ANS’s webinar on RIPB design in the NuScale US460
The American Nuclear Society’s Risk-informed, Performance-based Principles and Policy Committee (RP3C) has held another presentation in its monthly Community of Practice (CoP) series. Former RP3C chair N. Prasad Kadambi opened the meeting with brief introductory remarks about the RP3C and the need for new approaches to nuclear design that go beyond conventional and deterministic methods. He then welcomed this month’s speaker: Kent Welter, a member of the ANS Standards Board and chief engineer of testing and analysis at NuScale, who presented “RIPB Design and Licensing of the NuScale US460 Small Modular Reactor.”
Some background: RP3C is a special committee created by the ANS Standards Board and chaired by Steven Krahn that provides guidance to ANS standards committees on the use of risk-informed, performance-based (RIPB) methods. The CoP is part of RP3C’s charter, which includes training and knowledge-sharing of RIPB principles to exchange ideas outside of the normal management and project processes. CoPs are used frequently by organizations to help break down barriers that impede the flow of information.
A cutaway of the NPM (Source: NuScale)
The reactor: Welter opened his talk with a design overview of NuScale’s SMR, the US460, a power plant featuring six 77-MWe integral pressurized water reactors called NuScale Power Modules (NPMs). The NPM consists of a single 15-foot-wide by 76-foot-long containment vessel that contains the reactor core, steam generators, and pressurizer. This design eliminates reactor coolant pumps, large bore piping, and a bevy of other systems required by conventional reactor designs. The NPM has a design life of 60 years and requires refueling (with standard LWR fuel) once every 18–22 months.
Shifting to a safety lens, Welter highlighted that in the case of an accident the NPM does not require any AC or DC power, operator actions, or additional water to keep the reactor cooled. These safety features could allow the emergency planning zone (EPZ) around the plant to be as small as the plant’s site boundary, roughly 40–60 acres. This is orders of magnitude smaller than a conventional nuclear power plant’s EPZ, which is typically in the range of 5–10 miles.
Finally, before discussing the US460’s RIPB case, Welter explained the NPM’s unique ability to “black-start,” meaning its ability to operate without power from the grid. “Because there is more than one reactor inside the reactor building, and they are cross-connected electrically, if there is a loss of off-site power, the plant can still remain on.” He went on to say that when the plant switches into “island mode,” one reactor becomes a duty reactor, providing the power for the plant’s entire load, and allowing the plant to be the first to provide power back to the grid.
The results of NuScale’s defense-in-depth adequacy evaluation, comparing conventional, large-scale LWR designs against the NPM (Source: NuScale
Risk informed: Welter encouraged the audience to watch some of his other CoP presentations where he details the RIPB design process at NuScale before giving a brief overview of the newest evolutions in the company’s design. These changes included the addition of language in their procedures that better aligns with the recently approved standard ANS-30.3, Light Water Reactor Risk-Informed, Performance Based Design.
Another new addition to NuScale’s design process is a defense-in-depth adequacy evaluation built on the company’s defense-in-depth philosophy. That philosophy is aimed at preventing or lessening the effects of accidents and malfunctions through a layered and multibarrier approach. Some highlights of that approach include creating built-in redundancies, ensuring that safety is not dependent on any single design feature, aiming for engineering solutions over administrative ones, and developing feasible and workable emergency plans.
Performance based: Welter, who is the chair of the ANS-30.3 Working Group, ensured that during the development of the NPM NuScale closely followed the advice and tools that the standard provides.
One of the methods provided by the standard that NuScale adopted was its first key concept: a hierarchy of functions, requirements, and physical systems at the plant, system, subsystem, and component levels.
This hierarchy helps make sure that functions and requirements “are identified at all levels of the system and the relationships between them are maintained,” Welter said. “When it made sense, we established specific, measurable, quantitative safety performance objective requirements within this hierarchy.” These objectives, he explained, were set for things at the system level and component level alike, allowing for the quantitative analysis at each tier of the hierarchy.
Go deeper: Welter’s presentation, available at the above link, goes further into a comprehensive deep dive on the safety case design features of the NPM, the US460, NuScale’s design process, the company’s licensing status, and much more.
