Now that the West Virginia Legislature lifted the moratorium on nuclear energy in the Mountain State, we’ve been hearing a lot about the alternate energy form.
You may remember that early this month, Rep. David McKinley paid a visit to Harrison County to oversee the signing of a deal between X-Energy and Amstead Graphite Materials. X-Energy has a Department of Energy contract to build a nuclear reactor in Washington State, but it won’t use rods. Instead, it will use “pebbles,” and West Virginia’s Amstead Graphite will provide material for the pebbles’ protective coating. And just this week, state legislators received a presentation on the pebble bed micro-reactor being built near Pittsburgh.
Since they keep coming up, we thought we’d take the time to explain how small-scale nuclear reactors and their pebbles compare to traditional nuclear power generation.
The first thing you need to know about nuclear power is how it generates electricity. In a traditional plant, nuclear fission (splitting atoms) generates tons of energy in the form of heat. The reactor core, where this process happens, is usually cooled by flowing water over or around it. The water absorbs the heat, becomes hot itself and produces steam. The steam then turns turbines, and the turbines are what generate the electricity.
Pebble bed modular reactors work on a similar premise, but with different parts, which makes it easier to construct them as small-scale plants or micro-reactors. Instead of uranium and ceramic pellets stacked in rods, PBMRs use billiard or tennis ball-sized “pebbles.” Each pebble is comprised of thousands of tiny TRISO spheres, or particles, and each particle is a miniscule piece of enriched uranium surrounded by silicon carbide and other carbon insulators, and these are all surrounded by graphite. Think of a glitter-filled bouncy ball after it’s been shaken up: Each piece of glitter would be a TRISO-coated particle, and the plastic shell and liquid inside would be the graphite (but solid, of course).
The pebbles go through the fission process like nuclear rods do. However, unlike rods, which sit stagnant until they are used up, the pebbles will constantly move through the reactor’s core. For PBMRs, the core is shaped like an elongated funnel: New pebbles will be constantly added to the top as used pebbles come out the bottom. But “used” is relative. Each pebble takes about six months to pass through the core, but each pebble can be used up to six times, so it lasts about three years. PBMRs also run at higher temperatures and, combined with the reusable pebbles, tend to be more efficient.
The other key difference between traditional nuclear plants and PBMRs is the way they are cooled. Usually, nuclear reactors are cooled with water. PBMRs are cooled with helium instead. The heated helium is then sent directly to the turbine to power it, eliminating the need for water completely. Even though PBMRs work at higher temperatures, they are supposed to be more resistant to meltdowns if the coolant fails. The pebbles can withstand heat upwards of 2,732 degrees Fahrenheit, significantly higher than traditional nuclear plants, and the system is designed so ambient air can cool the reactor in the absence of helium — albeit more slowly.
This all sounds well and good, but pebble bed reactors aren’t 100% risk-free. The risks are just a little different. Instead of meltdowns, there’s a possibility of combustion if the graphite interacts with oxygen. Many PBMRs depend on the passive cooling system (air cooling) if the helium coolant fails instead of having active backup systems the way most nuclear plants do. And experts still aren’t completely sure what to do with pebbles that are completely used up, as they can’t be as easily repurposed as nuclear rods and have much greater mass.
As West Virginia ponders a future using nuclear power, these are all things to consider.
