Nuclear Waste Reduction through Advanced Reactor and Fuel Cycles

by Kevin Fischer

Abstract

This article analyzes three technologies that possess the possibility to help contribute to the success of nuclear energy in the U.S. First, thorium fuel cycles have the potential to improve light water reactors (LWRs), the current U.S. reactor standard. Using India's thorium program as a reference, this essay examines the benefits of thorium-based cycles, mainly considering their more efficient burns. Then the article focuses on advanced reactor designs with the capability to solve the most significant long-term nuclear waste issues. The two types of advanced reactors discussed are: integrated fast reactors (IFRs) and deep-burn modular helium reactors (DB-MHRs). Aside from their energy and waste benefits, these reactors are also inherently safer because of passive cooling, making nuclear disasters highly unlikely. The conclusion stresses the important role that these technologies can play in the future of nuclear energy in the U.S.

Introduction

Global movements against greenhouse gases have put developed countries in a difficult position, in which they are pressured to reduce greenhouse gas emissions drastically. Yet there are currently few effective alternatives to the cheap sources of electricity: coal and natural gas. By 2020, the demand for energy is expected to be 60% higher than in 2001. Nuclear technology offers the prospect of clean, abundant power for many generations.¹ The International Atomic Energy Agency (IAEA) believes that, to meet the rising demand for emission-free energy, nuclear power will be necessary.² If this is case, however, problematic issues do inhibit the growth of nuclear power in the areas of proliferation, safety, and waste management. Any viable nuclear solution must address these issues.

This paper examines three promising technologies that can contribute to a comprehensive nuclear solution, focusing both on technologies for immediate implementation as well as technologies for implementation in the near future. These technologies are not the only solutions, since many of them are best suited for a specific subset of variables, but as a whole, they could help make nuclear power a more attractive solution for the energy crisis.

The first part of the paper focuses on technologies that can be implemented immediately. A new type of thorium-based oxide could replace much of the uranium in the current fuel rods for light water reactors; the only change necessary for this fuel is to replace the fuel cores themselves. This would allow for more efficient burns, producing smaller volumes of less toxic waste. Next, the essay discusses two advanced reactor designs, the integrated fast reactor (IFR) and the deep-burn modular helium reactor (DB-MHR), which could augment the current fleet or possibly even replace it, making nuclear power more viable in the long term by providing a more comprehensive waste solution. These reactors possess the unique ability to use as fuels both hazardous plutonium and spent light water reactor fuel.

Thorium Fuels in Light Water Reactors (LWRs)

The current nuclear fuel standard is uranium because of its unique ability to be easily fissioned, but ultimately, uranium only exists in finite quantities on the earth. Thorium, however, speculated to be three to four times as abundant as uranium, could be used to augment world uranium supplies, making them last longer. ³ Currently, the industry standard reactor is the light water reactor (LWR), which can immediately take advantage of thorium-based fuels. Since only one percent of uranium is utilized in LWR fuel cycles, clearly, the status quo of fuel cycles cannot provide power for many generations because such an inefficient use of limited uranium will not last forever. A study by the Advanced Nuclear Energy Products, a division of the Idaho National Engineering and Environmental Laboratory, attempted to re-engineer a new fuel for LWRs that would cost less, be more resistant to proliferation, output more stable waste and less high-level waste, and have longer refueling cycles. Their ultimate solution was a mixed-oxide (MOX) core, comprised of a 70% weight thorium dioxide (ThO2) and 30% weight uranium dioxide (UO2) mixture, as opposed to the current LWR fuel, comprised of 100% UO2.¹ Th02-UO2 fuel proves far superior to UO2 fuel on a number of levels.

