Mark Bronson, Julio Diaz, Thomas A. Edmunds,
Leonard W. Gray, and David C. Riley
Excess Fissile Materials Disposition Program
Lawrence Livermore National Laboratory

Thomas L Rising
Los Alamos National Laboratory

The feed materials projected to be dispositioned by the Fissile Materials Disposition Program (FMDP) contain a broad distribution of plutonium sources, plutonium isotopes, uranium, and a broad spectrum of tramp impurities. The isotope ²⁴⁰Pu is of interest for the purpose of denaturing weapons grade plutonium; ²⁴¹Am is of particular interest as a source of gamma and neutron dose to the operators. The materials categories include pits and clean metal, impure metal, plutonium alloys, clean oxide, impure oxide, mixed uranium/plutonium oxide, alloy reactor fuel (e.g., the plates from the ZPPR at INEL), and oxide reactor fuels (e.g., the un-irradiated FFTF fuel). In addition, some materials which have been stabilized and treated in some fashion by the EM Program will also come to Immobilization for disposition. These materials will be transformed into plutonium-uranium oxides, and the impurities and the isotopics will be equalized somewhat by blending prior to being fed to one of two possible immobilization forms

The Excess Fissile Materials Disposition Program’s Record of Decision (ROD) published in January of 1997 by DOE/ MD creates a window of opportunity for increased coordination between the EM and MD programs. Some materials that fully comply with STD-3013 will be very difficult to process by either the MOX or the immobilization options. Conversely, some materials that do not satisfy STD-3013 because they are less than 50 wt% Pu or are not in a long term storage container, would make excellent feeds for the immobilization option. Additionally, there are several subsets of materials that will be difficult to process by any of the MD disposition options. These must have further stabilization or conditioning prior to being received by the disposition program. A decision must be made about where and when and by whom this conditioning should occur. The anticipated feed stocks, their isotopics and anticipated impurities, their treatment in the plutonium conversion operations (e.g., decladding to remove the reactor fuel from fuel assemblies, HYDOX for metals alloys, calcination to remove water and carbon, etc.) and their blending for downstream immobilization, all must be considered.

Between 1944 and 1994, the United States used reactors at Hanford and Savannah River to produce 103.4 tonnes of plutonium. Some of this plutonium was burned to make higher isotopes (Am, Cm, Bk, Cf, etc.) and some was used in weapons testing. Three different Programs of the Department of Energy must deal with the safe management and disposition of this plutonium now that the "Cold War" has ended. Defense Programs will maintain a strategic reserve of plutonium for long-term defense needs. That material which is declared excess to programmatic needs will either be disposed of as waste in the Waste Isolation Pilot Plant (WIPP) by Environmental Management Programs (EM) or dispositioned in accordance with the "Spent Fuel Standard" by the Materials Disposition Program (MD).

During World War II and the Cold War, the United States developed a massive industrial complex to research, produce, and test nuclear weapons. This complex included nuclear reactors, chemical processing plants for targets and fuels, weapons manufacturing (metal machining) and assembly plants, chemical recycle plants for returned weapons, and laboratories that manufactured tens of thousands of nuclear warheads.

Weapons production stopped in the late 1980s, initially to correct environmental and safety problems, but was later ended indefinitely because of the end of the Cold War. This abrupt halt to nuclear material production facilities left a legacy of plutonium, sufficient to fabricate thousands of nuclear weapons, in a variety of chemical and physical forms, packaging configurations, and geographical locations. These materials must be stabilized, safe-guarded, and dispositioned.

In 1989, the Department of Energy established the Environmental Restoration and Waste Management program, now called the Environmental Management program, to consolidate ongoing activities and accelerate efforts to deal with the inactive production facilities and sites, and the accumulated waste, contamination and the residual backlog of special nuclear materials.

The Environmental Management program encompasses

• Remediation of the environment and facilities that have been contaminated with radioactive materials and hazardous chemicals
• The stabilization and safe storage of special nuclear materials such as plutonium and highly enriched uranium
• The management and storage of spent nuclear fuel
• Management of waste produced by ongoing Department missions.

It is assumed that Environmental Management will dispose its plutonium by one of two paths:

• Preparation of materials at plutonium concentrations below proliferation concerns for disposal in the Waste Isolation Pilot Plant (WIPP)
• Transfer of the plutonium in a stabilized form to the Materials Disposition program.

As part of their agreement to make reductions in nuclear weapons, the United States and Russia have undertaken joint programs to develop and implement approaches for ensuring the secure management and eventual disposition of fissile materials rendered surplus by arms reductions. These endeavors have been endorsed with highest priority by the presidents of both countries. To demonstrate the U.S. government’s commitment, President Clinton declared in March, 1995 approximately 50 metric tons (MT), including about 38 MT of weapon-grade material, surplus to U.S. Russia has also indicated that a similar quantity of plutonium will be made surplus.

