United States Nuclear Regulatory Commission - Protecting People and the Environment

Morning Report for September 20, 1999

                       Headquarters Daily Report

                         SEPTEMBER 20, 1999

                    REPORT             NEGATIVE            NO INPUT
                    ATTACHED           INPUT RECEIVED      RECEIVED

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

MR Number: H-99-0086

                           NRR DAILY REPORT ITEM
                            SIGNIFICANT EVENTS

Subject: Scram on Loss of Condenser Vacuum
         and Recovery with Complications

On August 24, 1999, the NRR Events Assessment, Generic Communications and
Non-Power Reactors Branch classified this event as a Significant Event
for the NRC Performance Indicator Program. The basis for this
classification is the number of complications that resulted and produced
unnecessary burdens on licensee personnel.

On June 15, 1999, the Hatch licensee manually scrammed Unit 2 in
anticipation of receiving an automatic scram signal on a turbine trip due
to loss of condenser vacuum (Licensee Event Report 50-366/99-06). Several
factors complicated recovery from the event: (1) failure of automatic
realignment of two Unit 2 4160-volt buses because a transfer blocking
relay was out of calibration; (2) an arcing ground fault on a 600-volt
bus fed from Unit 1, tripping several safety-related Unit 1 600-volt
circuit breakers, which actuated some Unit 1 engineered safeguards
features but did not otherwise interfere with Unit 1 operation; (3) loss
of Unit 2 main feedwater pumps requiring use of the Unit 2 reactor core
isolation cooling system to maintain reactor level; (4) failure of a Unit
2 inboard main steam isolation valve to close due to an alternating
current solenoid valve being stuck in the energized position; and (5) a
leak in the Unit 2 residual heat removal service water system because of
a broken 3/4-inch vent line. The licensee complied with the required
limiting condition for operation and brought Unit 2 to cold
shutdown about 36 hours after the manual scram.

The NRC Region II Office conducted a special inspection of the
circumstances of this event (NRC Special Team Inspection Report
50-321(366)/99-10). The inspectors concluded that the licensee missed two
opportunities to improve early detection of loss of condenser vacuum
following similar events at this unit in 1995 and 1997.

The licensee took several corrective actions for this event, adding level
instruments in the circulating water pump pits, promulgating an operating
order on condenser operating parameters, repairing the grounded
conductor, replacing the 4160-volt fast transfer blocking relay,
shortening the broken vent line, and instituting the checking of all MSIV
solenoid valves for audible buzzing, an indication of undesirable
presence of debris.

The risk assessment for this event is low in spite of the number of
complications, which were generally related to balance of plant
equipment. All emergency core cooling systems were operable and the scope
of the event remained within plant design parameters. A calculation using
the conservative simplified plant analysis risk model indicated that the
conditional core damage probability is less than 1E-6.

A review of 1998-1999 event notifications using the search string
HEADQUARTERS      MORNING REPORT     PAGE  2          SEP. 20, 1999
MR Number: H-99-0086 (cont.)

"vacuum" disclosed ten reports with similar initiating conditions but
with no similar complications. Consequently, the Hatch 2 event appears to
be an isolated occurrence.

The susceptibility to electrical noise of breaker trip devices, similar
to ones that tripped some of the breakers in this event, is addressed in
Information Notices 96-62 and 93-75. On 18 Aug 99, this event was briefed
to senior NRC management because of the number of factors that were
confusing to licensee operators.

Contact:  C. Vernon Hodge, NRR
Page Last Reviewed/Updated Thursday, March 29, 2012