Neutron detection made easy for SMRs

Above: A local power range monitoring (LPRM) system measures neutron flux

The emergence of the small modular reactor (SMR) sector is prompting capacity ramp ups across the supply chain as well as new designs for key equipment areas such as neutron detection. Reuter-Stokes, which has been designing and manufacturing neutron detection devices since the 1950s, is embracing these new opportunities.

Rod Martinez, Reuter-Stokes Vice President, says: “in 1986 we became the OEM manufacturer of neutron sensors for boiling water reactors. Since then, we’ve been making neutron detectors for BWRs and PWRs and our company has expanded to not only provide neutron detection but auxiliary equipment. We’ve made the sensors that these companies need. With the emergence of SMR we’re helping these companies that are new names in the industry to design sensors too.”

Reuter-Stokes has already made market in-roads, recently signing a contract with NuScale to design and manufacture unique neutron detectors for the company’s SMR design.

Although currently the NuScale’s is the only SMR with design approval from the U.S. Nuclear Regulatory Commission, Reuter-Stokes is clear in its ambition to become the go-to supplier for neutron detectors right across the SMR industry by leveraging the company’s experience. Daniel Schreiner, Nuclear Product Manager, for Reuter-Stokes, explains: “From a Reuter-Stokes perspective we want to work right across the sector. Our detectors are essentially reactor technology agnostic. They can work with any kind of reactor, just as long as it generates neutrons.”

The first NuScale project to use the detectors, which are being developed in partnership with Paragon Energy Solutions, will be the Carbon Free Power Project (CFPP) in Idaho Falls, Idaho, which will see the first VOYGR SMR power plant installed. It is scheduled to begin generating power in 2029. Reuter-Stokes’ detector assemblies will be responsible for monitoring the fission rate while Paragon will develop the signal processing electronics and also the associated reactor protection system. “Our partnership with Paragon, where customers can realise our combined offering of detection technology and control system technology is unique,” says Martinez.

Detector decisions

For neutron detection, the major difference between large-scale boiling water reactors and pressurized water reactors (BWRs and PWRs) is the physical location of the neutron detector. That location drives the geometry and the size.

“For the boiling water reactor fleet our detectors are located inside the nuclear core right next to the fuel bundles. Since they’re very close to the fissioning fuel the detectors can be quite small. On a pressurized water reactor our detectors are located outside of the nuclear core, maybe one or two meters away so they see significantly less neutron flux. Therefore, the neutron detectors are substantially larger and could be up to, say, 40 inches (100 cm),” says Schreiner.

For small modular reactors, companies are developing reactors based on both PWR and, less frequently, BWR technologies. “While the majority of the market is going the PWR route there are some boiling water reactors currently being designed,” says Martinez, adding: “Just about everything else is a pressurised water reactor, such as NuScale, Holtech and Rolls Royce but there’s also some others that are being developed. While they’re well understood and have been theoretically worked on for decades we’re getting to the point where we’re going to construct new fourth generation plants like high-temperature gas reactors or liquid metal reactors so there’s quite a bit of variation out there. Every one has their own pros and cons.”

As Schreiner observes: “We would love to make just a generic detector that worked for everyone but what we’re seeing is that the different SMR developers are different enough that we are going to have to develop bespoke detector designs for each developer. A NuScale reactor detector design is not going to work for an X-Energy reactor detector design and so on. Each one of these reactors is different and is going to require a little bit of engineering work to custom fit our detectors to that specific technology. They’re potentially each going to need a separate design and qualification effort.” For example, on the large gigawatt-scale reactors all of the Reuter-Stokes detectors are of the fission chamber type. However, for small modular reactors, because of the differing technical parameters like temperature and neutron flux, it is possible to use ionization chambers in some instances as well. Because ionization chambers do not contain uranium they don’t require special nuclear material considerations that affect things like logistics regulations. That potentially is another benefit particularly associated with SMR technologies which are designed to be easily transported to site.

Nonetheless, the challenge of new reactor designs new market entrants is picked up by Martinez, who says: “They’ve done a lot of research, but are now in a commercialization phase addressing the challenges and developing the sciences to deliver an SMR facility and all the benefits that brings. We are helping these companies design what they actually need for their particular reactor design from a sensor side with practical experience. We’re already well established and we can help to make the discovery phase and design phase a bit easier. Luckily, that’s one of our sweet spots.”

Phased detector development

To ease the design and qualification process, Reuter-Stokes has launched a three phase programme for SMR developers. “We’ve developed this phased process to try to help in terms of risk management and cost management,” says Schreiner. The problem is that SMR developers don’t always have all the answers when considering neutron detection. “We can’t go directly into detailed design without really understanding what the requirements are for these detectors, the technical specifications and the acceptance criteria for example,” Schreiner says. However, he adds: “Since we’ve been in business for so long, we do have a large catalogue. We have thousands of different detector designs that are both fission chamber-based
and also ionization chamber types. When a new customer comes in and details the neutron flux, the temperature, and the sensitivity required our physicists can go into this database and find two or three detectors that are close. We can use that as the foundation for a custom product offering. We’re not starting from scratch with detailed engineering, we’re starting from maybe anywhere from the 60 to 80 percent complete phase.”

This allows a two to six month front-end engineering effort to be launched – phase one preliminary design – working with the SMR developer and the SMR fuels engineer and talking directly to detector physicists to explore issues such as the location of the detector, what the environment looks like at that specific location and then which detectors from the catalogue could work for this application.

“Once that process is complete now both sides have a really good understanding of what this technical specification looks like. Then the SMR developer can publish its technical specification and we’ll provide a fixed-term quote on phase two, which is detailed design and building a prototype detector to validate all of the physics models and manufacturing processes,” says Schreiner.

Once phase two is complete the process can move to qualification with the relevant nuclear regulatory authorities. “It’s a very rigorous and methodical qualification process for a nuclear safety-related detector to ensure that the detector has been qualified for the safe operation start up and shut down of a nuclear reactor. It’s a huge investment both in design and time and in dollars to get that qualification. Reuter-Stokes’ experience of going through the qualification process helps when it comes to new designs and new types of sensors,” explains Schreiner. He concludes: “It’s mitigating risk and making sure that we control costs as we progress through the phases so that we’re constantly incorporating the lessons learned as we continue to work on the project with the customer”.

The pressure is on all SMR developers to keep costs down as low as possible and Martinez makes a key observation: “The most important thing in terms of making things cheaper is you got to think about it early in the process. If we can have our detector physicists talking directly to the fuels engineer and they can work out where these detectors go before the SMR design is locked in that allows the widest range of detectors that we can choose from our off the shelf database.”

It’s simple steps like this that will ensure the supply chain can support SMR development in making its ambitions a reality.