Why Chloride Salts Are Not Suitable for Thermal Spectrum Nuclear Reactors

Chlorine and its compounds, such as sodium chloride, are considered for use in certain nuclear reactors. However, their use in thermal spectrum nuclear reactors poses significant challenges, making them unsuitable for such applications. This article explores the reasons behind this limitation and highlights the advantages of other options like fluoride salts for fast reactors.

Neutron Absorption in Chlorine

Chlorine, specifically the isotope Chlorine-35, plays a crucial role in nuclear reactions. Its thermal neutron absorption cross section is about 50 barns at room temperature, while it reduces to approximately 10^-3 barns at 1 MeV. In comparison, Fluorine-19 has a thermal neutron absorption cross section of 10^-3 barns at room temperature. The disparity between these values indicates a higher probability of neutron absorption in chlorine, making it less ideal for use in thermal spectrum reactors.

Tradeoffs Between Chloride and Fluoride Salts

When considering the choice between chloride and fluoride salts for fast reactors, several factors come into play. Sodium chloride (NaCl), commonly known as table salt, is a simpler, more readily available choice compared to exotic and potentially more costly fluoride salts. The tradeoff between cost and efficiency is a key consideration.

The MCSFR (Elysium's Modular Chemical Salt Fast Reactor) represents a simple design that eliminates the need for graphite moderation and the complex optimization of isotopic mixtures. This simplification allows for a broad range of fuel types, including decommissioned nuclear waste materials. While this reactor prioritizes simplicity and versatility over efficiency, the issue of neutron absorption remains critical.

Neutron Absorption Specifics

Chlorine-35 comprises about 75% of natural chlorine and has a notable thermal neutron absorption cross section of about 40 barns. Additionally, its fast resonance integral is approximately 15 barns. In contrast, hydrogen’s cross section is 0.3 barns. Although this value may seem small, the cumulative effect of hydrogen in some reactors necessitates the use of heavy water, underscoring the importance of this figure.

In addition to neutron absorption, there might be secondary issues like corrosion. Nonetheless, the primary concern is the probability of parasitic absorption of neutrons, which is too high to use chlorine salts in nuclear reactors. Even if the fast absorption aspect were acceptable, fast reactors require thermal fissions to sustain criticality. The thermal absorption cross section of chlorine would be prohibitive even in these scenarios.

Alternative Options for Fast Reactors

David MacQuigg suggests that chlorides for fast reactors might be proposed, with the effective fast cross section being much less than 15 barns. This is seen as a more serious issue rather than a showstopper. However, the practical limitations and the need for high efficiency in fast reactors generally favor the use of fluoride salts over chloride salts.

The advantages of fluoride salts in fast reactors include lower neutron absorption, fewer corrosion issues, and better control over neutron moderation. These factors make fluoride salts a preferred choice for fast reactor designs.

Conclusion

In conclusion, while chlorine and its compounds may have applications in certain nuclear reactor designs, their use in thermal spectrum nuclear reactors is not advisable due to the high probability of neutron absorption. The design and operation of such reactors require precise control over neutron moderation and absorption, which makes chloride salts a poor choice. For fast reactors, chloride salts are not without merit, but the complexities and limitations mentioned above make fluoride salts a more suitable and reliable option for these advanced reactors.