Neutron experiments add confidence to nuclear reactor safety

Canadian Nuclear Laboratories is a leader in sciences that are foundational to reactor safety—including the ability to predict the lifetimes of critical components used in nuclear power stations around the world, especially those in CANDU reactors.

Source: Canadian Neutron Beam Centre (CNBC)
Image: The Embalse nuclear generating station in Argentina is a Canadian-designed heavy water reactor. (SNC Lavalin)

One of the distinct advantages of CANDU reactors is the fact that they don’t require enriched fuel to operate. That’s because the CANDU design uses heavy water, which enables the use of natural (i.e., unenriched) uranium as fuel.

In the latter half of the 20th century, while other nuclear vendor nations were focusing on light water reactors (which do depend on enriched fuel), Canada developed its heavy water technology. Today, there are several CANDUs operating overseas and a fleet of CANDU-like heavy water reactors in India—and Canada remains a leader in the science needed to support the safe operation of reactor components exposed to heavy water.

In specific instances, there is good reason to believe that heavy water behaves similarly to light water in reactor conditions. Therefore, Canada has been able to rely on some of the scientific knowledge generated in the light water reactor industry when developing its own heavy water technology. A good example of this is the science that informs the lifetime expectancies of reactor components, such as the heat exchanger tubes in a reactor’s steam generator. These tubes, which are made of a nickel–chromium alloy, convert water into steam using the heat produced in the reactor’s core.

Lifetime predictions for heat exchanger tubes inform nuclear operators of when these critical components may need maintenance or replacement to avoid erosion of the reactor’s safety margin. The rate of corrosion in heat exchanger tubes is believed to be identical regardless of whether light water or heavy water is present. Therefore, the lifetime predictions for these tubes in heavy water reactors rely on data produced from studies that used light water.

Illustration of a heavy water reactor. The tubes in the heat exchanger use hot heavy water under high pressure from the reactor’s core to convert light water into steam. The steam, in turn, rotates the turbines to generate electricity.

Dr. Ha prepares an experiment in a chemistry lab at Canadian Nuclear Laboratories (image: CNBC).

“But could the corrosion behaviour actually be different in heavy water versus light water conditions?” questions Hung (Harry) Ha, a research scientist at Canadian Nuclear Laboratories (CNL) in Chalk River, Canada. “Why not study it in heavy water, to be sure?” Such a questioning attitude often leads to scientific advances, as well as to improvements in safety culture.

Therefore, as part of a project initiated by CNL, Ha examined how nickel behaves in heavy water. Notably, nickel has very good corrosion resistance properties due to the fact that it reacts with oxygen in water to create protective oxide films on its surface. Further, these protective films grow thicker under harsh and corrosive conditions, which further protects the material. “It’s like when the weather gets colder, you put on a thicker coat,” says Ha. “It acts in its own defence.”

Using neutron beams at the NRU reactor in Chalk River, Ha set out to determine whether the protective films on pure nickel behave the same in heavy water as in light water. In particular, he used neutron reflectometry to determine the exact thickness of the films’ surface layers, which can be extremely thin (i.e., less than a nanometre thick). The presence of heavy water in these experiments actually aided the detection of the protective films by improving the contrast between the films and the surrounding water.

Importantly, since one of the benefits of neutron observation is that it can take place in real time and under realistic conditions, Ha’s experiment was able to mimic the conditions found in the heat exchanger tubes of heavy water reactors. Specifically, voltages were applied to simulate the range of mild to harsh conditions that the nickel might actually face in operation.

The experiments confirmed that the protective films on pure nickel indeed behaved similarly in heavy water as they do in light water, thereby adding confidence in the accuracy of the lifetime predictions for heat exchanger tubes in heavy water reactors. The findings were published in 2017 in the Journal of the Electrochemical Society (doi:10.1149/2.0171713jes).

Today, Ha is extending his study to examine nickel–chromium and iron–nickel–chromium alloys. The results from the pure nickel experiment will serve as a baseline for the interpretation of these further experiments, which aim to determine whether the behaviour of the protective films on these nickel alloys differs in heavy water as compared to light water conditions.