The U.S. Department of Energy contracted Shaw AREVA MOX Services LLC to design, construct, and operate a Mixed Oxide (MOX) Fuel Fabrication Facility (MFFF) at its Savannah River Site in South Carolina. The MFFF will convert depleted uranium and excess weapons-grade plutonium stockpiles, equivalent to approximately 17,000 nuclear weapons, into MOX fuel assemblies that will be used in U.S. nuclear power plants by 2018.
The bilateral Strategic Arms Reduction Treaty (START) signed in July 1991 was an agreement to dismantle 80% of U.S. and then-USSR strategic nuclear weapons in existence. START may have signaled the end of the Cold War, but it also ushered in a new problem: potential uncontrolled access to large stockpiles of surplus weapons-grade (WG), highly enriched uranium (HEU) and plutonium. The challenge was how to safely dispose of these surplus nuclear materials to prevent their future use in nuclear weapons.
In 1992, General Brent Scowcroft, then the national security advisor to President George H.W. Bush, requested that the National Academy of Sciences (NAS) recommend disposition options to reduce the potential loss by theft of these materials, particularly the plutonium. (See the sidebar for important differences between each of the nuclear materials. The early work associated with the “megatons-to-megawatts” program focusing on uranium is described in “DOE Project Converts Weapons-Grade Uranium to Fuel for Browns Ferry” in the December 2006 issue of POWER, available in our archives at https://www.powermag.com.)
The outcome of the NAS study was that excess WG plutonium should be as difficult to acquire for a nuclear weapon as the reactor grade (RG) plutonium in spent fuel from civilian nuclear reactors. The “MOX option,” selected by the NAS as the best disposition alternative, blends WG plutonium from dismantled nuclear weapons with depleted uranium (a byproduct of uranium enrichment) to create mixed-oxide (MOX) fuel for irradiation in a commercial nuclear reactor. The plutonium becomes part of the spent fuel, thus no longer making it usable for a nuclear weapon.
The current inventories of surplus WG plutonium to be processed are based on the Plutonium Management and Disposition Agreement (PMDA) originally signed in 2000 by the U.S. and Russia, and reaffirmed in 2007 and 2010. The PMDA commits each country to dispose of no less than 34 metric tons (~75,000 pounds) of excess WG plutonium and irradiate it as MOX fuel in commercial nuclear reactors. The combined amount, 68 metric tons, represents enough material for approximately 17,000 nuclear weapons. To implement this agreement in the U.S., the Department of Energy’s (DOE) National Nuclear Security Administration (NNSA), under the Office of Fissile Materials Disposition, contracted the construction of a Mixed Oxide Fuel Fabrication Facility (MFFF).
POWER recently discussed the project and timetable with Kelly Trice, president and chief operating officer of Shaw AREVA MOX Services LLC, which is responsible for the design, licensing, and construction of the MFFF. He described the 17-acre MFFF as a “plutonium processing and fuel fabrication plant” designed to convert surplus WG plutonium inventories and depleted uranium into MOX fuel assemblies. It is the first facility of its kind in the U.S.
Trice pointed out that the completed MOX fuel assemblies will look like standard pressurized water reactor (PWR) and boiling water reactor (BWR) fuel assemblies. He indicated that of the 34 metric tons of plutonium coming from the U.S., 10 metric tons of plutonium oxide are already available for processing and the remaining 24 metric tons from the weapons programs will arrive later in the program.
Following irradiation in a reactor, the resulting spent fuel contains WG plutonium in a nonproliferent form. No reprocessing or subsequent reuse of the MOX spent fuel is planned. Once the fuel cycle use is completed, the MOX spent fuel will be permanently stored in a geologic repository. The MFFF will be shuttered when the plutonium disposition goals are met.
DOE and NRC Licenses Required
The contract to build the MFFF at the DOE’s Savannah River Site (SRS) near Aiken, S.C., was awarded in March 1999 to Shaw AREVA MOX Services LLC. The DOE looked at many sites, but the SRS’s existing security infrastructure and experience with handling plutonium gave it an edge in the selection process.
Licensing of the MFFF is following a two-step process. The first step required submitting a Construction Authorization Request to build at the SRS in February 2001. The Nuclear Regulatory Commission (NRC) issued the construction authorization on Mar. 30, 2005. The second stage requires NRC staff review of the license application submitted on Sept. 27, 2006. The license would authorize the possession and use of byproduct and special nuclear material. The NRC review verifies that the structures, systems, and components are constructed, installed, and can be operated properly.
