Copper coatings for minimization of retention and permeation of implanted tritium in aluminum alloy 6061
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I. INTRODUCTION
TRITIUM (3H) is produced conventionally in nuclear reactors.[1] The use of such nuclear reactors is becoming increasingly more controversial because of their potential impact on the environment, safety, and public health.[2] For this reason, the U.S. Department of Energy has started to evaluate an accelerator-based tritium production technique as an alternative to production in nuclear reactors.[3] Accelerator production of tritium involves energizing protons in a linear accelerator and directing them onto a heavy-metal target such as tungsten, which is located in what is referred to as the target and blanket assembly.[3] This results in the production of spallation neutrons. The target and blanket assembly also contains about 10,000 tubes of Al-6061 (3.05m long, 6.35-mm i.d., 1.27-mm wall thickness) surrounded by cooling water and filled with 3He gas at a pressure of about 0.78 MPa.[4] The interaction of spallation neutrons with 3He gas inside the Al-6061 tubes results in the production of tritium via the 3He(n,p)3H nuclear reaction. The partial pressure of tritium inside the tubes will be about 1300 Pa. Recently, Cowgill and Causey[4] pointed out the fact that the 3He(n,p)3H reaction produces tritons (tritium ions) with a kinetic energy of about 192 keV. This energy is sufficiently large to cause 10 to 15 pct of the produced tritons to be implanted into the Al-6061 tube walls to a depth of about 1.9 mm. This energetic implantation of tritium will lead to two major problems. First, if a significant portion of the implanted tritium were retained in the tube walls, it would become difficult to meet the production goals. Second, if the implanted tritium permeates through the thickness of the tube wall, it will enter into the surrounding cooling water, M.Y. INAL, Graduate Student, and M. ALAM, Associate Professor, are with the Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801. Manuscript submitted November 12, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
creating an environmental hazard. Both of the previously stated outcomes are unacceptable. It is, therefore, necessary to devise strategies to minimize both the retention and the permeation of implanted tritium in and through the tube walls, respectively. The release, permeation, and retention behaviors of hydrogen ions implanted in different materials have been studied in detail.[5] Depending on the energy, the tritium ions will be implanted to a certain depth below the inside surface of the Al-6061 tube. As the concentration of tritium increases below the surface, it will begin to migrate both toward the inside surface (surface exposed to 3He gas, migration distance 5 1.9 mm) and the outside surface (surface exposed to coolant, migration distance ,1.27 mm). The steady-state profile of tritium in the tube wall is shown schematically in Figure 1. The implantation, release, and permeation fluxes are represented by the symbols JI , JR , and JP , respectively. Mass-balance criteri
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