academic review by Professor Zahra Mohaghegh and Dr. Seyed A. Reihani. The evidence was evaluated with respect to the two criteria presented in Figure 3.3 (i.e., LOCA Relevancy and Debris Generation Relevancy). The reviewers were sent copies of the evidence tables and asked to provide any relevant information they may personally have in relation to the two evidence- seeking criteria. The resulting qualitative feedback helped eliminate some of the sub-
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Upon completion of the internal academic review, the evidence tables were sent for external review to three nuclear industry experts. The nuclear industry experts were selected due to their many years of experience working with PRAs involving LOCA frequencies. These experts provided their opinions regarding each component and sub-component based on the information collected from the evidence seeking process of the investigative procedure in addition to their professional experiences. In the screening provided by experts, some
contributors were identified as ‘indirect’ contributors to LOCA. These were internal components such as turning vanes, thermal barrier, and suction deflector, or sub-components such as
flywheels, framing, etc. that can add stress, for example, to pressure boundary components, but their failure would not directly cause the occurrence of a LOCA.
Subsections 3.3.1. – 3.3.6. report the results of the expert screening for each of the six categories: RCP, pressurizer, steam generator, reactor vessel, ECCS, and CVCS. In each category of components, two sets of examples of review comments are included in the subsections to demonstrate the type of information provided by the experts. The remaining comments can be found in Appendix B. The first set of examples of comments provided in each subsection are on the “agreement” expressed by the expert regarding the importance and the potential contribution of the sub-component to debris generation and the GSI-191 issue. This has benefitted this research and helped identify the sub-component as a potential non-piping concern for GSI-191. The second set of examples of comments relate to “disagreement” expressed by the experts regarding the importance and the potential contribution of the sub- component to debris generation and the GSI-191 issue and has helped this research eliminate subcomponents regarding potential concerns for GSI-191 from further consideration.
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The review comments from the academic and industry experts have assisted in the identification process of the potential importance of non-piping components for GSI-191. The items identified in each category are not based on any ranking. They only show items identified as being potential contributors to the GSI-191 project.
3.3.1 REACTOR COOLANT PUMP
After receiving the academic and industry reviews from the experts, the following 6 RCP sub-component categories were identified as having the potential to make a significant
contribution for non-piping components: - Pump shaft
- Pump closure (studs, bolts, main flange, and nuts) - Pump body/casing
- Flywheel
- Framing and support - Thermal barrier
Examples of the review comments include:
Pump closure (studs, bolts, main flange, and nuts): “Potential for large LOCA here. Probably the most important issue for reactor coolant pump (because seal package failure has been experienced).”
Turning vane bolts and cap screws: “Not an issue for the GSI-191, because the bolt fragments are too heavy. If the flow through the reactor pressure vessel isn’t strong enough to push the fragments out, then in the case of a LOCA, the flow on the containment floor will not push the fragments to help clog the sump strainer.”
121 3.3.2 PRESSURIZER
After receiving the academic and industry reviews from the experts, the following 5 pressurizer sub-component categories have been identified as potentially important non-piping components for the GSI-191 project:
- Spray head
- Manway bolts/studs - Thermal/heater sleeves
- Power-operated relief valves (some plants) - Walls/vessel shell.
Examples of the review comments include:
Spray head: “This is interesting because it is talking about the vessel walls. We should look into this and find out what the exposure may be. The pressurizer has a large volume of liquid in it and there would be a very large break potential (much bigger than a pipe).”
Instrument nozzles: “Instrument nozzles addressed in bottom-up approach. PWSCC susceptibility exists only for B&W and CE plants. Current fleet has implemented mitigation.”
3.3.3 STEAM GENERATOR
After receiving the academic and industry reviews from the experts, the following 2 steam generator sub-component categories have been identified as potentially important non- piping components for the GSI-191 project:
- Primary manway cover, bolts, and studs - Support bolts and embedded anchor studs.
122 Examples of the review comments include:
Support bolts, embedded anchor studs: “Steam generator supports failures could result in greater load on the connected piping. So this is something to consider.” Primary divider plate: “This is not a GSI-191 concern because it is internal to the
primary system.”
3.3.4 REACTOR VESSEL
After receiving the academic and industry reviews from the experts, the following 8 reactor vessel sub-component categories have been identified as potentially important non-piping components for the GSI-191 project:
- Reactor vessel flange - Instrument tubes
- Control rod drive mechanisms and housings - Thimble tubes
- Thermal shield - Nozzle safe ends
- Closure heads (torus, dome and cladding) - Nozzles.
Examples of the review comments include:
Control rod drive mechanism housings: “CRDM housing failures, especially the drive shaft housing could result in debris generation.”
123 3.3.5 EMERGENCY CORE COOLING SYSTEM
After receiving the academic and industry reviews from the experts, the following 3 ECCS sub-component categories have been identified as potentially important non-piping components for the GSI-191 project:
- Strainers, suction grating - Sump, thermowell
- High pressure injection system.
Examples of the review comments include:
Strainers, suction, grating, and sump: “If the ECCS suction strainers are weakened by corrosion or have additional buildup of corrosion prior to the need for recirculation, they could fail mechanically (allowing excess debris bypass to the core) or collapse and prevent pumping.”
Seals: “ECCS equipment requiring seals would not result in debris generation that would cause sump blockage.”
3.3.6 CHEMICAL VOLUME AND CONTROL SYSTEM
After receiving the academic and industry reviews from the experts, no CVCS sub- component categories were identified as potentially important non-piping components for the GSI-191 project. The consensus of the reviewers for the CVCS category has shown to be that
“Some CVCS equipment and piping is connected to RCS pressurized systems. This equipment could create debris.” However, “CVCS is another system located out of RCS pressure boundary between the first and second valves. The potential for debris generation could be an issue, but they are isolable-LOCA.”
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Therefore, since there was not any evidence of the potential for an unisolable-LOCA to occur from a subcomponent in the CVCS category, this category was eliminated from the investigative research.