concern that presently exists with spent nuclear fuel from most other reactors that arises with burying or storing their spent fuel in a geological repository, as they could possibly be used as a plutonium mine at some future date. "Despite the million-fold reduction in radiotoxicity offered by this scheme, some believe that actinide removal would offer few if any significant advantages for disposal in a geologic repository because some of the fission product nuclides of greatest concern in scenarios such as groundwater leaching actually have longer half-lives than the radioactive actinides. These concerns do not consider the plan to store such materials in insoluble Synroc, and do not measure hazards in proportion to those from natural sources such as medical x-rays, cosmic rays, or natural radioactive rocks (such as granite). These persons are concerned with radioactive fission products such as technetium-99, iodine-129, and cesium-135 with half-lives between 213,000 and 15.7 million years" Some of which are being targeted for transmutation to belay even these comparatively low concerns, for example the IFR's positive void coefficient could be reduced to an acceptable level by adding technetium to the core, helping destroy the long-lived fission product technetium-99 by nuclear transmutation in the process. (see more Long-lived fission products) Efficiency IFRs use virtually all of the energy content in the uranium fuel whereas a traditional light water reactor uses less than 0.65% of the energy in mined uranium, and less than 5% of the energy in enriched uranium. Carbon dioxide Both IFRs and LWRs do not emit CO2 during operation, although construction and fuel processing result in CO2 emissions, if energy sources which are not carbon neutral (such as fossil fuels), or CO2 emitting cements are used during the construction process. A 2012 Yale University review published in the Journal of Industrial Ecology analyzing life cycle assessment emissions from nuclear power determined that: Although the paper primarily dealt with data from Generation II reactors, and did not analyze the emissions by 2050 of the presently under construction Generation III reactors, it did summarize the Life Cycle Assessment findings of in development reactor technologies. Fuel cycle Fast reactor fuel must be at least 20% fissile, greater than the low enriched uranium used in LWRs. The fissile material could initially include highly enriched uranium or plutonium, from LWR spent fuel, decommissioned nuclear weapons, or other sources. During operation, the reactor breeds more fissile material from fertile