Liquid metal cooled reactors heat transfer

Heat Transfer in Liquid-Metal Cooled Fast Reactors Alexander Sesonske... 

Liquid metal fuel reactors are classified on the basis of their heat-transfer characteristics (Fig. 18-1) [21]. If heat is transferred within the core the reactor is said to be internally cooled. If heat is transported by the fuel to the primary heat exchanger external to the core, the reactor is externally cooled. The term "integral reactor" implies an externally cooled system, but one so compact that the reactor and primary heat exchangers can be placed in the same container. 

HEAT TRANSFER IN LIQUID-METAL COOLED FAST REACTORS... 

Thermal analysis is a major feature of the design of liquid-metal cooled fuel elements and assemblies in a fast reactor core. A brief discussion of a typical thermal design approach is therefore useful for the identification of many of the heat transfer questions in the core area. 

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. 

Sodium is used as a heat-transfer medium in primary and secondary cooling loops of liquid-metal fast-breeder power reactors (5,155—157). Low neutron cross section, short half-life of the radioisotopes produced, low corrosiveness, low density, low viscosity, low melting point, high boiling point, high thermal conductivity, and low pressure make sodium systems attractive for this application (40). 

Cooling by a liquid metal, such as sodium, has been universally adopted for fast breeder reactors. The main reason is that so many potential coolants are unsuitable because of the need to minimize moderation of the fast neutron spectrum in the core. Water (light or heavy) and organic coolants are therefore eliminated. Consideration has been given to the use of steam or helium as fast reactor coolants, but the heat transfer characteristics of a liquid metal are so much more favorable, particularly for a reactor of high power density, that neither steam nor gas cooling has been adopted for current fast breeder designs. 

Among liquid metal candidates, mercury (Hg), sodium-potassium (NaK) alloy, sodium (Na), lead (Pb), and lead-bismuth eutectic (Pb-Bi) have been considered and used to build and operate liquid metal nuclear systems. However, liquid Na became the most smdied and used option mainly because it allowed, together with the selection of an appropriate fuel type (e.g., metal or oxide fuel), for a lower doubling time. On the other hand, hquid Hg was abandoned due to its toxicity, high vapor pressure and low boiling temperature as well as poor nuclear and heat transfer properties. More recently, in the framework of Generation IV, the development of fast reactors cooled with liquid metals considers hquid Na but also liquid Pb and liquid Pb-Bi as coolant... 

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