ENS 47310
ENS Event | |
|---|---|
13:54 Sep 30, 2011 | |
| Title | 24-Hour Nrc Bulletin 91-01 Report Involving a Loss of One Leg of Double Contingency |
| Event Description | Description: At 0854 CDT on 09-30-11, the Plant Shift Superintendent (PSS) was notified that water was observed in the #5 withdrawal position scale pit during the completion of the monthly test of the C-310 scale pit water detection system alarm module. The alarm module was being tested per procedure, when the module was found with the visual alarm (a red light) on at the local panel in the #5 withdrawal position room. In response to the alarm, the scale pit hatch was opened and the water detection sensor cable was observed to be at least partially submerged. Immediate investigation found the sump pump breaker to be tripped; when the breaker was reset the pump actuated and water was immediately removed. At the time of the occurrence, product withdrawal was in progress in the #3 and #4 withdrawal position room, and no cylinder was present in the #5 withdrawal position room. The source of the water was found to be a leaking steam condensate valve above the #5 withdrawal room ceiling. The water had drained to the concrete pad outside the building and then along the scale cart rails, eventually finding its way into the #5 scale cart pit where it gradually accumulated. Because the C-310 Scale Pit Water Detection Alarm did not function as credited, it is in violation of NCSE 032 (NCSA 310-004). Since one leg of double contingency was lost, this is being reported to the NRC as a 24-hour Event Report in accordance with NRC BL 91-01 Supplement 1.
The NRC Senior Resident Inspector has been notified of this event. PGDP Problem Report No. ATRC-11-2610; PGDP Event Report No. PAD-2011-17 SAFETY SIGNIFICANCE OF EVENTS: The safety significance of this event is low, even though the event made it possible for the level of pre-existing water to exceed the safe geometry limit. Although it is normal case for overall PGDP operations to have assay up to 5.5 wt.% 235U, the actual assay of product withdrawal operations during the period in question remained no higher than 2.0 wt. % 235U. At that actual assay, the depth of water necessary to support a criticality would have been more than 7.21 inches, which might have been credible but in itself would have remained a very unlikely possibility due to the slow ingress rate and high probability of detection and mitigation by personnel performing routine activities in that area. POTENTIAL CRITICALITY PATHWAYS INVOLVED (BRIEF SCENARIO(S) OF HOW CRITICALITY COULD OCCUR): In order for a criticality to happen, a significant breach in the process system integrity would have to occur. After the breach, fissile UF6 and its reaction products would have to react with pre-existing water to form fissile solution. There would have to be a sufficient depth of water in the pit to support a criticality (e.g. more than 3.68 inches of water at 5.5 wt. % 235U). CONTROLLED PARAMETERS (MASS, MODERATION, GEOMETRY, CONCENTRATION, ETC.): The two process conditions relied upon for double contingency are mass and geometry. ESTIMATED AMOUNT, ENRICHMENT, FORM OF LICENSED MATERIAL (INCLUDE PROCESS LIMIT AND % WORST CASE CRITICAL MASS): Product withdrawal assay at the time of the event was no higher than 2.0 wt% U235. However, no UF6 release occurred. NUCLEAR CRITICALITY SAFETY CONTROL(S) OR CONTROL SYSTEM(S) AND DESCRIPTION OF THE FAILURES OR DEFICIENCIES: Geometry is controlled in the second leg of double contingency by limiting the level of pre-existing water that might be present in the scale pit. Water accumulation is considered normal case in the NCSE by acute or by chronic sources. The NCSE credits the Scale Pit Water Detection Alarm to provide detection of chronic water accumulation in the scale pit. The alarm is set to actuate before the water level exceeds 2.5 inches in the pit. The geometry parameter limit is 3.68 inches assuming the worst-case possible enrichment of 5.5 wt.% 235U. The alarm is credited to provide early indication, and result in prompt mitigation, of water ingress to the pit before the NCS parameter limit is exceeded. Since the alarm was not functional, and the ingress rate was sufficiently slow that ingress was not easily detectable, there was no reliable means in place to detect and mitigate the ingress of water into the pit. The sensor and local panel light performed their intended function; however, it is the ACR audible and visible alarms that are controlled as AQ-NCS equipment and not the local panel light and buzzer. With the alarm out of service, continued ingress of water to the pit could have resulted in exceeding the geometry parameter limit for water depth before detection and mitigation. NCS entered the scale pit for inspection shortly after notification of the discovery and after the water had been drained. NCS observed that the water level at the lowest point in the pit may have reached 2.5 inches. Based on those inspections, it is likely that the water level remained below the 3.68-inch level, but there was no definitive way to prove the maximum height that might have occurred throughout the period of time when the alarm was not functional. Therefore, for conservatism it is assumed credible that the geometry parameter limit was violated during the lime the alarm was not functional. CORRECTIVE ACTIONS TO RESTORE SAFETY SYSTEMS AND WHEN EACH WAS IMPLEMENTED: The sump pump was immediately activated by resetting its breaker, which restored the water level in the pit below the 2.5 inch administrative limit, thus removing the hazard of this incident. The #5 withdrawal position scale pit will be checked twice per shift beginning on 9-30-11 in accordance with procedure due to the ACR alarm being out-of-service. |
