The Rise and Fall and Rise Again of Nuclear Power

Since its discovery in 1938, nuclear fission has been studied and tested by scientists trying to harness the power generated to run everything from submarines to powering electrical grids. The U.S. Department of Energy (DOE) estimates that nuclear power contributes nearly 20% of all electricity generated in the U.S. today. Valves and actuators used on nuclear plants must meet stringent guidelines and specifications set by the Nuclear Regulatory Commission (NRC), ASME Boiler and Pressure Vessel Code (BPVC) and other regulatory agencies and requirements to ensure they are safe and fully seal to avoid leaks or create safety concerns. This article will give an overview of the technology, current state of the industry and delve into specific challenges that nuclear applications create for valve and actuator manufacturers. 

How does it work?

Nuclear fission occurs when the nucleus of an atom is bombarded by neutrons and protons and other particles and then splits into two or more smaller nuclei creating a chain reaction. This chain may release enormous amounts of energy in the form of heat and radiation that can be harnessed for electricity. This is accomplished in many modern reactors when the heat is used to boil water to temperatures of over 500°F and create steam that is then used to drive steam turbines to generate electricity. This very clean process doesn’t generate fossil fuel emissions like many other forms of power generation do but does generate nuclear waste from spent fuel that must be handled and disposed of very carefully. 

Uranium is the mineral most often used for nuclear fuel, and estimates are that one pound of uranium has as much energy as 3 million pounds of coal — making it an incredibly efficient fuel source and process despite the safety risks. Radiation is measured in “half-life” or the length of time it takes a material to lose half of its radioactivity. U-238, the most common form of uranium, has a half-life of 4.5 billion years, and uranium U-235 of about 700 million years. With that in mind, it’s critical that there are safe and effective disposal and storage mechanisms for spent fuel to make nuclear a viable option.

According to the World Nuclear Association, recycling today is largely based on converting U-238 to plutonium. This plutonium can then be used again as fresh fuel for fission power generation, as is currently the case in Japan, France, Russia and China, but other countries are not yet reprocessing the materials. By reprocessing used material to recover uranium and plutonium, operators not only avoid wasting valuable materials but also eliminate some of the waste stream. To date, World Nuclear estimates about 30% of spent fuel from nuclear power plants has been reprocessed.

In the U.S., the Nuclear Waste Policy Act of 1982, as amended, declares that the federal government is responsible for providing a place to permanently store high-level radioactive waste, while those who generate the waste are responsible for all costs of permanent disposal. This Act has been amended over time, and the U.S. government has designated Yucca Mountain, NV, as the permanent site for nuclear waste for the U.S., although the facility is still on hold so there is currently no waste at the site, or a completed facility for storage.  Other sites may be considered in the future but must have crystalline rock formations and particular geological qualities to deem them suitable for permanent and safe storage.

Use and recycling of nuclear fuel

Nuclear fuel is solid and is used as small ceramic pellets of enriched uranium oxide, encased in metal cladding to form fuel rods. The rods are bundled together into assemblies that are placed into the reactor. Once used, the rods are placed in steel-lined concrete pools of water to cool, then put into dry storage casks made of concrete and steel for protective shielding. The fuel can then be reprocessed to recycle and separate out the radioactive components for reuse or to be permanently stored. Whether it will be stored or transported to a recycling and reprocessing facility, it is placed in the casks that are built to withstand fire, water immersion, impact and punctures, keeping the radiation safely contained. 

In total, since the first nuclear power plants went online, it is estimated the U.S. has produced 90,000 metric tons of spent fuel, an amount that if stacked would cover one football field at a depth of less than 10 yards — a seemingly small amount relative to the power generated.

Spent nuclear material can be reprocessed using three different methods: hydrometallurgy, which uses an aqueous solution to dissolve metals and then separate them using electrolysis; electrometallurgy uses electrical current to separate the metals in solid form; and pyrometallurgy uses heat to initiate separation of the metals. Reprocessing often uses a combination of these technologies, and today the most current process is called PUREX, a plutonium uranium extraction process. It uses nitric acid to dissolve the fuel elements then chemical separation via solvent extraction. This very complex process requires multiple facilities and technologies but is the most commonly used process. This chart shows the flow of fuel through the PUREX process. 

Current state of nuclear energy

Because of a couple of high-profile nuclear disasters such as Fukushima, and a push toward more long-term sustainable technology, many countries decided to limit their use of nuclear and move toward renewables such as solar and wind. Among these is Germany, where nuclear capacity has been decommissioned with the last reactors shut down in 2023. However, many other European nations are pursuing more nuclear capacity, including government subsidies for low- or no-interest loans being considered in France. 

The U.S. has the most nuclear reactors currently in service with 94. The World Nuclear Association cites reactors in 32 countries totaling 440 operational. But with the extreme costs and frequency of delayed schedules to build new reactors or retrofit existing ones, many countries are hesitant to build new capacity. There is hope being placed in a new class of reactors called Small Modular Reactors (SMR). These SMRs can be installed on a very small footprint and are designed to generate around 300 MW, or enough energy to power an average of 220,000 homes in the U.S. or to easily power a specific facility or plant.

The most frequently cited plants under development are planned to power data centers and other highly energy intensive operations, with agreements recently signed by Amazon, Google and others to independently power their massive data centers on site without the need for major utility infrastructure. These small plants are currently in development and are planned to go online as soon as 2035. Google is working with Kairos Power and Amazon with X-energy for its SMR designs, however, only NuScale has received design approval from the U.S. Nuclear Regulatory Commission for its SMR design.

