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

Abstract

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.

Heat Pipe as a Passive Cooling System Driving New Generation of Nuclear Power Plants

“…Some Molten Salt Reactors (MSRs) where the fuel is dissolved in the coolant also use flibe containing isotopicallyseparated lithium-7. In a recent published paper by Zohuri et al [4], we examine the use of heat exchangers that contain multiple heat-pipes for transferring (1) heat from the primary salt coolant to steam or air-Brayton power cycles and (2) FHR and MSR decay heat from the primary coolant to the environment. Heat pipes can turn on at preset temperatures and thus reduce the risk of freezing the liquid salt coolant if the reactor shuts down.…”

“…In this published paper [4], a clean flibe salt coolant is also proposed for high-magnetic-field fusion machines. For fusion, flibe with lithium-6 is the only salt coolant option because of the requirement to generate sufficient tritium to fuel the reactor.…”

“…As a consequence, it is proposed to use flibe liquid-salt immersion blankets to adsorb the heat from the fast neutrons and breed tritium fuel (Figure 12). The blanket has four functions: (1) it slows down neutrons and converts that energy into heat, (2) the neutrons are used to breed tritium, (3) the salt acts as the primary radiation shielding to protect the magnets and (4) high-velocity flibe salt cools the first wall that separates the fusion plasma from the salt [4].…”

“…One of the aspects of heat pipe application and its integration in Generation Four (GEN-IV) type reactor as means of heat transfer system was suggested by Zohuri et al was for two type of high temperature reactors such as a Molten Salt Reactor (MSR) and a Fluoride-salt-cooled High-temperature Reactor (FHR) and it is published in Nuclear Technology journal as a critical review article, just recently [4]. The Generation IV (GEN IV) nuclear reactor design and the technology is known as nuclear micro reactors that are currently under development by many nuclear manufacturer and the advantages of nuclear micro reactor applications as sources of renewable energy, their use in military applications and department of defense requirements.…”

Fusion Blankets and Fluoride-Salt-Cooled High-Temperature Reactors with Flibe Salt Coolant: Common Challenges, Tritium Control, and Opportunities for Synergistic Development Strategies Between Fission, Fusion, and Solar Salt Technologies

“…Sodium heat pipes are proposed to transfer heat from the flibe salt to the power cycle 52,53 with three additional objectives: (1) isolate low-pressure flibe reactor system from highpressure power cycle, (2) prevent accidental freezing of flibe salt, and (3) capture tritium. Each heat pipe (Fig.…”