ML20198J503
| ML20198J503 | |
| Person / Time | |
|---|---|
| Site: | Framatome ANP Richland |
| Issue date: | 10/16/1997 |
| From: | Maas L SIEMENS POWER CORP. (FORMERLY SIEMENS NUCLEAR POWER |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-91-001, IEB-91-1, LJM:97:087, LJM:97:87, NUDOCS 9710210074 | |
| Download: ML20198J503 (3) | |
Text
. -.
- ' ' SIEMENS October 16,1997 LJM:97:087
- U.S. Nuclear Regulatory Commission Document Control Desk
](y' ( 7 g p Washington, DC 20555 -
Gentlemen:
Subject:
Followup to NRC Operations Center Report No. 32949 - Bulletin 91-01 Report of Discovery of Moisture in a Moderator - Controlled Process
- On September 17,1997, Siemens Power Corporation (SPC) reported an incident which met the 4-hour reporting criteria of NRC Bulletin 91-01. The incident involved the discovery of excessive levels of moisture in a moderator-controlled piece of process equipment, specifically a pyrohydrolysis vesselin the Dry Conversion Pilot Plant at SPC's Richland, Washington nuclear fuel fabrication facility. SPC internal procedures require a 30-day follow-up report to all Bulletin 91-01. reportable conditions or events. This letter meets this requirement.
Backoround One of the operating processes in SPC's nuclear fuel fabricating facility is a Dry Conversion" pilot plant. A primary c.smponent used in this process is a pyrohydrolysis vessel. This vesselis used to convert UF. to UO by reacting the UF with steam and other gases at elevated 2
temperatures. This vessel has two significantly different sections. The lower section of the vessel contains the majority of the material during processing and is favorable geometry while the upper section contains very little material and is not favorable geometry. Controlling the-amount 'of moderator in the vessel and ensuring an adequate temperature before steam is supplied to the vessel are the key criticality safety controls for this process vessel. The process
. design is intended to limit the material in this vessel to less than 1.0 wt% moisture.
Incident.Descriotion and immediate Corrective Actions At approximately 2150 hours0.0249 days <br />0.597 hours <br />0.00355 weeks <br />8.18075e-4 months <br /> on Tuesday, September 16,1997, following some maintenance activities, a technician observed moist-looking powder inside a disassembled spool piece which connects the Dry Conversion Pilot Plant pyrohydrolysis vessel and rotary airlock (RAL) to the screw conveyor that transports powder from the pyrohydrolysis vessel discharge to the calciner
- feed hopper. Operations sent samples of this material along with other samples of powder
/ O removed from the vessel earlier in the shift to the SPC Analytical Services Laboratory for
[5]
Siemens Power Corporation d
Nuclear Division 2101 Horn Rapids Road Tel:
(509) 375-8100 Engineering & Manufacturing P.o. Box 130 Fax:
(509) 375-8402 Richland, WA 99352-o130 lfll h
9710210074 971016 PDR ADOCK 07001257l' C
PDR.
U.S. NRC LJM:07:087 October 16,1997 Page 2 moisture content analysis. Sample results received on September 17,1997 at 0930 hours0.0108 days <br />0.258 hours <br />0.00154 weeks <br />3.53865e-4 months <br /> indicated that some of the material removed from the lower portion of the pyrohydrolysis vessel exceeded the 1 wt.% moisture limit (all sample results indicated moisture levels of less than 2 wt.% moisture).
The pyrohydrolysis vessel was shut down and in the process of being cleaned out at the time the evidence of excess moisture was observed. Operations contacted Criticality Safety and was instructed to halt all pilot plant operations pending management / Criticality Safety direction.
Subsequent to receipt of analytical results confirming excess moisture levels in the pyrohydrolysis vessel, a notification call was made to the NRC Operations Center per the requirements of Bulletin 91-01. This notification was made based on the initial indication, which was subsequently confirmed (see Incident Causes, below), that the moisture was introduced via a pathway not evaluated in the applicable criticality safety analysis (CSA).
At no time were conditions approached that would have made a criticality event possible. The material in the pyrohydrolysis vessel was only marginally over the 1 wt.% moisture limit (1.09 wt.% in the vesselitself). The lower portion of the pyrohydrolysis vesselis geometrically safe and SPC CSAs show that the upper portion of the pyrohydrolysis vessels at this facility would be substantially sub-critical (K,n < 0.93) when completely filled with UO, at 20 wt% moisture (over 200 kg of water). Corrective actions to re-establish double contingency were to simply remove any remaining material from the vessel that could exceed the moderator limit.
Incident Causes This incident was investigated by a formal SPC incident Investigation Board (llB) utilizing the Taproot
- analysis methodology. The Board confirmed the unanalyzed moisture migration pathway underlying the decision to notify the NRC per Bulletin 91-01, i.e. the backflow of calciner steam to the pyrohydrolysis vessel via the interconnecting screw conveyor. This backflow was facilitated by a negative pyrohydrolysis vessel pressure relative to the calciner and the fact that the pyrohydrolysis vessel was in a shutdown condition, thereby offering cool internal surf aces for moisture condensation. Underlying root causes were identified as two fold.
Firstly, controls were not properly identified due to the faulty operational assumption that a residual " powder plug" in the interconnecting transfer conveyor would mair'tain a seal between the pyrohydrolysis vessel and the calciner, keeping calciner gases from backflowing to the pyrohydrolysis vessel. Secondly, procedures contained ambiguous instructions in that in at least one location they appeared to allow for calciner purge gases to be left on while the pyrohydrolysis vessel was in a shutdown (cool) and ventilated condition.
Followun Corrective Actions The Dry Conversion Pilot Plant is currently shut down and will not be restarted until proper corrective actions havu been completed. These corrective actions as imposed by the llB include (1) evaluation of engineered barriers to prevent recurrence of drawing moisture back into the pyrohydrolysis vessel from the calciner. These engineered barriers would likely be similar to the barriers being applied in SPC's new Dry Conversion Facility, which are considered to be adequate; and (2) revision of all Dry Conversion Pilot Plant Standard Operating Procedures to
c., 4. ' !
A U.S. NRC -
LJM:9"':087
. October 18,1997 -
Page 3 reflect the engineered barriers and to provide clarification of any ambiguous operational steps.
-Prior to restart the Dry Conversion Pilot Plant CSA will also be appropriately rewed to fully reflect the incident pathway implicated by this event and any criticality safety barriers
= implemented to address this contingency.
Generic Imolications Locking beyond the Dry Conversion Pilot Plant itself, the llB considered the following generic implication issues, namely:
Are there any vessels in SPC's ammonium diuranate (ADU) conversion and powder -
e preparation areas where steam is injected that are connected to cold vessels where condensate could form?
~ What are the differences between the Dry Conversion Pilot Plant and SPC's new Dry j-
. Conversion Facility that will preclude this from occurring in the new facility?.
. As documented in the llB report, the' generic implications investigation failed to identify any plausible situations in SPC's ADU facilities where steam injected into one vessel will cause moisture to be present in another vessel which would result in a criticality safety concern.
Furthermore, the investigation identified the existence of ample engineered barriers in the new
.f acility to preclude this scenario from occurring.
If you have c,uestions regarding this incident or SPC's responses to it, please contact me on 509-375-8537.
Very truly yours,;
vhW W
L. J. Maas, Manager Regulatory Compliance
/pg Enclosures -
cci E. W. Merschoff, NRC Region IV, Arlington TX F. A. Wenslawski, NRC Region IV, Walnut Creek Field Office
.