The first advantage is the high energy conversion ratio. ThO2 has a much higher conversion ratio; thus, more thorium is fissioned. This also means the production of fewer long-lived actinides, which are radiologically dangerous. Therefore, ThO2-based fuels have a lesser degree of radiotoxicity and produce less spent fuel than their uranium-based counterparts. New core designs from the National Mexican University improve fuel economy by taking advantage of the U233 build up in a lattice design. Ultimately, the spent fuel discharged by reactors fueled with ThO2-UO2 is approximately 2.78 times less radiotoxic as the spent fuel from UO2. It also is considerably safer than UO2+PO2 (potassium dioxide) and Mixed Oxide (MOX) fuels.⁴

This new fuel form allows for nearly a two-fold improvement in refueling times. Current LWR plants operating on UO2 must refuel about every 18 months, but the ThO2-UO2 would allow the same plants to refuel every 24-36 months, according to a study led by J. Stephen Herring for the Advanced Nuclear Products division of the Idaho National Engineering and Environmental Laboratory. These longer cycles not only greatly reduce operating costs, but also significantly lengthen the time intervals between when workers must expose themselves to large doses of radiation during refueling. If all LWRs switched over, they would save about one billion dollars a year; also, the U.S. government would save over one hundred million dollars annually from not having to store as many spent bundles of fuel.¹ Furthermore, the reactor never needs to shut down during fueling because, as the fissile material fissions, new fissile materials are created.⁵

Also, according to Herring's study, the ThO2-UO2 is nearly completely proliferation resistant, producing 3.2% less plutonium than the UO2 fuel cycle. Unlike the plutonium from the UO2 cycle, which is relatively easily separable for weapons grade material, ThO2-UO2 is naturally harder to use for weapons. The fuel does not work well for weapons because it glows so hot that it usually melts any surface it contacts. Assuming that this fuel could be processed, it would only yield 10% of the design blast, which is not a good option for weapons; there are easier ways to acquire weapons-grade materials.¹

The ThO2-UO2 waste also holds up much better in the environment, and is much more suited for the proposed Yucca Mountain underground nuclear waste repository, mainly because of ThO2. The thorium oxide used in the fuel is the highest oxidation state of thorium; thus, the thorium's crystal structure would barely change during its extended storage. This drastically differs from that of uranium, which, after being discarded, continues to oxidize at a rate of around one percent each year. This is important because this would make the spent fuel more stable inside the canisters and more resistant to corrosion, allowing it to last longer, safely, in the repository.¹

Finally, ThO2-UO2 seems economically feasible. Currently, the fuel costs about ten percent more than UO2 fuel, but in the long run, in terms of both saved costs and the predicted comparative costs of thorium and uranium, it appears the more economical option. More uranium is mined currently, accounting for the cost difference. However, the researchers in the Herring study suspect that, when uranium reserves run short over the long term, the balance will reverse, causing thorium to cost less than uranium.¹

Pioneering More Advanced Thorium Techniques

Replacing thorium in current LWRs only covers the improvements that can be made with little immediate cost difference. This only represents a small portion of viable thorium research. A study by the National Atomic Energy Agency attempted to design reactors that would specifically make use of thorium’s unique deep burn advantages. Successful in their endeavor, they managed to conceive of reactor designs that, based on their models, would allow for ten years of burn-up without on-site refueling. These long-life cycles, around eight to ten times as long as current life cycles, were designed for high-temperature gas-cooled reactors, pressurized water reactors, and boiling water reactors. The ten year life cycle was achieved through the careful and meticulous design of the graphite moderator and fuel composition. Such design allows for the high internal conversion ratios necessary to achieve the ten year goal.⁶

India, a country with little uranium, has been forced to adopt nuclear reactors mainly to thorium fuel cycles. The current system in India already uses its thorium reserves through a three-stage reactor process representing a more long-term thorium-based solution. Since thorium has no fissile isotope, its initial stage needs to employ a fissile material. In the first stage, pressurized heavy water reactors (PHWRs) use natural uranium. During the second stage, plutonium from reprocessing spent PHWR fuel is employed in fast reactors to help generate the fissile material for the thorium stage. In the process of the third stage, the reactors burn a mix of Th-U233 for fuel. India has pioneered the field of thorium fuel, but it is still in its early stages and requires some perfecting before it is ideal.³

