Taking a Fresh Look at Nuclear Energy
According to Jacopo Buongiorno, TEPCO Professor and Associate Department Head of Nuclear Science and Engineering and Director for the Center for Advanced Nuclear Energy Systems, the rationale for taking a fresh look at nuclear energy comes from the desire to decarbonize the economy.
The Electric Power Research Institute several years ago conducted a study which showed that converting the full fleet of cars and light trucks in the United States to electric vehicles or plug-in hybrids would require between 150-200 gigawatts of power. The current fleet of nuclear reactors in the US provides 100 gigawatts power, and was built in about two decades. “If we’re serious about reducing the use of fossil fuels, nuclear energy can help,” says Jacopo Buongiorno, TEPCO Professor and Associate Department Head of Nuclear Science and Engineering and Director for the Center for Advanced Nuclear Energy Systems.
The rationale for taking a fresh look at nuclear energy, in addition to energy independence and reliability, comes from the desire to decarbonize the economy. Fossil fuels account for 80% of the overall energy supply worldwide. Associated with them is the emission of carbon dioxide into the atmosphere. To reduce the consumption of fossil fuels, small contributors such as nuclear, solar and wind have to grow by an order of magnitude. “How we do that is an issue of scalability, which has two dimensions,” says Buongiorno. The first dimension is temporal: how quickly can we get to zero carbon capacity in the power sector? The second is spatial: how much land is needed to bring that capacity to the power sector? In terms of these dimensions, nuclear energy is appealing not only as a proven technology for reducing carbon, but also a proven technology for doing so in a relatively short period of time. And while intermittent energy sources like solar and wind require utilization of an enormous amount of land, a single square mile of land can accommodate a nuclear plant that would power a city five times the size of Boston.
As director of the Center for Advanced Nuclear Energy Systems, Buongiorno is responsible for coordinating and promoting research and development projects that address such issues as the high cost of building nuclear power plants; the potential for accidents, which is a top concern for the public; disposal of highly radioactive waste; and, finally, the possibility of proliferation of nuclear weapons. “Improving the economics and safety of nuclear power plants in particular are goals for which technology solutions are more possible and within reach of what we typically do at MIT.” Both the offshore floating nuclear power plant project and the project on the use of nano-engineered surfaces in nuclear reactor fuel address the question of how to make nuclear power plants safer, more efficient and more economically attractive.
The offshore nuclear power plant concept integrates a nuclear reactor in a floating platform of the type commonly used in oil and gas offshore operations. The economic philosophy behind the concept includes a shipyard construction approach. The entire plant is built in a shipyard, reducing construction time and cost, floated out to and moored at a site located offshore, and connected to the electrical grid by means of a power cable. At the end of its lifetime the whole plant is brought back to a centralized shipyard where decommissioning is done. The concept also affords the possibility of moving the power plant from one market to another during its lifetime. “If the local market conditions change such that nuclear power in that particular market is no longer profitable for whatever reason in ten or twenty years, there’s the possibility of moving the plant to a different market, to a different location where it would be profitable.”
Buongiorno says the safety philosophy for the offshore floating nuclear plant concept is derived directly from the lessons learned at Fukushima. As part of the American Nuclear Society Special Committee on Fukushima, he studied the accident in detail. “From a technical point-of-view the offshore floating design eliminates earthquakes as accident initiators: if the bottom of the ocean moves, that energy is not transferred to the plant itself. It is also immune to tsunami waves, which are very low amplitude waves in the relatively deep waters where the plant would be sited. The cylindrical hull design of the platform combined with a robust mooring system ensure that the whole structure does not experience excessive motion during severe storms.”
“Another feature of the offshore concept is the ability to provide cooling to the reactor indefinitely and passively—as we say—which means without the need for electric power and external intervention. The nuclear reactor is below the water line and thus it is fairly easy to connect, via heat exchangers, to the ocean.” Buongiorno explains that the ocean water as an infinite heat sink, provides emergency cooling to the reactor, mitigating problems related to lack of cooling of the fuel, which can lead to fuel damage and ultimately lead to release of radioactivity into the environment.
The economic and safety improvement of current nuclear reactors is another project Buongiorno is working on. In nuclear reactors the energy that is generated in the nuclear fuel rods by fission is removed by boiling heat transfer. “Anything we can do to make that boiling heat transfer mechanism more efficient is beneficial to both safety and economics. How do we do it? We engineer at the micro and nano scale the surface of the cladding (the outer surface of the nuclear fuel rod) to increase the limit at which we can operate our nuclear fuel rod.” What Buongiorno means by limit is as follows: think about boiling heat transfer as a surface, where bubbles are generated. The more energy put through the surface the more bubbles are generated per unit time and per unit area. At a certain point these bubbles merge and create a continuous vapor film. “We want to delay the occurrence of this vapor film on the surface,” he says. “And we do it by creating a special texture that is hydrophilic and porous on the surface.”
Establishing a continuous vapor film can result in the physical damage of the fuel rod itself, with consequent release of radioactivity from the fuel. “But, by pushing the limit higher, we have created margin to that particular failure,” says Buongiorno. “Now, you can use that margin two ways: either it gives you flexibility in operation, because now you can do certain maneuvers with the reactor without getting too close to the failure limit, or you can operate the plant at a higher steady power.” The same reactor now operating at a higher power density means there is more power, more electricity to sell to customers.
In terms of the big picture, Buongiorno thinks we should invest in nuclear as a way to reduce the risk of failing to meet the carbon emission reduction targets. “Could you do all of this with solar and wind? The answer is maybe. It requires developing affordable energy storage and transmission technology. There are some people who think that can be done. But do you want to take the risk of embarking on this big investment and twenty years from now say, ‘I failed’.” Given the current rate of global warming, Buongiorno believes we need to have in the mix of energy sources one that is more proven and scalable than solar and wind.