Nuclear Energy for Our Challenging Future
The U.S. Energy Information Administration's International Energy Outlook 2011 predicts a 53 percent growth of global energy consumption between 2008 and 2035, with fossil fuels, primarily coal and oil, representing up to 78 percent of the increase.
Fossil fuels are a finite resource, and their use pollutes the Earth and reportedly changes the climate to our detriment. With the world's population surpassing 7 billion people and hungry for ever more energy, the United States, along with the rest of the world, needs to explore multiple cleaner energy sources, including solar, wind, and nuclear power.
For its own national security, the United States needs to rely more on domestic energy sources. Nuclear power could be an important part of a homegrown energy portfolio for generating the nation's electricity. Solar power and wind-generated power can contribute, but only nuclear power can reliably and cleanly provide large quantities of electricity.
But nuclear power creates radioactive nuclear waste, which needs long-term deep geological storage and which must be kept secure against the diversion of its fissile materials to weapons work (proliferation). So the Department of Energy's Fuel Cycle Research and Development program is looking for ways to expand nuclear energy use while reducing waste production and proliferation risks. Fast reactors, coupled with a new closed-fuel cycle, may be the answer. The Los Alamos Neutron Science Center (LANSCE) will be central to U.S. decision making about whether fast reactors are the answer the nation wants when addressing questions about the energy future.
India's Prototype Fast Breeder Reactor (PFBR) is pictured here under construction at Kalpakkam. India expects to commission the reactor this year. As a "breeder," this fast reactor will produce more fissile material than it consumes. India will use the surplus material in fuel for additional breeders, whose construction is scheduled to begin in 2017. –International Atomic Energy Agency
What a Little Neutron Can Do
A fast reactor is actually a fast-neutron reactor, "fast" being the energy of the reactor's neutrons. Fast reactors use neutrons with a high kinetic energy—1 million electronvolts. Thermal reactors (most of today's power reactors) use 0.025-electronvolt "slow" neutrons.
Neutrons initiate fission in a reactor's fuel, and they maintain it—a neutron splits an atom's nucleus, releasing energy, new radioactive isotopes ("fission products"), and additional neutrons, which strike more nuclei, causing new fissions and releasing more neutrons in a self-sustaining process: a neutron chain reaction.
Neutron energy determines how a reactor consumes its fuel. Thermal reactor fuel is a combination of two uranium isotopes—U-235 (5 percent) and U-238 (95 percent)—and the thermal reactor uses that fuel very inefficiently: its slow neutrons can fission only the U-235, not the U-238. When a slow neutron strikes a U-238 nucleus, rather than splitting the nucleus, the neutron gets captured inside the nucleus ("neutron capture"), changing U-238 into U-239 and beginning the transmutation (change) of one element into another. First, U-239 decays into neptunium, which then decays into plutonium, but the neutrons do not stop there. Rather, some slow neutrons get captured by the newly made plutonium, which then transmutes through decay into americium. Further, neutron capture followed by decay transmutes some americium into curium. All these transmutation products have higher atomic numbers than uranium and are therefore called "transuranics."
The fission products also can capture slow neutrons, and every neutron captured is a neutron removed from the fission chain reaction. Thus, the fission products and transuranics eventually "poison" a thermal reactor; that is, when too many have built up in the fuel, the chain reaction can no longer be sustained. At that point the fuel is called "spent" and must be replaced. Unfortunately, that happens well before all the U-235 has been consumed.
Fast neutrons can do much more. Their high energy lets them fission all uranium isotopes, including U-238, and even the transuranics that build up in the spent fuel from thermal reactors. For that reason, much of the thinking about fast reactors links them to the reprocessing of spent fuel for recovering of fissionable materials, which is what the closed fuel cycle is all about. Whether the United States eventually uses fast reactors for nuclear energy depends largely on a U.S. shift to the closed fuel cycle.
"If you use an open fuel cycle, you don't use about 99 percent of the energy available in uranium. Does it make sense to restrict nuclear power to a system that does not fully use the energy resource, does not tap all the energy that's available in the system?"
–Eric Pitcher, LANSCE Division Office
Open or Closed?
The United States currently uses the "open" fuel cycle, wherein the fuel is put through a thermal reactor once and then discarded, with most of its uranium unused.
