Small Modular Reactors: ‘Sealing’ the Deal

Small modular reactors (SMRs), defined by the International Atomic Energy Agency (IAEA) as a reactor less than 300 MW, are fast emerging as a promising option for nuclear energy. They offer flexible power generation capabilities and enhanced safety qualities.

Their modular designs and interchangeable components allow them to be factory produced. Additional units can be added incrementally should a capacity increase be required. Examples of potential applications include remote sites such as maritime shipping locations and military installations, where a single SMR could power an entire community.

Electrical penetration assemblies (EPAs) are a small but important component for nuclear power generation, especially in the operation of SMRs. Electrical and instrumentation cables must be fed into the reactor through its containment structure to monitor and control it. These containment structures are entirely sealed-off constructions designed to safely contain the reactor. The EPAs must provide a leak-tight pass-through for the necessary cabling while simultaneously maintaining the pressure boundary integrity of the entire containment structure. This is of the highest importance and is the most essential function of the EPA: maintaining the pressure boundary in the event of a severe accident helps to prevent radiation and gas leakage, and keeps monitoring systems and reactor control systems intact.

EPAs are manufactured using one of two standard sealing material options. The first option is the use of organic, polymer-based sealing materials, such as epoxy or polyetheretherketone (PEEK). The second option is the use of inorganic materials such as glass. The seal must remain leak-tight over the operational lifetime of the reactor in order to maintain effective safety levels and avoid costly downtime for maintenance and replacement.

The seals are exposed to radiation, humidity, and fluctuating temperatures and pressures. With organic materials, degradation is an unavoidable issue. Polymers naturally age, leading to a deterioration in the seal performance over time. Such degradation can have catastrophic consequences in the event of a severe accident. As such, recently-published IAEA guidelines—“Assessment of Equipment Capability to Perform Reliably under Severe Accident Conditions”—recommend choosing inorganic materials over certain types of less-durable polymer-based materials in instrumentation and control components, such as EPAs, for nuclear power plants.

At Fukushima, for example, the accident was deemed by the U.S. Nuclear Regulatory Commission to have been caused by a long-lasting complete loss of power, due to common-cause failure of the electrical equipment after the March 2011 tsunami.

The high temperature and pressure conditions of the accident far exceeded the design capabilities of the electrical penetrations and door hatch seals, resulting in leakage paths for radiation and hydrogen. This hydrogen leakage resulted in hydrogen explosions and ultimately loss of secondary containment. The electrical penetrations at the site were reported to have organic epoxy-based polymer seals. When put to the ultimate test, these seals could not withstand the temperature and pressure levels and allowed for the leakage of hydrogen gas, further worsening an already dire situation.

Benefits of Glass-to-Metal Sealed Penetrations

International safety requirements for electrical penetrations are substantially lower than the safety margin of the nuclear power plant containments. This means there is potential for penetrations to be a weak link in the safety equation if they utilize less-durable organic, polymer-based sealing materials. In order to mitigate the risk of seal failure due to the degradation of organic compounds, it is advisable to consider inorganic compounds that will not degrade over time. It is here that glass-to-metal sealed (GTMS) penetrations (Figure 1) present a viable alternative thanks to superior reliability, safety, and lifecycle cost. These characteristics provide nuclear plant managers a more favorable chance of avoiding daunting expenditures associated with a plant shutdown that can cost nearly $1 million per day of lost operation.

SCHOTT AG in Landshut - Produktion von Grossdurchfuehrungen im Werk I Foto: SCHOTT AG / HRSchulz
1. Quality check. A SCHOTT technician does a quality check of a glass-to-metal sealed (GTMS) electrical penetration assembly. GTMS penetrations are made in a process that fuses superheated glass and specially selected metal to create a hermetic seal. Courtesy: SCHOTT

GTMS penetrations are constructed using only inorganic, non-aging sealing materials. The production is as unique and important as the material selection: GTMS penetrations are made by a manufacturing process that fuses superheated glass and specially-selected metal to create a hermetic seal. The strength of the seal allows for the safe conduction of electricity and data through the fire-protective, pressure-resistant, and tightly sealed containment walls of a nuclear reactor while still maintaining a gas-tight barrier. Should an accident occur, a glass seal is able to survive significantly elevated temperatures of more than 700F and pressures greater than 5,800 psi for an extended period of several hours. The inorganic materials do not age or degrade over time and are unaffected by routine exposure to temperature and radiation.

The fact that glass seals are maintenance-free represents a significant advantage in the form of reduced total cost of ownership. Operational costs are lower compared to organic polymer seals, which wear out in as little as 25 years, well before the lifespan of the reactor, and require costly downtime for complicated replacements.

EPAs in SMRs: Painting a Picture for a Viable Nuclear Future

SMRs present the unique arrangement of incorporating sets of electrical penetrations in separate areas: in the containment and in the first loop of the reactor itself.

Historically, for the controllers or the fuel rods, these mechanisms were on the outside. Now, with these new designs, they are placed inside. This requires penetrations that can manage high pressures and high temperatures in a smaller containment, so there is a need for penetrations that are smaller and can withstand maintained exposure to high heat and elevated pressure. As such, EPAs with polymer seals that have inferior temperature and pressure resistance for SMRs are not feasible. GTMS EPAs are the only viable option for this application.

One significant innovation of SMRs is that many components can be manufactured quickly and efficiently, and easily put into operation by utilizing a strong connector. Advanced EPAs can connect 120 electric conductors. The unit is simply plugged in, locked, and ready for operation. Supplying individual wires to a junction box can take hours of strenuous work. By choosing a plug-and-play option, the process can be streamlined.

As advancements continue in smaller, modular reactor technology across a wide range of applications, the safety elements will be front-of-mind for manufacturers, operators, and governments. High-quality EPAs are necessary for these nuclear power plants to function safely and reliably.

The combination of unique and innovative assembly and operation of SMRs combined with the superior safety and performance of GTMS EPAs represents a viable option for the advancement of nuclear power in the years to come. ■

Thomas Fink is general manager at SCHOTT Electronic Packaging.