Information Notice 1980-20, Loss of Decay Heat Removal Capability at Davis-Besse Unit 1 While in a Refueling Mode

Loss of Decay Heat Removal Capability at Davis-Besse Unit 1 While in a Refueling Mode

ML031180340
Person / Time
Site:	Davis Besse, 05000000
Issue date:	05/08/1980
From:	NRC/OI
To:
References
IN-80-020, NUDOCS 8002280671
Download: ML031180340 (7)

SSINS No.: 6870

UNITED STATES Accession No.:

NUCLEAR REGULATORY COMMISSION 8002280671 OFFICE OF INSPECTION AND ENFORCEMENT

WASHINGTON, D. C. 20555 May 8, 1980

IE Information Notice 80-20

LOSS OF DECAY HEAT REMOVAL CAPABILITY AT DAVIS-BESSE UNIT 1 WHILE IN A

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 Cl; 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 Protec- tion System (RPS) and Safety Features Actuation System (SFAS) were being ener- gized 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 Injec- tion; Level 3 - Low Pressure Injection; Level 4 - Containment Spray; and

IE Information Notice No. 80-20 May 8, 1980 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 refuel- ing 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 activi- ties 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 suscep- tible 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

IE Information Notice No. 80-20 May 8, 1980 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 Informa- tion 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.

_

Enclosure A

DAVIS-BESSE EVENT OF APRIL 19. 1980

STATUS OF DAVIS-BESSE 1 PRIOR TO LOSS OF POWER TO BUSSES E-2 D F-2:

1. Refueling mode with RCS temperature at 9 0 7F and level slightly beltw

vessel head flange. Bead detensioned with bolts in place. Hanway cover

om top of OTSG removed. Tygon tubing attached to lower vents of RCS hot

leg for RCS level indication. Decay heat loop 2 in uervice for RCS

cooling.

2. All non-nuclear instrument (NNI) power and Static Voltage Regulator YAR

supplied from 13.8 KZV Bus B via EBBF2. 13.8 K Bus A energized but not

connected. RPS and SFAS Channels 1 and 3 being supplied from YAL

3. Equipment Out of Service

a. Source Range Channel 2 - Surveillance

b. Emergency Diesel Generator 1 - Maintenance.

c. Decay Beat Loop 1 - Maintenance.;

4. Breakers for containment spray and EPI pumps racked out.

SEQUENCE OF ELE!S

TIME EVENT CAUSE/C010ENTS

2:00 p.m. Loss of power to Ground short on 13.8 KV breaker EBBF2 Busses E-2 and f-2 which caused breaker to open. This

(non-essential 480 interrupted power to busses E-2 and F-2 VAC) which were supplying all non-nuclear

instrument (NNI) power, channels 1 and 3 of the Reactor Protection System (RFS) and

.the Safety Features Actuation Signal (SPAS),

the computer, and much of the control room

indicators.

2:00 p.=. SFAS Level 5 (recircu- Two out of four logic tripped upon loss

lation mode) actua- of Busses E-2 and F-2. Actuation caused

tion. 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.?. Decay Reat (low Operator turned off only operating DR

pressure safety in- pump to avoid spillage of RCS water to

jection) flow secured containment via the tygon tubing for RCS

by operator 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/COCIENTS

2:44 p.m. . Attempt to reestab- Started DH pump 1-2 then stopped it when

lish DE flow 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 sequentually as efforts

to (480 VAC) F-2, F-21, progressed to isolate ground fault.

4:06 p.m. 1-22, and F-23

4:25 p.m. DE flow restored DE pump 1-2 started after venting. RCS

temperature at 1700°F.DH flow bypassing

°oj15 6 1 Incore TC's being taken and maximum

4:46 p.m. Containment sump Precautionary measure to assure containment

pump breakers sump eater from BWST remained in aontainment.

opened Incore TC's range from 161 to 164 F.

5:40 p.m. Computer returned to Incore TC's range from 158 to 1600F.

service.

6:24 p.m. D8 flow directed RCS cooldown established at less shan 25aF

-0p through cooler per hour. RCS temperature at 150 F. Incore TC's

range from 151 to 158°F.

9:50 P.M. Power completely RCS temperature at approximately 1150P.

res tored

STATUS OF DAVIS-BESSE I AFTER RECOVERY FROM LOSS OF POWER TO BUSSES E-2 AMD F-2:

1. Refueling mode with RCS temperature at 115aF and level slightly below vessel head

flange. Read detensioned with bolts in place. Manway cover on top of OTSG

removed. Tygon tubing attached to lower vents of RCS hoc leg for RCS level

indication. Decay heat loop 2 in service for RCS cooling.

2. Bus E-2 being supplied from 13.8 EtV Bus A via breaker HAAZ2 and Bus F-2 being

supplied from 13.8 IV Bus B via breaker HBBF2.

3. Decay heat 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 DE-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 zUp.

5. Equipment Out of Service:

Emergency Diesel Generator 1 - maintenance

6. Breakers for containment spray and BPI pumps racked out.

IE Information Notice No. 80-20 Enclosure

May 8, 1980

RECENTLY ISSUED

IE INFORMATION NOTICES

Information Subject Date Issued To

Notice No. Issued

80-19 NIOSH Recall of Recircu- 5/6/80 All holders of a power

lating-Mode (Closed-Circuit) reactor OL, Research

Self-Contained Breathing Reactor License, Fuel

Apparatus (Rebreathers) Cycle Facility License

and Priority I Material

License

80-18 Possible Weapons Smuggling 5/5/80 All power reactor

Pouch facilities with an OL,

fuel fabrication and

processing facilities

and Materials Priority I

licensees (processors

and distributors)

80-17 Potential Hazards Associated 5/5/80 All radiography

With Interchangable Parts Licenses

On Radiographic Equipment

80-16 Shaft Seal Packing 4/29/80 All power reactor

Causes Binding In Main facilities in your

Steam Swing Check And Region with an OL or CP

Isolation Valves

80-15 Axial (Longitudinal) 4/21/80 All Light Water Reactor

Oriented Cracking In Facilities holding power

Piping reactor OLs or CPs

80-14 Safety Suggestions From 4/2/80 All power reactor

Employees facilities with an

OL or CP

80-13 General Electric Type SBM 4/2/80 All light water reactor

Control Switches - Defective facilities holding power

Cam Followers reactor OLs or CPs

80-12 Instrument Failure Causes 3/31/80 All holders of power

Opening of PORV and Block reactor OLs and CPs

Valve

80-11 General Problems with ASCO 3/14/30 All holders of Reactor

Valves in Nuclear Application OL, CP, fuel fabrica- Including Fire Protection tion and processing

Systems facilities