Do New Nuclear Reactor Designs Change The Nuclear Waste Issue?

Not particularly because there is, and always will be, such a small volume of nuclear waste, no matter what designs are built or how many of them we build. However, with the advent of new nuclear reactor designs, the question…

Not particularly because there is, and always will be, such a small volume of nuclear waste, no matter what designs are built or how many of them we build.

However, with the advent of new nuclear reactor designs, the question of how they each affect the ultimate disposal of their nuclear waste looms large.

Currently 440 nuclear power reactors are operating in 30 countries. About a dozen new reactors began operations in the last year or so, but there will be as many as 16 permanent shutdowns that accompany them. 174 reactors have been permanently shut down worldwide, about half of those are in Western Europe. Globally, the amount of used or spent fuel in storage is approaching 450,000 metric tons of uranium (MTU), sometimes referred to as metric tons of metal (MTM). Somewhat less than half of that amount has been reprocessed to make new fuel, mainly by France, India, Russia and the UK, although a number of other countries have reprocessed in the past. China and Japan are planning to begin reprocessing. The form of the nuclear waste does make a difference in terms of repository performance, but most regulatory guidelines require nuclear waste to have less than 1% free liquid for transportation – so that should make little difference between various reactor types. MORE FOR YOUHow Green Is Wind Power, Really? A New Report Tallies Up The Carbon Cost Of RenewablesWhy Won’t Environmentalists Speak Out Against Forced Labor For China-Made Solar Panels?Amtrak Doesn’t Just Need Money The real difference will be found in the relative amounts of short-lived versus long-lived radionuclides (radioactive elements) in the waste, which will derive mainly from the fission products (like the broken pieces of the uranium nucleus) versus actinides (the heavy elements in the last row of the periodic table including uranium, plutonium, americium and neptunium) produced during reactor operations from the bombarding neutrons (see A Nuclear Primer). Solid versus liquid fuels, pebbles versus rods, fast reactors versus thermal, and any of the various reprocessing methods versus just once-through operations, will have some effect on these ratios. As an example, compared with conventional reprocessed waste in Europe, the ORIENT-cycle reprocessing method produces about ten times less HLW canisters than traditional reprocessing like the PUREX process. But as Alex Cannara points out the biggest difference in these ratios comes from using thorium instead uranium or plutonium as the fuel. Long-lived actinides are about 10,000 lower using Th. These ratios will affect the disposal density of packages in the repository, and thus the repository size, because their decay heat will determine the disposal geometry with respect to any thermal limitations of the specific rock and required separations between disposal packages. Advanced reactor designs, like X-energy’s, uses layered pebbles instead of rods. X-energy The benefit is mostly financial, not environmental. The size of the repository doesn’t really change the performance. But the cost of building and operating deep geologic repositories is large. If one could shrink the repository ten-fold, the cost savings would be significant and could more than pay for the most optimal reprocessing methods. This is an argument for reprocessing once-through fuel in the United States – the size reduction of any final repository could be up to 50 times, according to the Organization for Economic Co- operation and Development (OECD) and International Atomic Energy Agency (IAEA) studies on alternative reactor waste. Decay heat is a major input for the design of underground repositories. For disposal in granite, clay and tuff formations the maximum allowable disposal density is determined by thermal limitations and most scenarios have waste canisters separated by about 8 feet. HLW arising from advanced fuel cycle schemes generates considerably less heat than the spent fuel arising from the reference Pressurized Water Reactor (PWR) once-through scheme, although the separated fission products have to be handled or used in some industrial process like food or blood irradiation. This lower thermal output of reprocessed waste allows a significant reduction in the total volume of the final repository, that is, you can pack the waste packages closer together. Separation of cesium and strontium from the waste reduces the required repository size even further since they are the most heat-producing nuclides. For example, in the case of disposal in a clay formation, the volume needed for disposal is reduced by a factor 3.5 through a fully-closed cycle scheme as compared with the reference PWR once-through scheme and by a factor of 9 through a scheme including separation of cesium and strontium. After 50 years of cooling, variations in decay heat do not exceed a factor of four for any of the fuel cycle scenarios considered in the OECD study. After 200 years, the decay heat of HLW would be reduced by a factor of up to 30 in all minor actinide-burning schemes as compared to the reference PWR once- through scheme. Extending the cooling time from 50 to 200 years will result in a drastic reduction of the thermal output of HLW from advanced fuel cycle schemes and, consequently, of the repository size needed. Massive salt host rocks, like the Salado Formation that hosts America’s only operating deep geologic nuclear repository (WIPP), are a special case. The thermal conductivity of massive salt is about five times that of crystalline rocks, meaning the thermal loading can be much higher becuase the salt conducts heat away much faster. Furthermore, the unique creep-closure property of massive salt, that provides such amazing performance at WIPP, also goes as the sixth power of the temperature, making it work even better the hotter the waste. Recently, Deep Borehole Disposal of spent fuel has become a serious disposal option and one to which some reactor designs are particularly suited, such as pebble bed reactors or molten salt reactors where the fission products can be removed from the molten fuel in real time. The waste from these designs do not have a width or volume limitation, making it very easy to package the waste to accommodate an optimal-size borehole or other repository geometries. OECD considered two main pathways for waste minimization: multiple recycling of uranium/plutonium and minor actinides in fast reactors, and the use of thorium-based fuel in thermal reactors to increase utilization of natural resources and minimize waste streams containing minor actinides. They found that both pathways would reduce the radiotoxicity of waste products relative to a once-through fuel cycle, and would generally reduce the quantities of long-lived wastes and their resulting heat loads on repository systems. The IAEA study indicated that full fissile recycling or full actinide recycling can reduce the radiotoxicity of the waste by a factor of 100 to 200, and reduce the critical timescale from over 100,000 years to less than 1,000 years. There are a lot of unknowns in these alternative scenarios and reactor designs. Some designs operate at much higher temperatures than current reactors, and require longer service lives and high burnup for fuels. Some use fast or epithermal neutrons and use very aggressive coolants, like molten metals or molten salts. So new, sometimes exotic, materials are needed which may introduce different or greater amounts of activation products (materials made radioactive from being bombarded with neutrons during operation). So additional R&D will certainly be required to adapt the fuel reprocessing techniques to the new fuel types and to develop suitable immobilization matrices for their waste streams. All things considered, new reactors and various reprocessing schemes will change the waste type and will affect repository size and design to some degree, but the amount of nuclear waste will still be very small no matter what we do, particularly since most of these new designs get more energy out of the same amount of fuel. This will still require only a single repository no matter what the designs or number of reactors we end up building. And don’t forget – the total volume of nuclear waste produced worldwide since WWII is the same as the volume of toxic chemical waste produced by the coal industry worldwide every hour.