ENS 47310: Difference between revisions

v t e 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.

ENS 47310 +/-
Where
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
v • d • e

v t e Emergency Notification System
Paducah Gaseous Diffusion Plant with Response
WEEKMONTHYEAR2012-05-08T21:16:00 [Table view]
[ENS list for Paducah Gaseous Diffusion Plant]

@@ Line 16: / Line 16: @@
 | event date = 09/30/2011 08:54 CDT
 | last update date = 10/01/2011
-| title = 24-Hour Nrc Bulletin 91-01 Report Involving A Loss Of One Leg Of Double Contingency
+| title = 24-Hour Nrc Bulletin 91-01 Report Involving a Loss of One Leg of Double Contingency
 | event text = 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

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