MW(e) two fluid MSBR reactor. This reduces the cost of the initial fissile startup charge, and allows more reactors to be started up on any given amount of fissile material. More efficient breeding. The thorium blanket can effectively capture leaked neutrons from the core region. There is nearly zero fission occurring in the blanket, so the blanket itself does not leak significant numbers of neutrons. This results in a high efficiency of neutron use (neutron economy), and a higher breeding ratio, especially with small reactors. One weakness of the two-fluid design is the necessity of periodically replacing the core-blanket barrier due to fast neutron damage. ORNL chose graphite for its barrier material because of its low neutron absorption, compatibility with the molten salts, high temperature resistance, and sufficient strength and integrity to separate the fuel and blanket salts. The effect of neutron radiation on graphite is to slowly shrink and then swell it, causing an increase in porosity and a deterioration in physical properties. Graphite pipes would change length, and may crack and leak. Another weakness of the two-fluid design is its complex plumbing. ORNL thought a complex interleaving of core and blanket tubes was necessary to achieve a high power level with acceptably low power density. ORNL chose not to pursue the two-fluid design, and no examples of the two-fluid reactor were ever constructed. However, more recent research has questioned the need for ORNL's complex interleaving graphite tubing, suggesting a simple elongated tube-in-shell reactor that would allow high power output without complex tubing, accommodate thermal expansion, and permit tube replacement. Additionally, graphite can be replaced with high molybdenum alloys, which are used in fusion experiments and have greater tolerance to neutron damage. Hybrid "one and a half fluid" reactor A two fluid reactor that has thorium in the fuel salt is sometimes called a "one and a half fluid" reactor, or 1.5 fluid reactor. This is a hybrid, with some of the advantages and disadvantages of both 1 fluid and 2 fluid reactors. Like the 1 fluid reactor, it has thorium in the fuel salt, which complicates the fuel processing. And yet, like the 2 fluid reactor, it can use a highly effective separate blanket to absorb neutrons that leak from the core. The added disadvantage of keeping the fluids separate using a barrier remains, but with thorium present in the fuel salt there are fewer neutrons that must