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Part 2 of 2 Parts (Please read Part 1 first)
        In order to safely squeeze more power out of nuclear reactors, much better modeling of the processes leading to CHF is needed. Baglietto says, “Previous models were based on clever guesses, because it was impossible to see what was actually going on at the surface where boiling took place, and because models didn’t take into account all the physics driving CHF.”
       When he undertook the CASL project, his goal was to create a comprehensive, high fidelity model of boiling heat transfer processes up to the point where CHF occurs. This required the creation of very accurate models of the actual physical movement of bubbles, boiling and condensation that take place on the surface of the cladding of the nuclear fuel rods. There are tens of thousands of such rods in a typical nuclear power reactor.
       Baglietto made use of existing knowledge of the complex heat transfer process in the reactors for his work but he also acquired new experimental data to verify his results. He got assistance from Norman C. Rasmussen, Assistant Professor of Nuclear Science and Engineering, and Jacopo Buongiorno, the TEPCO Professor and associate department head for nuclear science and engineering at MIT.
       A physical model was constructed which included electrically simulated heaters, surrogate fuel assemblies and transparent walls. This allowed Baglietto’s team to collect details on the process of going from boiling to CHF. Baglietto says, “You’d go from a situation where nice little bubbles removed a lot of heat, and new water re-flooded the surface, keeping things cold, to an instant later when suddenly there was no more space for bubbles and dry spots would form and grow.”
       These experiments resulted in the confirmation of a fundamental fact. Baglietto’s first models suggested that during boiling, evaporation does not exclusively account for significant heat removal. Simulation experimental results showed that when bubbles slid, bounced around with other bubbles and left the heating surface, they carried away more heat than evaporation.
       W. David Pointer is the group leader of advanced reactor engineering at the Oak Ridge National Laboratory. He is not part of the CASL research team. He said, “Baglietto’s work represents a landmark in the evolution of predictive capabilities for boiling systems, enabling us to model behaviors at a much more fundamental level than ever possible before. This research will allow us to develop significantly more aggressive designs that better optimize the power produced by fuel without compromising on safety, and it will have an immediate impact on performance in the current fleet as well as on next-generation reactor design.”
       Experts says that Baglietto’s findings will be quickly put to work improving nuclear fuels. It is hoped that in the future, Baglietto’s research may be able to allow better cladding for the nuclear fuel rods that would be more accident tolerant, more resistant to impurities. This will improve wettability which makes surfaces more prone to contact with water and less likely allow the formation of dry spots.
        Baglietto says, “If fuel performs five percent better in an existing reactor, that means five percent more energy output, which can mean burning less gas and coal,” he says. “I hope to see our work very soon in U.S. reactors, because if we can produce more nuclear energy cheaply, reactors will remain competitive against other fuels, and make a greater impact on CO2 emissions.”