Evaluation of Aluminum-Boron Carbide Neutron Absorbing Materials for Interim Storage of Used Nuclear Fuel

Abstract

The objective of this work was to understand the long-term corrosion behavior of Boral® and Bortec® neutron absorbers during deployment in a used nuclear fuel dry cask storage environment for several hundred years. Corrosion effects were accelerated by flowing humidified argon through an autoclave at temperatures between 300 and 570°C. Humidity levels ranged from 0.028 to 0.58 mass fraction with flow rates ranging from approximately 0.2 scfh to 1.5 scfh.

Results from corrosion testing at temperatures between 300 and 570°C with varying humidity have shown that both Boral® and Bortec® develop new aluminum-boron-carbon phases. The phases formed are consistent at 300 and 400°C. Different formations were observed at 570°C. The samples also showed boron depletion at all temperatures.

It is predicted that two mechanisms control the changes in Boral® and Bortec®. The phase changes observed result from the interaction of boron carbide with aluminum. These interactions result in boron and carbon diffusing into the aluminum matrix. The other series of interactions occurring between the sample and the water in the humidified argon. Boron on the surface rapidly reacts with the water to form B2O3 which can be volatilized. The loss of boron at the surface creates a concentration profile that can result in the continued diffusion of boron to the surface. The water will also react with the aluminum to form Al2O3. Aluminum oxide formed in humid conditions has some porosity and can degrade away and allow for continued oxidation of aluminum.

Additional research is required to determine the suitability of these materials for use in dry cask storage. The new phase formation may affect mechanical properties and adversely affect the fuel baskets structural integrity. Boron redistribution may cause localized areas of boron depletion, additional testing needs to be conducted to determine boron diffusion in these materials in the absence of humidity. In the presence of humidity the boron will leach from the samples. Cask humidity levels need to be determined to be able to predict how much boron may be lost.