In the unlikely event of an unprotected transient overpower or unprotected loss-of-coolant flow, overheating of the reactor core may lead to partial core meltdown in sodium-cooled fast reactors. Hence, studying the interaction of high-temperature melt with sodium, known as the molten fuel-coolant interaction (MFCI), is significant. In this study, to understand MFCI, experiments were conducted by dropping the high-temperature melt generated by a thermite reaction into a stainless steel interaction vessel containing sodium, maintained at 400 degrees C. The sodium vapor generated during the MFCI process due to the melt-sodium superheat was captured using real-time X-ray imaging.For characterizing the sodium vapor from a typical X-ray image, a standard phantom was designed with cylindrical cavities that simulate sodium vapor. While the lateral dimensions of the vapor could be estimated from two-dimensional images, information on the depth had to be assessed from changes in the image intensity. Towards estimating the vapor depth, two approaches were adopted, one based on a mathematical approach arrived at by analyzing the X-ray image of the voids in sodium phantom in stainless steel interaction vessel and another based on normalizing the intensity distribution of the image, which was used to estimate the reduction in the sodium path length (RISL) while vapor was formed during MFCI.These two approaches, which were initially adapted to estimate the depth of the argon bubbles in the simulated experiments, were found to be in close agreement with each other. However, the difference between the lateral dimension estimated from the full-width at half-maximum and the RISL estimated by these two approaches showed a constant ratio. Similarly, the lateral and depth dimension of the sodium vapor formed during the MFCI process were evaluated.This paper presents the details of the phantom used, the methodology adapted for estimating the linear attenuation coefficient of sodium at 400 degrees C, the determination of the RISL, and the correction ratio estimated for mapping of the sodium vapor depth.
This paper discusses the challenges associated with managing the criticality hazard of the formation of Pu-bearing solids during the transfer of two feeds of plutonium nitrate liquors on the Sellafield Site during post-operational clean out. The cessation of reprocessing operations at Sellafield Ltd. lead to a requirement to transfer to a vitrification facility two final Pu-bearing liquor feeds via routes that have historically only handled effluent with trace quantities of plutonium.Three mechanisms for creating plutonium solids were identified that could undermine the safe concentration of the liquor under both normal and fault scenarios. Existing solids, which were found to be present in a transfer route tank, had the potential to associate with plutonium. In addition, due to the presence of oxalic acid in the liquors, conditions that could lead to the precipitation of plutonium oxalate could be realized. Similarly, the potential to precipitate Pu hydroxide (colloids) from the liquor required consideration.The key control identified to protect against plutonium solid formation was to ensure that the liquors maintained a very high nitric acid molarity for the duration of the transfer. This hazard management strategy was challenging to implement due to various operational factors.
