A US nuclear reactor has switched from Regular to Premium fuel with Southern Nuclear inserting four Lead Test Assemblies (LTAs) into the Vogtle Unit 2 reactor in Waynesboro, Georgia that contains uranium enriched above 5% – a US commercial first.
Nuclear fuel is one of those things that we tend to pretty much take for granted. You have a nuclear reactor, it needs fuel, you put fuel in. QED. The general assumption is that filling a reactor with uranium and plutonium is like shoveling coal into a boiler furnace – a straightforward operation that, in the case of nuclear, is just a bit more involved.
Let’s leave aside how complex coal is (and, believe me, it is) and concentrate on nuclear fuel. In order for nuclear fission to occur, the nuclear ore not only needs to be refined, it needs to be enriched. That is, the natural percentage by weight of the uranium isotope 235 to 238 must be increased.
Natural uranium is only about 0.7% uranium-235 (U₂₃₅) and only a very few reactor designs can use this to any degree. The problem is that the balance that consists of uranium-238 (U₂₃₈) isn’t fissionable. That is, the atoms won’t split properly to produce energy, while those in U₂₃₅ do. Worse, in order for U₂₃₅ to produce a nuclear chain reaction, there has to be enough of it in a high enough concentration to sustain the reaction. In the case of most modern nuclear reactors, this concentration is about 3 to 5% U₂₃₅ to U₂₃₈, which is called Low-Enriched Uranium (LEU)
US Department of Energy
All this is basic high school physics, so you should have been paying attention in class. What isn’t is that this enrichment can vary with different kinds of reactors, achieving different results.
For example, nuclear submarines use Highly Enriched Uranium (HEU) fuel enriched to 20 to 93% U₂₃₅ to U₂₃₈. The latter is so high it, put simply, gets into the weapons-grade enrichment class.
One reason for doing this is to allow for reactors that can pump out a lot of power on demand, and I mean a lot. The aircraft carrier USS Enterprise had nuclear reactors that shoved it through the water so hard in tests that the standing orders were never to go at full throttle. Ever.
Other reasons are that this highly enriched fuel means you can build a submarine that never needs to be refueled. In fact, modern nuclear boats can’t be refueled; there’s no provision for it in their design. If you have to do so, it means cutting open the submarine with torches to get at the reactor. This fuel also means you can build very compact reactors that are very efficient and less prone to thermal and radiation damage than commercial reactors.
Working with the Westinghouse Electric Company, Southern Nuclear is streets away from anything like that. What they are doing is pushing the envelope a bit so that conventional reactors can have the benefits of next-generation reactors that are being built that use High-Assay LEU (HALEU) fuel where the enrichment is five to 20%.
Until now, HALEU reactors have been either research jobs or exotic new designs using molten salt technology and other advances. Developed as part of the US Department of Energy (DOE) Accident Tolerant Fuel Program, this commercial reactor test aims at finding new enriched fuels that can be burned in a conventional reactor to improve performance, increase fuel life, and reduce waste.
US Department of Energy
This isn’t simply a matter of adding more U₂₃₅ to the mix. When a fuel is highly enriched, a new recipe is needed to make the uranium pellets that go into the fuel rods. LEU fuels are essentially a uranium ceramic, but highly enriched fuels are a uranium-zirconium or uranium-aluminum alloy.
They also incorporate what are called burnable “nuclear poisons,” which are elements like boron-10 and gadolinium-157 that absorb neutrons and dampen nuclear reactions. This keeps the energy output even throughout the fuel’s life. Early on, the poisons hold back the reactions, but over time the poisons burn up, allowing the remaining fuel to keep blazing away at the same rate.
The experimental fuel for the test was made by Westinghouse. Called ADOPT, it’s an improved uranium dioxide (UO₂) design that’s doped with chromia (Cr₂O₃) and alumina (Al₂O₃). This produces greater uranium density, improves thermal conductivity, and reduces gas released by the fission process that can damage fuel. In addition, the interaction of ADOPT fuel with the metal cladding on the fuel rods reduces the chance of cladding failure.
According to Westinghouse a variant called LEU+ ADOPT fuel can handle enrichments of up to 8%, greatly improving conventional reactor performance with reduced needs for refueling. The ADOPT fuel pellets were produced at Westinghouse Springfields Fuels Ltd in Lea, Lancashire, England, with uranium powder provided by the Idaho National Laboratory. After being fabricated into four fuel rod assemblies, these were inserted into the Vogtle Unit 2 power plant where they will be monitored on how they respond to intense radiation exposure through multiple reactor cycles.
“This achievement is a significant step forward for not only the resiliency of the entire US operating fleet, but future nuclear technologies,” said Pete Sena, Southern Nuclear Chairman, President and CEO. “Our goal is to operate our units for longer periods with higher output, and with higher enriched fuel, we’re even better positioned to meet the growing energy demands of the state of Georgia.”
Source: US Department of Energy