United States Nuclear Regulatory Commission - Protecting People and the Environment

Morning Report for October 20, 2000

                       Headquarters Daily Report

                         OCTOBER 20, 2000

                    REPORT             NEGATIVE            NO INPUT
                    ATTACHED           INPUT RECEIVED      RECEIVED

REGION I                               X
REGION II                              X
REGION III                             X
REGION IV                              X

Licensee/Facility:                     Notification:

                                       MR Number: H-00-0060
Westinghouse                           Date: 10/20/00



A potential generic concern that could introduce loose parts into the
reactor coolant system of Westinghouse PWRs has been identified at
Vogtle, Unit 1. Plants using Westinghouse fuel containing Vantage Plus
upper nozzle leaf springs could be affected. The screws holding down the
upper nozzle leaf springs have been cracking due to primary water stress
corrosion cracking; the screws are manufactured of heat treated Inconel
600 a material of known susceptibility. The cracking and ultimate
fracture occurs in the screw shank immediately beneath the screw head.

This failure mode has been known for over a year and broken screws have
been found during refueling; what makes the Vogtle occurrence novel is
that a screw head was found lodged in a steam generator hot leg tube
stub. The following information was provided by Westinghouse through its
owner's group contact during October 5 and 11, 2000 telephone calls.

Before the Vogtle occurrence, Westinghouse had believed that, even if a
screw fractured, the head would remain captured by a lock wire inside the
spring screw cover bracket. The cover bracket is secured to the fuel
assembly upper orifice plate by a lock screw, tack welds or both. Each
screw is held in the hole within the bracket by a welded lock wire across
the top of the hole. However, at Vogtle, the bracket was found out of
position on top of the nozzle spring. The tack welds had failed
presumably from vibration after the screw heads fractured. One of the two
screw heads was still inside the bracket, the other was the one found in
the steam generator wedged in a steam generator tube hot leg stub.

Two other brackets were found out of position, one on top of the outlet
nozzle plate, the other not found until it was in the upender so the
exact position it had moved to is unknown. Both of these brackets still
had both screw heads captured inside. Other fractured screws have been
identified at Vogtle but the brackets were still in place which prevents
the heads from getting loose.

To summarize a complex failure mode in order for a screw head to get
loose in the primary system, five things have to happen. The screw heads
have to fracture; the bracket tack welds then have to fail; the bracket
then has to be raised up in order to clear the spring top; the screw head
has to vibrate free of the lock wire; and finally, the screw head to drop
out the bottom of the bracket socket hole. This is apparently what
happened at Vogtle.

Westinghouse stated that geometry and clearance considerations preclude
the entire bracket assembly from leaving the area of the core upper
internals. Since the screw head is too large to fall through the upper
orifice plate into the core, it must either remain in the upper internals
area or be swept into the outlet plenum and into the hot leg nozzle.
According to Westinghouse, the high cross flow velocities at the outlet
MR Number: H-00-0060 (cont.)

of the fuel channels make it highly probable that any screw head that
falls out of a bracket will be swept into the hot leg and eventually to
the steam generator inlet plenum rather than remaining in the core where
it could interfere with rod motion.

They made several points as to the extent of facilities at risk from this
failure mode. The only fuel assemblies that have had this problem are
those with Vantage Plus springs; these springs place larger stresses on
the screws that other types due to Vantage Plus spring's higher k value
and greater compression. The only spring screw bracket mounting type that
has failed is the type without a lock screw and with only one or two tack
welds as opposed to the three tack welds that are more common. Last,
screw fracture has only been detected in assemblies that have completed
two cycles of operation.

The only plants starting up in the next two weeks that are susceptible to
the problem are Watts Bar and Vogtle and both of these will have replaced
all vulnerable upper orifice plates. Other plants with the problem are
inspecting vulnerable fuel looking for indications of broken screws or
displaced brackets. Westinghouse is examining various inspection methods
for detecting cracked or fractured screws but much work remains to be
done. Another call with Westinghouse and the owner's group is scheduled
later in October to discuss next steps for both owners and Westinghouse.

Westinghouse proposed the following general safety argument. The
population at risk for loose parts is limited to second cycle assemblies
with Vantage Plus springs and tack welded spring screw brackets.
Experience has shown, even for the assemblies at risk, only a limited
number of screws have failed. For most of these, the bracket mounting has
not failed; when it has failed, either the bracket has not moved thus
capturing the screw head or, If the bracket did move, the lock wire
continued to restrain the screw head. The screw heads are slightly larger
than the internal diameter of a steam generator tube so the screw head
must be peaned in order to fit into the tube. Therefore the head will not
go up the tube with enough velocity to rupture the tube catastrophically
although it could wedge and vibrate, eventually causing the tube to leak;
this, though serious, is more an economic than a safety problem.

Contact:    Edward Goodwin, NRR
            Email: efg@nrc.gov
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