Part 1 of 2 Parts
I have often said that nuclear power reactors are the most expensive, complex and dangerous way to boil water ever invented by the human race. The nuclear reactions in the reactor core generate enormous heat. This results in a maelstrom of boiling, bubbling and evaporation on the surface of the fuel rods in the core. The heat is ultimately transferred from this core activity through the cooling system to create steam which is used to drive the turbines that generate the electricity. Scientists have spent a lot of time trying to analyze and predict the physical processes involved in this heat transfer but have only had limited success.
Emilio Baglietto is an associate professor of nuclear science and engineering at MIT. He is thermal hydraulics lead for the Consortium for Advanced Simulation of Lightwater Reactors (CASL). This is an initiative that was started in 2010 with support from Department of Energy’s Consortium for Advanced Simulation of Light Water Reactors for the purpose of designing predictive modeling tools to assist the improvement of current and future nuclear power reactors. In addition, CASL is working on insuring that nuclear reactors can be economically viable as a major source of power for our world. Baglietto’s team includes Etienne Demarly who is a doctoral candidate in nuclear science and engineering at MIT and Ravikishore Kommajosyula who is a doctoral candidate in mechanical engineering and computation at MIT.
Baglietto has recently made an important breakthrough in characterizing the physical processes of heat transfer. His team is using a modeling system called computations fluid dynamics (CFD). They have created new CFD models that accurately capture the basic physics involved in the boiling of coolants in nuclear reactor cores. It is now possible to model the rapidly evolving heat transfer phenomena at the microscale in a variety of reactor designs under different operating conditions. Baglietto says, “Our research opens up the prospect of advancing the efficiency of current nuclear power systems and designing better fuel for future reactor systems.”
A critical issue for the work of CASL involves something called critical heat flux (CHF). Baglietto says that CHF “represents one of the grand challenges for the heat transfer community.” CHF describes a situation where, during boiling, there is a sudden loss of contact between the boiling liquid and the heating element. This is what happens in the core of a nuclear reactor. When power levels change in a reactor, CHF can suddenly appear. The boiling reaches a crisis where a vaporous film covers the surface of the heating element which consists of nuclear fuel rods. Dry spots form on the surface of the fuel rods and quickly reach very high temperatures.
Baglietto explains that, “You want bubbles forming and departing from the surface, and water evaporating, in order to take away heat. If it becomes impossible to remove the heat, it is possible for the metal cladding to fail.” “We want to allow as much boiling as possible without reaching CHF,” says Baglietto. “If we could know how far we are at all times from CHF, we could operate just on the other side, and improve the performance of reactors.”
The allowed power setting for operation of a commercial nuclear power reactor set by nuclear regulators are far below the point at which CHF might occur. On a practical level, this means that most power reactors are operating way below their potential energy levels.
Please read Part 2