Technical Bulletin
Tri Nuclear Corp.
Re: Failure of a Bracket on a Tri Nuclear Pump in Service at Peach Bottom Station
This Technical Bulletin is Tri Nuclear’s response to the failure at the Peach Bottom Nuclear Power Station in September 2004 of an anti- rotation bracket on a PP-260 pump used with the Tri Nuclear UFV-260 Filter Vacuum.
Since 1990, Tri-Nuclear has used the all stainless steel Grundfos pump in our Underwater Filter/Vacuum units Models UFV-100, UFV-260 & UF-600. Over the years Tri Nuclear has made improvements to the design of the pump assembly. One area of improvement was made on the anti rotation brackets, which also serve as pump cover guides.
From 1990-1992, Tri Nuclear used four ¼” round bars fillet welded to the underside of the pump cover plate and to the pump body. (Fillet welds were small to minimize heat distortion on the cover plate, minimizing any bypass flow around the pump cover). These round bars center and guide the pump into the pump tube housing and also prevent rotational startup kick in the pump tube during pump startup.
In 1992, an improvement was made for attaching the 1/4” guide rods to the pump cover. Four 1/4” holes are drilled through the pump cover allowing the guide rods to penetrate the cover plate. Plug welds are made on the top cover to secure the rods to the cover plate. The lower end of the guide rods continue to be welded to the pump body.
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· Peach Bottom Nuclear Power Station contacted Tri Nuclear about a guide bracket found in the UFV-260 housing in September 2004. The failure mode of the pump anti rotation/guide bracket was not known at that time.
· Peach Bottom Nuclear Power Plant has been a consistent user of Tri Nuclear UF/UFV Equipment over the years and based on information they provided to us, it is probable that the failure of the pump bracket was from a 1990-1992 earlier design pump.
· Calculations indicate that it would take over 1,400 lbs of tension force to break the 1/4” guide rods on our current design. It is not known what force would break the former design guide rods since the size of the welds was not specified.
Recommended Action
A brief visual inspection of the pump cover assembly should be performed. See table below:
|
Pump Vintage |
Distinguishing Characteristics See Figure 1 & 2 |
Inspection Point |
|
1990-1992 All PP-100/260/600 pumps |
Flat yellow power cable No plug welding shown on the top pump cover plate where the four round bar anti rotation guide rods penetrate. |
Looking from the underside of the pump cover plate, inspect the four 1/4” round bar fillet welds attached to the cover plate and to the pump. |
|
1992- May 2001 All PP-100/260/600 pumps |
Flat yellow power cable Four 1/4in plug welds, 90 degrees apart found on the top pump cover |
Inspect the plug welds for integrity |
|
May 2001-Present All PP-100SC, PP-260SC & PP-600SC pumps |
Round black power cable Four 1/4in plug welds, 90 degrees apart found on the top pump cover |
Inspect the plug welds for integrity |
Figure 1 Figure 2
Conclusion
The failure mode of the 1/4” guide rod breaking off of the pump cover plate allowed the rod to fall down in the bottom of the pump tube housing. We have analyzed the possibility of the loose part coming out of the system. The only mode for the loose part to pass through the system is through the pump intake. The Grundfos pumps have built-in strainers attached to the pump intake that will prevent the 1/4” rods from passing through. Therefore, any failure of the guide rods from the pump cover would insure the loose part would remain in the pump housing until it is physically removed.
The Tri Nuclear Underwater Filter/Vacuum systems have been a standard in the nuclear industry for over 25 years. Tri Nuclear understands the severity of the problem that occurred at Peach Bottom. Our QA dept. has added an additional inspection point for all pump covers to ensure the quality and integrity of the anti rotation guides and their welds.
If you have technical questions or if we can be of any further service, please contact us.
Best regards,
James Warden
President