Says Eric Pitcher, of the LANSCE Division Office, "If you use an open fuel cycle, you don't use about 99 percent of the energy available in uranium. Does it make sense to restrict nuclear power to a system that does not fully use the energy resource, does not tap all the energy that's available in the system?"
The open cycle also generates a lot of waste that, because it contains the transuranics, is very long lived. Many of the transuranics have exceedingly long half-lives. Plutonium-239 (Pu-239) has a half-life of 24,000 years, while Pu-242's half-life is more than 300,000 years. Americium-243 has a half-life of 7,000 years, and neptunium-237 lasts almost forever, with its half-life of 2 million years. With such things as part of the mix, spent fuel must be stored indefinitely in a deep geologic repository. To compound the problem, no such repository exists yet; for now, the spent fuel stays in interim storage.
"All the nuclear materials that have come out of our reactors are still stored where the reactors operate," say Pitcher. "A couple of reactors have lived out their useful lives and been closed down, and all that's left is a large green field and a building that stores the spent nuclear fuel. There's nothing else left. No reactor, nothing."
There are different approaches to a closed fuel cycle. The ultimate closed cycle would be one in which only fast reactors were used. In such a scenario almost all of the fuel would be consumed, leaving mostly the fission products. (The fission products will always be waste.) But the possibility of that great a commercial use of fast reactors is far in the future.
The most common closed cycle—used in Great Britain, France, Japan, and Russia—involves extracting the spent fuel's plutonium and mixing it with uranium to form a new, mixed-oxide fuel, or MOX. The use of MOX reduces the volume of waste destined for geological storage, but the waste still contains the long-lived "minor" actinides (the elements with atomic numbers 89 to 103, excluding uranium and plutonium). So the MOX process results in what is really only a partly closed cycle.
In a fully closed cycle, the uranium would be separated and recycled into new fuel for thermal reactors and the plutonium and other transuranics recycled together into fuel for fast reactors. There would still be waste—the short-lived fission products and traces of long-lived actinides created by transmutation inside the fast reactor.
"But the volume of those wastes would be much smaller than the volume of waste we have now," says Pitcher, "and that would greatly reduce the number of geologic burial sites you'd need for a large fleet of reactors supplying electricity."
The fully closed fuel cycle would also reduce the risk of proliferation—the diversion of plutonium to weapons production. The plutonium would be burned (fissioned in a reactor) instead of sitting in storage, currently onsite dry-cask storage (steel containers, surrounded by concrete) and because it would never be separated from the other transuranics, so there would be no pure plutonium stream. The MOX process does include a pure plutonium stream. Concerns about the proliferation risk of that stream are a major reason the MOX process has not been used in the United States.
Workers at the Fukushima power plant struggle with the damage wrought by the 2011 tsunami. The highly radioactive spent fuel from the plant's reactors threatened to add to the disaster when water was lost from the cooling pools where it was stored. –Eco Watch
An additional risk is inherent in the first step of storage—placing the spent fuel rods in cooling pools before moving them to dry-cask storage. The cooling pools can be vulnerable in a disaster such as the March 2011 tsunami that damaged the Fukushima nuclear reactors in Japan. During that event, water was lost from the cooling pools, raising fears that the spent fuel rods would overheat enough to release large amounts of radioactive material.
Says Pitcher, "The pools might have had a lot less spent fuel if Rokkosho [Japan's new reprocessing plant, still coming on line] had been up and running." Rokkosho is a MOX plant, so it represents only a partly closed fuel cycle, but the point is still a good one. Rods that were cool enough could already have been out of the pools and into reprocessing.
In a fully closed fuel cycle, a chemical process—UREX, still being developed—would separate the uranium into one stream and the plutonium into another, with the other transuranics. The uranium would be reprocessed (for example, enriched in U-235) and turned into new fuel for thermal reactors. The plutonium and other transuranics (including the minor actinides) would become fuel for fast reactors. In both cases, the fuel would be used, reprocessed, and reused more than once in a continuous recycling strategy until all that is left for disposal as waste are the fission products and a trace of actinides created by the fast reactors from the minor actinides in a process called "transmutation."
Materials Test Station
Pitcher is the project manager for the proposed Materials Test Station (MTS), a new fast-neutron irradiation facility planned for construction at LANSCE. MTS will help researchers answer questions about a fast-reactor nuclear power future for the United States.
