Document: NUREG-0800
Document ID: 846ded8e-04ea-4031-8f64-62690ca967b6
Document Type: srp
Title: ANTICIPATED TRANSIENTS WITHOUT SCRAM
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0705/ML070570008.pdf
Revision Date: 2023-06
Chapter: 15
Section ID: 15.8
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Content:
REVIEW RESPONSIBILITIES Primary - Organization responsible for review of transient and accident analyses for PWRs/BWRs Secondary - Organization responsible for the review of instrumentation and control systems I. AREAS OF REVIEW An anticipated transient without scram (ATWS) is an anticipated operational occurrence (AOO) as defined in Appendix A to 10 CFR Part 50 followed by the failure of the reactor trip portion of the protection system specified in General Design Criterion (GDC) 20. Since protection systems (e.g., the reactor trip system) must satisfy the single-failure criterion, multiple failures or a common mode failure must cause the assumed failure of the reactor trip. The probability of an AOO, in coincidence with multiple failures or a common mode failure, is much lower than the probability of any of the other events that are evaluated under SRP Chapter 15. Therefore, an ATWS event cannot be classified as either an AOO or a design-basis accident. The failure of the reactor to shut down during certain transients can lead to unacceptable reactor coolant system pressures, fuel conditions, and/or containment conditions. Typical AOOs that may result in unacceptable conditions following a pressurized-water reactor (PWR) scram failure are loss of feedwater, loss of load, turbine trip, inadvertent control rod withdrawal, loss of alternating current power, and/or loss of condenser vacuum. For a boiling-water reactor (BWR), AOOs with failure to scram that could lead to unacceptable conditions include closure of main steamline isolation valves, or turbine trip with bypass available if unmitigated unstable power oscillations are allowed to grow. 15.8-2 Revision 2 - March 2007 Safety issues associated with an ATWS have been evaluated since the early 1970s. During NRC evaluations of vendor models and analyses addressing ATWS events, the agency formally identified the ATWS as Unresolved Safety Issue (USI) A-9, “Anticipated Transients Without Scram.” The agency