Can We Use Heavy Water in a Light Water Reactor? If Not, Why?

When considering the application of heavy water (D2O) in a light water reactor (LWR), it is important to understand the fundamental properties and the potential impacts on the reactor's performance. Heavy water, with its deuterium atoms, behaves differently from ordinary water (H2O) in terms of neutron moderation. This difference arises from the higher mass of deuterium compared to hydrogen. Let's explore these aspects in detail.

Neutron moderation is a critical process in nuclear reactors, where fast neutrons emitted by fission are slowed down to thermal energies to increase the probability of fission reactions. When heavy water is used as a moderator, it takes about twice as many collisions for a neutron to slow down from fission source energies to thermal energies compared to ordinary water. This property can significantly affect the reactor's performance and safety.

Effects on Neutron Moderation

In a light water reactor, ordinary water is used both as a moderator and coolant. The reduced neutron moderation in heavy water could lead to under-moderation, causing more neutrons to be absorbed by uranium-238 (U-238) instead of undergoing fission reactions. This could result in increased production of plutonium within the reactor, which might impact the fuel cycle and lead to changes in the cycle length.

However, these effects are not permanent. During the first cycle, the reactor cycle length might be shortened due to the higher absorption rate in U-238. However, at equilibrium, the increased production of plutonium might offset this effect. Additionally, the smaller absorption in heavy water offers a significant advantage, as it could potentially reduce the fuel cycle costs or even improve them.

Thermal Properties and Turbines

The primary coolant in a Pressurized Water Reactor (PWR) does not flow directly through the turbine, but instead passes through a heat exchanger that boils water, which flows through the turbine. The thermodynamic properties of heavy water are not significantly different from those of ordinary water, so the heat transfer process should not be substantially affected.

In a Boiling Water Reactor (BWR), the cooling water is the working fluid that also flows through the turbine. Here, the use of heavy water could potentially cause issues with the turbines. The heavier molecules might flow differently around the turbine blades, potentially leading to damage or performance issues. More research would be needed to assess the exact impact on the turbines.

Chemical and Corrosion Aspects

Water chemistry plays a crucial role in nuclear plants, and even though heavy water (D2O) and ordinary water (H2O) are chemically identical, their physical properties differ. Heavy water might cause different corrosion patterns and water chemistry issues, which need to be carefully managed to ensure the long-term reliability of the reactor.

Conclusion

In summary, while heavy water can be used in a light water reactor, its use would have some significant impacts on neutron moderation, fuel cycle, and potentially on the turbines in a Boiling Water Reactor. A PWR might work fine with heavy water, but the turbines in a BWR would require careful consideration and potential redesign to ensure optimal performance and safety.

Keywords: heavy water, light water reactor, neutron moderation