Building a Thorium Reactor Without Plutonium or Uranium: An Overview

The concept of constructing a thorium reactor without utilizing plutonium or uranium is both intriguing and complex. While it is technically feasible to design such a reactor, several technical and practical challenges must be addressed. Let’s delve into the details.

Introduction to Thorium Reactors

Thorium-based reactors represent a promising pathway towards sustainable energy production. Unlike traditional reactors that rely on fissile materials like uranium or plutonium, thorium is a fertile material. This means that thorium itself cannot sustain a chain reaction; instead, it needs to be exposed to neutrons to transform into a fissile material, specifically uranium-233 (U-233) in the case of thorium-232 (Th-232).

Technical Challenges

Neutron Source Requirement: For a thorium-based reactor to achieve its first criticality, a neutron source is essential. This is because thorium-232 needs to capture neutrons to convert into protactinium-233 (Pa-233), which then decays into uranium-233. Both Pu-239 and U-235 are commonly used as initial neutron sources due to their fissile properties. Once U-233 is produced, it can sustain the chain reaction.

Molten Salt Reactor Design: One workaround is to use a molten salt reactor (MSR) design. In such reactors, thorium is dissolved in a molten salt, allowing it to be easily exposed to the neutron flux. This process can take about a month for the thorium to transform into U-233, which then becomes the sustaining fuel of the reactor.

Alternative Neutron Sources: A theoretical approach involves the use of a Rubbiaatron, a device hypothesized by Carlo Rubbia that could generate the necessary neutron flux with a particle accelerator. However, this technology has yet to be demonstrated at a prototype level, making it an untested solution.

Practical Considerations

Challenges of Maintenance: The transformation of thorium to U-233 is a slow process. This means that the reactor needs to operate continuously for an extended period (months) before achieving criticality. During this time, the neutron source must be maintained to ensure the thorium is being converted efficiently.

Operational Safety: The use of plutonium or uranium as a neutron source poses inherent safety risks. However, if the reactor is designed with robust safety features, such as subcritical conditions (meaning the reactor is always subcritical and only becomes supercritical by plugging in the neutron source), the risk can be mitigated. In the event of an emergency, unplugging the neutron source immediately halts the reaction.

Conclusion

While a thorium reactor can be built without directly utilizing plutonium or uranium, certain practical and technical challenges must be overcome. The neutron source requirement, the need for prolonged operation, and the development of specific reactor designs are key considerations. Moreover, the development of alternative neutron sources like the Rubbiaatron remains a theoretical concept and requires further research and development.

Keywords: Thorium reactor, Fissile material, Molten salt reactor