Historically, the initial research concerning reactors at Oak Ridge National Laboratory by a team headed by Alvin Weinberg, during and after the war, the technical-military-political choices are very well related in the documentary movies: “Thorium, the hidden face of nuclear”.
2/ Physical principle
The natural thorium Th 232, unlike the uranium, is not fissionable and can’t therefore lead directly to the production of energy.
However, it is called fertile because it can lead to fissile elements. In fact, the Th 232 is able to absorb a slow thermal neutron, resulting from, for example, the controlled fission of an atom of U 235 or Pu 239 and results in Th 233, which, by two successive emissions of β–, gives protactinium Pa 233, then uranium U 233, which is fissile.
Th 232 + n –> Th 233 –> Pa 233 + β– –> U 233 + β–
For example, the thorium associated with U 235 becomes a nuclear fuel. It’s all the more interesting since the regeneration ratio, i.e the ratio of the number of produced fissile atoms to the number of atoms consumed, is 1.25 greater than 1.03, ratio of the couple U 238-Pu 239.
The U 233, bombarded by neutrons, breaks into two lighter nuclei, together with a release of thermal energy, recovered by turbines activating an electricity generator. This fission releases two or three neutrons witch, in turn, will trigger the next fission.
The liquid fuel UF4 in a MSFR is combined to molten salts, Lithium fluoride LiF. and Beryllium fluoride BeF2. These salts play simultaneously the roles of heat transfer, moderator and containment barrier.
3/ Current reesearch
Daniel Heuer and his team, Laboratoire de Physique Subatomique et de Cosmologie, CNRS Grenoble, are at the origine of the project MSFR shortlisted by the Generation IV International Forum (see the presentations by Daniel Heuer and Elsa Merle-Lucotte).
Academic research centers, in Europe, besides the LPSC in Grenoble, include : Technology University in Delft, Karlsruher Institut für Technologie, Paul Scherrer Institut in Geneva, IPN in Orsay, IRSN, CEA Saclay, etc…