Information Notice No.80-20 – Loss of Decay Heat Removal Capability at Davis-Besse Unit 1 While in a Refueling Mode
SSINS No.: 6870
Accession No.:
8002280671
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
May 8, 1980
Information Notice No. 80-20
LOSS OF DECAY HEAT REMOVAL CAPABILITY AT DAVIS-BESSE UNIT 1 WHILE IN A
REFUELING MODE
Description of Circumstances:
On April 19, 1980, decay heat removal capability was lost at Davis-Besse
Unit 1 for approximately two and one-half hours. At the time of the event,
the unit was in a refueling mode (e.g., RCS temperature was 90F; decay heat
was being removed by Decay Heat Loop No. 2; the vessel head was detensioned
with bolts in place; the reactor coolant level was slightly below the vessel
head flanges; and the manway covers on top of the once through steam
generators were removed) (See Enclosure A, Status of Davis-Besse 1 Prior to
Loss of Power to Busses E-2 and F-2 for additional details regarding this
event.)
Since the plant was in a refueling mode, many systems or components were out
of service for maintenance or testing purposes. In addition, other systems
and components were deactivated to preclude their inadvertent actuation
while in a refueling mode. Systems and components that were not in service
or deactivated included:
Containment Spray System;
High Pressure Injection System;
Source Range Channel 2;
Decay Heat Loop No. 1;
Station Battery 1P and 1N;
Emergency Diesel-Generator No. 1;
4.16 KV Essential Switchgear Bus C1; and
13.8 KV Switchgear Bus A (this bus was energized but not aligned).
In brief, the event was due to the tripping of a non-safeguards feeder
breaker in 13.8 KV Switchgear Bus B. Because of the extensive maintenance
and testing activities being conducted at the time, Channels 1 and 3 of the
Reactor Protection System (RPS) and Safety Features Actuation System (SFAS)
were being energized from only one source, the source emanating from the
tripped breaker. Since the SFAS logic used at Davis-Besse is a
two-out-of-four input scheme in which the loss (or actuation) of any two
input signals results in the actuation of all four output channels (i.e.,
Channels 1 and 3, and Channels 2 and 4), the loss of power to Channels 1 and
3 bistables also resulted in actuation of SFAS Channels 2 and 4. The
actuation of SFAS Channels 2 and 4, in turn, affected Decay Heat Loop No. 2,
the operating loop.
Since the initiating event was a loss of power event, all five levels of
SFAS were actuated (i.e., Level 1 - High Radiation; Level 2 - High Pressure
Injection; Level 3 - Low Pressure Injection; Level 4 - Containment Spray;
and
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Information Notice No. 80-20 May 8, 1980
Page 2 of 3
Level 5 - ECCS Recirculation Mode). Actuation of SFAS Level 2 and/or 3
resulted in containment isolation and loss of normal decay heat pump suction
from RCS hot leg No. 2. Actuation of SFAS Level 3 aligned the Decay Heat
Pump No. 2 suction to the Borated Water Storage Tank (BWST) in the low
pressure injection mode. Actuation of SFAS Level 5 represents a low level in
the BWST; therefore, upon its actuation, ECCS operation was automatically
transferred from the Injection Mode to the Recirculation Mode. As a result,
Decay Heat Pump No. 2, the operating pump, was automatically aligned to take
suction from the containment sump rather than from the BWST or the reactor
coolant system. Since the emergency containment sump was dry, suction to the
operating decay heat pump was lost. As a result, the decay heat removal
capability was lost for approximately two and one-half hours, the time
required to vent the system. Furthermore, since Decay Heat Loop No. 1 was
down for maintenance, it was not available to reduce the time required to
restore decay heat cooling.
MAJOR CONTRIBUTORS TO THE EVENT:
The rather extended loss of decay heat removal capability at Davis-Besse
Unit 1 was due to three somewhat independent factors, any one of which, if
corrected, could have precluded this event. These three factors are:
(i) Inadequate procedures and/or administrative controls,
(ii) Extensive maintenance activities; and
(iii) The two-out-of-four SFAS logic.
Regarding inadequate procedures and/or administrative controls, it should be
noted that the High Pressure Injection Pumps and the Containment Spray Pumps
were deactivated to preclude their inadvertent actuation while in the
refueling mode. In a similar vein, if the SFAS Level 5 scheme had been
by-passed or deactivated while in the refueling mode, or if the emergency
sump isolation valves were closed and their breakers opened, this event
would have been, at most, a minor interruption of decay heat flow.
Regarding the extensive maintenance activities, it appears that this event
would have been precluded, or at least ameliorated, if the maintenance
activities were substantially reduced while in the refueling mode. For
example, if the maintenance activities had been restricted such that two
SFAS channels would not be lost by a single event (e.g., serving Channels 1
and 3 from separate sources), this event would have been precluded.
Likewise, if maintenance activities had been planned or restricted such that
a backup decay heat removal system would have been readily available, the
consequences of the loss of the operating decay heat removal loop would have
been ameliorated.
