Heat-Pipe Heat Exchangers for Salt-Cooled Fission and Fusion Reactors to Avoid Salt Freezing and Control Tritium: A Review

The fluoride-salt-cooled high-temperature reactor and some proposed fusion reactors use clean fluoride salts as reactor coolants that have melting points above 450°C and generate tritium. Tritium diffuses through most hot metals, thus methods to capture tritium and prevent its release to the environment are required. Molten salt reactors (MSRs) use fluoride or chloride salts with high melting points where the fuel is dissolved in the coolant. MSR systems produce volatile fission products (Xe, Kr, etc.) and some produce significant tritium. We examine the use of heat exchangers with multiple heat pipes for salt-cooled fission and fusion systems that serve four functions: (1) transfer heat from primary coolant to power cycle, secondary loop, or environment; (2) provide the safety function of a secondary loop by isolating the reactor salt coolant from the high-pressure power cycle; (3) stop heat transfer if the reactor coolant approaches its freezing point to prevent blockage of the primary loop; and (4) block tritium escape to the environment with recovery of the tritium. Each of these capabilities in some form has been demonstrated in a heat pipe system, but not all the functions have been demonstrated in a single system because there has been no need for all of these capabilities in a single system. We review the status of heat pipe technology and the limits of heat pipe technology as the starting points for decisions on the development of such heat pipe systems.