MFFF construction officially started on Aug. 1, 2007 (Figure 1). Overall, the concrete structure at the main plant is about 88% complete and 12 of 18 buildings are finished. The Waste Solidification Building, expected to be completed in 2013, is forecasted to treat 150,000 gallons of waste and solidify approximately 600,000 gallons of low-level radioactive waste streams from the MFFF for ultimate disposal.
|1. Complex construction project. The Mixed Oxide Fuel Fabrication Facility at the DOE’s Savannah River Site near Aiken, S.C., recently entered its sixth year of construction. Courtesy: Shaw AREVA MOX Services LLC|
In addition to the 2,400 personnel on site, an additional 800 people are employed in 42 states by suppliers to the construction project. Trice noted that both large and small businesses are benefiting from the construction activities. “We actually do about 58% of our subcontracts with small businesses. To date, we have subcontracted about $900 million in small business awards.”
Trice proudly acknowledged the team’s achievement of a new safety milestone over the summer. “We have also just crossed 11.5 million safe work hours without a lost work day accident, which is a significant accomplishment for a project of this size, complexity, and importance.”
Cold-startup testing is scheduled to begin in 2016, followed by fabrication of the first fuel assemblies slated to begin in 2018. The MFFF will be licensed for 20 years and is expected to operate into the 2030s.
Building Fuel Assemblies
The MFFF is composed of two main process operations: the aqueous polishing process to remove impurities, such as americium and gallium, and the MOX process, which converts the plutonium and depleted uranium into fuel pellets, fuel rods, and fuel assemblies (Figure 2).
|2. Pair of processes. The MFFF will house two main process operations: aqueous polishing and the MOX process. Source: Shaw AREVA MOX Services LLC|
The process will begin with incoming plutonium and depleted uranium received in their respective shipping containers and inventoried according to the MFFF material control and accounting and radiation protection programs. The material would then be moved to the aqueous polishing (AP) area.
Aqueous Polishing Process. Before combining the surplus WG plutonium with depleted uranium to produce ceramic pellets for MOX fuel rods, the plutonium oxide will be purified using an AP process, equivalent to the process the French nuclear industry has successfully used for over 30 years. The AP process removes impurities such as gallium, americium, aluminum, and fluorides and consists of three major steps: dissolution, purification, and conversion.
Plutonium oxide is dissolved in nitric acid in the first step. Next, a solvent extraction process removes impurities and purifies the material. Then plutonium is separated from the uranium. The solid and liquid materials removed are recycled to reduce waste volume. The final step converts the purified plutonium stream back to an oxide powder by precipitation and calcination. The oxide powder is then homogenized, sampled, and stored in cans for future production of MOX fuel pellets.
MOX Process. The MOX process is a mechanical process and consists of four major steps: powder master blend, pellet production, fuel rod production, and fuel assembly production.
In producing the powder master blend, polished plutonium oxide is mixed with depleted uranium oxide and recycled powder/pellet material. This mixture is micronized in a ball mill and mixed with additional depleted uranium oxide and recycled material to produce a final blend with the required plutonium content. A lubricant and pore-former are added to control density.
Next, the final powder blend is pressed to form “green” pellets, which are then sintered in a furnace to obtain the required ceramic qualities. The sintering step removes organic products dispersed in the pellets and the previously introduced pore-former. The sintered pellets are ground to a specified diameter and then inspected to verify dimensions, density, markings, and appearance. The MFFF will produce upwards of 70,000 pellets each day.
The fuel rods are then assembled in gloveboxes by arranging the pellets in a long tray and inserting the pellets into a zirconium alloy tube (the fuel rod), loaded to an adjusted pellet column length, pressurized with helium, welded, and then decontaminated. Each fuel rod contains approximately 360 pellets. The decontaminated rods are then removed from the gloveboxes and placed on racks for inspection.
In the final step, fuel assemblies are manufactured by inserting the individual rods into a metallic structure referred to as the fuel assembly skeleton. A typical MOX PWR fuel assembly contains 264 fuel rods in a 17 x 17 array, is 13 feet in length, and weighs about 1,500 pounds. Each MOX fuel assembly is subjected to a final inspection prior to storage and shipment.
The MOX fuel fabrication facility will have the flexibility to produce fuel for PWRs and BWRs as well as for the new-generation reactors.
Fresh MOX fuel assemblies will be stored in the assembly storage vault. The assemblies will be transferred to the shipping and receiving area and loaded into an NRC-approved MOX transportation package and then loaded onto a secure transport vehicle for shipment to a commercial nuclear reactor.
