Tests with ceramic waste form materials made by pressureless consolidation.

A multiphase waste form referred to as the ceramic waste form (CWF) will be used to immobilize radioactively contaminated salt wastes recovered after the electrometallurgical treatment of spent sodium-bonded nuclear fuel. The CWF is made by first occluding salt in zeolite and then encapsulating the zeolite in a borosilicate binder glass. A variety of surrogate CWF materials were made using pressureless consolidation (PC) methods for comparison with CWF consolidated using a hot isostatic press (HIP) method and to study the effects of glass/zeolite batching ratio and processing conditions on the physical and chemical properties of the resulting materials. The data summarized in this report will also be used to support qualification of the PC CWF for disposal in the proposed federal high-level radioactive waste repository at Yucca Mountain. The phase composition and microstructure of HIP CWF and PC CWF are essentially identical: both are composed of about 70% sodalite, 25% binder glass, and a 5% total of inclusion phases (halite, nepheline, and various oxides and silicates). The primary difference is that PC CWF materials have higher porosities than HIP CWFs. The product consistency test (PCT) that was initially developed to monitor homogeneous glass waste forms was used to measure the chemical durabilities of the CWF materials. Series of replicate tests with several PC CWF materials indicate that the PCT can be conducted with the same precision with CWF materials as with borosilicate glasses. Short-term (7-day) PCTs were used to evaluate the repeatability of making the PC CWF and the effects of the glass/zeolite mass ratio, process temperature, and processing time on the chemical durability. Long-term (up to 1 year) PCTs were used to compare the durabilities of HIP and PC CWFs and to estimate the apparent solubility limit for the PC CWF that is needed for modeling. The PC and HIP CWF materials had similar disabilities, based on the release of silicon in long-term tests. These tests and analyses indicate that CWF made using the PC and HIP methods should be equally acceptable for disposal. The same waste loading can be used in PC CWFs and HIP CWFs. The disposition of radionuclides is the same in PC and HIP CWFs. One minor difference is that radionuclide and halite inclusions are fairly uniformly distributed in the binder glass phase of the PC CWF, whereas they are segregated near the sodalite domains in the HIP CWF. This is an advantage of the PC CWF, since the aggregation of halite inclusion lowers the effective durability of the surrounding glass, due to the greater exposed glass surface area after the halite dissolves