ML19254D306

LER 79-025/01T-0:on 791003,water Rod Lower End Plug Wear Was Discovered on Two Assemblies.Caused by Flow Excitation of Water Rod Lower End Plugs by Cross Flows within Lower Tie Plate Flow Vol.Insp Program Will Be Developed

ML19254D306
Person / Time
Site:	Vermont Yankee
Issue date:	10/17/1979
From:	Conway W VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
Shared Package
ML19254D307	List: ML19254D306
References
LER-79-025-01T, LER-79-25-1T, NUDOCS 7910230624
Download: ML19254D306 (2)

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VTVYS1 05000271 LER 79-25/lT EVENT DESCRIPTION AND PROBABLE CONSEOUENCES As previously reported in RO 79-25/1P, October 3, 1979, evidence of lower end plug (LEP) wear was discovered on the water rods associated with two assemblies while perfoiw.ing refuel outage inspections of 8x8R fuel. Examination of fuel rods iocated in the general area of the water rods revealed no indication of end plug wear.

The wear is thought to be due to flow excitation of the water rod LEPs by cross flows within the lower tie plate (LTP) flow volume. In the 8x8R design, the LEPs extend into the LTP internal flow volume; in 8x8, they did not. Wear does not appear to be present either on fuel rods, which fit into a tapered recess in the LTP, or in 8x8 water rods which have the shorter LEPs.

The chief concern associated with this discovery is capture rod LEP wear. Should the capture rod LEP wear to approximately 38% material loss, the square end plug would be worn sufficiently to permit possible rota-tion. Potential could exist then for a loss of positive spacer posi-tioning. To our knowledge, however, no rotational driving force is present.

The observed wear on the several R5 (Reload 5 6x8R Bundles) water rods inspected ranges from about 2.5 to 13% material loss. These R5s have exper ienced approximately 9 1/2 months of full flow core residence.

Two additional data points, one at 30 months service (LTA6), and one at 26 months service (LTA7) show about 15% and 39% loss respectively. It is to be noted that the LTA-7 water rod was observed to be worn to the point of freedom to rotate. However, grid spring markings on this water ro r'. indicated that no rotational movement had, in fact, occurred.

Analysis of L..m data would predict that a typical water rod LEP would be worn to potential for rotation (%38% wear) somewhere in excess of 40 months service. With a conservative continuous, or linear wear model, the average R5 water rod will accumulate less than 20% wear by EOC7.

It is concluded that few if any R5 water rods will be worn to the potential for rotation by EOC7 and therefore there are no consequences or potential consequences to the health and safety of the public.

CAUSE DESCRIPTION AND CORRECTIVE ACTION A special test procedure has been developed which modifies four 8x8R fuel bundles such that the water rod is replaced in each bundle with a special capture rod, thereby, leaving the modified bundle with two cap-ture rods. The special capture rods were modified to the extent of having a shorter end ping to eliminate the flow-induced excitation and wider spaced grid retaining tabs to allow insertion of the special rod.

A longer upper end plug will preclude the lifting of the special rod from the LTP once the upper tie plate is attached. These four bundles, two of which were previously irradiated, will be loaded in the core fer opera-tion during Cycle 7. An inspected program will be developed for imple-mentation at EOC7; action will be taken as deemed necessary.

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