UC Berkeley

Deep Burn Modular High Temperature Rectors (DBMHR) have been proposed as a means to reuse the transuranic (TRU) content of commercial spent nuclear fuel (CSNF). By fissioning greater than 60% of the initial TRU load DBMHR's transmute much of the fuel inventory into shorter-lived fission products. This use of a DBMHR to recycle CSNF offers remarkable benefits including the extraction of additional electricity without the need for additional raw fuel materials, added proliferation resistance by utilizing up to 99% of the 239Pu content in the initial load, and a reduction of the radiotoxicity of the subsequent spent fuel. Two central features of the DBMHR design are the TRISO fuel particles and the all graphite core This is important from a repository perspective because pure graphite is reported to be "one of the most chemically inert materials" known, and offers the potential to serve as an ultra-durable matrix for the sequestration of radionuclides (over geologic time periods) generated as a result of the nuclear fuel cycle.

In this study we evaluate the performance of DBMHR spent fuel (DBSF) for final geological disposition. The Yucca Mountain geological repository (YMR) is used as the environment for this study because of the completeness of the data sets necessary to conduct this investigation and because the regulations associated with the YMR provide a clear basis for evaluating the performance of the DBSF. A study of DBMHR fuel cycles shows a radiotoxicity benefit from the recycling CSNF in a DBMHR. Additionally, models developed to evaluate the release and transport of radionuclides from TRISO fuel particles and DBSF in a geological repository environment, including a novel model for the transport of an arbitrary length decay chain through an arbitrary combination of fractured and porous transport segments, demonstrate the efficacy of the DBMHR fuel cycle in reducing the environmental impact from the geological disposition of DBSF relative to CSNF. Calculations of the far-field transport of radionuclides released from DBSF are made by the newly developed TTBX computer code (a multi-region extension to the to the existing single region TTB computer code) which implements a numerical inversion of the Laplace-transformed analytical solutions to the radionuclide transport equation. This is done to evaluate the exposure of the target population. Results indicate compliance of the fuel form with regulatory standards related to exposure via groundwater for all cases studied by many orders of magnitude.

In our studies of the repository behavior of DBSF we have seen here that graphite is an extremely robust material that has the potential to serve as a highly durable matrix for the sequestration of high level nuclear materials. The long lifetime of the graphite matrix (3 x 10^6 years at a minimum) allows many of the short-lived fission products to decay away before they are transported to the biosphere. Additionally, lifetime estimates for the graphite matrix greatly exceed the projected lifetime of many other matrices currently being considered such as UO2 or borosilicate glass. We have seen that in the YMR the long graphite lifetime assures that radionuclides are released congruently with the oxidation of the graphite (which oxidizes extremely slowly). This removes uncertainties associated with the solubilities of the radionuclides and assures that radionuclides will always be present at levels which are at or below their solubility limit. The remarkable performance of graphite in a geological repository highlights its utility to serve as a matrix for the disposition of nuclear material and the need for further detailed material studies of the performance of graphite in repository environments.