United States Nuclear Regulatory Commission - Protecting People and the Environment

Morning Report for August 16, 2000

                       Headquarters Daily Report

                         AUGUST 16, 2000

                    REPORT             NEGATIVE            NO INPUT
                    ATTACHED           INPUT RECEIVED      RECEIVED

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

Licensee/Facility:                     Notification:

Philadelphia Electric Co.              MR Number: H-00-0049
Limerick 1 2                           Date: 08/16/00
Dockets: 50-352,50-353



On April 4, 2000, a potential design problem with a floating pilot poppet
on an Atwood & Morrill (A&M) main steam isolation valve (MSIV) at
Limerick 1 was discussed on the R-I/NRR morning conference call. The
Limerick licensee had contacted A&M after an inboard MSIV failed a local
leak rate test and subsequent licensee investigation discovered the pilot
poppet had separated from the stem. The floating pilot poppet was a
design modification made by A&M to allow the pilot end of the stem to
have a small amount of movement to compensate for a loss of clearance due
to wear over a period of time.

The floating pilot poppet design includes a pilot poppet that is an
internally threaded, cup-shaped forging with a hard surfaced seat and
wear surfaces on its diameter. The poppet is held onto the stem with a
split ring and pilot poppet nut. The end of the stem is machined to
accept the split ring and fits inside the pilot poppet. The poppet nut is
slid over the stem and then the split ring is installed in the groove of
the stem. The pilot poppet is then installed on the stem with the split
ring fitting inside it below the internal threads. The poppet nut is then
slid down the stem and screwed into the pilot poppet. As the nut tightens
it draws the pilot poppet up until the pilot poppet, split ring, and nut
are locked together. A set screw is installed through the side of the nut
and tightened against the pilot poppet threads to act as a secondary
locking device. The set screw was ineffective in the failed MSIV at
Limerick, allowing the poppet nut and pilot poppet to become unthreaded
to the point that they separated.

The Limerick licensee stated that A&M noted that River Bend had a similar
failure of a pilot poppet in an MSIV in April 1999. Because of the
potential generic implications of the two similar failures, NRR staff
contacted the A&M Vice President (VP) of Engineering to discuss the

During a conference call between NRR staff and A&M on May 5, 2000, the
VP-Engineering confirmed that River Bend had experienced a similar
failure of the pilot poppets on two MSIVs in April 1999. As a result, A&M
sent a letter to the River Bend licensee providing a solution to the
problem on July 30, 1999. The letter included a sketch showing where to
add a weld to attach the pilot poppet nut to the pilot poppet. The weld
provides a more positive means for locking the nut in place. A&M sent a
copy of the letter to all of its potentially affected customers,
including the Limerick licensee in July 1999. The Limerick licensee was
aware of the letter prior to the MSIV failure and the engineering
department had determined that the weld should be implemented once the
Limerick MSIVs began experiencing the pilot poppet failures. The Limerick
licensee will now implement the weld modification on its MSIVs.
MR Number: H-00-0049 (cont.)

While discussing the floating pilot poppet modification with A&M, the NRR
staff learned that several enhancements and modifications have been made
by A&M over the last ten years to address ongoing functional and leakage
issues associated with MSIVs in BWRs. Most BWR licensees have
incorporated some or all of the modifications. In addition to the
floating pilot poppet, the key modifications are:

Nose Guided Poppet - the nose of the poppet was redesigned to have a
cone-shaped leading edge to help guide the poppet into the valve body

Anti-Rotation Stem and Poppet - locks the poppet to the stem, and the
stem to the external bottom spring plate, to prevent rotation of the
poppet or stem, which could cause wear on the valve body rib while the
valve is in the open position (see Information Notice 94-08, "Potential
for Surveillance Testing to Fail to Detect an Inoperable Main Steam
Isolation Valve," Accession #9401260242),

Improved Stem Guidance System - helps eliminate stem breakage due to side
loading and provides increased stem guidance,

Poppet Backseated Cover - locks the poppet to the valve cover while the
valve is in the open position, thus eliminating rib wear caused by poppet
movement while the valve is open.

The failure of the pilot poppet is not a significant safety concern. If
the pilot poppet separates from the stem, it is captured in the assembly
and will not prevent the MSIV from closing. However, the MSIV could
experience leakage in excess of TS limits. The vendor recommendation to
weld the nut to the pilot poppet will prevent separation of the pilot
poppet from the stem.

Contacts:   Art Burritt, R-I6   610-327-1344
            Dave Skeen, NRR     301-415-1174
            Kamal Naidu, NRR    301-415-2980

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