Several other companies are in varying degrees of “preapplication activities” according to the NRC, so it is likely that in the next decade other designs may achieve NRC design approval. Globally, Rolls-Royce, Westinghouse, GE-Hitachi and Holtec are also developing SMR technology, with Romania’s Nuclearelectrica planning to deploy its first NuScale reactor by 2028 and Poland by 2029. This is an ever-evolving technology and market and it is expected to continue on this path for the foreseeable future.

Flow control products in nuclear applications

Within nuclear power plants are a wide variety of valves, some specific to these applications. For example, reactor coolant system valves are often gate or globe valves that must operate under extreme pressure and temperature. Containment isolation valves require fast and reliable actuation to isolate containment if there is an emergency. Steam system valves encounter very high temperatures and must be resistant to creep, which is a gradual and permanent deformation of metal components. This creates leaking conditions and is often caused by damaged seals or debris or contamination in the flow.

Among these applications, globe, gate, ball and check valves are used in various systems and they must be resistant to a variety of conditions including:

• Thermal and mechanical stress: exposure to prolonged high temperatures and pressures that could lead to wear, fatigue and deformation.
• Corrosion or erosion: high velocity flow or aggressive chemicals used in the process can compromise seal and valve integrity.
• Leakage: small leaks could be catastrophic and lead to significant safety issues at the worst and while system inefficiencies pose less risk, they can still cause potential damage to the systems. 
• Actuator malfunctions: actuators are used throughout the plants to operate valves. If their integrity is compromised or they fail in any way, this could create a catastrophic safety condition.
• Radiation exposure: components that are exposed to the radiation from the fuel rods are at higher risk of degrading which can affect valve or actuator performance.

To address these needs, specific materials and design requirements and constraints are imperative in the nuclear industry. Materials including Inconel or other nickel-based alloys, stainless steel or other alloys are application specific. Nickel-based alloys are often required for high-temperature and high-pressure areas of the plant with the components ability to withstand harsh and corrosive environments, radiation and hydrogen embrittlement. 

Reactor buildings are often very tight quarters, so size constraints for valves and all equipment are very specific. Valve sizes must be carefully specified based on the application, flow rates and the space allowed for the valve in the physical footprint of the plant. Often valves must be custom engineered to fit into applications properly, but all must still meet the requirements of the ASME BPVC Section 3, and be NRC certified for use in U.S. plants. 

Nuclear power plants follow rigorous maintenance schedules and must adhere to the defined life of the valves and equipment. While some valves will need to be replaced at set intervals, others may be built to last for the entire operational life of the facility with regular maintenance. ASME code mandates inspection intervals for valves in working plants, and advanced diagnostics can be used to predict failures or required maintenance based on vibration monitoring, thermal imaging and other non-destructive testing methodologies are employed to validate the safety of equipment while in operation.

New technology in nuclear

Along with SMRs, Generation IV reactors, such as sodium-cooled fast reactors (often called Natrium), gas-cooled reactors and molten salt reactors are under development by companies including TerraPower (cofounded by Bill Gates and GE Hitachi). These reactors use liquid sodium instead of water for heat transfer. Molten salt is also used for energy storage, which allows plants to store energy that is produced until it is needed based on load. Power output can be adjusted based on grid demand more easily with these reactors, and the higher boiling point of sodium reduces the risks of coolant boiling and pressurization. The chart below was derived from the World Nuclear Association and GE Vernova’s websites.

Feature	Natrium reactor	Boiling water reactor (BWR)
Coolant	Liquid sodium, allowing operation at higher temperatures without high-pressure systems.	Water, which boils within the reactor vessel to generate steam to power the turbines.
Pressure	Operates at near-atmospheric pressure due to sodium’s high boiling point, reducing the risk of pressure-related incidents.	Requires high-pressure systems to prevent boiling water within the reactor vessel, which necessitates higher pressure vessels and equipment and enhanced safety protocols.
Energy storage	Features an integrated molten salt energy storage system for flexible output and grid stability.	Lacks integrated storage so output is constant and can’t be easily adjusted based on grid demands.
Safety considerations	Sodium coolant operates at low pressure but is more reactive with air and water so still requires stringent containment measures.	Water as coolant is less reactive but requires higher pressure equipment and multiple safety systems to regulate and monitor pressure and temperature.

With this new generation of reactors, nuclear power appears to be gaining on its clean energy capabilities and offers advantages such as flexible output and the capacity for energy storage that traditional BWRs don’t offer today.

In addition to this new technology, additive manufacturing or 3D printing is finding applications within nuclear power applications. Oak Ridge National Laboratories working with Framatome 3D printed some brackets for use in a reactor that will be in service for 6 years then removed and tested.

Westinghouse and Carnegie Mellon have collaborated on spacer grids used to hold fuel and control rod assemblies. These parts are traditionally manufactured through stamping and welding, requiring a lot of precision machining and manufacturing. With 3D printing, the parts can be printed with a laser powder bed fusion process and produce these grids in one piece. This can reduce total part count, increase production and reduce cost of these components for nuclear plants, all while producing very complicated assemblies in just one operation.

While some aspects of the valve industry and the markets served are slow to change and still use technology invented centuries ago, the nuclear market is one in a constant state of change and refinement and is getting a new life thanks to a focus on sustainable and consistent power generation requirements. The once highly lauded technology has seen ebbs and flows but it appears to be on the upswing again.