Nuclear criticality safety analysis: recovery of old containers holding fissile material

This paper presents a nuclear criticality safety analysis of an operation to recover containers from a 30-year-old storage site. There are 145 containers containing waste from fuel elements with fissile material. The storage consists of 15 cement wells, 7 m deep and 0.4 m in diameter. Each well contains up to 10 60-liter containers stacked on top of each other. Waste containers contain mixtures of various fuel element sections and samples which have various shapes (rods, needles, bars, etc.), different physical or chemical forms (metal, oxide) and variable isotopic compositions. The analysis is based on an inventory of the quantities of fissile material and their composition, which are well known. Given the duration of storage and the possibility of dropping a container during handling, two methods of taking up containers are envisaged. The first one is handling the container as a whole (without opening the container), and the second is opening the container inside the well and recovering the waste directly into the container. For the recovery of the containers, nuclear critical safety of the operation is ensured by limiting the masses of handled fissile material. These masses must remain below the safe masses under normal condition (gripping of a container) or under an abnormal situation (falling of a container on the one below with grouping of the fissile material of the two containers). These safe masses are calculated at optimal moderation for homogeneous or heterogeneous media with reflection by concrete. Different criteria are used for normal and abnormal situations. The paper presents the method for determining a reference fissile medium that bounds all fissile material present in the two containers without being too penalizing. It takes into account the nature of the fissile material (oxide or metallic, homogeneous or heterogeneous), ²³⁵U enrichment and the moderating material. The paper also presents calculations of critical and safe mass limits of fissile material for all the reference fissile media. For each grouping of two containers, the paper presents the calculations of an equivalent mass of fissile material in both containers and then verifies that it is below the safe mass limit. Thanks to this method, it is possible to handle a majority of containers (90%) as a whole (without opening) thus saving time and reducing worker exposure for the operation all the while ensuring nuclear criticality safety control.