United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
.

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 
                                                               Page 3 of 3 

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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