pressures that the transport subsystem can tolerate and still remain reliable. Higher pressures are associated with higher gas density which in turn requires a smaller and less expensive pipe and insulation for the transport. For example, if a fluid pressure of 1000 atmospheres (15,000 psi) could be tolerated by the transport subsystem instead of the 100 atmospheres (1500 psi) considered here, then the pipe internal diameter distribution in the high pressure system would be smaller than the considered system by a factor of 10°'4 = 2.5 for the same pump work allowance. The high pressure system would have thicker walled pipes than calculated in this study. There is an increasing penalty as the thermal power transferred increases (except for chemical transport). This effect is relatively mild for the steam and water heat transport subsystems with less than a factor of one-half increase in in cost for a factor of 100 increase in delivered power. The NaK and helium systems are more sensitive to total thermal power. Average pipe temperature is a strong cost parameter. System 5 (chemical) operates at ambient temperature and is the cheapest. Table 7 presents selected results for the 5 fluid systems at the 3 sizes for the minimum subsystem cost design. The smallest and largest pipe sizes (outer diameter and wall thickness) and insulation thickness are shown to indicate the magnitude of the physical parameters. The percent heat leak and percent pumping power are also shown. The reason for the high cost of the NaK and helium energy transport subsystem is obvious from these results. Both pipe size and losses are large. Furthermore, system 3 uses expensive stainless steel pipe. Hitec fluid was also evaluated as an energy transport fluid and was found to be only 10 to 15% more expensive than pressurized water at the lower temperatures (200°C to 315°C). Hitec was about 1/2 the average cost of the NaK and He systems at higher temperatures (480°C to 600°C). Since it is being considered by several
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