The Nuclear Waste Disposal Problem

What, then, are our options for disposing of nuclear waste? Since our focus is on evaluating fission reactors as a viable source of energy in the future, we will examine the properties of and disposal options for SNF, and ignore storage of defense waste (from decommissioned nuclear warheads, etc.).

One option that nuclear proponents discuss is the use of breeder reactors to recycle the waste. On the surface, recycling sounds like a good choice from an environmental standpoint, as it would reduce the amount of waste that needs to be disposed of, and it would reduce the required amount of environmentally harmful Uranium mining. However, the Carter administration chose in 1977 to ban the use of breeder reactors due to the enhanced risk of nuclear proliferation (breeder reactors produce Plutonium, which is ideal for making nuclear bombs). France uses breeder reactors to recycle their fuel, but I’ve been told by experts at Vanderbilt that breeder reactors are so complex that they frequently break down and have poor safety records [1]), so France has started to decommission their plants. Breeder reactors are not a panacea to the waste disposal problem.

Geological storage is widely considered to be the safest method for storage of SNF [2]. Until recently, the goal was to isolate SNF from the surface environment for at least 10,000 years, which was considered long enough for the total radiation level to decrease to acceptable levels. However, a court ruling in 2006 (?) increased the mandatory safe storage duration to 1,000,000 years. Considering humans have yet to build any structure that has lasted more than 5,000 years, there clearly is no way to guarantee that a HLNW disposal structure could maintain its integrity and confine the waste for one million years.

Yucca Mountain is a logical choice to store SNF because it is so dry. The primary objective of SNF storage is to keep the waste away from water. Why? Because water is the strongest known solvent, and it is mobile. The fear is that water would dissolve the waste and transport it a densely populated area such as Las Vegas, which is where groundwater from Yucca Mountain was originally thought to flow. Yucca Mountain has the lowest water table in the continental U.S.; to get well water there, you would have to drill a well 2,000 feet deep. The idea was to bury the waste 1,000 feet deep so that 1,000 feet of rock would protect it from the groundwater below and any infrequent precipitation events at the surface. Furthermore, it was discovered that Yucca Mountain is in an isolated hydrologic basin, so even in the worst-case scenario where the waste contaminated the groundwater, it would still be isolated within that small, uninhabited basin. Yucca Mountain is located at the edge of the Nevada Test Site, where 928 atomic bombs were detonated between 1951 and 1992, so it is already contaminated by radiation. Finally, the low population density and suitable host rock (volcanic tuff) make Yucca Mountain well suited for disposal of SNF.

Evidence that geological storage of SNF is relatively safe comes from natural analogues such as the Oklo natural reactor in Gabon. In this location 1.7 billion years ago a natural uranium ore deposit formed. At that time natural uranium had a higher proportion of ²³⁵U, the fissile isotope, so the uranium did not have to be artificially enriched like today to generate a self-sustaining nuclear reaction. Isotopic analyses show that the ore body is highly depleted in ²³⁵U, and has the same proportions of isotopes as SNF, so we infer that the ore body acted as a natural fission reactor (http://www.ocrwm.doe.gov/fact/Oklo_Natural_Nuclear_Reactors.shtml). In fact, 15 separate reactors have been discovered at the site. When the reactors were active 1.7 BYBP, groundwater acted as neutron moderator, slowing neutrons so that they could fission ²³⁵U nuclei. The heat released by fission reactions caused the groundwater to boil off, which shut down the chain reaction. Groundwater would then fill up the reactor again, and the cycle repeated. The fission reactions consumed 6 tons of ²³⁵U, producing 15,000 megawatt-years of energy over 500,000 years and heating rocks to ~400°C. Yet in the 1.7 BY since the reactors stopped operating, the original uranium and all of the fission-product nuclides have remained immobile, even though the host rocks are permeable and were likely often filled with flowing water. This is very strong evidence that SNF can be stored safely underground.

Some of my own research can be applied to the problem of safe SNF storage. To answer the question of what material can safely immobilize the components of SNF, geologists look to nature for the answers. They look for minerals that can hold high concentrations of radioactive elements like uranium and thorium for long periods of time. The mineral that holds the longevity record, the Methuselah of all Earth materials, is zircon (ZrSiO₄). The oldest solid material ever found on the surface of the earth is a 4.4 BY old fragment of a zircon crystal. How do we know it is 4.4 BY old? Zircon concentrates uranium in its structure, and once a zircon crystal grows it traps the uranium so that it can’t escape. Over time, the uranium decays to lead at a very low but constant rate, so that today we can measure the proportions of uranium and lead isotopes and estimate the amount of time elapsed since crystallization. This “isotopic clock” works because zircon also traps the lead after it forms from uranium decay, and because zircon does not incorporate any lead when it forms. Zircon can last 4.4 BY because it is very stable and therefore insoluble in natural waters, as shown by measurements made by myself and others. All of this suggests that zircon would be a good “wasteform” for storage of uranium in SNF. The problem is that zircon actually incorporates < 1 wt.% uranium in it structure, and we need something that can incorporate much higher concentrations. Another problem is that over time high radiation levels destroy the zircon structure [3], turning the zircon crystals into glass, which is much more soluble in natural waters and therefore much less effective at immobilizing the uranium [4].

