of a PG&E Corporate Security investigation, it has been concluded that the two most likely scenarios are that the source was inadvertently disposed of as dry radioactive waste or that the source is still in the radiologically controlled area (RCA) in some yet to be identified location. Another scenario discussed, but considered less likely, was that an individual either inadvertently or intentionally exited the RCA with the source, which was on his/her person and went undetected.
The source was subsequently found on February 1, 2011 inside Unit 3, but outside the RCA, in a Control Technician (CT) Measurement and Test Equipment (M&TE) locker when a CT who was obtaining equipment from the locker observed it.
A Technical Review Group (TRG) met to investigate the discovery, but was unable to determine why the source had not been located previously, particularly when considering the frequency at which the locker is accessed and that the locker underwent a thorough search in the June 2010 timeframe. The TRG did confirm that a radiation survey of the M&TE locker was not performed during the search for the missing source. The TRG agreed that it would be appropriate for corporate security to revisit the investigation that was conducted following the report that the source was missing. The results of this further investigation did not give any insights into the re-appearance of the source.
(iv) Exposures of individuals to radiation, circumstances under which the exposures occurred, and the possible total effective dose equivalent (TEDE) to persons in unrestricted areas:
The calculated effective dose equivalent (EDE) is for the less likely scenario (i.e., an individual left the RCA with the source on his/her person and because the dose rate was so low it went undetected). The resultant TEDE would be several millirem with the assumptions made about time and source location on the individual.
MicroShield was used to finalize the dose assessment above by assuming that the source, which was enclosed in an approximately three-inch diameter plastic case, remained on the person in direct skin contact for a period of 48-hours. The DDE calculation yielded a dose rate of 0.21 mrem/hr to the 1 cm depth, and a DDE of 10 mrem. The EDE calculation yielded a dose rate to the abdomen (the highest EDE compartment) of 0.00033 mr/hr and an EDE/TEDE of 0.001584 mrem.
(v) Actions that have been taken, or will be taken, to recover the material An exhaustive search for the missing source was commenced and continued for approximately one month. Multiple locations were checked and rechecked without success. The search included item-by-item inspection of the contents of numerous bags of dry radioactive waste. A portable gamma spectrometry system (the Canberra In Situ Object Counting System (ISOCS)) was utilized in searching the dry radioactive waste. It was hoped the source would be found by identifying its Cadmium-109 gamma, which is not a constituent of the HBPP waste stream. This proved unsuccessful. No record that the source had been retired was located, nor was the source found in the source disposal drum.
(vi) Procedures or measures that have been, or will be, adopted to ensure against a recurrence of the loss or theft of licensed material:
Unit 3 Radiation Control Procedure RCP-6D, "Inventory and Controls for Radioactive Sources," was revised to require the following:
- Locking sources in designated storage lockers when not in use
- Logging sources in and out
- Peer checks when logging sources in anctout C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event: NA D. Other Systems or Secondary Functions Affected: None
E. Method of Discovery
Performance of the quarterly source inventory per Unit 3 Radiation Control Procedure RCP-6D, "Inventory and Controls for Radioactive Sources.
F. Operator Actions: None G. Safety System Responses: None
III. Cause of the Problem
A. Immediate Cause
Radioactive Source HBS-498 could not be located during the performance of the quarterly inventory on June 24, 2010.
B. Root Cause
Radiation Protection procedures were inadequate to ensure control of radioactive sources.
IV. Assessment of Safety Consequences
There are no safety consequences as a result of this event because of the minor dose consequences to any individual who might have removed the source from the RCA, as assumed in the initial dose assessment.
The source was subsequently found on February 1, 2011 inside Unit 3, but outside the RCA as assumed in the initial dose estimate. The source was located in a Control Technician (CT) Measurement and Test Equipment (M&TE) locker when a CT obtaining equipment from the locker observed it. The final EDE calculation yielded an EDE/TEDE of 0.001584 mrem.
Therefore, it can be concluded that the dose consequences to any individual woilld have been negligible.
V. Corrective Actions
A. Immediate Corrective Actions
HBPP sources were locked in storage lockers and a policy to log sources in and out was established. This event was discussed with Radiation Protection and Instrument Technicians, as were the new policy requirements to store and lock sources in the designated storage lockers when not in use, and to log sources out and in prior and following use, respectively.
B. Corrective Actions to Prevent Recurrence
Procedure RCP-6D, "Inventory and Controls for Radioactive Sources," was revised to require the following:
- Locking sources in designated storage lockers when not in use
- Logging sources in and out
- Peer checks when logging sources in and out
VI. Additional Information
A. Failed Components: NA
B. Previous Similar Events
License Event Reports:
18-inch segments of a single spent fuel rod. An extensive investigation did not definitively locate these segments. Due to the wide disparity in radiation levels between the two types of material (fuel rod segments versus a calibration standard), the more generic issue of radioactive "source" control was not identified during the cause analysis of the 2003 event.
Notifications (Problem Reports):
1204330 (2/4/1999): A Co-60 whole body counter check source was inadvertently removed from the site by a radiation protection technician. The source was normally used outside of the RCA. No potentially generic impacts to sources located within the RCA were identified.
1206771 (8/19/1999): A whole body counter source was missing. The source was not marked as "radioactive" and was discarded in the trash, where it was later recovered. Corrective actions were limited to notification of the vendor and marking the source as radioactive.
C. Industry Reports
INPO Nuclear Network Event OE 22090 (12/27/2005): "Loss of Radioactive Sources due to Inadequate Administrative Controls" Five radioactive sources, including two Am-241 sources, were discovered missing during an annual inventory. There were no administrative controls, such as logging out or in of sources, for RP use