Conceptual Design of Molten Salt Loop Experiment for MIT Research Reactor

Abstract

Molten salt is a promising coolant candidate for Advanced High Temperature Reactor (AHTR) Gen-IV designs. The low neutron absorption, high thermal capacity, chemical inertness, and high boiling point at low pressure of molten salt coolants could potentially lead to enhanced safety and lower cost of AHTR designs as compared with conventional Light Water Reactors. Improved economics are expected to be a result of the higher possible operating temperatures (700oC), improving thermal efficiency, availability of process heat for industrial applications, and reduced containment costs. Improved safety margins arise from the use of highly robust TRISO particles fuel in either pebble or graphite block form, greater thermal inertia, low pressure and high boiling point of molten salts relative to water cooled reactor designs. Currently, one of the main challenges associated with further advancement of AHTR design is predicting reactor core materials’ interactions with molten salt coolant over long time scales in a radiation environment. In the Fall of 2010, the Nuclear Engineering Design Project Course (22.033/33) undertook the challenge to design a molten salt test loop to be installed in the MIT Research Reactor (MITR) that would recreate anticipated AHTR operating conditions and fill the knowledge gap in understanding of materials behavior in such environment. In addition to simulating neutronic, thermal and chemical conditions similar to those of AHTR, the test loop must also meet the safety and operating requirements of the MITR. During the course, a preliminary design was developed that features an annular in core molten salt flow channel to maximize the volume available for testing materials’ samples and maintaining the salt temperature at 700oC and flow velocity at 6 m/s, while avoiding boiling at the outside surface of the loop, as prescribed by MITR safety requirements. A number of additional requirements were addressed by the students including reactivity insertion, power peaking, tritium production, shielding, and others. The design tasks were subdivided into four key areas of neutronics, thermal hydraulics, chemistry and materials, and instrumentation and control. The molten salt chosen was LiF-BeF[subscript 2] (FLiBe) with lithium enriched in [superscript 7]Li isotope up to 99.995% because this salt is the leading coolant candidate for AHTR. Hastelloy-N was chosen as the material in contact with the molten salt due to its high resistance to corrosion, good material properties at high temperature and extensive use in previous experiments. The presence of corrosion products, free fluorine and production of tritium in the molten salt were found to be important phenomena challenging the loop design. Therefore, various methods for the salt chemistry control and tritium release were evaluated and resulted in a design of multi-component system for monitoring the salt conditions, maintaining redox potential and removing the impurities and tritium from the salt. Another challenge was managing the loop operation given the relatively high freezing point of the salt at about 460oC. Procedures were developed for start-up, steady-state, shutdown and transient operation of the loop. The thermal hydraulic analyses indicate that 1.8 kW of strap heating along the loop outside the core section and a 1.5 to 2 kW pump were required, depending on final design choices. In addition, preliminary cost estimates of constructing the loop experiment at MITR were performed. The main constraints on the choice of the loop’s individual components and diagnostics were: 1) the ability to function at the designed operating temperature, pressure, and flow rates; 2) the ability to function in a nuclear radiation environment; and 3) the necessity to meet MITR safety requirements. A database of vendors for the loop’s components, instrumentation, and diagnostics was compiled. To support further work on the molten salt test loop an electronic library of references was compiled as well. Finally, a number of potential accident scenarios were examined and their effects on the safety and operation of MITR were evaluated and found to represent no danger to the public or interfere with normal operations. Minor leakages of either the reactor water or the molten salt coolant inside the loop were found to be self-sealing with little to no effects on the safety and operation of MITR. A complete failure of the loop’s heating and pumping systems was found to lead to FLiBe’s cooling and freezing inside the loop, with the freezing time ranging from several minutes to ~1 hour depending on the choice of the loop thermal insulation material.