Molten Salt Reactors: A New Beginning for an Old Idea

Molten Salt Reactors have seen a marked resurgence of interest over the past decade, highlighted by their inclusion as one of six Generation IV reactor types. The most active development period however was between the late 1950s and early 1970s at Oak Ridge National Laboratories (ORNL) and any new re-examination of this concept must bear in mind the far different priorities then in place. High breeding ratios and short doubling times were paramount and this guided the evolution of the Molten Salt Breeder Reactor (MSBR) program. As the inherent advantages of the Molten Salt concept have become apparent to an increasing number of researchers worldwide it is important to not simply look to continue where ORNL left off but to return to basics in order to offer the best design using updated goals and abilities. A prime example being the trend towards removal of graphite moderation from the central core, as evident in recent French work on the Thorium Molten Salt Reactor (TMSR) and Russian efforts towards the Molten Salt Actinide Recycler and Transmuter (MOSART). Another major change to the traditional Single Fluid, Molten Salt Breeder Reactor (MSBR) design and the primary subject of this presentation is a return to the mode of operation that ORNL had in mind for the majority of its MSR program. That being the Two Fluid design in which separate salts are used for fissile ²³³UF₄ and fertile ThF₄. Oak Ridge abandoned this promising route due to what was known as the “plumbing problem”. It will be shown that a simple yet crucial modification to core geometry can in fact solve this problem and allow the great advantages of the Two Fluid design that ORNL had sought for many years. It will also be shown that this updated design can be started on Low Enriched Uranium with a simple transition to a pure Th-²³³U cycle which removes the need for shipping proliferation sensitive material and relieves the constrictions on large scale start up due to limited supplies of Pu or ²³³U. In addition, another promising route laid out by ORNL was simplified Single Fluid converter reactors that could obtain far superior lifetime uranium utilization than LWR or CANDU without the need for any fuel processing beyond simple chemistry control. Updates and potential improvements to this attractive concept will also be explored.