Liquid lithium coolant

It is feasible to breed more tritium in a lithium cooled reactor than is used in the reaction. The excess tritium can be used to start other reactors or in a reactor using some coolant other than lithium that prevents it from breeding its own tritium. Nature has been kind with the properties of lithium. It is an excellent choice for transferring heat from the reactor and it is the raw material needed for the continual production of more fuel. Both these functions can be provided by the use of liquid lithium as the blanket material. The isotopic composition of the lithium may be adjusted to provide the proper balance of lithium 6 and lithium 7 to optimum heat transfer and production of tritium. The lithium can also be diluted with metallic sodium or potassium to aid in adjusting the tritium production rate. 

However, the principal disadvantages at present to the use of liquid lithium as a coolant appear to arise from chemical properties. A review of the published literature reveals that liquid lithium is highly corrosive. However, these data are subject to question, in view of the fact that the amount of contained impurities was not accurately reported. Lithium is highly reactive with most of the major constituents of the surrounding atmosphere—oxygen, nitrogen, and water. The lithium compounds of these elements therefore are usually present in lithium as impurities. All of these compounds of lithium can be expected to react with most materials of construction at elevated temperatures. To the authors knowledge, literature published to date does not cover the rate of corrosion by molten lithium metal in relation to the contained impurities. 

Data on the properties of lithium alloys as liquid metal coolants have not been reviewed by the authors. However, with this approach, it may be possible to make use of lithium s excellent physical properties and overcome some of its adverse chemical properties. 

Based on its physical properties alone, lithium metal would appear to have no equal as a liquid metal coolant. However, because of corrosion caused at elevated temperatures by impurities in commercially available lithium and by the metal iself, lithium has found only experimental use as a liquid metal coolant. 

The optically transparent liquid-salt coolant is a mixture of fluoride salts with freezing points near 400°C and atmospheric boiling points of 1400 C. The reactor operates at near-atmospheric pressure, and at operating conditions, the liquid-salt heat-transfer properties are similar to those of water. Several different salts can be used as the primary coolant, including lithium-beryllium, sodium-beryllium, and sodium-zirconium fluoride salts. 

Lastly we will consider the reactor advantages of the Z-pinch, especially the gas embedded pinch if it is formed in a high pressure ( 100 Atmos.) D-T gas bubble immersed in a vessel of liquid lithium. The liquid lithium will act as one or possibly both electrodes, return conductor, first wall, moderator, breeder, and coolant, and so relax the conditions on wall loading and blanket thickness which restrict the economic operation of other magnetic fusion systems. 

Lithium and its compounds may be used in fusion reactors in either liquid or solid form. Liquid Li is an excellent coolant with low density and viscosity, and with high heat capacity and thermal conductivity (Table 1). Consequently, it is used in many designs as a combined breeding material and coolant. However, hot molten Li can react violently with water or air under certain conditions. Hence, either strict engineering design must preclude large scale Li - air or water reactions, or another form of Li must be used. Both approaches have been studied. 

Molten lithium metal is a potential candidate for the coolant to be circulated through the blanket. Lithium is a light metal with a low melting point (186 degrees Celsius). In the liquid state, it has a high specific heat and thermal conductivity. These properties make it an excellent heat transfer material and thus, a good choice as a means of removing heat from the reactor. When lithium is used in the blanket for heat transfer it also serves as the primary absorber of the 14,100 keV neutrons from the D + T reaction. 

Since the element lithium is essential for the fusion reaction, some concepts for reactor blankets are based on the application of liquid metals, lithium or the eutectic alloy LiivPbgj, as blanket fluids or also as reactor coolants. 

Graphite compounds of rubidium and cesium seem to be more stable and to be formed easily. This is of technical importance since the absorption of the fission elements rubidium and cesium by graphite immersed in liquid sodium will be applied to remove them from the sodium coolant of a fast neutron reactor The formation of a lithium compound of this type has never been observed. 

Unlike other fuel forms, the ratio of fuel and moderator to liquid coolant is fixed in a pebble-bed reactor. This places major constraints on the choice of coolant (it most likely will require the use of a salt with enriched lithium-7 and beryllium) and other core design parameters. While this salt is more expensive, it has very low parasitic neutron capture, which combined with the very small excess reactivity and large cylindrical core, would provide high fuel utilization. Initial studies on a liquid-salt-cooled pebble-bed reactor have been conducted at Delft University in the Netherlands. 

The concentration of lithium-7 in the reactor coolant system varies according to a pH control curve as a function of the boric acid concentration of the reactor coolant system. If the concentration exceeds the proper value, as it may during the early stages of core life when lithium-7 is produced in the core at a relatively high rate, the cation bed demineraliser is used in the letdown path in series with the mixed bed demineraliser to lower the lithium-7 concentration. Since the build up of lithium is slow, the cation bed demineraliser is used only intermittently. When letdown is being diverted to the liquid radioactive waste system, the purification flow is... 

Molten salt fluorides, which are proposed as coolants for MSR, have promising thermophysical and thermal hydraulic properties. Molten salts, similar to liquid metals, have a low vapor pressure even at high temperamres, which is attractive compared to water and gaseous coolants. The salts are less chemically reactive than Na. In addition, salts can provide moderation due to their light element composition, like fluorine (F), lithium (Li), and beryllium (Be) in ELiBe. 

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