The remainder of this section describes the approach that was used in our analyses to obtain consistent mass and area estimates among the various power system options. 3.1 Description of System Models The mass and area estimates presented in this paper were derived from models developed at Sandia. The objective of these models was to develop a consistent set of estimates based on design approximations and to a lesser extent on known component masses. For example, design approximations are used to determine reactor fuel mass. The fuel masses required to meet criticality limits, fuel damage limits, burn-up limits, thermal limits, etc. are calculated and the fuel mass selected is the largest of these cases (Ref. 11, 12, 13, 14, 15, 16). The mass of the radiation shield for the power conditioning system is also based on design approximations, but power conditioning mass and volume are estimated from known component values (Ref. 10). The level of detail in each of the models is consistent and some of the subsystem models used in the various system models are the same. As a result, we believe the relative comparison between the various power systems is quite good. Further, the SP-100 program's own detailed mass estimates indicate that our absolute values should be reasonably representative of actual mass. Another important characteristic of the models developed for each system is that all of the subsystems are contained in one model. Therefore, the interactions between the various subsystems are modeled. For example, the thermal power required from the reactor is not known until the efficiency of the electrical system is known. This is further complicated by the fact that each component of the electrical system cannot be designed until the efficiency of the other electrical components is known. By having all of the subsystem models contained within a single model, these interactions are taken into account. The final, major characteristic of the models is that the mass of the power systems can be optimized at each power level. The optimization is based on the important system parameters. In the OTR concept, for example, the amount of power obtained is directly proportional to the surface area of the reactor core. Therefore, the model varies the height to diameter ratio of the core as specified by the user. The system mass is calculated for as many ratios as desired and the code selects the optimum ratio based on minimum total power system mass.
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