ML20248H138
| ML20248H138 | |
| Person / Time | |
|---|---|
| Site: | Paducah Gaseous Diffusion Plant |
| Issue date: | 06/01/1998 |
| From: | Polston S UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GDP-98-1035, NUDOCS 9806080074 | |
| Download: ML20248H138 (4) | |
Text
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USEC A Clobal Energy Company June 1,1998 GDP 98-1035 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001 Paducah Gaseous Diffusion Plant (PGDP)
Docket No. 70-7001 l
Event Report ER-98-08 l
Pursuant to 10 CFR 76.120(d)(2), enclosed is the required 30-day written report for the C-360 Water Inventory Control System (WICS) actuation. This was initially reported on May 4,1998 (NRC No.
34177).
There are no new commitments identified in this report. Any questions regarding this matter should be directed to Larry Jackson at (502) 441-6796.
S'ncerely, Steve Polston General Manager Paducah Gaseous Diffusion Plant
Enclosure:
As Stated cc:
NRC Region 111 Office NRC Resident Inspector - PGDP 9906060074 990601 PDR ADOCK 07007001 0,
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P.O. Box 1410, Paducah, KY 42001 Telephone 502 441-5803 Fax 502-441-5801 http://www.usec.com Offices in Livermore, CA Paducah, KY Portsmouth, OH Washington. DC
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Docket No. 70-7001 GDP 98-1035 Page1of3 EVENT REPORT ER-98-08 DESCRIPTION OF EVENT The C-360 autoclave Water Inventory Control System (WICS) provides the means to limit the water inside an autoclave such that upon an accidental UF release from a UF cylinder, overpressurization 6
6 of the autoclave or a nuclear criticality cannot occur. This is accomplished by two electrically powered ultrasonic probes located slightly below the autoclave in the three-inch drain line. When the condensate level in the autoclave drain system covers these probes, they send a signal to remove air from two steam isolation valves and the vent steam valve. Isolation of steam from the autoclave removes the source of water. The WICS is designated as a "Q" safety system and is required to be operable by Technical Safety Requirement 2.1.4.3 when in Mode 5 (heating). In C-360, unlike the autoclaves in the C-333-A and C-337-A feed vaporization facilities, autoclaves No. I and No. 2 share components of a common steam supply and condensate drain system, as do autoclaves No. 3 and No. 4. These autoclave pairs which share common steam / drain system components will be referred to as companion autoclaves.
On May 3,1998 at 2157 hours0.025 days <br />0.599 hours <br />0.00357 weeks <br />8.207385e-4 months <br />, steam was applied to autoclave No. I to heat cylinder AK-1765 for sampling. Similarly, at 2258 hours0.0261 days <br />0.627 hours <br />0.00373 weeks <br />8.59169e-4 months <br />, steam was applied to companion autoclave No. 2 to heat cylinder AK-793. A one hour minimum delay is required by procedure for companion autoclaves because the steam and drain systems have some common components. The intent of the time delay is to reduce the condensate load on the common drain components to help prevent a WICS actuation.
At 2330 hours0.027 days <br />0.647 hours <br />0.00385 weeks <br />8.86565e-4 months <br />, a high drain alarm was received on autoclave No. 2 (WICS actuation). Both primary and secondary alarms were received, which indicates that this was not a spurious actuation. All components functioned per design and alarm response procedure CP4-CO-AR8360 was entered.
In accordance with Safety Analysis Report Table 6.9-1, Criteria J.2, NRC was notified of this safety system actuation on May 4,1998 (Reference Notification Worksheet No. 34177).
CAUSES OF EVENT A.
Direct Cause 1
The condensate drain system requires positive autoclave steam pressure in order to force condensate out the drain during steam heating of the cylinder. Gravity flow in itselfis not suflicient to expel the condensate from the autoclave. The direct cause of the WICS actuation was insufficient positive autoclave steam pressure to force the condensate through the drain system, allowing the condensate to reach the probe level. The WICS actuation occurred approximately 32 minutes into the heat cycle. According to a review of the autoclave pressure i
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c Docket No. 70-7001 GDP 98-1035 Page 2 of 3 strip chan, the WICS actuation occurred after the autoclave steam pressure peak. Thus, full condensate flow with dropping autoclave pressure led to a level increase in the drain piping causing a WICS actuation.