Regarding the two-out-of-four SFAS logic used at Davis-Besse, even under
normal conditions, it appears that this type of logic is somewhat more
susceptible to spurious actions than other logic schemes (e.g., a
one-out-of-two taken-twice scheme). This susceptibility is amplified when
two SFAS channels are served from one source. Consequently, when the source
feeding SFAS Channels 1 and 3 was lost, all five levels of SFAS were
actuated. As stated
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Information Notice No. 80-20 May 8, 1980
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previously, this particular event would have been precluded if SFAS Channels
1 and 3 were being served from separate and independent sources. In a
similar vein, this specific event would have been precluded by a
one-out-of-two taken twice type of logic that requires the coincident
actuation of or loss of power of an even numbered SFAS Channel and an odd
numbered SFAS Channel.
Since each LWR can be expected to be in a refueling mode many times during
its lifetime, licensees should evaluate the susceptibility of their plants
to losing decay heat removal capability by the causes described in this
Information Notice. No specific action or response is requested at this
time. Licensees having questions regarding this matter should contact the
director of the appropriate NRC Regional Office.
Enclosures:
1. Davis-Besse Event of
April 19, 1980
2. List of Recently Issued
IE Information Notices
.
Enclosure A
DAVIS-BESSE EVENT OF APRIL 19, 1980
STATUS OF DAVIS-BESSE 1 PRIOR TO LOSS OF POWER TO BUSSES E-2 AND F-2:
1. Refueling mode with RCS temperature at 96F and level slightly
below vessel head flange. Head detensioned with bolts in place. Manway
cover on top of OTSG removed. Tygon tubing attached to lower vents of
RCS hot leg for RCS level indication. Decay heat loop 2 in service for
RCS cooling.
2. All non-nuclear instrument (NNI) power and Static Voltage Regulator YAR
supplied from 13.8 KV Bus B via HBBF2. 13.8 K Bus A energized but not
connected. RPS and SFAS Channels 1 and 3 being supplied from YAR.
3. Equipment Out of Service
a. Source Range Channel 2 - Surveillance
b. Emergency Diesel Generator 1 - Maintenance.
c. Decay Heat Loop 1 - Maintenance.
4. Breakers for containment spray and HPI pumps racked out.
SEQUENCE OF EVENTS
TIME EVENT CAUSE/COMMENTS
2:00 p.m. Loss of power to Ground short on 13.8 KV breaker HBBF2
Busses E-2 and F-2 which caused breaker to open. This
(non-essential 480 interrupted power to busses E-2 and
VAC) F-2 which were supplying all
non-nuclear instrument (NNI) power,
channels 1 and 3 of the Reactor
Protection System (RPS) and the
Safety Features Actuation Signal
(SFAS), the computer, and much of
the control room indicators.
2:00 p.m. SFAS Level 5 (recircu- Two out of four logic tripped upon
lation mode) actua- loss of Busses E-2 and F-2. Actuation
tion. caused ECCS pump suction valves from
containment sump to open and ECCS
pump suction valves from Borated
Water Storage Tank to close. During
valve travel times, gravity flow
path existed from BWST to
containment sump.
2:02 p.m. Decay Beat (low Operator turned off only operating
pressure safety in- DH pump to avoid spillage of RCS
jection) flow secured water to containment via the tygon
by operator tubing for RCS level indication and
open SG manway.
2:33 p.m. Partial restoration
of power Power to Bus E-2 and SFAS channels
1 and 3 restored along with one
channel of NNI. This restored all
essential power for ECCS.
.
- 2 -
TIME EVENT CAUSE/COMMENTS
2:44 p.m. Attempt to reestab- Started DH pump 1-2 then stopped it
lish DH flow when it was determined that air was
in suction line. Pump secured to
prevent damage.
3:34 p.m. Source Range
Channel 2 energized.
4:00 p.m. Restoration of Busses Busses restored sequentially as
to (480 VAC) F-2, F-21, efforts progressed to isolate ground
4:06 p.m. F-22, and F-23 found.
4:25 p.m. DH flow restored DH pump 1-2 started after venting.
RCS temperature at 170F.DH flow
bypassing cooler. Incore TC's being
taken and maximum is 170F.
4:46 p.m. Containment sump Precautionary measure to assure
pump breakers containment sump water from BWST
opened remained in containment. Incore TC's
range from 161 to 164F.
5:40 p.m. Computer returned to Incore TC's range from 158 to
service. 160F.
6:24 p.m. DH flow directed RCS cooldown established at less than
through cooler 25F per hour. RCS temperature
at 150F. Incore TG range from
151 to 158F.
9:50 p.m. Power completely RCS temperature at approximately
restored 115F.
***
STATUS OF DAVIS-BESSE 1 AFTER RECOVERY FROM LOSS OF POWER TO BUSSES E-2 AND
F-2:
1. Refueling mode with RCS temperature at 115F and level slightly
below vessel head flange. Head detensioned with bolts in place. Manway
cover on top of OTSG removed. Tygon tubing attached to lower vents of
RCS hot leg for RCS level indication. Decay beat loop 2 in service for
RCS cooling.
2. Bus E-2 being supplied from 13.8 KV Bus A via breaker HAAE2 and Bus F-2
being supplied from 13.8 KV Bus B via breaker HBBF2.
3. Decay beat loop filled, all tags clear. Maintenance work restricted so
restoration of system will be less than two hours.
4. ECCS pump suction valves (DH-9A and DH-9B) from containment sump closed
and breakers racked out. This will prevent the suction of air into the decay.
- 3 -
heat loop during a level 5 actuation (recirculation mode) when there is no water in the sump.
5. Equipment Out of Service:
Emergency Diesel Generator 1 - maintenance
6. Breakers for containment spray and HPI pumps racked out.
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