Novel designs for nuclear reactors, being drawn up by researchers at MIT and a new research institute in the United Arab Emirates (UAE), could decrease the risk that nuclear fuel could be diverted for use in nuclear weapons.
When nuclear materials are in use inside a nuclear reactor, they’re too hot to steal, says Youssef Shatilla, a professor at the Masdar Institute, in the UAE. The greatest danger comes when fuel is being manufactured, when enrichment facilities can be used to make weapons-grade materials, or when nuclear materials are in transit, during either delivery or waste removal. To lessen the first danger, the government of the UAE plans to lease its fuel from other countries rather than making its own fuel. As a result, it won’t have the technology to enrich uranium for making nuclear weapons.
The MIT and Masdar researchers are working on the second problem. They’re designing new reactors that would need to be refueled far less often than conventional ones–once every 15 to 30 years rather than every 5 years. This would decrease the frequency of deliveries and the chances that the materials could fall into the wrong hands. “If you look at how you can divert nuclear material so it can be used in a weapons program, it is when the nuclear fuel is outside of the reactor core, when it’s relatively cool and people can manipulate it,” Shatilla says. “Our strategy is to keep the fuel inside the core as long as we can.” The new reactors would have the added benefit of producing at least one-third of the waste of existing plants.
The new designs are part of an effort by the UAE to convince the international community to approve its plans to build nuclear reactors to generate electricity. The UAE and other Middle Eastern countries want to build nuclear power plants as a way to meet fast-growing domestic electricity demand. This would let them export oil and gas rather than burning it to generate electricity. “You cannot stay on course burning your own precious resources to generate electricity,” Shatilla says. “In 30 to 40 years, oil and gas will be very expensive commodities–too expensive to burn.”
To decrease the frequency of refueling, the researchers at MIT and Masdar are investigating ways to get more energy out of a given amount of fuel. One way to do this, says Mujid Kazimi, a professor of nuclear engineering at MIT, is to increase the concentration of uranium-235, the isotope of uranium that undergoes fission to create the heat that drives nuclear power plants. Currently, nuclear fuels contain less than 5 percent uranium-235, but this can be enriched to about 20 percent without making the material suitable for use in weapons. However, increasing the enrichment level poses a couple of challenges. Manufacturing plants that make fuel pellets from enriched uranium will require new safety precautions, Kazimi says. What’s more, the fuel will need to be modified to ensure that the reactions don’t proceed too quickly. The presence of so much “fissionable material,” Kazimi says, could lead to a relatively quick chain reaction that would use up fuel too quickly. By incorporating materials known as burnable poison that absorb neutrons emitted during fission to slow down the reactions, the fuel could slowly generate heat over 15 years or more, he says.
Another way to increase the amount of energy that can be extracted from nuclear fuel is to promote the creation of more fissionable material within the reactor itself. In ordinary nuclear power plants, some of the neutrons released during fission are absorbed by uranium-238, a material that does not undergo the process. When this happens, it triggers a series of reactions that produce other types of fissionable material that can generate heat in a nuclear reactor. Essentially, these reactions turn uranium-238 into fuel, allowing the plant to operate longer between refueling. Researchers have long known how to increase this fuel production within the reactor, even to the point that certain reactors can produce more fuel than they consume. But again, the danger is that creating too much new fuel could provide materials for nuclear weapons. So the researchers are investigating ways to increase fuel production, but not so much that it becomes a nuclear proliferation risk. The result would still be both more energy from a given amount of fuel and less waste.
Finally, Kazimi and Shatilla are designing the new plants to operate at higher temperatures than conventional reactors, thereby increasing the efficiency with which they convert heat energy into electricity. This would also make nuclear plants more useful as a source of heat for chemical reactions, such as hydrogen generation. Toward this end, the pair is investigating unconventional materials for coolants, such as molten salts, which are less corrosive at high temperatures than the water that is commonly used. The researchers are also studying the use of superheated steam, which involves boiling water to create steam, and then heating the steam to yet higher temperatures. The higher temperatures yielded will also require new materials in the core, such as a silicon carbide ceramic that Kazimi has been developing. This silicon carbide is made in the form of a mesh that can stretch without breaking as the reactor heats up and cools down.
Kazimi notes that the research project is still only one year old and that final designs could be several years away. Ultimately, Shatilla says, the goal is to produce designs in which “there is no possible pathway to divert nuclear material into a weapons track, and then at the same time produce nuclear power with the environment in mind.” If the project is successful, he says, the designs could be useful in many more places than just the Middle East.