In this article, I explore the responses of the Open Source community to the Fukushima disaster by describing the development of collaborative technologies to measure radioactive contamination across Japan. Based on participant observation at Tokyo Hacker Space, this article addresses an emergent form of technoscientific expertise which is not confined to or legitimized by established institutions, but, rather, is distributed and organized around transnational networks of field specialists, companies, educational institutions, and volunteers. Both Tokyo Hacker Space, as an independent laboratory, and Safecast, as a spin-off project, advanced practices of hardware engineering and Open data management, which are examined in this article as manifestations of bricolage. As a concluding argument, I suggest that expertise and responsibility are interpretive keys for understanding the possibilities and impossibilities of collaboration and coordination in the context of Open Source-based responses to the Fukushima crisis. Responsibility has become an open question for Safecast and Tokyo Hacker Space, as both have championed “openness” as an ethical point of departure, a projected outcome, and a technical orientation in the context of the Fukushima disaster.