A better candidate for storage of uranium and thorium is the mineral monazite, which is a rare earth element phosphate (REEPO₄). Although the geological evidence suggests that monazite is not quite as durable as zircon, it can hold much higher concentrations of Th (up to 10 wt.% ThO₂) without experiencing significant radiation damage and still last for billions of years. In the laboratory, I have studied the solubility of monazite in natural waters at elevated temperatures and pressures, and found its solubility to be very low at near-neutral pH. In field studies, I have investigated the stability of monazite in rocks, and have developed methods for using monazite to date the infiltration of water into rocks [5]. Although this research was “pure science” because the primary objective was to develop a better understanding of how the Earth works, it has implications for storage of SNF. History shows that most technological advances were enabled by research in pure science, and since it is primarily advances in technology that fuel the economic engine, particularly in the U.S., and that in the future may provide answers to how our society may become sustainable, it would be unwise for the U.S. to stop investing in pure science.

I am confident that further research into durability of crystalline wasteforms and the geology of potential waste disposal sites will give us the technological ability to safely dispose of SNF in the future. However, we do not and may never have the political or societal will to deal with the problem. Even if we as a society face the situation, agree on a site, and fund the building of a facility, it will take too long to make nuclear power a short-term fix to our energy needs. Abandoning Yucca Mt. means that we won't have a SNF disposal site for at least 20 years. Given the possibility that they will be stuck with more SNF in the future, utility companies are less likely to start building new power plants. In addition, since it takes about 20 years to build a new reactor, U.S. capacity to generate electricity through nuclear fission is unlikely to increase for at least 30 years.

To sum up, what are the advantages of nuclear power plants? They have near-zero CO₂ and pollutant emissions. What are the disadvantages? Radiation is released to the environment at every stage of the nuclear fuel cycle. There is a very small but real risk of nuclear reactor accidents (e.g., Chernobyl). Terrorists or hostile countries could steal enriched uranium destined for fission reactors or plutonium from breeder reactors to make nuclear bombs. The U.S. has no safe SNF disposal facilities, and won’t have any for at least twenty more years. We have a limited supply of minable uranium, so nuclear power is a non-renewable energy source (we have enough U ore to deploy 1000 new reactors in the next 50 years and maintain for 40 years [6]). Finally, nuclear power is not cost-effective. In a nutshell, nuclear power is a very complicated, expensive, centralized form of energy production that requires a lot of government involvement (regulation and oversight), has a very vocal opposition, and big potential problems, while decentralized, renewable energy sources pose fewer risks and may be more cost effective.

In general, I am advocating a move from centralized to decentralized, from hard path to soft path, from non-renewable to renewable, and from fossil fuels to alternative energy sources. Nuclear is centralized, and we don't have a solution to the waste problem, so I am not recommending it as an energy source, unless it is the only way we can eliminate fossil fuels.

1. Charman, K., Brave Nuclear World? Part II. World Watch Magazine, 2006: p. 12-18.

2. Macfarlane, A.M. and R.C. Ewing, eds. Uncertainty Underground: Yucca Mountain and the Nation's High-Level Nuclear Waste. 2006, The MIT Press: Cambridge, Massachusetts. 431.

3. Farnan, I., H. Cho, and W.J. Weber, Quantification of actinide [agr]-radiation damage in minerals and ceramics. Nature, 2007. 445(7124): p. 190-193. http://dx.doi.org/10.1038/nature05425

http://www.nature.com/nature/journal/v445/n7124/suppinfo/nature05425_S1.html

4. Grambow, B., Nuclear Waste Glasses - How Durable? Elements, 2006. 2: p. 357-364.

5. Ayers, J.C., et al., In situ oxygen isotope analysis of monazite as a monitor of fluid infiltration during contact metamorphism: Birch Creek Pluton aureole, White Mountains, eastern California. Geology, 2006. 34(8): p. 653-656. http://geology.geoscienceworld.org/cgi/content/abstract/34/8/653

6. Ansolabehere, S.e.a., The Future of Nuclear Power: An Interdiscplinary MIT Study. 2003, Massachusetts Institute of Technology. p. ix-x, 1-16.