Liquid-cooled reactors

There are reports on the use of solid state displacement reactions, too. The joints were made from Tic and Si powder processed at 1400 C, and their shear strength at room temperature reached 50 MPa Nevertheless, these joints were weak compared to SiC/SiC composites, so strength improvement is required for their applications to gas/liquid cooled reactor systems. Additionally, irradiation behavior of these joints will also require to be studied... 

Peak Reactor Temperatures Are Lower In Liquid-Cooled Reactors Than Gas-Cooled Reactors... 

Section 5 reviews the applicability of sodium fast reactor refueling to an LS-VHTR. Both the LS-VHTR and sodium-cooled fast reactor can be described as high-temperature, low-pressure, liquid-cooled reactors that require control of the chemical composition of the gas space above the liquid. Because of the functionally similar characteristics of these two reactor classes, many of the technical characteristics associated with refueling a sodium fast reactor are directly applicable to a LS-VHTR. 

Temperature drops. The peak temperature of the fuel is fixed by the need to avoid fuel failure. Temperature drops occur from the fuel to reactor vessel wall, from the vessel wall to silo wall, and from the silo wall into the earth. Liquid cooling (reactor coolant and secondary salt) minimizes the first two temperature drops. This allows for higher silo temperatures, which, in turn, allow greater heat rejection to the ground. 

A simultaneous flow of a gas and a liquid, a gas and a solid, two different liquids, or a liquid and a solid is described as a two-phase flow. Among these types of two-phase flow, gas-liquid flow is the most complex flow due to the deformability and the compressibility of the phases. The analysis of the two-phase flow is very important for liquid-cooled reactors. Two-phase flow occurs in the BWR core and in the steam generator of the PWRs. In order to analyze reactor systems with liquid-vapor mixtures, it is necessary to predict liquid-vapor density, pressure drop across a given channel length, flow stability, maximum flow rates, and heat transfer rates. As the liquid is vaporized, the mixture of vapor and liquid flow gives rise to interesting flow and heat transfer challenges. 

AHTR A Low-Pressure, High-Temperature Liquid-Cooled Reactor... 

AHTR design basis. As a starting point, the S-PRISM facility design was used as a basis for the AHTR. This included using the same size reactor vessel. Because the AHTR is also a low-pressure liquid-cooled reactor, it is a reasonable starting assumption to assume the same fundamental limitations in facility and vessel design. 

As in a liquid cooled reactor, the equipment sizes are small relative to those for gas cooled reactor concepts. 

Some industrial examples (Table 2) demonstrate these differences between gas-cooled and liquid-cooled nuclear reactors. The General Atomics gas-cooled HTGR (the GT-MHR) has a AT across the reactor core of 359 C, while the British Advanced Gas-Cooled Reactor (Hinkley Point B) has a AT of 355 C. Liquid-cooled reactors t5q)ically have much-smaller temperature increases across the reactor core. The Point Beach pressurized-water reactor (PWR) has a AT across the reactor core of 20 C, while the French liquid-metal fast reactor (Super Phenix) has a AT of 150 C. A liquid-cooled reactor can deliver all of its heat with small temperature differences (20 to 150 C) between (1) the hottest temperatures in the reactor coolant, piping, and heat exchangers and (2) the maximum temperature of the chemical reagents in the H2 production facility. 

If heat is needed at 750 C, the maximiun temperature of the gas coolant in a gas-cooled reactor and heat exchangers may exceed 1000 C whereas that of the liquid coolant in a liquid-cooled reactor will only be 20 to 150 C higher—depending upon the design. This can significantly reduce the high-temperature demands on materials. Liquid-cooled reactors include the AHTR, MSR, and LFR... 

Pressure. The chemical reactions for H2 production go to completion at low pressures, while high pressures reverse the desired reactions. Low-pressure reactor coolants are preferred that (1) minimize material strength requirements for heat exchangers between the reactor and chemical plant and (2) avoid the potential for overpressurization of the chemical plant in the event of heat exchanger failure. These consideration favor the use of low-pressure liquid cooled reactors the AHTR, VHTR, and MSR. 

Another factor, unique to MSRs, down rates this reactor in comparison with other liquid-cooled reactors in terms of isolation. In an MSR, the fuel is dissolved in the molten salt. The fission process produces tritium, the radioactive form of H2. This places an additional requirement on the intermediate heat transfer loop to ensure that tritium does not reach the H2 production facility. Significant work has been conducted to develop methods to ensure tritium does not cross the heat exchanger. Most of this work is associated with development of fusion reactors that have very large tritium inventories. It is unclear how serious this issue is. 

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