LMFR, externally cooled

The two-fluid externally cooled LMFR (Fig. 18-2) is somewhat more complex because it has a physically separate core and blanket, but higher conversion ratios are possible. The blanket can be made in a variety of ways, making use of either solid or liquid blanket materials. In exploiting the LMFR concept to the full, a fluid blanket consisting of a slurry of ThBi2 or Th02 in bismuth is used. 

The LMFR readily lends itself to a wide variety of designs and arrangements. The concepts proposed to date may be classified according to type as being internally or externally cooled and either compact or open arrangement of cycle. Such classification has been brought about in an attempt to present designs which minimize bismuth and uranium inventories. 

Externally cooled LMFR. In an externally cooled LMFR the fuel is circulated through the core to an external heat exchanger, where the heat is removed l>y the secondary fluid. This type provides the simplest core design, re< uiring. simply an assembly of graphite pierced with holes for circulation of liquid-metal fuel. The major problems of heat transfer are essentially removed from the core design. 

Fig. 23-1. Externally cooled compact arrangement LMFR for closed-cycle gas turbine.

In the open-cycle externally cooled, two-fluid LMFR, the bismuth-uranium solution serves as the primary coolant as well as the fuel. In the reactor itself, there is no actual heat transfer. Instead, the solution acts as a transporter of heat to an external heat exchanger. In evaluating bismuth as a primary coolant, it is helpful to make a comparison between it and three other coolants sodium, a typical alkali metal coolant LiCl-KCl eutectic, a typical alkali halide salt mixture and water. (The salt eutectic used here would not be a suitable primary coolant for a thermal reactor. Its heat transfer properties, however, are typical of salt coolants.)... 

General description. The two-fluid externally cooled LMFR concept consists of a relatively small core surrounded, for the most part, by a blanket containing fertile material. The core is composed of high-density, impervious graphite through which vertical channels arc drilled to allow circulation of the fuel coolant. The fuel in the core is dissolved or dissolved and suspended in liquid bismuth. The fluid fuel also acts as coolant for the core system. The required coolant to moderator ratio is obtained by proper size and spacing of the fuel coolant channels. 

Economic considerations were essential to the optimization studies required to establish the reference designs pre.sented in Sections 24-2 and 24-4. An important objective of this study is the economic comparison of energy costs for the single-fluid and the two-fluid externally cooled LMFR. A brief summary of energy costs for the optimum design in each concept is presented in Table 24-12. 

Design of equipment for FPV removal. In the LMFR, the fuel would flow continuously through several parallel loops to external heat exchangers for cooling. Degassing equipment would, in all probability, be located in each of these loops. For a 500-Mw reactor, if all heat-exchange... 

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