process operability, and sustainability, given the situation that exists for a nation at a particular time.

In many cases some systems may offer more promise on some of these criteria, while others look better with respect to other criteria, making trade-offs inevitable. Whether more emphasis should be given, for example, to saving money or to reducing environmental impact is not a technical decision but one based on values, which must ultimately be made by society, through a political process. The role of designers and technical experts is to make clear the choices and trade-offs that need to be made, outline the benefits and downsides of each of the leading approaches, and do their best to ensure that the decisions ultimately made are well informed and carefully considered.

Criteria for Comparison

Each of the key criteria mentioned above can be specified in more detail, so as to provide more detailed guidance to those designing and assessing these systems.

Economics. Each system can be compared based on its life-cycle electricity cost. Additional criteria may include the degree of uncertainty of those cost estimates; the system’s contribution to the costs of spent fuel and nuclear waste management; initial capital costs and the resulting level of financial risk in implementing and operating a system; the variability and reliability of the electrical output; and the system’s attractiveness or unattractiveness to the private sector (along with the scope of required government subsidies or regulations needed to make the system competitive).

Safety. Each system can be compared based on the overall risk of a significant accident it poses (including both the probability and the consequences of the various types of plausible accidents in the system); accident reports by regulatory agencies and others can provide insight into risks. Radiation doses to the public and industrial safety during normal operations are also considerations, though these risks are low for most proposed systems. Because the risks of significant accidents may be difficult to estimate rigorously and compare among systems that have never been built, decision makers may choose to focus on the degree to which known risk factors are present and how they are addressed (such as positive coefficients of reactivity, which can result in power excursions), or the degree to which known safety factors are present (such as “passive safety systems”).

Security. Thorough security comparisons would examine how difficult it would be for adversaries to cause a major radioactive release through sabotage, or through the theft of material that could be used to make a nuclear device. Systems that continuously maintain the nuclear materials in their cycle in forms that could not be used in weapons without either isotopic enrichment or extensive chemical processing using heavy shielding rank better on this criterion. Reactors with greater degrees of inherent safety and widely separated redundant safety systems so that they would be more difficult to sabotage simultaneously are also more inherently secure, according to this measure.

Proliferation resistance. The proliferation resistance of alternative nuclear systems depends on how difficult it would be for a nation or a subnational group to use a facility or material to make a nuclear explosive device. No chemical processing facility can be constructed to make it impossible to change its product streams, but it can be designed to make changes costly, lengthy, and detectable. Proliferation resistance can be judged by criteria related to the material streams and the processes, including the extent to which (a) access to the material,

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