Issue 72: Control Rod Drive Guide Tube Support Pin Failures

DESCRIPTION

Background

This issue was raised in Board Notification No. 82-81[1]and addressed the failure of control rod drive (CRD) guide tube support pins in W reactors. The first pin failure at a U.S. plant occurred at North Anna 1 in 1982

where a portion of a broken support pin caused extensive damage to a steam generator.[2]Before this event, CRD guide tube support failures were reported in French and Japanese reactors dating back to 1978.[3]

CRD guide tube support pins are used to align the bottom of the CRD guide tube assembly into the top of the upper core plate. Two support pins are bolted into the bottom plate of each lower guide tube and are inserted into the top of the upper core plate in a manner that provides lateral support while accommodating thermal expansion of the guide tube relative to the core plate. The pins are about 3.5 inches long and about 0.5 inch in diameter, and the pin assembly includes (1) a bolt section to which a nut (sleeve) is threaded to anchor the pin to the guide tube; (2) a collet that rests against the guide tube; and (3) a leaf spring section. The guide tube support pins are made from a nickel alloy (Inconel X-750).

Following a study of the CRD guide tube events, W concluded that the pin failures were caused by stress corrosion cracking (SCC) due to solution heat treatment below 2000F and torquing of the nuts above 140 ft- lb.[4]

Safety Significance

The principal safety concerns related to this issue are (1) possible failure of control rod(s) to insert on command due to misalignment of the guide tube to the upper core plate; and (2) damage to the fuel or safety components due to loose parts in the RCS. Control rod failure was deemed to be most acute for reactors with upper head injection. Fuel damage was deemed unlikely since the flow of the RCS is upward, away from the fuel, and toward the heat exchangers.

Possible Solution

The solution proposed was to replace the CRD guide tube support pins sensitive to SCC with pins of improved design and manufacture. Improvements consist of (1) solution heat-treatment of the pins at 2000F for longer periods of time; (2) reducing the torque on the nuts to the 130 to 140 ft-lb range; (3) changing the transition between the bolt and the collet from a circular radius to a "parabolic" shape, thereby reducing the potential for cracking in this area; (4) lengthening the pin to preclude the chance of guide tube misalignment if a break should

occur; and (5) other design changes to reduce stresses imposed during the manufacturing process.[5]^,[6]

CONCLUSION

Since this issue was raised, CRD guide tube support pins susceptible to SCC were replaced with guide pins of

improved design at operating W reactors.[7]Thus, the safety concerns of this issue were addressed by the staff and the issue was DROPPED from further consideration.