A major consideration in tine design of tiie reactor plant is the 

 refueling cycle, which may have a significant effect on cost. In addition to 

 the direct cost of the refueling operation, there are costs for retrieving and 

 transporting the power plant to a refueling site, providing refueling facilities, 

 and being without the plant during refueling operations. Full-power refueling 

 cycles based on full-time operation range from more than 10 years for 30 kw 

 to about 1 year for 3,000 kw, depending on the particular plant selected. 

 Selecting larger reactors for smaller power plants can generally extend the 

 refueling cycle, but this must be traded off against increased acquisition costs, 

 weight, and size. A larger refueling cycle is also possible by increasing the 

 efficiency of converting thermal energy to electric energy. 



Radioisotope Energy. The radioisotope power supply considered for 

 the study program employs a steam Rankine cycle conversion system for 

 obtaining electrical power from the heat produced by the decay of radio- 

 isotope fuel. Only the 30-kw power level was investigated because of the 

 high fuel inventory cost, estimated at $70,000/kw, and fuel use cost, estimated 

 at $12,000/kw/yr. The break-even power level is dependent on many factors, 

 but in general is substantially less than 100 kw. For larger radioisotope plants, 

 extrapolation from the figures for the 30-kw plant should provide a reasonable 

 estimate since the major investment and operating costs are either due to the 

 fuel or proportional to it. 



The 30-kw radioisotope power plant of interest here is completely 

 self-contained within its own pressure hull. The hull is equipped with cable 

 connections for supplying power and for start, stop, or monitoring functions, 

 as required. The plant is self-regulating and has a nearly constant output 

 voltage and frequency from no load to full load. Table 5 presents tentative 

 system data on the plant. 



At the 30-kw power level, the minimum size of the pressure hull is 

 determined by the requirement for adequate hull area for the transfer of 

 waste heat. Each hull is sized to provide adequate heat transfer for operation 

 at all depths from 600 feet to its design depth and at a maximum ambient 

 temperature for each depth. Heat is transferred from the power plant hull to 

 the sea by natural convection for both reliability and safety. 



The 30-kw radioisotope power plant is estimated to weigh 14,000 

 pounds, including all machining and shielding. Radiation levels at the outer 

 surface of the pressure hull are limited to 200 milliroentgens/hour maximum, 

 with considerably lower levels for access and maintenance. 



Spherical pressure hulls will result in a minimum hull weight-to- 

 displacement ratio. However, the hull shape is not critical. For design 

 considerations and feasibility of fabrication, it appears that hull shapes made 

 of ring-reinforced cylindrical sections and hemispherical heads would be 

 desirable. 



