Synthesis and Properties of Metallic Technetium and Technetium - Zirconium Alloys as a Radioactive Storage Waste Form to Stabilize the Technetium Waste Stream of the UREX+1 Process

摘要

In the AFCI program the UREX+1 process is proposed as one of the most promising technique to separate TRU (transuranic elements) form LWR spent nuclear fuel in the years to come. The application of UREX+1 results in good separation of the 5f-transuranics from the 4f-lanthanides, reduced waste volumes by eliminating the uranium content, and reduced waste package costs. Technetium-99 will be separated together with uranium and iodine within the first process steps. After the separation of uranium, technetium and iodine must be immobilized by their incorporation in a suitable waste storage form. Based on recent activities within the AFCI community, a potential candidate as waste storage form to immobilize technetium-99 is to alloy metallic Tc-99 with excess metallic zirconium. Alloying metallic Tc-99 with zirconium has potential advantages in terms of the future reuse of Tc-99 and its potential transmutation, compare to the stabilization of Tc-99 in rock-forming mineral-type oxides. The synthesis of technetium[IV] based spinel-type oxides, and perovskite-type oxides as potential candidates for geological waste storage is known since 1969. However, Tc-99 is abundant in a variety of nuclear waste streams and has a long half-life, about 200,000 years. Released into the environment, Tc-99 is extremely damaging, traveling up the food chain, and causing cancer in humans. Due to the mobility of technetates it is believed that Tc-99 could cause long-term exposure problems for geological repositories to come, after the anticipated failure if engineered barriers in 10,000 to 100,000 years. Therefore, providing a waste storage form for Tc-99 waste streams which allows transmutation of Tc-99 into stable isotopes or less toxic radioisotopes strongly promotes the AFCI program and the future separation of TRU elements by applying the UREX+1 process. However, only few thermodynamic data in the binary metal system technetium– zirconium exist, and only few data are available on the synthesis of technetium-zirconium alloys and on their potential performance under temporary or geological storage conditions.We intend to systematically investigate the binary metal system technetium-99 – zirconium for the first time. We propose to investigate the synthesis of metallic technetium as well as its alloys with zirconium. In order to provide valuable data to the AFCI program, we also propose to determine the thermodynamic equilibrium phases as well as their performance under the scenario of a geological repository. Therefore, we propose to address the following research tasks:Task 1: Synthesis of metallic Tc[0]-99 applying up to three different procedures.Task 2: Characterization of micro-structure, nano-structure and crystal structure of Tc-99 metal.Task 3: Synthesis of alloys in the binary system technetium – zirconium.Task 4: Determine thermodynamic equilibrium phases at 1000 °C to 1600 °C.Task 5: Determine the binary phase constitution (phase diagram) of technetium and zirconium.Task 6: Investigation of Tc-corrosion and Tc-leaching of binary Tc-Zr phases at elevated temperature (200 ºC) and elevated pressure (20 MPa).