What are the most challenging technical issues?

The greatest technical challenge is to be able to determine the isolation of the host rock formation from the human environment. We address this in two complementary ways. First, we obtain the most detailed data on the host layer and on the rock above it and below. Then we run an advanced simulation (using the state-of-the-art code TOUGH2) to model potential leakage paths to the surface. Two mechanisms are important: advection (flow of deep brine) and diffusion (molecular motion of the waste) and these two mechanisms interact. We need to run many variations to take into account faults and fissures, including known dislocations, unknown ones, and potential future ones. Typically, we need to run several hundred independent simulations with variations in the parameters to take into account the unknowns. We have already done this for generic geology, and published the method and results in a peer-reviewed journal, but once we have a site, we need to model the specifics of that location. The second method of determining isolation uses measurements of natural radioisotopes, typically of Cl-36 and I-129, but including several others, which were created by natural uranium and thorium fission in the rock. From such measurements, we can calculate the million-year prior migration history of these radioisotopes. These measurements require sampling of the rock entrained brines at depth, possibly done with a small pilot hole prior to the drilling of the repository. If the measurements indicate strong isolation, that is, isolation from the surface that is 100,000 to a million years or more, then that would give us even greater confidence than obtained from the modeling alone. These measurements require the highest standard of care and the use of the most sensitive radioisotope methods, including Accelerator Mass Spectrometry (AMS). These methods are challenging, but we believe we can do them; as our team includes the original inventor of the AMS method.