Document: NUREG-0800
Document ID: f1629507-6318-4e71-94a6-9d6664ba96a3
Document Type: srp
Title: of the plant safety analysis report (SAR).  Although NRC did not endorse the annexes of
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0706/ML070660327.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7.6
CFR Part: 
CFR Title: 

Content:
c software may have been well tested and extensively used in the past, while the operational code is likely new for this application. The reviewer’s judgement should be used. A non-software watchdog timer is critical in the overall diagnostic scheme. A software watchdog will fail to operate if the processor freezes and no instructions are processed. The reviewer should look for a hardware watchdog timer whose only software input is reset after the safety processor completes its function. Even then, the reviewer should look to ensure that there is no possibility of a software failure causing a jump to the reset function, thereby nullifying the effectiveness of the watchdog timer. 5.8 Information Displays (IEEE Std 7-4.3.2-2003 Clause 5.8) In the past, information displays only provided a display function, and therefore required no two- way communications. More modern display systems may also have included control functions, and therefore the reviewer should ensure that incorrect functioning of the information displays does not prevent the safety function from being preformed when necessary. This is the same issue as in subsection 5.6, “Independence”, and similar methods are appropriate. If the communications path is one-way from the safety system to the displays, or if the displays and controls are qualified as safety related, the safety determination is simplified. Two-way communications with non-safety control systems have the same isolation issues as any other non-safety to safety communications. In addition, however, the reviewer should ensure that inadvertent actions, such as an unintended touch on a touch sensitive display can not prevent the safety function. 5.9 Control of Access (IEEE Std 7-4.3.2-2003 Clause 5.9) Guidance is provided in Subsection 5.9 of SRP Appendix 7.1-C and Regulatory Guide 1.152, Revision 2. 5.10 Repair (IEEE Std 7-4.3.2-2003 Clause 5.10) Guidance is provided in Subsection 5.10 of SRP Appendix 7.1-C. Appendix 7.1-D-19 March 2007