PLENTIFUL ENERGY

Figure 14-6 Comparison of excess reactivity requirements
between IFR and thermal reactors
The thermal-hydraulic, mechanical, and neutronic design constraints do not fix
the core design parameters since there are more degrees of freedom in the IFR core
design than in the thermal reactors. The core design process, therefore, is one of
continued iteration to balance the tradeoffs to meet the overall core performance
goals.
A typical core, as illustrated in Figure 14-4, will be made up of fuel pins about 1
cm in diameter, enclosed in hexagonal ducts about 20 cm across, running the full
height of the core and blankets. There are two hundred or so pins in each duct; the
ducts channel the coolant flow and allow flow to be controlled by each individual
duct. The coolant exit temperatures are controlled in this way to maximize thermal
efficiency. For this, the core exit temperatures need to be as high as possible, and
uniformly so from every duct, but kept within the limits placed on fuel pin cladding
temperature (550 to 600 o C).
The reactor core region fuel pins are about 20 percent enriched in fissionable
materials. The blanket regions surrounding the core are made up of assemblies of
pins containing uranium only, generally somewhat larger in diameter than in the
core, as satisfactory pin temperatures can be still be maintained at the much lower
powers they generate. Their principal purpose is breeding, but they can also give
some latitude in design of the safety-related properties of the core if, as is
sometimes done, they are made part of the core itself. Mostly, however, the blanket
assemblies will ―blanket‖ the core, surrounding it. Their purpose is to catch the
neutrons leaking from the core which otherwise would be wasted. The thicker the
blanket, the more neutrons captured, and the higher the breeding. But as the radial
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