High-Efficiency Fuel for Light Water Reactors (LWRs)

The use of advanced fuel instead of conventional solid cylindrical fuel in LWRs has been under investigation by Professor Kazimi and Dr. Pavel Hejzlar. A DOE-supported project investigated the ability to raise the core power of pressurized water reactors (PWR) while maintaining or improving thermal margins via adoption of annular fuel with internal cooling. A special issue of Nuclear Technology was devoted to the results of this effort in October 2007.

A new project supported by Korea Atomic Energy Research Institute (KAERI) was started to examine the application of the annular fuel to Korean Reactors. The effort included assessment of the shutdown margin and effects of partial blockage of the internal coolant due to crud. TEPCO supported a scoping analysis of the use of several advanced fuel designs to raise the power in a boiling water reactor (BWR). The project investigated smaller pins of solid geometry, annular fuel, twisted cross-shaped fuel, and the use of uranium hydride instead of uranium oxide as fuel. The three approaches, with some careful design choices, may be able to raise the power up to 30 percent. In addition, hydraulic tests were conducted and showed that the twisted cross-shaped fuel could reduce the core pressure drop below that of the current fuel, because the twisted rods’ contact points along the axial length eliminate the need for grids for mechanical support. The current focus is on determining the design of an experiment to determine the critical power associated with this new fuel design.

Professor Neil Todreas is investigating the use of inverted fuel concept for LWRs. In a typical fuel rodded design, the fuel rods are surrounded by coolant, while in this design it is the fuel that surrounds the coolant. Hence, the inverted fuel concept designation. The inverted core configuration uses hydride fuel in vertically-oriented hexagonal blocks (U-Th-ZrH1.6 or Pu-Th-U-ZrH1.6) perforated by coolant channels arranged in a triangular lattice. A cylindrical Zircaloy clad forms the walls of each coolant channel, and a Liquid-Metal (LM) gap separates the clad from the fuel. Each channel is provided with multiple short twisted tape inserts (TT) aimed at enhancement of critical heat flux (CHF) . A hexagonal duct made of Zircaloy or stainless-steel surrounds each fuel prism and a LM gap separates the inner surface of the duct from the outer surface of the fuel. The inverted fuel configuration yields lower fuel temperature than a typical rodded core having the same fuel volume fraction and coolant pressure drop. However, the addition of TTs reduces this temperature difference, by an extent depending on the TT geometric characteristics. Prediction of pressure drop, heat transfer coefficient and CHF at high pressure in presence of short TTs is currently under investigation. Neutronic and structural analyses as well as the thermal hydraulic ones will determine the extent of possible power density upgrade.