The responsibility for U. S. fissile materials disposition resides with the Office of Fissile Materials Disposition, created in 1994 to manage DOE activities relating to the management, storage, and disposition of surplus fissile materials. The goal of the plutonium disposition program is to "make the plutonium as unattractive and inaccessible for retrieval and weapons use as the residual plutonium in the spent fuel from commercial reactors." This goal, referred to as the "Spent Fuel Standard," was originally stated in a report by the U.S. National Academy of Sciences (NAS) Committee on International Security and Arms Control(Ref. 1).

MD has been conducting development activities and evaluations of promising disposition technologies leading to a choice of the best technologies for implementation. In the Record of Decision (ROD) for the Storage and Disposition of Weapons-Usable Fissile Materials issued in January 1997 (Ref. 2), DOE announced its decision to pursue two alternative technologies for Pu: (1) irradiation of Pu as mixed-oxide fuel in existing power reactors, and (2) immobilization of Pu into large solid forms containing fission products as a radiation barrier. The immobilization alternative will be used for the disposition of impure Pu materials and could, if desired, be used for the larger quantity of pure Pu materials.

• to convert metals and alloys to oxide through the hydride-oxidation technique,
• to dejacket fuel unirradiated elements, to grind materials to proper sizes,
• to calcine materials, and
• to leach soluble salts from materials but on a very limited capacity, and
• to blend materials on a 40 to 50 kg Pu basis.

There are a number of ways to sub- divide the anticipated feedstock that is assumed to be coming to immobilization. The EIS divides the feedstock into two cases, a 50 tonne case in which all Pu comes to immobilization and a 18.2 tonne case in which the Pu from clean metal and pits go to MOX feed. These two cases are given in Table I.

The categories specified in Table 1 cover large amounts of materials with significant variations in properties. These can be subdivided into subcategorize so that the individual sub-categories, which will become feed streams, will have less variation within them.

The compositional data for many of the current streams is limited because the only data required to be recorded on the streams is the amount of fissile material, for material control and accountability (MC&A) purposes. The available data come in one of four forms: engineered materials data, material specifications, sampling data, and engineering knowledge. The level of knowledge decreases as one moves down the list.

Engineered materials are well known because they have been designed to meet certain criteria. The feed streams that fit into this group are pits, the alloy reactor and oxide reactor fuels. With the exception of ²⁴¹Am, U, and Ga, the tramp impurities in Pits are very low, typically in the <100 ppm range. The alloy reactor fuel is dominated by the ZPPR fuel. ZPPR fuel is of predominately two types: Pu-Al alloy with 1% aluminum and Pu-U-Mo alloy with compositions of 28%:68%:3%; 20%:78%:2%; and 28%:68%:2%. The plutonium isotopic varies from 5 to 27% ²⁴⁰Pu. The total uranium content is 7400 kg. The oxide reactor fuel is dominated by the FFTF fuel. The mixed oxide FFTF fuel was manufactured but not irradiated in a reactor. The plutonium consists of 12 to 26% ²⁴⁰Pu with the total plutonium compositions being 13 to 27% with depleted uranium oxide (4000 kg) being the remainder. Tramp impurities are nominally less than 100 ppm for each impurity element.

The metals will have to be converted to oxide and the ceramic reactor fuel pellets will have to be ground to size. No other processing is required. This is excellent blend stock for the Immobilization Process.

The material specification data is given as a range of allowable values for the impurities. Material specification values were available for the clean metal and clean oxide categories. Clean metal generally contains less than 100 ppm of any given chemical impurity. The metal can be either weapon-grade, fuel-grade, or reactor-grade. The specification for the clean oxide is that the impurities are less than 3 wt%.

In general, the U/Pu oxides at Rocky Flats are mixtures of enriched uranium(500 kg) and plutonium oxides and those at Hanford are primarily materials that were being prepared as FFTF fuel, hence high purity PuO₂ and UO₂ (800 kg). In general the U/Pu oxides are also fairly high purity materials. Although these are not specification materials because of the uranium content, so far as immobilization is concerned, the tramp impurities are of about the same range.

Approximately one tonne of impure metal is listed in the impure metals category not because of tramp impurities but because of the ²⁴⁰Pu content. Otherwise this appears to be very high quality plutonium metal. Another tonne or so of impure metal has been produced at SRS under the present clean-out program. This metal is not being analyzed so it cannot be placed in the clean metal category. There is no reason to believe that this material is grossly contaminated with tramp impurities. In general, these two materials are as pure as the clean oxide; for the purposes of Immobilization processing, the materials can be placed into the clean category or specifications materials.

