By David Jones, Senior Vice President, AREVA Back End Division North America
At the end of the Cold War, the United States and Russia began cooperating to counter the proliferation of weapons of mass destruction, especially seeking a means to reduce the “clear and present danger” from theft of global stockpiles of surplus weapons-grade plutonium. After evaluating the options, a National Academy of Sciences (NAS) report in 1994 identified two options for plutonium disposition: Permanently downblend the surplus weapons-grade plutonium with uranium into mixed oxide (MOX) nuclear reactor fuel, or immobilize and store the plutonium through vitrification and deep geologic burial.
Noting that the immobilization option was not acceptable to the Russians – they stated that the process was insufficient in rendering the plutonium unusable for a nuclear weapon, and the plutonium was a valuable energy source – the report recommended the MOX fuel option.
In 2000, Russia and the United States signed the Plutonium Management Disposition Agreement (PMDA) requiring each country to permanently dispose of 34 metric tons of surplus weapons-grade plutonium.
The U.S. Department of Energy (DOE) then undertook its own evaluation and researched multiple plutonium disposal methods, ultimately identifying five options:
• Downblending and irradiating plutonium as MOX fuel in Light Water Reactors (LWRs)
• Irradiating plutonium fuel in Fast Reactors
• Immobilization (ceramic or glass form) and storage with high-level waste
• Downblending and storage in secure facility
• Deep borehole disposal
After initially choosing a two-path strategy primarily using downblended MOX fuel irradiation with a smaller amount of immobilization storage, the DOE re-evaluated these two options again in 2002 and selected the MOX fuel path as the single best option to fulfill the agreement’s nonproliferation commitments. This path also offered the added benefits of producing income and low-carbon energy through the creation of nuclear fuel for commercial American reactors.
During this time, the National Nuclear Security Administration (NNSA) contracted with a consortium, now named CB&I AREVA MOX Services LLC, to design, build, and operate a Mixed Oxide Fuel Fabrication Facility, also called the MOX Project. The decision was made to build a single facility on the Savannah River Site (SRS) in Aiken, South Carolina, based on a combined design of AREVA’s La Hague used fuel reprocessing facility and its MELOX MOX fuel manufacturing facility, both located in France. The U.S. Nuclear Regulatory Commission (NRC) granted a license to construct the project, and has licensing and oversight authority during the facility’s operation and decommissioning.
Begun in 2007, the MOX Project facility is more than 65 percent complete as of March 2015, with some sections nearing 90 percent of their final construction. Currently, the MOX Project has more than 4,000 suppliers across 39 states.
Upon completion, the MOX Project will be a key component in the United States’ nonproliferation agreement process. Similar to the French facilities’ processes, the MOX Project will take surplus weapons-grade material consisting of 93 percent plutonium, remove the impurities, and downblend the plutonium 239 with uranium oxide 238 to achieve a 5 percent or less plutonium concentration to form MOX fuel pellets for commercial reactor fuel assemblies.
When operational, the facility will be capable of permanently converting 34 metric tons of U.S. surplus weapons-grade plutonium into safe, stable, and secure nuclear reactor fuel to reliably power American businesses, industry, and homes. To put this energy impact into perspective, the MOX Project’s output could generate enough power for 15 million homes for a year and indirectly create more than 4,000 American jobs.
Successful U.S. Test of MOX Fuel
In the United States, AREVA joined with Duke Energy, one of America’s largest nuclear power producers, to test MOX fuel assemblies in the Catawba Unit 1 nuclear power facility in York, South Carolina.
In 2003, AREVA began fabricating four MOX fuel assemblies at its facilities in France from surplus U.S. weapons-grade plutonium. The four assemblies were loaded in June 2005 into the Catawba reactor and brought into operation for long-term in-core testing through three 18-month cycles.
After the second cycle, the MOX fuel assemblies were removed and closely inspected based on established safety parameters. Upon evaluation, the fuel was verified to be within the safety requirements, and this inspection provided the necessary qualification data.
MOX fuel produced from U.S. surplus weapons-grade plutonium had been successfully, permanently converted into nuclear fuel and then safely used to generate electricity within an American nuclear reactor. This testing validated the use of MOX fuel as yet another fuel source for commercial nuclear reactors.
|The MOX Project is 65% complete at the Savannah River Site in Aiken, S.C.|
Forty Years of MOX Fuel in Europe
Originally developed in the United States, mixed oxide processing was halted in America in 1977 as part of a nonproliferation effort. The technology continued developing in Europe, however, where MOX fuel manufactured from nuclear reactors’ used fuel has become the final component of a successful closed cycle nuclear fuel strategy in Europe and Asia for more than 40 years. Globally, MOX fuel produces more than 10 percent of all nuclear-generated electricity.
In Europe, uranium and plutonium are recovered from the used nuclear fuel and reprocessed to manufacture new MOX fuel, a mixture of uranium and plutonium oxides.
Within a reactor, nuclear fuel assemblies release energy through the fission of uranium nuclei while generating power. This activity also naturally produces plutonium, as well as fissioning plutonium. In essence, every nuclear reactor eventually operates with a mix of uranium and plutonium fuel during the end of the fuel cycle.
