ready resource for further production of weapons by the states holding them, also constitute a potential source of materials that could be used for the fabrication of nuclear weapons by non-nuclear weapon states and even terrorist groups. Any assessment of the potential future availability of NEM, moreover, must include not only military stocks of these materials but also the NEM in research reactors and the growing quantities of it in civilian nuclear power programs.

The degree of transparency that countries have permitted concerning their nuclear weapons inventories and NEM has tended to increase over time, albeit with some reversals and persistent regional exceptions, because the countries agreeing to the increased transparency have regarded these changes as bringing bigger benefits than liabilities for their security. The benefits include improved opportunities for monitoring that can increase confidence in the verification of agreements and reduce the uncertainties in assessing potential threats that can feed worst-case scenarios. The liabilities against which such benefits must be balanced include the danger of revealing sensitive information that could aid proliferators or increase a nation’s vulnerability to attack. Both the United States and Russia have moved slowly and cautiously to share information regarding even well-known aspects of their nuclear arsenals and remain reluctant to provide each other with information they regard as closely related to the details of weapon design. Countries with small nuclear arsenals have particularly acute concerns about sharing information regarding the locations of their nuclear weapons.

Most of the measures and technologies for transparency and monitoring assessed in this study were developed in the course of continuing efforts to find ways to limit the risks from existing nuclear arsenals and their proliferation to additional countries and groups. Attempts to limit numbers and characteristics of nuclear weapons and their delivery systems have often been controversial; belief in the value and practicality of nonproliferation efforts has also varied over time and across groups. Verification issues frequently have been at the heart of these controversies, with debates focusing on the likelihood of cheating and the capability to assess compliance in fundamentally adversarial circumstances. On a number of occasions, however, the bilateral or multilateral political will needed to increase transparency and the availability of suitable monitoring technologies for these purposes allowed the completion of measures and agreements—including some previously thought unattainable—in both the arms control and nonproliferation domains. Controversies about what is desirable and feasible in these

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