Throughout the DOE complex there are thousands of tons of spent nuclear fuel stored underwater in pools. The fuel was intended for short-term storage in water before it was to be reprocessed. However, the fuel will no longer be reprocessed, and extended storage in water has caused many of the aluminum- and steel-clad elements to degrade, exposing the uranium fuel. When the uranium is exposed to water, uranium hydride forms, and the reaction occurs as follows,

It turns out that the uranium hydride reacts very rapidly with oxygen in what is known as a pyrophoric reaction. The reaction is highly energetic and can produce a large amount of heat.

A technique known as passivation can be used to treat the corroded fuel elements and prevent the occurrence of a pyrophoric reaction. The passivation process essentially allows a small amount of oxygen to come in contact with the corroded fuel element, forming a protective oxide layer over the exposed fuel. This oxide layer separates the uranium hydride from the air and prevents a pyrophoric reaction from occurring. The figure below shows a container with fuel elements. A mixture of 98% Helium-2% Oxygen is injected into the canister through a downcomer pipe. The oxygen reacts with the exposed corroded fuel, and the inert helium merely acts as a transport mechanism.
 

 The purpose of our project is to visualize and understand the flow field within the canister and to ensure that oxygen is transported to all of the exposed areas of the fuel. Most classical flow visualization techniques use inert particles that can be photographed as they flow past some body. We propose to investigate flow visualization methods where the tracking particles react with the surface of the body, just as the oxygen would react with the corroded fuel. The project will also use the Matched-Index-of-Refraction (MIR) facility (see picture below) at the Idaho National Engineering and Environmental Laboratory (INEEL) to measure the velocity field within the canister and between the fuel elements.
 

Computational models of the flow field will also be constructed. The results of those models will be compared to the experimental results. The information gained from this research will assist the DOE in designing safe and efficient fuel treatment and storage facilities.
 
 

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