What is ANEEL and why thorium could change nuclear energy

The Idaho National Laboratory has completed a two-year testing program of ANEEL fuel, developed by Clean Core Thorium Energy. The fuel is a hybrid mixture of thorium and HALEU (high-assay low-enriched uranium), with a U-235 enrichment level of approximately 5–20%. If we were to list technologies currently considered most urgently needed, next-generation nuclear fuel would likely not rank high for most people. Public attention is usually focused on areas such as artificial intelligence or medical breakthroughs. However, experimental work in nuclear materials continues in specialized research facilities.

In this case, a two-year experiment conducted at an underground laboratory in Idaho has recently been completed, and its results were of interest to researchers involved in the project. The work relates to ongoing efforts to evaluate alternative nuclear fuel concepts, which could influence future reactor designs and fuel efficiency, although practical deployment would require further validation. Before going into the technical details, it is useful to provide context, as discussions around “breakthroughs” in nuclear technology often circulate in simplified or overstated forms.

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What is problematic about conventional nuclear fuel?

Nuclear power plants are, in essence, highly complex steam engines. The basic principle is the same as in early steam locomotives: heat water, produce steam, and drive a turbine. The difference is that instead of burning coal, the heat comes from nuclear reactions inside a reactor contained within a protective structure.

Most modern reactors use uranium-235 as fuel. It is typically enriched to around 5%. This isotope is responsible for sustaining the chain reaction, while the rest of the system is focused on control, cooling, and containment. This approach is generally effective in terms of energy output and stability. However, challenges become more apparent when considering the back end of the fuel cycle, particularly spent nuclear fuel and long-lived radioactive waste, which require careful handling, storage, and long-term management.

This is where the real issue begins. After nuclear fuel has been “spent” in a reactor, it still contains plutonium, americium, and a range of other transuranic elements. Some of these remain hazardous for tens of thousands of years – not hundreds or even thousands. To put that timescale into perspective, it is comparable to the period when humans were first creating cave paintings of animals. In other words, we are dealing with materials that outlast recorded history by a significant margin.

This long-lived radioactive waste is one of the main arguments raised by critics of nuclear power. And, importantly, the concern is not without technical basis, since it involves both long-term safety and the challenge of permanent containment.

Thorium: an element that was largely set aside

Thorium is a naturally occurring radioactive metal, discovered in 1828 and named after the Norse god of thunder. It is roughly four times more abundant in the Earth’s crust than uranium, meaning it is relatively widespread in nature. In the mid-20th century, thorium was seriously considered as a potential basis for nuclear energy systems. However, during the Cold War, development efforts largely shifted toward uranium-based reactors. One key reason was that uranium reactors produce plutonium, which can be used in nuclear weapons programs.

Thorium fuel cycles do not efficiently produce weapons-grade materials, which is often described as proliferation resistance. This characteristic made thorium less attractive in a military-driven development context at the time. Paradoxically, this same limitation is now viewed as an advantage in civilian nuclear energy discussions, where non-proliferation and fuel cycle safety are important considerations.

After that, interest in thorium gradually declined. For decades, it remained on the margins of nuclear research – considered too expensive for large-scale deployment and not easily compatible with existing reactor designs. This situation persisted until the American company Clean Core Thorium Energy began working on it in a more systematic way, revisiting thorium-based fuel concepts within modern reactor development frameworks.

ANEEL: what it is and why it matters

ANEEL stands for Advanced Nuclear Energy for Enriched Life. The name is somewhat promotional, but the concept itself is straightforward: it is a hybrid nuclear fuel composed of thorium and HALEU (high-assay low-enriched uranium), with an increased concentration of uranium-235. The goal of the developers was not to create a completely new reactor technology, but rather a fuel that could potentially be used in existing reactor designs with minimal modification.

In practical terms, this approach is often compared to replacing batteries in a device rather than replacing the entire system. The idea is to improve fuel characteristics while avoiding the need for a full redesign of nuclear power infrastructure.