The Thorium-Uranium Extraction (THOREX) process is less well defined than the mature Plutonium-Uranium Extraction (PUREX) process; nor are the physics of thorium reactions as well understood. For instance, thorium is harder to process than uranium because it does not dissolve as easily. The Indira Gandhi Centre for Atomic Research has produced the most promising results for THOREX research and continues to be the leader in thorium-reprocessing technology. Although there remain some technical hurdles to overcome, thorium is necessary for a nearly sustainable energy infrastructure dominated by nuclear energy

Integral Fast Reactors (IFRs)

While thorium-based fuels present a short-term solution by providing a new source of fuel for nuclear reactors, they ultimately contribute to a major issue for nuclear power: an abundance of hazardous waste. LWRs only use a small portion of the fissile material in their fuel. After it has been fissioned, the material is not reprocessed and is sent to a repository. These once-through cycles employ uranium resources very inefficiently.⁵ Initially, PUREX was used to reprocess LWR spent fuel, but in the 1970s, the PUREX process was halted because U.S. experts began to view the plutonium separated during the process as a proliferation threat.⁷ Since 1984, one possible solution has been in development in the Argonne National Laboratory: the integral fast reactor (IFR). This reactor possesses the unique ability to recycle its plutonium and LWR fuel until it is fissioned away.² The IFR’s hard neutron spectrum causes all actinides and plutonium isotopes to fission at the same efficiency level. Even at LWR plants that recycle spent fuel, only a few cycles are practical, whereas an IFR can completely destroy plutonium and all the most dangerous radioactive isotopes. The IFR introduces a safer solution that lowers processing costs, fissions plutonium, is completely proliferation resistant, and uses resources efficiently.²

Pyroprocessing involves a pyrometallurgical and electrochemical process that efficiently recovers actinide elements, while recovering no plutonium. Because no plutonium is separated in the process, the fuel can be considered proliferation resistant.

A major advantage of the IFR comes from pyro-processing, a new method of processing fuel developed for the IFR. Pyroprocessing involves a pyrometallurgical and electrochemical process that efficiently recovers actinide elements, while recovering no plutonium. Because no plutonium is separated in the process, the fuel can be considered proliferation resistant. An electrorefiner separates the spent fuel mixture into useful fuel through electrochemical means. Two cathodes are present in the mix; the impure fuel is put on the anode. When a powerful electric current is applied, pure uranium is collected at one cathode and a mix of plutonium, americium, neptunium, curium, uranium, and some rare-earth fission products collect at the other cathode. The other few remaining products remain in the salt solution. This separation process is quite accurate and, through thermodynamics, can be predicted very precisely.⁷ During this step, dangerous transuranic isotopes with long-lived radiological toxicity can be dumped in the refinery along with weapons -grade plutonium and actinides recovered from the spent fuel from LWRs.² The material that collects at the cathode is then injected into closed-end molds and rapidly cooled. After being capped, the rods are inspected.

The pyroprocessing facility, conceived by the Chemical Technology Division of the Argonne National Laboratory, led by J. J. Laidler, was housed at the same site as the reactor. Their designs made it a completely remote facility requiring no human intervention. Since the plant and its operations are completely contained and isolated in a highly shielded remote facility, it is considerably safer than other plants that utilize reprocessing techniques. Furthermore, this makes the plant even more proliferation resistant.⁷ Along the same line of thought, the plant is a “zero-release” plant since everything, including the coolant, is self-contained. This means that there will be no radioactive wastes from corrosion; the only waste released has been processed by the plant. Furthermore, the plant only produces high-level, low volume waste.⁸

A unique and attractive characteristic of IFRs is that they can use spent LWR fuel as fresh fuel after a round of processing. Estimates from J. J. Laider's study predict that approximately 40,000 tons of LWR spent fuel will be in retrievable storage in the U.S.⁸ This is a prime fuel source for IFRs. Not only could IFRs employ this as a source, but it also seems necessary for them to do so. Predictions from the 2002 joint study led by C. Rodriguez, sponsored by General Atomics and the Los Alamos National Laboratory, show that the planned Yucca Mountain repository lacks sufficient capacity for all the spent fuel that will have accumulated by the time it is finished. In fact, these researchers believe that the U.S. will need a new Yucca Mountain-sized repository every 20-30 years.⁹ One percent of LWR spent fuel contains material that can be used for fuel in IFR reactors and 96% of the spent fuel can be employed as makeup feed to the IFR. Currently, there is enough output from LWRs in the U.S. to provide 1500MWe (Megawatt Electric) of IFR generating capacity annually.⁷