The metals will have to be converted to oxide. The oxide may require grinding and / or calcination. Blending to adjust the ²⁴⁰Pu content is required for this material. No other processing is required. This is excellent blend stock for the Immobilization Process.

For residues in general, the only data required was the fissile content for MC&A purposes. Data on tramp impurities was not required and is not recorded. However, a number of samples of various materials have been sampled and analyzed and coupling the sample results with process knowledge, gives fairly good representations of what is contained within the residues.

1. Of the 2.3 tonnes of impure plutonium oxide at Rocky Flats, approximately 1.6 tonnes is in the >75% category (pristine oxide is ~88.2% Pu). The major impurities appear to be stainless steel corrosion products along with Ca and Mg. Since no one element dominates the 13 wt% impurities, one to one blending with any plutonium oxide high in uranium will bring the impurities well within the proposed specifications. Acceptable ceramic product has been prepared using 13 wt% impurities in the plutonium feedstock.
2. The approximately 1.6 tonnes of impure plutonium oxide at Hanford averages about 65% Pu oxide and about 31% U oxide. This material is a mixture of fuels-grade and weapon-grade ²⁴⁰Pu. Since the tramp impurities average only 4 wt%, this is high grade blend stock for tramp impurities.
3. Chlorinated plutonium oxide resulted from the electrorefining operation in Building 371. Electrorefined plutonium metal and salt (KCl-NaCl) was heldup on the lip of the tilt-pout furnace. After a number of runs, this build-up was oxidized by burning out the crucible. This generated a plutonium oxide contaminated with KCl-NaCl salt. On the average, this material contains about 70 wt% Pu (~78 wt% PuO₂) and about 22 wt% of a 50:50 mixture of NaCl-KCl. Apparently about 350 kg of this material was transferred to Hanford during the MMEC Program and about 350 kg of this material remains at Rocky Flats. A simple leach with water will remove the majority of the soluble salts leaving a reasonably pure PuO₂ feedstock for Immobilization. The Immobilization facility has designed into it a small line to wash the NaCl-KCl salts from this material. This should generate about 700 kg of fairly high purity PuO₂. It is also highly probable that vacuum distillation would remove sufficient NaCl-KCl to prepare acceptable feedstock for immobilization. There are individual data for the Rocky Flat’s Chlorinated Oxide sent to Hanford in References 3 and 4.
4. Fluoride Residues. The bulk of these se residues appear to be from the fluorination cabinet and contain varying amounts of PuF₄ (commonly called pink cake) or from the metal reduction cabinet; there are 141.5 kg of Pu contained in about 316.5 kg total mass. Many of the residues are near 70 to 75% Pu; pure PuF₄ is 75.9% Pu. The presence of fluoride results in a high neutron emission rate caused by the alpha-neutron reaction between the Pu alpha particle and fluorine nucleus. Immobilization cannot accept this amount of fluoride. The higher assay materials could possibly be bomb reduced to metal and the metal shipped to Immobilization. Another route would be to hydrolyze the material with steam and then calcine the resulting plutonium oxide; although this may not remove all of the fluoride, it should remove a sufficient amount to make the material acceptable to Immobilization. A third route would be to dissolve these plutonium fluorides and purify them either by solvent extraction or anion exchange such as through the canyon needs evaluation presently being performed by EM.

Data for some of the lower-grade residues comes from sampling of the streams. Some of these data pertain to individual samples, while some of it is a composite of several samples. The composite data generally provide information on maximum, minimum, and average concentrations. There are composite data for the Rocky Flat’s Ash and Ash Heels presented in Reference 5. and in the Rocky Flats EIS. (Ref. 6)