| Where | |
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| Paducah Gaseous Diffusion Plant Paducah, Kentucky (NRC Region 2) | |
| License number: | Gdp-1 |
| Reporting | |
| Response | |
| Time - Person (Reporting Time:+10.87 h0.453 days <br />0.0647 weeks <br />0.0149 months <br />) | |
| Opened: | Billy Wallace 00:46 Oct 1, 2011 |
| NRC Officer: | Steve Sandin |
| Last Updated: | Oct 1, 2011 |
| 47310 - NRC Website | |
Paducah Gaseous Diffusion Plant with Response | |
WEEKMONTHYEARENS 483982012-10-10T18:05:00010 October 2012 18:05:00
[Table view]Response Double Contingency Control for Empty Cylinders Not Maintained ENS 479112012-05-08T21:16:0008 May 2012 21:16:00 10 CFR 76.120(c)(2), Response Autoclave High Pressure Isolation System Failure ENS 474832011-11-28T15:30:00028 November 2011 15:30:00 Response 24-Hour Report Concerning a Localized Loss of a Geometry Criticality Control Contingency ENS 473102011-09-30T13:54:00030 September 2011 13:54:00 Response 24-Hour Nrc Bulletin 91-01 Report Involving a Loss of One Leg of Double Contingency ENS 471762011-08-19T13:15:00019 August 2011 13:15:00 Response 24-Hr Loss of Criticality Control Report Required Under Bulletin 91-01, Supplement 1 ENS 465922011-02-03T15:30:0003 February 2011 15:30:00 Response 24 Hour Notification Under Bulletin 91-01 Concerning Leakage of Moderating Liquid ENS 463122010-10-06T16:06:0006 October 2010 16:06:00 Response 24 Hour Notification Under Bulletin 91-01 Concerning Cooldown Verification of Uf6 Cylinders ENS 461052010-07-17T19:07:00017 July 2010 19:07:00 Response 24 Hour 91-01 Bulletin Report on Loss of One of Two Criticality Control Contingencies ENS 454922009-11-12T17:15:00012 November 2009 17:15:00 Response Violation of a Nuclear Criticality Safety Control ENS 450422009-05-04T10:07:0004 May 2009 10:07:00 Response 24 Hour 91-01 Bulletin Report on Loss of One of Two Criticality Control Contingencies ENS 445582008-10-09T20:37:0009 October 2008 20:37:00 Response 24-Hours Bulletin 91-01 Report Involving Failure to Visually Inspect Storage Cylinders ENS 442232008-05-18T22:21:00018 May 2008 22:21:00 Response Violation of a Nuclear Criticality Safety Approval - 24-Hour Report ENS 441512008-04-17T21:44:00017 April 2008 21:44:00 Response 24-Hour Nrc Bulletin 91-01 Report Involving an Excessively Large Container Found in a Fissile Control Area ENS 441002008-03-26T19:57:00026 March 2008 19:57:00 Response 24 Hour Report of Degraded Criticality Control ENS 440562008-03-11T19:57:00011 March 2008 19:57:00 Response Two Cylinders Identified with Weight Change Prior to Wash ENS 432322006-09-23T03:00:00023 September 2006 03:00:00 Response Reported Under Nrc Bulletin 91-01 24 Hour Notification ENS 421652005-11-22T18:05:00022 November 2005 18:05:00 Response Improper Criticality Spacing for Waste Drum ENS 411582004-10-29T14:45:00029 October 2004 14:45:00 Response Criticality Control Report ENS 410202004-09-04T18:45:0004 September 2004 18:45:00 Response Violation of Nuclear Criticality Safety Control in the Process Building ENS 408732004-07-14T18:00:00014 July 2004 18:00:00 Response Criticality Control 24-Hour (Bulletin 91-01) Report ENS 408602004-07-08T20:50:0008 July 2004 20:50:00 Response 24 Hour Nrc Bulletin 91-01 Loss of Criticality Control ENS 407232004-05-03T21:00:0003 May 2004 21:00:00 Response Criticality Control 24-Hour (Bulletin 91-01) Report ENS 407002004-04-23T13:45:00023 April 2004 13:45:00 Response Criticality Control 24-Hour (Bulletin 91-01) Report ENS 408052004-04-23T10:53:00023 April 2004 10:53:00 Response Paducah Gaseous Diffussion Plant - Response Bulletin 91-01 24 - Hour Report ENS 406412004-04-04T19:30:0004 April 2004 19:30:00 Response 24-Hour Nrc Bulletin 91-01 Loss of Criticality Control Notification ENS 404552004-01-16T21:27:00016 January 2004 21:27:00 Response 24 Hour Notification for Bulletin 91-01 Loss of Criticality Safety Controls Involving Waste Drum ENS 404472004-01-13T19:00:00013 January 2004 19:00:00 Response 24 Hour Notification for Bulletin 91-01 Loss of Criticality Safety Controls Involving Waste Drum ENS 403562003-11-25T09:45:00025 November 2003 09:45:00 Response 24 Hour Nrc Bulletin 91-01 Notification Regarding Loss of Double Contingency Protection ENS 401512003-09-11T13:45:00011 September 2003 13:45:00 Response Double Contingency Was Not Maintained for Non-Fissile Nams. ENS 401392003-09-07T12:50:0007 September 2003 12:50:00 Response Criticality Controls Degradation at Paducah ENS 401092003-08-27T14:00:00027 August 2003 14:00:00 Response Procedural Deficiency Omission 2012-05-08T21:16:00 | |