In particular, this concept is being evaluated for pressurized heavy water reactors. These systems use heavy water (deuterium oxide) instead of ordinary light water as both a coolant and neutron moderator. As a result, they can operate with natural or near-natural uranium, typically containing about 0.72% uranium-235. However, this also places constraints on fuel utilization efficiency. In effect, only a limited portion of the potential energy in the fuel is extracted under standard operating conditions.

In May 2024, the Idaho National Laboratory loaded 12 experimental ANEEL fuel rods into the Advanced Test Reactor. The experiment was designed around three target burnup levels: 20, 40, and 60 gigawatt-days per metric ton of uranium. Burnup is a measure of how much energy is extracted from a given amount of nuclear fuel. In general, higher burnup values indicate more efficient fuel utilization.

Eight of the fuel rods reached the first two burnup targets already last year. The remaining four recently exceeded the highest target of 60 gigawatt-days per metric ton. Notably, the reactor conditions were intentionally more demanding than those typically found in commercial operation. This can be compared to a test drive where a vehicle is deliberately driven at high speed over rough terrain to identify failure points under extreme stress rather than normal use conditions.

It didn’t break.

“This surprised even us”

The key figure reported by the developers is that ANEEL fuel demonstrated a burnup level more than eight times higher than conventional fuel used in heavy water reactors. Eight times higher is not a marginal improvement or a minor optimization. It represents a different order of magnitude in fuel utilization compared to standard benchmarks.

The CEO of Clean Core Thorium Energy, Mehul Shah, frames the objective in pragmatic terms: not to develop entirely new reactor types, but to integrate thorium into the existing nuclear fuel cycle. If ANEEL can deliver comparable or higher energy output than fuel used in light water reactors, it could, in principle, support broader deployment without requiring large-scale investment in new reactor infrastructure.

This is important because building a new nuclear reactor typically takes 15–20 years and requires billions of dollars in investment. In contrast, replacing the fuel in an already operating reactor is a fundamentally different proposition. It avoids the need for large-scale construction projects and long regulatory timelines associated with new reactor builds, focusing instead on modifications within existing infrastructure.

What about waste?

One of the main arguments in favour of thorium-based fuel cycles is the difference in waste composition compared to conventional uranium fuel. In theory, a thorium fuel cycle produces fewer long-lived transuranic elements than traditional uranium-based fuel. As a result, the radiotoxicity of spent fuel can decrease more quickly over time, potentially reaching comparatively lower levels on the scale of a few hundred years rather than thousands.

This difference is not only technical. It also affects practical considerations such as long-term waste storage requirements, cost of geological repositories, and public perception of nuclear safety. Time horizons of a few hundred years are still significant in engineering terms, but they are more manageable to conceptualize and plan for than timescales extending into tens of thousands of years, which are associated with conventional spent nuclear fuel.

Here it is important to make a key clarification, which the developers themselves do not hide: thorium fuel is not a “magic solution” and it is not waste-free. The International Atomic Energy Agency explicitly notes that the thorium fuel cycle still requires complex infrastructure for fuel reprocessing and material handling. Certain isotopes produced during the reaction remain radioactive and must still be managed appropriately.

The difference compared to conventional uranium fuel is mainly quantitative rather than absolute: there is generally less long-lived radioactive waste, and some of it has shorter effective timescales. However, it still requires careful engineering, regulation, and long-term control.

Safety: higher melting point and reduced accident risk

Another potential advantage worth noting is that thorium-based fuel has a higher melting point and generally improved thermal characteristics compared to conventional uranium fuel. In practical terms, this means fuel elements may tolerate higher temperatures and provide a greater margin under overheating conditions or during abnormal reactor operation scenarios.

In a hypothetical scenario where something goes wrong, an ANEEL-based fuel system may provide a larger thermal safety margin before conditions become critical. This does not imply that such a reactor is inherently “fail-safe” or immune to severe incidents, since all nuclear installations depend on properly functioning safety systems and strict operational control. However, the design characteristics can offer additional buffer under certain abnormal conditions.

