Renewed interest in nuclear power and advanced nuclear reactors places a focus on efforts by the Nuclear Regulatory Commission to review and license reactor designs. One part of that licensing process is quantifying potential risks in order to better ensure that the reactors will be safe once they enter service.

To help in that task, the Energy Department’s Sandia National Laboratories in New Mexico developed what they said is a standardized screening method to determine the most important radioactive isotopes that could leave an advanced reactor in the event of an accident. 

The Sandia team recently tested the method with a conceptual design for a heat pipe reactor.

A heat pipe reactor uses an inert gas or liquid metal to cool the core. This means the reactor could potentially require less water than conventional light-water reactors.

Fluid in the heat pipes also would need no moving parts–such as valves or pumps–to regulate the core’s temperature. This means they would not require electricity to ensure the reactor’s safety.

Isotope inventory

The Sandia team’s screening method started with an inventory of radioactive isotopes produced by a nuclear reactor and sorted them to determine which isotopes would pose the most risk to humans and the environment in the case of an accident.

Radioactive isotopes are unstable forms of elements. They release energy in various forms of potentially harmful radiation as part of the process of becoming more stable isotopes.

For example, naturally occurring radon-222 is a product of the decay of uranium and in turn decays into polonium-218, releasing alpha radiation. This decay process is harmful if it occurs in someone’s lungs, which is why homeowners often are urged to test for radon gas in their houses.

In the case of the heat pipe reactor, Sandia researchers started with a list of more than 1,200 radioactive isotopes, and threw out the isotopes that decay quickly.

For example, rhodium-106, a form of the element rhodium with 45 protons and 61 neutrons, has a half-life of 29.9 seconds. So, after 30 seconds, half of the initial amount of rhodium-106 will have become palladium-106, a stable metallic element with 46 protons and 60 neutrons. After 60 seconds, only a quarter of the initial amount of rhodium-106 will remain. Researchers said that even if an accident were to occur rapidly, scant rhodium-106 would remain to affect people or the environment.

The team next removed very rare isotopes, specifically those that made up less than 0.0001% of the inventory’s total radioactivity.

These first two screening methods were similar to those used in the 1970s and 1980s to determine radioactive isotopes of interest for light-water reactors. Using these two screenings, the team produced a narrower list of 110 radioactive isotopes for further study.

The team then took a set amount of each of the radioactive isotopes and determined the resulting radiation doses using values from the Environmental Protection Agency. The dose is a numerical representation of the health impacts of exposure from that radioactive isotope. 

The researchers used advanced computer codes like Maccs to calculate the transport of radioactive isotopes through the environment and also the hazard posed by the isotopes on bone marrow and the lungs. Those body parts offer a good picture of radiation exposure’s overall health impacts.

Calculating health impacts

Combining the dose values with the proportion of that isotope present in the heat pipe reactor inventory enabled the team to calculate both short-term and long-term health impacts of the studied radioactive isotopes. They next compared the dose values with equivalent doses of iodine-131 and cesium-137.

Iodine-131 is the radioactive isotope that scientists say poses the most short-term risk from an accident at conventional nuclear power reactors. That is why potassium iodide tablets — which block the body’s intake of radioactive iodine — are distributed near U.S. nuclear power plants. Cesium-137 is the isotope that represents the long-term risk from an accident at a conventional nuclear power reactor.

The Sandia researchers found more than a dozen radioactive isotopes present in the heat pipe reactor inventory that could pose short-term health risks on par with iodine-131. Four radioactive isotopes could pose long-term health risks on par with cesium-137.

The researchers said that while their work suggest that these isotopes may be important for future assessment of heat pipe reactors, more study is needed to refine the heat pipe reactor inventory and determine the proportions of isotopes that could be released during a possible accident.

Read the technical report here.

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