The 1983 Salem ATWS Event

Delaware Bay, New Jersey

February 22-25, 1983

A safety minute from whatisnuclear.com

What happened

• Salem 1 was at 12% thermal power and rising after a refueling outage
• At 12:21 am Feb 25, 1983, low-low water level in 1 steam generator initiated a reactor trip signal in the reactor protection system
• Both redundant reactor trip breakers failed to open!
• Operators manually initiated a trip 25 seconds later from the control room
• Investigators discovered that the same event had occurred on Feb 22nd as well, without operators knowing the breakers had failed.

This was the first actual Anticipated Transient Without Scram (ATWS) event in *the commercial fleet, something that had been considered a possibility for at least 10 years. (Hasksin, 1994)

Investigation and root cause

• Both breakers failed due to mechanical binding in under-voltage trip mechanism
• Excessive wear from improper maintenance
• Inadequate attention to the importance of vendor-supplied information
• Absence of adequate preventive maintenance program
• Inadequate supply, control, and verification of information by the vendor

(Thompson, 2016)

Regulatory reactions

• NRC did not permit Salem plants to restart until technical and management corrective actions were addressed
• NRC issued Bulletin 83-01 (DeYoung, 1983)
• Fact finding task force sent out to investigate (NRC Fact-Finding Task Force Report on the ATWS Event at Salem Nuclear Generating Station, Unit 1, on February 25, 1983, 1983)
• NRC prepared two-volume NUREG-1000 summarizing the event
• NRC imposed a civil penalty of $850,000
• Previously identified problems at Salem and their failure to recognize the ATWS event on Feb 22 were the major violations

(Hasksin, 1994)

Regulatory impact

• ATWS rule proposed (NRC, 1984)
• ATWS rule adopted as 10 CFR 50.62 (10CFR50.62 – Requirements for Reduction of Risk from Anticipated Transients without Scram (ATWS) Events for Light-Water-Cooled Nuclear Power Plants., n.d.)
• Industry required to establish formal vendor interface programs
• PWRs required to have diverse equipment to initiate trips (AMSAC)
• BWRs required to have diverse recirc pump trip, alternate rod insertion circuitry, or install high capacity standby liquid control systems.

(Thompson, 2016)

References

1. Hasksin, F. (1994). Reactor Safety Course (R-800) (NUREG/CR-6042; Number NUREG/CR-6042, p. 242). US NRC. https://adamswebsearch2.nrc.gov/webSearch2/main.jsp?AccessionNumber=ML072740014
2. Thompson, J. (2016). Significant Operational Experience: 7 Major Industry Events with Regulatory Implications. https://adamswebsearch2.nrc.gov/webSearch2/main.jsp?AccessionNumber=ML16006A288
3. DeYoung, R. (1983). Bulletin 83-01: Failure of Reactor Trip Breakers (Westinghouse DB-50) to Open on Automatic Trip Signal (Bulletin IEB 83-01; Number IEB 83-01). US NRC. https://www.nrc.gov/reading-rm/doc-collections/gen-comm/bulletins/1983/bl83001.html
4. NRC Fact-Finding Task Force Report on the ATWS Event at Salem Nuclear Generating Station, Unit 1, on February 25, 1983 (NUREG-0977; Number NUREG-0977). (1983). (Number). Nuclear Regulatory Commission, King of Prussia, PA (USA). Region I. https://doi.org/10.2172/6391289
5. NRC. (1984). Reduction of Risk from Anticipated Transients Without Scram (ATWS) Events for Light-Water-Cooled Nuclear Power Plants. US Federal Register, 49(124), 26036. https://archives.federalregister.gov/issue_slice/1984/6/26/26023-26045.pdf#page=14
6. 10CFR50.62 – Requirements for Reduction of Risk from Anticipated Transients without Scram (ATWS) Events for Light-Water-Cooled Nuclear Power Plants. Retrieved April 13, 2024, from https://www.ecfr.gov/current/title-10/part-50/section-50.62