B.
Root Cause Preliminary indications an: that the root cause of the insufficient positive pressure was a wrong size orifice on the temperature control valve, TCV-217. Drawing 15B-14443-K24 requires TCV-217 to have a flow coefficient, C,, of 20, which corresponds to an orifice size of one inch.
The steam control valve was removed for inspection from autoclave No. 2. The steam contml
. valve is a Masoneilan, two-inch body, Model 35-35212. The diameter of the installed seat orifice measured about %-inch. Based on conversations with the vendor, a %-inch orifice provides a C, of only 10, in contrast to the required C, of 20.
A review of strip charts indicates that the steam pressure characteristics of autoclave No. 2 are significantly different from properly operating autoclaves during the heat cycle. A typical heat cycle for properly operating autoclaves in C-360 begins with the steam contml valve going to the full open position. Withinjust a few minutes, the autoclave pressure reaches 5 psig. As temperature approaches 220 F, the control valve will start to close. In contrast, on autoclave No. 2, a typical heat cycle begins with the steam control valve opening fully, bringing the autoclave pressure to approximately 2.5 psig. The pressure increases slowly from this point and i
' may even decrease. After approximately 30 minutes, the peak pressure of 5 psig is reached, and the control valve starts to close as the temperature approaches 220" F.
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'Ihe exact time when this valve with the incorrect orifice was installed could not be determined.
Previous C-360 autoclave system engineers, maintenance front line managers, and maintenance L
mechanics were interviewed and do not remember this valve being replaced within the last several years. A search of the Maintenance data base, which reflects maintenance work back 1
to late 1996, showed no maintenance records on this valve. Therefore, the cause of the incorrect valve orifice size is' attributed to lack of formality in work control practices and configuration management at the time ofinstallation. Under current procedures, procurement i
. of the "Q" valve would be controlled by an Engineering Specification Data Sheet and repair of the valve would be controlled by a~ work package and quality control inspections. Temperature control valve, TCV-217, on autoclave No. 2 was replaced with the proper one-inch orifice valve. Following replacement of the valve, steam was applied to autoclave No. 2 without a-cylinder in the autoclave; the strip chart showed a pressure curve indicative of a normally operating temperature control valve. Autoclave No. 2 was declared operable on June 1,1998.
h An inspection of C-360 autoclaves No. I and No. 4 revealed that their steam control valves also
. had a %-inch orifice, rather than the required one-inch orifice. The temperature control valve
l Enclosure I l
Docket No. 70-7001 GDP 98-1035 l
Page 3 of 3 on C-360 autoclave No. 3 will also be inspected. The temperature control valves on autoclaves l
No. I and No. 4 have been replaced with the proper one-inch orifice valve. Autoclave No. I was declared operable on June 1,1998; autoclave No. 4 has not been returned to service at this
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time.
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An incorrect orifice size is not considered a generic concern on temperature control valves on autoclaves in the feed vaporization facilities based on two factors: (1) these autoclaves have displayed acceptable performance over the last year with no WICS actuations occurring from a valid signal; (2) the steam pressure charts have acceptable characteristics. Additionally, all l
valves of this type in Stores were inspected and had the proper one-inch orifice.
l The investigation is continuing. Target date for a supplemental report is August 28,1998, t
i CORRECTIVE ACTIONS l
To be detennined i
L EXTENT 'OF EXPOSURE OF' INDIVIDUALS TO RADIATION OR TO RADIOACTIVE MATERIALS l
'Ihere was no exposure ofindividuals to radiation or to radioactive materials. This event was simply an actuation of the WICS safety system due to excessive water in the drain; the safety system performed as designed.
L LESSONS LEARNED j
l To be determined l
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