Separately, there is the previously mentioned aspect of proliferation resistance. Waste products from a thorium-based cycle are generally less suitable for use in nuclear weapons compared to conventional fuel cycles. In the current geopolitical context, this is considered a relevant strategic factor in nuclear fuel development discussions.

What is holding back widespread adoption?

It would be misleading to present this technology as a straightforward upgrade. The thorium fuel cycle still lacks widespread commercial deployment, which is a major practical limitation. This means that much of the supporting ecosystem would need to be developed almost from scratch. That includes fuel production chains, regulatory and licensing frameworks, and logistics for handling and processing materials. Building this infrastructure would require significant investment and time, likely spanning many years before large-scale implementation could be feasible.

There is also the issue of HALEU availability. High-Assay Low-Enriched Uranium is a required component of ANEEL fuel, but its production is currently limited. Expanding HALEU manufacturing capacity is a separate and substantial industrial challenge.

Following the laboratory phase, ANEEL fuel rods are now undergoing detailed post-irradiation examination. The next step is expected to involve demonstration testing at operational commercial nuclear power plants. This represents a significantly higher level of complexity and regulatory responsibility compared to controlled laboratory environments.

And what about Ukraine?

This question naturally arises, given that before the full-scale war, nuclear power accounted for roughly half of Ukraine’s electricity generation. Nuclear power plants remain a critical part of the country’s energy system.

However, there is an important technical limitation. Ukraine operates VVER-type reactors (water–water energetic reactors), which are light-water reactor designs originally developed in the Soviet Union.

ANEEL fuel, on the other hand, was primarily designed with heavy water reactor applications in mind. Because of this mismatch in reactor physics and fuel requirements, direct use of ANEEL in Ukraine’s current nuclear fleet is not feasible without significant redesign or broader changes in reactor technology and fuel qualification processes.

Looking at this only in terms of “compatible or not compatible” would be too narrow. The broader idea behind more efficient fuel concepts – lower long-lived waste production, improved safety margins, and potential use in existing reactor infrastructure – can be relevant in the context of long-term energy system reconstruction. This is particularly important given that Ukraine is already considering small modular reactors as one of the possible directions for future energy development, where different technological platforms and fuel strategies may be applied.

Another dimension is operational security in conflict conditions. The situation at the Zaporizhzhia Nuclear Power Plant has demonstrated how critical stable cooling systems and external power supply are for nuclear safety. In this context, reactor designs and fuel systems with greater inherent safety margins and reduced sensitivity to external disruptions are not purely theoretical considerations, but part of broader discussions around energy security and resilience.

What comes next?

To summarize, ANEEL is not a technology on the level of concepts like teleportation or fusion energy. It is not a distant future system that might only become relevant decades from now. Instead, it is a practical fuel concept that has already undergone controlled testing and produced results that exceeded initial expectations. At the same time, there is always a substantial gap between successful laboratory-scale experiments and full commercial deployment across operating nuclear power plants worldwide. This transition typically involves years of regulatory review, significant capital investment, and the development of industrial supply chains for production and certification.

The nuclear industry is inherently conservative in its adoption of new fuel technologies, largely due to the safety requirements and long operational lifecycles of reactors. As a result, progress tends to be gradual rather than rapid, even when early experimental results are positive.

There is, however, a broader sense that something may be shifting. Not because of a single experiment in Idaho, but because such work is part of a wider effort to reconsider how nuclear energy is designed and operated – toward higher efficiency, improved safety margins, and potentially reduced long-term waste burdens. Whether this leads to a meaningful transition will depend on much more than laboratory results. It requires industrial scaling, regulatory approval, and sustained investment over many years.

Historically, technologies that once seemed speculative have sometimes become routine over time. Whether thorium-based approaches follow a similar trajectory remains an open question. For now, they sit at the intersection of established nuclear engineering and ongoing experimental development, competing for attention in a field that is also driven by advances in areas like artificial intelligence and space exploration.

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