elements, which are hexagonal blocks) and the CANDU. A third, the spectral-shift-control reactor, is also worth mention. The prismatic HTGR has the lead in the United States by virtue of the licensed prototype unit (Fort Saint Vrain). Further development of the reactor requires only that this unit be scaled up to fully commercial size. Most generic problems have been identified. The CANDU coded by pressurized heavy water is fully commercial in Canada, and Canadian affiliates of companies in the United States are CANDU designers and vendors. Nevertheless, these companies are skeptical of the ability of the CANDU to meet domestic licensing requirements in its present design. Possible points of difficulty (under the industry’s understanding of licensing philosophy) are the use of on-line refueling, thin-walled fuel cladding, and the design of the emergency core-cooling system.³⁴ If these design features must be changed, the CANDU would evolve in the direction of the British steam-generating heavy water reactor (SGHWR), a system that has been essentially abandoned in Great Britain as uneconomical. The SSCR is claimed to be a straightforward extension of LWR engineering, with its main point of development being an auxiliary unit: an in plant heavy water reconcentration unit that requires separate commercial development, but whose development may be considered independent of other reactor problems.

Two other systems seem to present more formidable development problems, but exhibit some development advantages. The organic-cooled CANDU has been developed in Canada to a point just short of prototype construction. One evaluation³⁵ suggests that its economic prospects are more favorable than those of the CANDU cooled by pressurized heavy water, but not by a margin sufficient to justify the cost of commercialization. However, if the United States were to undertake development of a CANDU for domestic use, there might be keen Canadian interest in joint development of the organic-cooled version.

The pebble-bed reactor is a version of the HTGR that has had successful prototype operation in Germany, and the vendor of the domestic HTGR (General Atomic) has access to its technology.

Capital Costs Under present economic circumstances, none of the advanced converters appears to be competitive with LWR’s. All types seem to involve appreciably higher capital costs. These capital costs must be counterbalanced by savings in fuel cycle costs.

The size of the capital-cost disadvantage for various systems is highly controversial; it is only natural that proponents of a concept present optimistic data, and it is extremely difficult to find economic evaluations for systems not yet built that are free of subjective judgments. With the