Foreword by the Guest Editors

Dear Reader, The Molten Salt Reactor (MSR) is one of the Generation-IV reactor designs excelling in the field of safety and sustainability. Many of the principles needed for the successful development and operation of the MSR were already investigated and demonstrated in the 1950s and 1960s at the Oak Ridge National Laboratory in various experiments, culminating in the Molten Salt Reactor Experiment (MSRE) which ran from 1965 to 1969. Unfortunately, its successor, the Molten Salt Breeder Reactor (MSBR), was never built due to financial restrictions. During the last decade, however, new designs evolved ranging from thermal breeder reactors similar to the MSBR to designs with an intermediate or fast neutron spectrum capable of actinide transmutation, but also of breeding U-233 from thorium. Current research includes structural materials, thermodynamics of molten salts, chemistry of the actinides (with a focus on the thorium fuel cycle), corrosion studies, multi-disciplinary numerical simulation codes and models, engineering of chemical processing schemes, and component and system design. Because of the outstanding thermodynamic properties, molten salts are also considered as a coolant in solid fuel reactors. Most of these designs, generally called Fluoride salt-cooled Hightemperature Reactors (FHR), employ TRISO fuel particles similar to those developed for the gas-cooled High Temperature Reactor.