Using Recycled Fuel
There are two types of fuel for nuclear plants: uranium oxide (the most common) and MOX, a mixture of uranium and plutonium. Plutonium has more available energy than uranium—analogous to adding a gallon or two of premium gasoline to a car’s tank of regular fuel (see sidebar).
In the U.S., there was substantial development work on MOX fuel technology in the 1960s and 1970s. That work culminated in a series of MOX fuel demonstration programs at five reactors: the San Onofre and Ginna PWRs and the Dresden, Quad Cities, and Big Rock Point BWRs. In each program, lead test assemblies were used to study the performance of MOX fuel rods. After several operating cycles, the MOX fuel had performed acceptably and similar to the co-resident uranium fuel.
The U.S. nuclear industry was poised to begin large-scale reprocessing of spent nuclear fuel and associated re-use of the separated RG plutonium. However, fearing worldwide nonproliferation consequences of separating large quantities of plutonium, the U.S. government decided against reprocessing spent nuclear fuel and stopped the development and deployment of U.S. MOX fuel technology. A more detailed history of U.S. attempts at reprocessing and recycling used fuel can be found in the August 2008 article, “How to Solve the Used Nuclear Fuel Storage Problem,” available at https://www.powermag.com.
Other countries continued their large-scale development and reprocessing of spent fuel. In the early 1980s, nuclear reactors in Germany began using substantial quantities of reprocessed plutonium in the form of MOX fuel. Other European reactors followed in France, Belgium, and Switzerland. International safeguards implemented in the MOX fabrication process have ensured that no proliferation has occurred from this process for nearly 40 years.
The MELOX facility in France has been operating since the mid-1990s and produces nearly all the commercial MOX fuel assemblies in the world. In fact, MELOX increased production in April 2007 to keep in step with market needs for MOX fuel. The mixture of uranium oxides and plutonium is obtained from recycling used fuel at AREVA NC’s La Hague plant, which currently handles nearly half of the world’s light water reactor spent nuclear fuel reprocessing capacity.
In the U.S., from June 2005 through May 2008, four MOX fuel test assemblies containing WG plutonium were tested at the Catawba nuclear plant located in Rock Hill, S.C. Nondestructive and destructive hot cell examinations of five fuel rods verified that the MOX fuel behaved as predicted on the basis of experience with uranium dioxide fuel and MOX fuel with recycled RG plutonium.
Trice noted that the MFFF is a general reproduction of France’s La Hague and MELOX plants. In addition, the Catawba experience has been translated into MFFF design features. “The processing technology is generally similar, but the scale of the MOX plant is much different. We operate about half of the production capacity of the plants in France. Also, we are using weapons grade plutonium as opposed to reactor grade plutonium. We use pieces of machinery that are similar to La Hague, but not exactly the same because we are not reprocessing spent nuclear fuel. At La Hague, they take reactor fuel, chop it up, dissolve it, extract the plutonium, then reprocess it into plutonium oxide, and then ship the oxide to the MELOX plant.”
Trice emphasized that although the two French facilities were designed and constructed according to French building codes, the MFFF design meets all U.S. regulatory requirements. It will also be a hardened facility, similar to a nuclear reactor. Security will be equal to the security measures currently in place at SRS.
Finding Customers for MOX Fuel
The Master Services Agreement, a result of the successful Catawba fuel-testing program, is a “very promising contract that would open up a third of the U.S. reactor fleet as potential users of MOX fuel,” according to Trice. “Several utilities have expressed interest, and that will create a substantial demand for the MOX fuel.” In the future, reactor licensees approved to use MOX fuel are still expected to run test assemblies for at least two operating cycles to gain operational experience and confirm computer models to predict fuel performance.
The NRC expects no significant interim storage differences between used MOX fuel and used uranium fuel. After the MOX fuel has been in a reactor for two operating cycles, it can be stored in fuel pools or dry-storage casks located at each reactor site. In the U.S., the used fuel will remain in interim storage until a permanent geological repository is available. If a repository is licensed, the used MOX fuel assemblies would be packaged into special containers and shipped directly to the repository by truck or rail using NRC-approved shipping packages.
Trice concluded that operation of the MFFF and disposing of surplus U.S. weapons-grade plutonium will demonstrate that the U.S. is living up to its nonproliferation commitments in a transparent and irreversible manner. In addition to these critical nonproliferation benefits, the U.S. MOX strategy will support additional DOE missions by consolidating materials, thereby reducing security and storage costs of surplus plutonium, estimated to be hundreds of millions of dollars annually, while generating clean energy.
— James M. Hylko ([email protected]) is a POWER contributing editor.