Today fast reactors exist only in Japan, Russia, India, and China, although they have been operated and eventually closed down in the United States, Great Britain, Germany, and France. They are not as widespread as proponents predicted in the 1970s, partly because uranium has remained abundant and cost effective but also because fast reactors are difficult to operate and expensive to construct. The operational challenges include reliability and safety—many of the fast reactors that are now closed were undergoing repairs for more time than they were operating and were finally permanently closed down because of accidents.
To date, fast reactors have used either uranium or MOX fuel. If they are to be used to transmute nuclear waste, as they would in the fully closed fuel cycle, scientists need to experiment with different types of fuel with different combinations of isotopes to find the ones that will burn as needed in a fast reactor. And they must create new alloys for structural materials that can withstand the extremely high heat and intense neutron radiation inside a fast reactor.
Despite the difficulties, research into better, more advanced designs for fast reactors is continuing because of fast reactors' efficiency and potential impact on the waste problem. In fact, the United States is leading an international collaborative effort—the 13-nation Generation IV International Forum—that is pursuing six new reactor designs, three of which are fast reactors.
In support of such new designs, all the questions about fast reactors must be answered. The role of MTS will be to help solve the puzzles about fuel and alloy for structural reactor materials. Experiments run by Los Alamos researchers are already revealing the properties of new fuels and alloys. "But," says Pitcher, "the success of those materials cannot be judged until they have been irradiated and tested in an environment mimicking the extreme conditions found in a fast-neutron reactor." MTS will provide that environment and thereby fill a huge gap: the United States currently has no fast-neutron facility where new fuels and materials can be tested.
Left in temporary storage above ground, containers of high-level radioactive waste must be checked periodically for leaks and for the internal buildup of gases that might rupture the container. This problem persists because of the absence of a permanent underground repository. Los Alamos scientists have developed a new laser process that penetrates such containers (for sampling) and then reseals them with a unique laser alloying technique that prevents cracks, permits the final seal to be certified, and allows the container to be resampled repeatedly.
This lack of a domestic facility means that, without MTS, researchers would be forced to travel to fast reactors abroad, enduring all the difficult logistics and costs inherent in conducting irradiation research: government-to-government negotiations, extensive and complicated export licensing requirements, transportation requirements, and other costly challenges.
As an example, negotiations for use of the French fast reactor Phenix (now closed) to test a small sample of fuel began in 2002 but were not completed until 2007; testing was completed in 2009, at a cost of almost $8.5 million. Today the samples are still awaiting shipment back to the United States for post-irradiation analysis. (They are expected back this year.)
Radioactive waste is categorized and managed in terms of its radioactive content and thermal characteristics. Wastes categorized as "high-level"—including spent nuclear fuel and byproducts of fuel reprocessing activities—must be immobilized and transported for isolation in engineered vaults or underground repositories. The wastes pose long-term hazards to people and the environment.
Scientific approaches for solidifying and immobilizing high-level wastes include vitrification in borosilicate glass.
A Cost-Effective Solution
Filling the fast-neutron facility gap without MTS could be very expensive. Building a new experimental fast reactor would cost more than $1 billion. Modifying a linear accelerator and building a new experimental facility and beam line would cost over $160 million. In contrast, building MTS at LANSCE will cost less than $100 million because MTS can take advantage of LANSCE's 800-million-electronvolt linear proton accelerator for the production of fast neutrons. And an experimental hall and beam line already exist at the accelerator's end, ready for MTS. Having the basic structures already in place makes MTS the most cost-effective and quickest solution—clearly the preferred alternative.
Pitcher is excited about the part MTS can play. "MTS will be an important facility for researchers, and it will be important for informing decision makers who are considering options for new nuclear energy systems and fuel cycles. We'll be able to show the performance specifications you'd get with the reprocessing option, the waste streams that would come from the reprocessing step, the volume and composition of the waste stream and, therefore, the size and number of repositories you'd need to build, if you wanted a future scenario where 30 percent of U.S. electricity was produced by nuclear power. And we can offer assessments of technology that could then be deployed under such a scenario."
He concludes, "Those who are passionate about the future of nuclear power believe that it depends strongly on the deployment of fast reactors and the use of reprocessing. I really believe in the need for MTS. It will help our government make those decisions and move nuclear power forward in this country."
In this issue...
- More on this article: LANSCE: Button-to-Boom
- More on this article: Supercomputer Testing at the ICE House