Title: Process specifications for critical mass control: Purex Plant. Revision No. 2
Abstract: This document is Revision No. 2 of the ``Process Specifications for Critical Mass Control -- Purex Plant``. It encompasses the changes in flowsheet, equipment and plant operating philosophy that have developed since the plant startup. It supersedes Document HW-44064 (Revision No. 1). Plutonium-239 and uranium-235, either as the metals or as dissolved or solid compounds are capable of spontaneous and violent nuclear reactions. Following initial dissolution of irradiated natural uranium fuel elements the Purex Plant is normally involved in the processing of liquid solutions containing these isotopes. However, in Purex Plant solutions, uranium-235 is invariably associated with a large fraction of neutron-absorbing uranium-238, a condition which precludes the possibility of a nuclear reaction. (These specifications are based on the premise that no enriched uranium is processed in the Purex Plant.) Plutonium-239, on the other hand, is separated from uranium by means of the Purex process; and even prior (process-wise) to its separation, the ratio of plutonium to uranium must be controlled during the off-standard conditions of plutonium rework or recycle. Critical mass control in the Purex Plant is concerned solely with prevention of nuclear reactions of plutonim-239, which has been produced by irradiation of natural uranium fuel elements.