1. Incinerator ash residues resulted from the combustion of feed materials during the operation of the residue recovery incinerator at Rocky Flats primarily in Building 771 Incinerator. This material may be unpulverized, pulverized or could have been leached several times through the cascade dissolver system (called ash heels). The unpulverized material is a mixture of course, granular, fine, and very fine particulate. It contains some miscellaneous tramp metal, bits of unburned feed materials, and carbon from the incomplete oxidation of the feed materials. The course material consists of fused ash clinkers or unburned materials. The ash contains measurable quantities of organic and carbon. The composition of the incinerator ash varies widely and was generally analyzed for plutonium only. A few analyses of incinerator have been made; the major components(according to the Rocky Flats EIS, Ref. 6) are silica (15 to 75%, but more typically 45 to 50%), and carbon (5 to 40%, but generally about 20%). The major metallic impurities are Al₂O₃, B₂O₃, CaO, Fe₂O₃, MgO, TiO₂, generally averaging from 1 to 6%. The ash heel are typically very similar in composition. The EIS states that the typical Pu analysis of incinerator ash as 1.8 to 3.8 wt%; other data bases gives individual can at much higher values. Again, according to the EIS (Ref. 6) Incinerator ash heels typically run from about 1.6 wt% to about 16.4 wt% Pu with an average of about 9 wt%. Other data bases indicate that there are a few containers that approach 30 wt% Pu. Incinerator ash (about 10 to 11 tonnes) and ash heels (about 9 tonnes) are therefore below the nominal cutoff limit of 30 wt% Pu in the present draft of the acceptance specifications. However, if the carbon were burned out, a large fraction of the containers of incinerator ash heel would be above 20 wt% Pu. The primary constituent of concern would then be SiO₂. Analysis of some of the initial samples suggest that B plus Si content of up to about 8 wt% actually produce a higher density ceramic with better leaching characteristics. Apparently, the SiO₂ is acting as a sintering aid. To reach this level of SiO₂ will require about 4 tonne of incinerator ash heels (after reburning to remove the carbon) for the 18 tonne Pu immobilization case and about 11 tonnes for the 50 tonne Pu immobilization case.
2. Graphite Scarfings and Fines. Graphite scarfings and fines residues were generated during plutonium foundry operations (Ref. 6). Graphite molds were mechanically cleaned to remove the mold coating (CaF₂) and plutonium embedded on the graphite surface. The matrix is mostly graphite containing small quantities of CaF₂, Ca and Mg metals or oxides with plutonium metal and oxides. These residues contain a mixture of granular, fine, and very fine particulates. The total bulk quantity of material is 946 kg containing 74.3 kg of Pu. The Immobilization Program cannot handle this amount of graphite. The graphite would have to be burned out of these residues prior to receipt by the Immobilization Program.

The fourth type of data is based on engineering judgment. The quality of this information is dependent upon the knowledge of the person(s) estimating the values. This type of data was used to estimate the composition of Hanford impure oxide (Ref. 7), DOR residue, ER Residue, MSE Residue (Ref. 8) and fluoride residues.

1. DOR Residues. The salt residues (Ref. 6) in this category were generated from several molten salt processes, including direct oxide reduction, molten salt extraction, salt scrub, and pyroredox. All of the salts in this category contain molten CaCl₂. Calcium chloride was used as a flux in the DOR process to promote coalascence of Pu metal during the reduction of PuO₂ by Ca metal. CaO, a byproduct of the reduction, is also present in the residue. The pyroredox process purified impure Pu metal. In the oxidation step, impure Pu metal was heated with ZnCl₂ in a CaCl₂/KCl flux. Pu and more reactive impurities were oxidized into the salt by the ZnCl₂; the resulting salt was reduced with Ca metal to yield Pu metal contaminated with Zn and Ca, which were removed with a vacuum melt operation. The residues total about 3 tonnes containing about 185 kg of Pu. Immobilization cannot accept the amount of chloride salts in these residues. The salts must be separated from the plutonium prior to transferring these residues to Immobilization.
2. ER Residues. The electrorefining salt residues (Ref. 6) are materials resulting from the electrorefining furnaces. The dominant components are primarily NaCl and KCl; they may also contain americium chloride, plutonium chloride, minor amounts of magnesium chloride and possibly small amounts of free sodium. On a limited bases, Pu-Np alloys were also electrorefined, hence these residues may also contain neptunium chloride. The form is a mixture of chunks, granular and fine particulate. The material contains relatively high concentrations of ²⁴¹Am and trace amounts of plutonium chloride and may also contain MgCl₂. There are about 7.5 tonnes of ER salt residues containing about 480 kg of Pu. Immobilization cannot accept the amount of chloride salts in these residues. The salts must be separated from the plutonium prior to transferring these residues to Immobilization.
3. MSE residues. Molten salt extraction was used to remove the daughter product ²⁴¹Am from Pu metal. Americium-241 has a low energy but intense gamma radiation that increases personnel exposure when handling this material. This category (Ref. 6) contains Ca, Zn, and K chlorides; or contains pulverized or unpulverized NaCl-KCl with either 8% MgCl₂ or 30% MgCl₂. The material may contain a mixture of chunks, granular and fine particulate. It contains relatively high concentrations of ²⁴¹AmCl₃ and Pu probable as PuCl₃. In general, these materials contain from 10 to 25 wt% Pu with a some what smaller amount of ²⁴¹Am. There are about 5.5 tonnes of MSE residues containing about 325 kg of Pu. Immobilization cannot accept the amount of chloride salts in these residues. The salts must be separated from the plutonium prior to transferring these residues to Immobilization.
4. SS&C Residues. Sand, slag, and crucible residues (Ref 6) were generated during bomb reduction and breakout of plutonium buttons. Some of these have been prepared for downstream processing by crushing and grinding. The slag is a non-homogeneous mixture of coarse chunks of CaF₂ which contains uncoalesced Pu metal, excess Ca metal, Mg metal; PuF₄, and MgO sand. The sand and crucible residue may contain uncoalesced metal, Ca and Ma metal, CaF₂ slag, and trace amounts of a pyrotechnic initiator that contains potassium iodide and sodium peroxide. The residue will range in size from chunks of the MgO crucible to grains of MgO sand. Their plutonium content is generally low, typically <10 wt%. There is about 3.4 tonnes of this material containing about 130 kg of Pu. particulates. The total bulk quantity of material is 946 kg containing 74.3 kg of Pu. The Immobilization Program cannot handle this amount of SS&C. Prior to receipt by the Immobilization Program, the SS&C would have to be processed; the canyon needs study has identified this material as needing to be processed in the canyon.
5. Scrub Alloy. The scrub alloy category includes approximately 700 kg of material containing about 200 kg of Pu. This is a mixture of Mg, Al, 241Am, and Pu generated during the salt scrub processing of MSE salts and the alloy processing of electrorefining anode heels. The Immobilization Program cannot handle this amount of SS&C. Prior to receipt by the Immobilization Program, the scrub alloy would have to be processed; the canyon needs study has identified this material as needing to be processed in the canyon.

