This article is a response to the challenge of nuclear non-proliferation for the 21st century. It describes the new landscape of nuclear risk and explains the role that the extraordinary developments in nuclear forensics science can play in generating an expansive deterrence to improve nuclear security by governments, and diminish the nuclear risk presented by terrorists and other non-state actors. Exploring the potential of nuclear forensics science, it provides the analysis to understand why that potential is not being realized. The article then proposes, as a practical and achievable foundation for a new and expansive deterrence, the establishment of an international nuclear forensics data bank, and the legal and institutional basis required. It explains the superiority of the proposed international data bank to alternative modalities of nuclear risk reduction such as the imposition of sanctions, revealing the guiding principles for effective nuclear containment in the twenty-first century.