Document: NUREG-0800
Document ID: dc63e8aa-2a74-4078-9e19-9a26ed1b4d8c
Document Type: srp
Title: Revision 6 – July 2012
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1105/ML110550791.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
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Functions As described in NUREG-0737, Supplement No. 1, “Clarification of TMI Action Plan Requirements,” sufficient information should be provided to the nuclear reactor operators to monitor (and thereby control) the following plant critical safety functions and conditions: 1. Reactivity control 2. Reactor core cooling and heat removal from the primary system 3. Reactor coolant system (RCS) integrity 4. Radioactivity control 5. Containment conditions 1.3 Combining RTS and ESFAS In addition to divisional independence, many earlier analog I&C architectures consisted of discrete and separate analog components in each echelon of defense. In digital systems, formerly discrete systems (e.g., the RTS and the ESFAS) could be combined into a single DI&C system. Digital systems that combine most, if not all, RTS and ESFAS functions within a single digital system using a limited number of digital components in both new NPP designs and upgrades to current operating plant systems could introduce new effects from single failures as well as CCF effects that do not exist in systems that use separate discrete components. While a single random failure could affect multiple echelons in one division, a CCF could affect multiple echelons in multiple divisions. However, the four echelons of defense described above are only conceptual and, with the exception of the monitoring and indication echelon of defense noted in Point 4 (see Section B.1.4, "Four-Point Position"), NRC regulations do not require nor does this guidance imply that RTS and ESFAS echelons of defense must be independent or diverse from each other with respect to a CCF. Plant responses to postulated CCF that could impair a safety function should be in accordance with the acceptance criteria of this BTP, regardless of the echelons of defense that may be affected. 1.4 Four-Point Position On the basis of reviews of the ALWR DC applications for designs that use digital safety systems, the NRC has established the following