ENS 44711
ENS Event | |
|---|---|
21:30 Dec 9, 2008 | |
| Title | As-Built Depth of Low Level Dissolver System Dissolver Trays Exceeded Maximum Design Depth |
| Event Description | Event Description: Planned upgrades to the Low Level Dissolver System in the BWXT's Uranium Recovery Facility were undergoing a safety review as part of the change management process. One of the upgrades involved replacement of the dissolver trays to utilize trays of the same length, width, and depth throughout the dissolution process. During the evaluation process a Recovery process engineer questioned differences between the tray heights in the current safety basis and the change review documentation. The ISA summary listed the tray geometry as an Item Relied on For Safety (IROFS). This limit was the same as that discussed in the text of the change management documentation.
Once it was determined that the discrepancy in the documentation also represented an as-built condition in the field, the Low Level Dissolution process was immediately shutdown pending further evaluation of the safety concern. The IROFS tray geometry was degraded. Evaluation of the Event: A review of related safety release documentation indicated the original trays installed in 1999 met the NCS requirements. The requirements limited the tray or allowed modification of the tray (drilling holes, cutting slots) to limit the solution height. Due to the corrosive nature of the process, the trays degrade over time and are replaced when a minimum wall thickness is achieved. The replacement trays were intended to be of like kind. The current fabrication drawing attached to the change management documentation shows no additional features to limit the solution height. The initial review of the accident scenario for a geometry upset indicated that although an IROFS was degraded, other IROFS (operator control of mass, operator control of moderator, and poisoning by the tray materials of construction) remained available to assure double contingency. This is a qualitative evaluation that does not analyze the magnitude of the upset condition. The original NCS analysis of the Low Level Dissolver trays conducted in 1999 evaluated tray height; the impact of a range of tray heights was not considered. The as-found condition in the field, including tray height, was subsequently modeled using BWXT's validated NCS codes and approved methodology. The normal operating condition was determined to be within the k-effective limit of 0.92 in BWXT's NRC License SNM-42 for a high enriched uranium (HEU) system. When a fully flooded moderation upset condition was analyzed, although the resulting k-effective was shown to be subcritical, it exceeded the safety limit of 0.95 in BWXT's license for a HEU system. This determination was reached at 4:30 pm on December 9, 2008. There was no immediate risk of a criticality or threat to the safety of workers or the public as a result of this event. A fully flooded condition in the Low Level Dissolver is extremely unlikely. The equipment is located on a mezzanine, well above the main process area. There are no water sources (e.g., sprinklers) in the area, and the dissolvers are not contained in water tight enclosures. However, with the degradation of the IROFS of tray height, and demonstration that the as-found condition exceeded the license limit for a single upset condition, double contingency could no longer be assured. Notification Requirements: BWXT is making this 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> report in accordance with 10 CFR 70, Appendix A, (b)(2) loss or degradation of items relied on for safety that results in failure to meet the performance requirement of 70.61. Status of Corrective Actions: An investigation of the root causes of this event is ongoing. Corrective actions will be determined as a result of the investigation. The Low Level Dissolution process remains shutdown. The licensee documented this event in their condition report CR#1027010. The licensee notified the NRC Resident Inspector.
The analysis of a fully flooded moderation upset condition showed the resulting k-effective exceeded the safety limit of 0.95 in BWXT's license for a HEU system showed a k-effective of 0.96. Notified R2DO (Ernstes), NMSS (Habighorst), and Fuels Group (via e-mail).
This report was made based on the assumption that the tray dimensions on the drawing were representative of as-built tray dimensions in the field. Subsequent to the report, measurements were taken of the Low Level Dissolver trays in use at the time of discovery. Using the same methodology used to determine the initial reportability of the event, the moderation upset was reevaluated for the as-built tray dimensions. The resulting k-effective was 0.94, which is below the safety limit of 0.95 in BWXT's license for a HEU system. Evaluation of the as-built tray dimensions demonstrated although there was a loss or degradation of Items Relied On For Safety, BWXT did not fail to meet the performance requirement of 10 CFR 70.61. BWXT is therefore withdrawing the 10 CFR 70, Appendix A, (b)(2) notification, #44711. The licensee notified the NRC Resident Inspector. Notified R2DO (Hopper), NMSS EO (Benner), and Fuels OUO Group (via email). |
| Where | |
|---|---|
| Bwx Technologies, Inc. Lynchburg, Virginia (NRC Region 2) | |
| License number: | SNM-42 |
| Reporting | |
| Part 70 App A (B)(2) | |
| Time - Person (Reporting Time:+18.45 h0.769 days <br />0.11 weeks <br />0.0253 months <br />) | |
| Opened: | Barry Cole 15:57 Dec 10, 2008 |
| NRC Officer: | Donald Norwood |
| Last Updated: | Dec 23, 2008 |
| 44711 - NRC Website | |
Bwx Technologies, Inc. with Part 70 App A (B)(2) | |
WEEKMONTHYEARENS 447112008-12-09T21:30:0009 December 2008 21:30:00
[Table view]Part 70 App A (B)(2) As-Built Depth of Low Level Dissolver System Dissolver Trays Exceeded Maximum Design Depth ENS 440352008-03-05T13:00:0005 March 2008 13:00:00 Part 70 App A (B)(2) Vacuum Daily Inspections Not Completed 2008-03-05T13:00:00 | |