A Method for Determining Material Attributes from Post- Detonation Fission Product Measurements of an Heu Device

An algorithm was developed that uses measured isotopic ratios from fission product residue following the detonation of a nuclear weapon to compute the original attributes of the nuclear material used in the weapon. While more accurate (and more computationally intensive) methods are being explored by others, the method described here could serve as a preprocessing step to a more detailed methodology (potentially saving on computational time). This would, in turn, expedite the process of determining where the device came from, eventually leading to which terrorist group perpetrated the event. This work was restricted only to Highly Enriched Uranium (HEU) devices; however, future efforts will consider plutonium devices as well. The attributes determined include original material uranium isotopics (at present considering only U, U, and U) and the type of enrichment process used to create the material (e.g., gaseous diffusion, gas centrifuge, etc.). The approach to developing this algorithm involved a simulation of the fission products and actinides present following a nuclear explosion and a detailed evaluation to determine valid ratios that could be used to work backward and achieve the original material in the device. The algorithm used was purely analytical, derived directly from burnup and radioactive decay equations. Thus, this methodology provided solutions with essentially no computational time required. INTRODUCTION One of the most crucial issues to national security in the United States is the ability to safeguard our country against nuclear terrorism. If national security was breached and a terrorist nuclear device was detonated in the United States, how quickly could we assess the site to determine what type of device was detonated, how powerful the device was and where it came from? Nuclear threats are not widely understood by the general population; therefore, if a terrorist device was ever detonated in our country immediate results must be produced in order to prevent mass hysteria. The objective of the algorithm developed here was to utilize post-detonation measured isotopic ratios in order to determine the pre-detonation material attributes within reasonable accuracy. More computationally intensive (and admittedly more accurate) methods are being developed elsewhere; however, these methods require extensive computational times in order to produce acceptable results. In effort to reduce the computational time required to compute the original material attributes, the method developed here uses an analytical approach which consisted inversions of the buildup and decay equations (all first-order ordinary differential equations). It is envisioned that this methodology could serve as a preprocessor step to a more computationally intensive and more accurate system.