During the time that the fuel spends in the reactor, the quantity of fission products increases and the number of fissile atoms decreases, reducing the performance of the fuel. The plutonium that is formed contributes to the energy released, but this does not fully compensate for the reduction in performance.
After three to four years, the fuel is no longer effective, but it still contains 96 percent of useful material (95 percent uranium, 1 percent plutonium). The plutonium produced over the service life of the fuel in the reactor has significant energy potential: a single gram of plutonium holds 1 MW of energy, the same that is produced by the combustion of 600 gallons of fuel oil or three tons of coal. The remaining 4 percent of used fuel material is made up of non-recyclable end waste, such as fission products and minor actinides.
First used in German nuclear reactors in 1972, MOX fuel was introduced to Swiss nuclear plants in 1984, in France in 1987, and in 1995 in Belgium. More than 42 reactors around the world have safely used MOX fuel to produce electricity.
This reactor experience has shown that MOX fuel is comparable to uranium fuel (UO2) in terms of performance, operational use, safety, and environmental impact. A few modifications may be required to load and operate a commercial nuclear reactor with MOX fuel, including additional control rods.
Nuclear safety authorities in a number of countries – Belgium, France, Germany, Japan, Switzerland and the United States – have evaluated MOX fuel’s behavior in power generation reactors, including in the event of a significant incident. They concluded that MOX fuel is as safe as “conventional” nuclear fuel made with low-enriched uranium.
MOX Fuel Manufacturing in Six Stages
The manufacture of MOX fuel is similar to that of the uranium oxide (UOX) fuels commonly used in nuclear reactors. In the U.S., the MOX Project is designed to permanently downblend the surplus weapons-grade plutonium into commercial nuclear fuel for PWRs, BWRs, and future fuel designs in five stages:
1. Plutonium preparation: Before surplus weapons-grade plutonium oxide can be used in mixed oxide fuel, the plutonium oxide must be purified using a chemical process called aqueous polishing. This step uses chemicals to remove impurities such as gallium, americium, and uranium, and ends with the purified plutonium converted back to plutonium oxide.
2. Mixing the powders: A primary mixture consisting of the plutonium oxide and depleted uranium oxide powders is blended with ground-up reject pellets. Depleted uranium is added to this primary MOX mixture to achieve the exact concentration specified by the utility customer (between 3 and 12 percent). This final mixture is known as the secondary mixture.
3. Sintering: The secondary mixture is compressed into MOX pellets, which are then heated to about 3,000°F in a high-temperature furnace, converting the pellet material into a ceramic. The MOX Project facility will produce around 70,000 pellets each day.
4. Grinding: The ceramic pellets are ground between two molds to achieve the required diameter, with only a one micron variance (3.93700787 × 10-5 inches). Any pellets failing to meet this exacting tolerance are rejected and ground up for reuse in the primary mixture stage.
5. Filling: The finished MOX pellets are then inserted into zirconium alloy tubes known as fuel rods. Each rod is about 13 feet long and contains approximately 330 pellets, depending on the customer specifications. The filled rods are then carefully cleaned and inspected.
6. Assembly: The final stage is to bundle the rods together in metal structures to form MOX fuel assemblies. A typical pressurized water reactor fuel assembly will contain 264 fuel rods in a 17 x 17 array weighing about 1,500 pounds. Following a final inspection, the MOX fuel assemblies are delivered to the customer.
More than 125 quality control procedures are carried out during the fabrication process.
Challenges and Choices for the Future
With the signature of the PDMA with Russia, the United States found itself, for the first time in decades, building a nuclear energy facility. Creating a first-of-a-kind combined MOX fuel facility from the ground up required rebuilding the country’s lapsed expertise in nuclear energy construction, training a nuclear-grade workforce, developing an extensive certified domestic supply chain, and managing complex project licensing.
Though the MOX Project progressed through each of these challenges, the impact to budget and schedule opened the project to new questions about selecting and funding the MOX fuel disposal option.
In 2013, the DOE began considering a new review of plutonium disposal options. This action also indicated the potential for an attempt to renegotiate the PDMA with Russia. A new agreement would be required because the MOX fuel disposal option is the only option fulfilling all of the existing agreement’s criteria for irretrievability of weapons-grade plutonium through isotopic degradation, thereby ensuring nonproliferation.
With the release of DOE’s Surplus Plutonium Disposition Options analysis in 2014, the MOX fuel disposal option again ranked as the best option for fulfilling America’s nonproliferation commitments with Russia in the most timely and efficient manner, but deliberations continue.
Construction also continues on the MOX Project facility, as does interest by U.S. utilities for using the MOX fuel. Unfortunately, there’s been an extended delay in signing the Master Fuel Contract. Without the DOE’s overarching agreement in place, the MOX facility must wait to negotiate with utilities on MOX fuel sale prices, terms and conditions.
Despite these challenges, the future of MOX fuel in the U.S. represents a singular means to honor America’s plutonium disposal agreement, deliver significant amounts of low-carbon electricity, and continue driving economic development across the country.
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