During the period 1987 to 1991, the Governments of the Russian Federation and the United States agreed that they no longer needed the existing number of nuclear weapons they each had on hand to meet their respective national security requirements. Based on this assessment, the respective governments signed various START agreements to mutually reduce their inventories of nuclear weapons. As a result of these agreements, each country had more of the key bomb-making material – weapons grade Plutonium (wgPu) – than was needed. In September 2000, the Russian Federation and the United States formalized their mutual interest in the reduction of their wgPu inventories. Specifically, each country agreed to dispose of 34 metric tons of wgPu so that it would be impractical for this material to be available for weapons, with a resultant reduction in the terrorist threat from this source.

A study by the National Academy of Science was conducted to evaluate the various ways in which this disposal could be accomplished. Based on the information in this study, as well as other sources, the United States and the Russian Federation agreed to use civilian nuclear power reactors in their respective countries to irradiate or “burn” the wgPu in nuclear fuel assemblies. When used in a reactor core, the wgPu fissions and transforms into a mixture of isotopes no longer suitable for making weapons. These isotopes and residual (non- fissioned) plutonium meet the “spent fuel standard”, i.e., the material is essentially equivalent to spent fuel which is removed routinely from existing power reactors. Material in this condition is extremely difficult to use as weapons and is considered to be sufficiently resistant to proliferation.

The approach described above is accomplished by mixing a small percentage of wgPu with depleted uranium, creating a mixture called mixed oxide (MOX). In a manner similar to that used for typical nuclear fuel, MOX is loaded into fuel rods that are fabricated into standard sized/configured fuel assemblies for use in nuclear reactors. These MOX fuel assemblies are supplied to a commercial nuclear power plant and used in an operating reactor alongside typical fuel. After a suitable period of operation, the spent MOX fuel assemblies will be removed from the reactor and stored like any other spent fuel. It should be noted that all commercial light water nuclear power reactors generate some plutonium in the fuel due to neutron irradiation of the U²³⁸ which is present in the fuel. This plutonium is known as “reactor grade” plutonium, which is impractical for producing weapons. Similarly, the residual plutonium in spent MOX fuel assemblies is reactor grade plutonium.

The Russian program for use of MOX fuel has the potential to utilize their pressurized light water reactors, fast reactors, or gas cooled reactor concepts to “burn” wgPu. The gas cooled reactor is still a developmental concept with an active Russian program aimed at generating a large scale prototype. MPR, as a contractor to the U.S. Department of Energy, is helping to facilitate the Russian light water reactor and fast reactor applications.

Even though both Pu²³⁹ and U²³⁵ will fission, they have different nuclear characteristics. Most light water reactors were originally designed to use only enriched uranium fuel. Therefore, they need to be modified to operate with MOX fuel. In addition, safety analyses must be recalculated. These analyses need to be revised for each different core configuration as the plant transitions to increased use of MOX fuel. MPR participated in the decision process regarding which modifications were necessary to accommodate MOX fuel in Russian reactors.

Cross Section View of BN-600 LMFBR Reactor

MPR also supports the Department of Energy as it works with the Russian Federation in its activities relative to the BN-600 fast reactor at Beloyarsk. This nuclear power plant, in contrast to light water reactors, is designed to also produce plutonium for use as a fuel in other nuclear reactors. The BN-600 core, as presently configured, has several rows of assemblies around the core in the form of a radial breeding blanket. The blanket is normally composed of depleted uranium with a high concentration of U²³⁸. Neutron flux irradiation of this blanket results in the production or breeding of plutonium. As part of the agreement between the United States and the Russian Federation to each dispose of 34 metric tons of wgPu, the RF agreed to replace the radial breeding blanket with non-breeding assemblies composed of stainless steel and boron to provide neutron shielding, reflection, and absorption. This approach reduces the production rate of plutonium in the reactor so that it will become a wgPu burner rather than a breeder after MOX fuel assemblies are inserted.

MPR is working with Russian counterparts to minimize the duration of each manufacturing activity leading to completion of the radial blanket replacement and introduction of MOX fuel into the BN-600 core. In addition to providing technical advice, MPR is also supporting transfer and implementation of the latest techniques and tools in project management to minimize the critical path for radial blanket replacement and initiation of MOX fuel irradiation. To this end, MPR, through a Russian subcontractor, is in the process of installing computer work stations, software, training, and networking while complying with Russian security requirements. Until the Russian team is operational, MPR provides current schedule maintenance and analyses.

Successfully disposing of 68 metric tons of surplus weapons plutonium will require the cooperation and good will of numerous contributors from both the United States and the Russian Federation. Once the program is accomplished, we will have taken a measurable step toward a reduction in the potential for proliferation of weapons grade Plutonium into the hands of terrorists.

For further information on this article, a copy of the latest MPR Profile or our engineering services, contact Larry Cundy.