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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 . 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 96
F 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 170 F.DH flow bypassing cooler. Incore TC's being taken and maximum is 170 F. 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 164 F. 5:40 p.m. Computer returned to Incore TC's range from 158 to service. 160 F. 6:24 p.m. DH flow directed RCS cooldown established at less than through cooler 25 F per hour. RCS temperature at 150 F. Incore TG range from 151 to 158 F. 9:50 p.m. Power completely RCS temperature at approximately restored 115 F. *** 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 115 F 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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