Furthermore, IFRs could be used to reduce the U.S.'s vast plutonium reserves. During the Cold War, many tons of plutonium were produced for warheads that have now been dismantled. Large, scattered stockpiles of plutonium are hard to guard; material not in use is difficult to control and monitor. It is a large proliferation risk, because even hundreds of years after the material has continuously decayed, much of it still remains suitable for weapons.¹⁰ Fast neutron reactors, such as the IFR, can efficiently fission plutonium. In 1997, the US had 1000 tons of plutonium with a growth rate of 70 tons a year. IFRs could be powered on this fuel, reducing the need to guard the plutonium safely and greatly reducing nuclear proliferation risks.² According to a study sponsored by the U.S. Department of Energy, performed at Argonne National Laboratory by W. H. Hannum and D. C. Wade, a single IFR would burn through only 50 tons per four years, so this supply could provide an energy source sufficient for many thousands of MWe.¹⁰

IFRs do need to release waste, but they produce significantly less waste than LWRs and that waste is friendlier to the environment. Most of the released heat comes from fission products with half-lives of 30 years or less, which is much less than that of the spent fuel from LWRs. This difference allows for repositories to be built to lesser specifications. A repository holding IFR waste would only need to house each waste package for a few hundred years versus a repository for LWR waste that would need to store each waste package for hundreds of thousands of years. Furthermore, no proliferation risk is associated with IFR fuel, so security would not need to be as tight. Such shorter holding times also decrease the risk of environmental contamination. Also, because the packages are cooler, they can also be placed closer together, increasing repository capacity.⁸ Clearly, IFRs can be part of a much longer-term strategy for U.S. nuclear energy, since they provide a cheap, efficient way to dispose of waste that is proliferation resistant. Ultimately, it appears that the research for IFRs is nearly completed and the only hurdle left is government funding and implementation.

Deep-Burn Modular Helium Reactors (DB-MHRs)

Another possible solution for nuclear waste disposal is the deep-burn modular helium reactor (DB-MHR), which possesses the capability to play a similar role as the IFR. The main difference between the two is that the DB-MHR reprocesses the spent fuel only once. The fuel is broken up into two categories: transmutation fuel and driver fuel. The initially fissile isotopes exist in the driver fuel and it powers the reaction; the driver fuel can be comprised of actinides, such as plutonium or americium. Nuclear scientists find this advantageous over previous deep burn reactors that had to use burnable poisons such as boron or erbium. The transmutation fuel contains the spent fuel from other reactors.⁹ Because the fuel is left in the reactor for significantly long periods of time and the rods are shuffled towards the outside of the core from the center, while the driver fuel is mixed in with the transmutation fuel, less reprocessing is necessary to achieve nearly the same effects as IFRs. Although these reactors are not as effective, they can process waste more quickly.⁹ The goal of the DB-MHR plants appears to be to process waste more simply from LWRs so that it can be used in conjunction with the current LWRs. IFRs, however, are capable of serving a different role. As discussed earlier, they are well suited for replacing LWRs as they are decommissioned, not just burning their used waste.

The DB-MHR solution is economically feasible, according to a 2002 joint study by C. Rodriguez, sponsored by General Atomics and the Los Alamos National Laboratory. A deep burn plant could recover 20% of the energy from the fuel in just one pass, while destroying 90% of the transuranic waste. Furthermore, the waste left has been irradiated; thus it can be safely placed into a repository because it is unusable for weapons. Reprocessing can be employed to destroy even greater percentages of the transuranic elements. However, these cycles do not make sense economically or environmentally because the proportion of irradiated material used in the reprocessing steps that must be disposed of relative to the material recovered to be fissioned grows with each subsequent process.⁹

