three basic options for the total cost estimates which are: (1) complete reliance on logistically supplied fuels; (2) solar power with fuel regeneration; (3) nuclear power with fuel regeneration. The costs of these options must be calculated separately because, as pointed out previously, the fuel regeneration plant size must meet different requirements if solar energy is used because of the lunar night cycle. During the lunar night periods, the fueled system must pick up the load normally carried by the solar system. Effect of Variables on Nuclear Power Systems From the above, it is evident that the cost attractiveness of the nuclear option for meeting the power demand of a lunar exploration system increases with greater power demand and longer base duration. Many factors affect the conditions under which this break-even occurs. For example, in the logistic cost comparisons leading to the breakeven curves, the average power demand on the nuclear power plant is greater than the corresponding power demand on the alternative non-nuclear power sources. This is so because the non-nuclear system supplies the power demand of the ultimate loads directly (e.g. mainly by fuel cells, with solar energy for the base proper during the lunar day). The nuclear plant supplies directly all the stationary electrical loads at the base, but must also supply indirectly the energy needs of lunar surface vehicles and other remote loads, by powering a chemical fuel regeneration plant. Based on logistic costs alone and the conditions inherent in Fig. 3, the base duration at which nuclear and non-nuclear power logistic costs break-even changes only from six months to four months, as the average power demand to the ultimate loads increases from 5 kWe to 40 kWe. However, when allowance is made for an apportioned amount of development costs for the nuclear and fuel regeneration plants, the break-even base duration becomes a more sensitive function of power demand. Thus, at a demand of five kilowatts, the apportioned development cost would lengthen the break-even base duration from six months to 11 months. At 40 kWe even an increase of the apportioned development cost by a factor of five would raise the breakeven base duration to only seven months (from a logistic cost break-even duration of four months). Figure 3 has been constructed on the basis of an ultimate base load pattern involving 50% direct electrical energy from the nuclear plant and 50% supplied chemically by regenerated hydrogen and oxygen. For a given average power demand at the ultimate base loads, the base duration required for nuclear plant logistic break-even will increase if the percentage of the ultimate baseload supplied by the regeneration plant is increased. Conversely, nuclear plant break-even time will decrease if the percentage supplied by regeneration plant decreases. At an average power demand of 30 kWe at the ultimate loads and the highest development cost shown in Fig. 3, a 0% load supply by the regeneration plant will reduce nuclear break-even time by 10%. Under the same conditions, a 100% supply by the regeneration plant will increase nuclear break-even time by 20%. Consideration of the relatively short nuclear power break-even time, based on total power cost, led us to conclude that nuclear power should be utilized very early during lunar base evolution as indicated in Fig. 2 even if the first nuclear unit is initially operated at a derated power level. Figure 2 also shows that nuclear power is not used in the Ref. [3] lunar base scenario until the power demand increases to 560 kWe with a 9-15 man crew. Reference [3] considered LEO mass penalties for solar versus nuclear systems, but did not compare total power costs for the two alternatives. Our results
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