05000339/LER-2014-002
| North Anna Power Station | |
| Event date: | 09-15-2014 |
|---|---|
| Report date: | 1-1-2014 |
| 3392014002R00 - NRC Website | |
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1.0 DESCRIPTION OF THE EVENT At 0900 on September 15, 2014, with Unit 2 defueled, debris that had the potential to be fuel fragments was located on the core plate (EIIS System — AC) directly below the B11 core location. Ten pieces of material, approximately 1/8" in diameter, were found. The material was near the edge of the outer flow hole and partially under the gap between the baffle plate and the core plate. Fuel assembly (EIIS System — AC) 4Z9 was located at the B11 location during Cycle 23. Video inspection of fuel assembly 4Z9 identified that the top springs of two fuel pins were dislodged.
Due to the fact that the fuel damage exceeded expected conditions, at 1454 on September 15, 2014, this event was reported as an eight hour report as per 10 CFR 50.72(b)(3)(ii)(A), any event or condition that results in the condition of the nuclear plant, including its principle safety barriers, being seriously degraded.
Detailed video inspections estimated that fifteen (15) fuel pellets were dislodged from fuel assembly 4Z9. For reference, the reactor core contains approximately 15 million fuel pellets. During efforts to identify and recover the fuel pellets, debris fragments estimated to represent five (5) fuel pellets were found in the damaged fuel assembly that is currently in the Spent Fuel Pool (SFP) (EIIS System — DA). In addition, an estimated three (3) pellets worth of material was retrieved by the foreign object search and retrieval (FOSAR) efforts in the reactor vessel and are now located in the SFP. The remaining seven (7) fuel pellets have already or are expected to granulate into fine particles that will dissolve in low flow areas of the primary plant systems or be removed by normal purification processes. However, since the specific location of the seven (7) fuel pellets is undesignated, a report was made at 1227 on September 30, 2014, pursuant to 10 CFR 74.11(a) for the loss of special nuclear material (SNM). At that same time, a report was made pursuant to 10 CFR 20.2201(a)(ii) because the seven (7) fuel pellets contain licensed material in a quantity greater than 10 times the quantity specified in Appendix C of 10 CFR 20.
10 CFR 20.2201(b) requires a written report after the initial notification for the occurrence of any lost, stolen, or missing licensed material that was reported under 10 CFR 20.2201(a)(ii) for licensed material in a quantity greater than 10 times the quantity specified in Appendix C of 10 CFR 20. The following topics are required to be addressed:
(i) A description of the licensed material involved, including kind, quantity, and chemical and physical form:
Fuel Pellet Description — Based on the review of the video of the recovered material, the possibility that these fuel pellets have remained intact is very low.
North Anna Power Station Unit 2 05000339 Type of Special Nuclear Material Uranium dioxide pellets initially enriched to 4.45% Length of fuel pellet 0.4 inches nominal Pellet diameter 0.3225 inches Total Uranium in the 7 fuel pellets 32.3 grams (Sept 2014) Total Uranium 235 in the 7 fuel pellets 0.4 grams (Sept 2014) Total Plutonium in the 7 fuel pellets 0.4 grams (Sept 2014) Total Fissile Plutonium in the 7 fuel pellets 0.3 grams (Sept 2014) Activity Level 266 Ci Average Burnup of Assembly 4Z9 46733 MWD/MTU Effective Full Power Days (EFPD) of Assembly 4Z9 1160 EFPD (ii) A description of the circumstances under which the loss or theft occurred:
The fuel pellet loss occurred as a result of baffle jetting on the fuel assembly. The affected fuel rods had their top springs dislodged and fuel pellets were able to escape the fuel rod. Fragments of fuel pellets were found within the associated fuel assembly and on the core plate. However, about seven (7) fuel pellets worth of material were not located and have already or are expected to granulate into fine particles that will remain in low flow areas of the primary plant systems or be removed by normal purification processes. The possibility of theft is not plausible because of the plant's radiation monitoring instrumentation, physical security measures, and the size and type of container required for transporting nuclear material of this nature.
(iii) A statement of disposition, or probable disposition, of the licensed material involved:
During efforts to identify and recover the fuel pellets, debris fragments estimated to represent five (5) fuel pellets were found in the damaged fuel assembly that is currently in the SFP. In addition, an estimated three (3) pellets worth of material was retrieved by the FOSAR efforts in the reactor vessel and are now located in the SFP. The remaining seven (7) fuel pellets have already or are expected to granulate into fine particles that will dissolve in low flow areas of the primary plant systems or be removed by normal purification processes.
(iv) Exposures of individuals to radiation, circumstances under which the exposures occurred, and the possible total effective dose equivalent to persons in unrestricted areas:
No unauthorized exposure to radiation occurred to the plant staff or members of the public because the fuel pellet fragments either remain in the SFP or granulated into fine particles that will dissolve in low flow areas of the primary plant systems or be removed by normal purification processes.