Rodriguez's team proposed a plant consisting of five DB-MHR critical reactors and one DB-MHR sub-critical unit with an accelerator and predicted that it could produce 1700 MW of power at 50% thermal conversion efficiency from irradiating 1500kg of waste transuranics a year. This process would release 150 kg of waste in ceramic-coated particles encased in graphite blocks. These particles are isolated from the environment as a result of their natural casing alone, making them ideal for storage.⁹

Also, these reactors are incredibly safe. Meltdown is impossible because the core would cool to the ground automatically by convection and conduction, if the coolant somehow escaped.⁹ Also, the non-reactive helium coolant actually has a negative temperature coefficient of reactivity. This means that an increase in coolant temperature causes a decrease in reactivity. These reasons, combined with a graphite core that causes high structural stability and heat resistance, a low power density over the core, and the triple isotropic-coated fuel particle that retain fission products even in severe accidents, make the DB-MHRs incredibly and inherently safe.¹¹ This, combined with a simulation performed by the Department of Nuclear and Reactor Physics at the Royal Institute of Technology, shows that DB-MHR technology is ideal for disposing of LWR and plutonium waste.¹¹

Conclusion

With a worldwide push towards greenhouse gas-free energy production, nuclear power is an important option for providing such clean energy. Nuclear power in its current form, however, is unsuitable for long-run operations. Three main technical challenges face nuclear power: making nuclear fuel last longer, keeping weapons-grade material out of the wrong hands, and developing a comprehensive waste management solution. Modifications can be made to current fuel cycles by replacing up to 70% of the uranium with thorium to make present reactors last longer and produce more manageable fuels, but a longer term solution is necessary since these reactors still produce a considerable amount of waste. Argonne National Laboratory offers one such solution, the integral fast reactor, which possesses the unique ability to fission nearly every isotope of actinides and plutonium with equal efficiency. When combined with an efficient recovery process, IFR technology could fission dangerous plutonium and waste fuels from LWRs that still have much energy potential. Deep burn-modular helium reactors offer similar capabilities but without much reprocessing; the main difference is their design intent. IFRs were developed to burn anything and replace all current reactors, whereas DB-MHRs were designed to burn and transmute spent fuel. DB-MHR waste is also slightly longer-lived than IFR waste since it is not fully transmuted. The DB-MHR two-burn cycle eliminates the majority of the waste and leaves material unwanted for nuclear weapons. Although these two advanced reactors still produce hazardous waste, that waste is much more suited for repository storage. Unlike the current output of LWRs, their waste lasts for hundreds of years versus hundreds of thousands of years and is significantly more stable. Also the waste is nearly proliferation resistant because of the low concentrations of plutonium comprised mainly of poor material for weapons. Furthermore, studies show each of these partial solutions to be economically viable and relatively cost effective. Their economic viability and capability to revolutionize commercial nuclear power by addressing issues of fuel longevity, waste management and nuclear proliferation concerns, could allow them to serve an integral role in providing tomorrow's energy. With working prototypes of this technology, only one problem is left: public opinion.

No new nuclear reactors have been commissioned since 1977. Since then, as plants' operating licenses expire, they have had an incredibly difficult time renewing their licenses. Until Congress can recognize well-reasoned arguments and overcome its fears of nuclear energy, such advanced reactor technologies will never be implemented in the nuclear cycle. Although some research in the thorium-reprocessing field is necessary, thorium core, IFR and DB-MHR technology are ready for deployment. New thorium-based fuel cores can be used immediately to replace the current uranium cores in LWRs. IFR and DB-MHR plants can be built to begin processing spent fuel and U.S. plutonium reserves. There are no technical limitations barring their implementation, just the barrier of misinformed fear.

Glossary of Acronyms

DB-MHR: Deep-Burn Modular Helium Reactor
IFR: Integrated Fast Reactor
IAEA: International Atomic Energy Agency
LWR: Light Water Reactor
MOX: Mixed Oxide
MWe: Megawatt Electric
PUREX: Plutonium-Uranium Extraction Process
PHWR: Pressurized Heavy Water Reactor
THOREX: Thorium-Uranium Extraction Process

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Nuclear Waste Reduction through Advanced Reactor and Fuel Cycles

by Kevin Fischer