The Tennessee Valley Authority (TVA) is considering a plan to burn mixed oxide fuel in two of its power reactors starting in 2018. While the utility hasn’t formally made up its mind, the decision process reached a new stage with the release this week by the Department of Energy (DOE) of a “Draft Surplus Plutonium Disposition Supplemental Environmental Impact Statement.”
In the document DOE says its “preferred alternative" is to convert surplus plutonium from the nation’s nuclear weapons program into Mixed Oxide fuel (MOX) at a facility being built in South Carolina.
The MOX fuel is proposed to be used in commercial nuclear reactors owned and operated by TVA. These units are the Sequoyah and Browns Ferry nuclear plants. A total of approximately 34 tonnes of material would be converted under this program.
MOX fuel is used in about three dozen nuclear reactors worldwide, but so far all the MOX that has been manufactured has come from recycling materials from spent nuclear fuel that was already used once in a commercial reactor. What’s new is that weapons grade plutonium will be used to make MOX fuel. It will be the equivalent of standard uranium fuel enriched to 3-5% U235. It cannot be used to make bombs and will not blow up.
TVA has said it has not yet made a formal decision to burn MOX fuel at the two reactors, but it did tell the TimesFreePress July 25 it has three criteria which must be met.
- It is operationally and environmentally safe
- It is economically beneficial to TVA’s customers
- The licenses for the two reactors can be modified and accepted by the NRC
If TVA decides to use MOX, it could eventually replace up to 40 percent of the fuel assemblies in the cores of its Sequoyah and Browns Ferry reactors. The two Sequoyah reactors are pressurized water reactors with 193 fuel assemblies each. The three Browns Ferry reactors are boiling water reactors with 764 fuel assemblies each.
The DOE’s MOX plant is expected to produce the equivalent of 1,700 PWR assemblies to dispose of 34 tonnes of surplus plutonium. At a projected output rate of up to 70 metric tons heavy metal per year, the MOX facility may produce more fuel than TVA’s five reactors could consume.
Two other nuclear utilities – Duke and Energy Northwest, are also considering using MOX fuel. One of the key issues all three utilities have is reliable fuel services. This means that if the utilities decide to use the MOX fuel, it must be ready when the reactors have their scheduled fuel outages. Energy Northwest has reliable fuel services lined up for a number of years. For this reason earlier this year it cancelled a study to be carried out by PNNL to look at the issue.
TVA won’t start out at the 40-percent core replacement level. The initial replacement level for the reactors will be about 8 assemblies of MOX fuel. Ramp up time to the 40-percent level depends on the DOE’s production schedule, how well the MOX works, and cost factors, among others. TVA does not expect to load MOX fuel before 2018.
Explaining MOX to the public
One of the challenges that TVA faces is that the public perceptions of using plutonium as fuel needs some explaining. TVA starts by describing that MOX is a mix of uranium and plutonium. MOX has about 4-percent plutonium oxide (of which 94 percent is Pu-239) and the rest is depleted uranium oxide.
Commercial nuclear fuel starts as uranium oxide. What many people do not know, is that plutonium is a normal byproduct in nuclear reactors that fission uranium.
Plutonium builds up in the fuel inside the reactors and eventually provides up to 40 percent of the core’s heat energy. Fission of plutonium produces this energy in the reactor at the end of the life of the fuel.
TVA managers point out they are not introducing a new element to a core. The plutonium is already there. Also, the thermal output of the reactor will not change as a result of using MOX fuel.
While Pu-239 is more energetic than U-235, the NRC license governs the use of MOX. Heat inside a core can be managed by blending different fuels just like mixing different types of wood in a fireplace.
Oak Ridge National Laboratory data presented by TVA to the Nuclear Waste Technology Review Board show little difference in decay heat loads between used MOX fuel and normal non-MOX fuel.
Thus the difference in heat load between used MOX and used uranium oxide fuel can be accommodated in spent fuel pool cooling or space requirements and in dry cask thermal design.
Overall, with TVA support as a cooperating agency, the DOE is on track to complete a supplemental environmental impact statement for MOX fuel use that will assess safety for workers, the public, and the environment.
The Nuclear Regulatory Commission licenses for all the reactors that are candidates to use MOX will have to be updated to address physical operating differences and any changes in safety requirements. Technically, at this point, TVA believes that the physical modifications needed for each reactor are manageable. Also, TVA expects the DOE’s MOX to cost less than uranium fuel.
A decision to proceed with engineering and licensing is currently expected to be made in 2013.
TVA is a public power provider for a seven-state region serving nine million people. In 2010, 36 percent of its power generation came from nuclear energy.
One element of its charter, which dates back to the New Deal programs between 1933 and 1936 of President Franklin Roosevelt, is to support national security missions. TVA built power plants to provide electricity for the Manhattan Project at Oak Ridge. It’s role as a potential customer for MOX fuel comes from this legacy.
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