(v) Actions that have been taken, or will be taken, to recover the material:
During efforts to identify and recover the fuel pellets, debris fragments estimated to represent five (5) fuel pellets were found in the damaged fuel assembly that is currently in the SFP. In addition, an estimated three (3) pellets worth of material was retrieved by the FOSAR efforts in the reactor vessel and are now located in the SFP. The remaining seven (7) fuel pellets have already or are expected to granulate into fine particles that will dissolve in low flow areas of the primary plant systems, or be removed by normal purification processes.
(vi) Procedures or measures that have been, or will be, adopted to ensure against a recurrence of the loss or theft of licensed material:
Westinghouse fabricated and delivered a low-enrichment RFA-2 fuel assembly armored with seven (7) stainless steel rods in place of fuel rods which could be affected by jets from baffle gaps for core location B11 in cycle 24. A similar modification to that of the reactor vessel upflow conversion design change that was performed on Unit 1, DC NA-95-001, will be developed and implemented on Unit 2.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS No significant safety consequences resulted from this event because the reactor coolant system activity levels during Unit 2 Cycle 23 were well within the requirements of Technical Specification (TS) 3.4.16, Reactor Coolant System Specific Activity. After Cycle 24 startup, the activity remains well within the requirements of TS 3.4.16. The health and safety of the public were not affected by this event.
3.0 CAUSE The direct cause of the event was due to baffle jetting. Baffle jetting is the process by which water on the outside of the core baffle plate is forced through small openings in the baffle seams and onto the fuel assemblies. During Unit 2 Cycle 23, baffle jetting caused two rods in assembly 4Z9, located in core position B11, to begin rotating and vibrating. This movement resulted in fuel rod wear and eventual mechanical failure and rod separation. Once separated, a maximum of 15 fuel pellets were released from the two affected fuel rods. The Root Cause of the failed fuel assembly was the change in material properties of the baffle plates and bolting due to aging mechanisms resulting in the gap widening at the baffle joint. Stress, temperature, and irradiation since initial plant start-up have resulted in relaxation, creep, and loss of pre-load in the bolting and baffle plates. The changes in material properties allowed the gap in the corner baffle joint, adjacent to location B11, to widen when subjected to the relatively high differential pressure, approximately 25 psi, associated with the baffle-barrel downflow configuration in North Anna Unit 2.
4.0 IMMEDIATE CORRECTIVE ACTION(S) Westinghouse fabricated and delivered a low-enrichment RFA-2 fuel assembly armored with seven (7) stainless steel rods in place of fuel rods which could be affected by jets from baffle gaps for core location B11 in cycle 24. Visible debris on the core plate from 4Z9 was found and retrieved. An inspection of the baffle was performed with no anomalies noted. An inspection of the fuel assembly that was previously located at B11 for fuel cycle 22 was performed and no indications of baffle jetting were noted. An inspection was performed of other cycle 23 fuel assemblies in other baffle locations for baffle jetting damage. Ten of the other assemblies exhibited some indications at the center injection locations ranging from slight marks on a mid-grid adjacent to rod 15 or rod 3 to some slight surface erosion or buffing of the grid at the same locations. Other than being in the proper location for where center injection would be expected to occur, it was not clear whether the indications were due to baffle jetting or to some other interaction such as fuel handling or wear from a bowed assembly rubbing against the baffle plates. The indications were reviewed by Dominion's Nuclear Analysis and Fuel (NA&F) Fuel Performance Analysis (FPA) group, and it was determined that no further action was required. Both AREVA and Westinghouse reviewed the video of 4Z9 and concluded that the cause was baffle jetting. A revised Reload Safety Evaluation (RSE) incorporating the replacement fuel assembly for location B11 was completed and approved. An Operability Determination, OD000600, was completed for baffle jetting.
5.0 ADDITIONAL CORRECTIVE ACTIONS No additional corrective actions were identified by the Root Cause Team.
6.0 ACTIONS TO PREVENT RECURRENCE A modification similar to the reactor vessel upflow conversion design change that was performed on Unit 1, DC NA-95-001, will be developed and implemented on Unit 2.
7.0 SIMILAR EVENTS Unit 2 has operated without indications of baffle jetting for 34 years and Unit 1 has operated without baffle jetting since 1996 when the upflow conversion was performed.
While Unit 1 did have baffle jetting issues prior to 1996, the baffle jetting issues were from the center joints. Whereas the Unit 2 baffle jetting was from a corner joint.
Additionally, Unit 2 has a different bolting configuration that made it less susceptible to the baffle jetting experienced on Unit 1.
8.0 ADDITIONAL INFORMATION Unit 1 continued operating in Mode 1, 100 percent power during this event.