Nuclear Fuel: Fission
One of the most important parts of a nuclear system is its fuel. Today’s large light water reactors use pellets made out of uranium oxide. These pellets are stacked in in fuel rods made out of zirconium. Multiple of these fuel rods are arrange in so called fuel assemblies. A typical option would be a 17×17 assembly.
n certain scenarios the zirconium cladding can become a problem. If it becomes too hot, it starts to act as a catalyst for splitting water. The created hydrogen can lead to explosions. This is what happened in the Fukushima-Daiichi nuclear power plant. 3 times.
There has been considerable work on improving these systems. They are collectively known as ATF, or accident tolerant fuels.
These aim to either replace the composition of the fuel pellets or the composition of the cladding.
Some of these are rather incremental ones. For example in an early phase of Westinghouse’s ATF program, the traditional UO2 pellets are doped with chromium (Cr3O2) and alumina (Al2O3)
This leads to an increased density of fissile material, higher thermal stability and a reduction of fission gas release in a transient scenario.
In later phases, new materials for fuel pellets will be investigated and ultimately commercialized. These include uranium silicide (U3Si2 ) and uranium nitride (U15N). The former will have a 17% increase in 235U density and produce 5x increase in thermal conductivity, the later will result in a 35% increase in 235U density and a 10x increase in thermal conductivity.
But there is also work on the cladding material. Framatome is working on chromium coated fuel rods.
Global Nuclear Fuel (GNF) is working on a new alloy for the fuel rods, called IronClad. It consists of an iron-chromium-aluminum alloy which showed outstanding oxidation resistance up to temperatures of 1500°C.
General Atomics developed a SiC-SiC material, initially for its Energy Multiplier Module. It is a ceramic material reinforced with flexible silicon carbide (SiC) fibers. It works like steel-reinforced concrete.
SiC produces 10 000 times less hydrogen at 1200°C than zirconium. Cladding made of SiC can withstand temperature of more than 1700°C.
Framatome and Westinghouse are aiming to produce just such claddings for later phases of their accident tolerant fuel programs.
LightBridge is developing a metallic zirconium-uranium fuel in a helical, multi-lobe form. This increases the surface area of the fuel and is supposed to enhance the coolant flow. The company claims that this fuel can be used for a 10% power uprate in existing power plants while simultaneously increasing the time between outages from 18 to 24 months.
Another use case is a newly designed pressurized water reactor: It could have a 30% power uprate (but without longer times between refueling). Although the enrichment for this fuel is in the 15-20% 235U range, it produces less plutonium than uranium oxide fuel. The net result should be a lower proliferation risk.
The higher level on enrichment poses challenges. Today only enrichments of up to 5% are commercially available.
Enrichments of up to 20% creates so called high-assay low enriched uranium. It is not considered too great of a threat from a proliferation perspective, but currently there are no commercial sources of it in the US, although at least two companies are capable of producing HALEU, Centrus and Urenco. The latter one exclusively for non-military purposes.
The capability of HALEU production is critical, because a lot of advanced reactor designs are dependent on these higher enrichment levels. A lot of these designs combine the HALEU with a decidedly different form of fuel.
It’s called TRistructural-ISOtropic (TRISO). It consists of a fuel kernel composed of a uranium containing material. UO2, UCO, UC and UN have been used for it. It’s coated with four layers of three different materials deposited through fluidized chemical vapor deposition. The first layer is a porous buffer made of carbon that absorbs and retains fission products. The second layer is made of dense, protective pyrolytic carbon (PyC). Around that is a ceramic layer of SiC. This retains fission products at elevated temperatures and gives TRISO particles more structural integrity. The last layer is made of a dense PyC again. These TRISO particles are tough. In fact, TRISO particles had been irradiated to previously unseen levels of burn-up and therefore fission product content. These particles were than tested at up to 1800°C and showed no to little damage and full fission product retention. These temperatures are well beyond anything that can be achieved in worst case accident scenarios. In other words, TRISO can’t melt down.
The TRISO particles are integrated into fuel elements, which are typically either spherical or hexagonal. The usage of cylindrical fuel elements, i.e. pellets for usage in pressurized water reactors has also been investigated. In the US, there are at least three companies trying to produce TRISO commercially, BWXT, X-Energy and Ultra Safe Nuclear Corporation.
The latter two are also developing reactor designs (gas cooled) to work with these fuels. But other companies are just waiting to get their hands on TRISO, like U-Battery, HolosGen or Kairos Power.
In fact, TRISO fuel is considered so safe by the Department of Defense, that they started “Project Pele” to assess mobile nuclear reactors that could be deployed to forwards facing bases. These will have to withstand direct, kinetic impacts without the release of radiation. Its high operating temperature makes it also a great candidate for nuclear propulsion systems in space. Yes, you read that right.
TRISO fuel is more complex and therefore costly to manufacture, but in a sense each particle comes with its own containment around it. This offers more “defense in depth” against the accidental release of fission products into the environment and should strengthen the safety case for most reactor designs relying on it. The fuel can also safely be used at very high temperatures to increase the efficiency of the power generation.
Heavy water moderated reactors have their own fuel. The fuel elements of a CANDU reactor are shorter than those of light water reactors. The fuel bundle in the picture will produce enough electricity for a Canadian family of four for about 100 years.
There is some research going on in that area, especially as India is trying to tap into its vast resources of thorium, which can be partially used in their IPHWRs.
A last area of new fuel systems is in liquid fuels. The idea is to dissolve the nuclear fuel in a salt, which simultaneously acts as primary coolant.These salts are then molten and circulated in the reactor core. Depending on the reactor system and especially neutron spectrum, different forms of salts are being considered. For fast spectra, chloride salts are envisioned, for thermal reactors, FLiBe, NaBe and FLiNaK are under active research. These salts generally have good thermal, neutronic and hydraulic properties and good fission product retention. Noble gases bubble out of the salt and can be easily collected, iodine and cesium form salts themselves that have boiling points well above the operating temperatures of these reactors. The salts expand easily when heated, creating in general strong negative temperature coefficients, i.e. if the reactor gets too hot, the nuclear reaction shuts down.
It remains to be seen, whether the increase in safety or operating temperature of the fuel itself can be translated into cost saving in the overall plant design and of course the cost of electricity.
A lot of companies are betting on it and our climate trajectory might depend on it.