The processing to occur in the Pu Conversion portion of the Immobilization Facility is shown in Table II. The Pu Conversion processes are HYDOX conversion of metal to oxide, halide wash, decladding and grinding. The HYDOX process involves hydriding the plutonium metal. The hydride is then converted to an oxide. The HYDOX process is used instead of furnace oxidation because during furnace oxidation a protective coating of oxide forms that slows down the oxidation process significantly. The hydride that forms during hydriding has a significant volume expansion. This breaks any protective coating that might form. Hydriding also hydrides the plutonium and uranium preferentially over other metals. Because the HYDOX process depends on the fact that plutonium reacts with hydrogen to form a hydride, there is a limit to how much impurity metals can be with the plutonium. If the impurity metals do not hydride, they could inhibit the hydriding reaction. The hydriding process has been tested on a number of materials. The largest amount of impurity tested to date was an anode heel that had about 11 wt% gallium. This can be used as a conservative estimate of how much metal impurities can be hydrided. Converting the 11 wt% to atom percent and allowing a margin for uncertainties results in a limit of 27 atom % non-hydridable metal impurities. The relative weight percent of the metal impurity varies depending upon the molecular weight of the metal. For low molecular weight metals, the weight percent is low. For high molecular weight metals, the weight percent is high. The wt% limit for some example metals are: 4 wt% Al, 6 wt% Ca, and 4 wt% Mg.

The Pu Conversion front-end of the Immobilization Facility will have a chloride wash station. This equipment is sized to handle the chlorinated oxide that was generated as a result of ER processing at Rocky Flats (Rocky Flats Chlorinated Oxides). It is currently sized too small to handle all of the salts that are present in the Complex. We assume they will be treated as part of the stabilization program.

The Pu Conversion front-end will also declad the oxide and alloy reactor fuels. It will also have size-reduction equipment to break down material sizes to that desired for the first stage immobilization process (~ 10 micron).

One can divide the Pu feedstock into three groups:

• Group I: Materials with purity far above what is required for immobilization. This group of materials contains about 45 tonnes of the projected 50 tonnes of excess Pu.
• Group II: Materials with impurities that can easily be blended into acceptable feedstock for immobilization or the immobilization facility has planned processes to make them acceptable blend stock. This group of materials is still under study but appears to contain about 2.5 to 4 tonnes of Pu. All processing of this group of materials is within the planned capability of the conceptual immobilization facilities.
• Group III: Materials with impurity levels to great to be blended into acceptable blend stock for immobilization or the impurities are in excess to the capacity of the immobilization facilities to process them into acceptable blend stock. These materials will have to be processed in other facilities prior to coming to the immobilization program. This group of materials is still under study but appears to contain about 1.5 to 2 tonnes of Pu. All processing of this group of materials is within the present capabilities of the existing facilities. Most of these materials have been previously identified under the on-going canyon needs study.

1. National Academy of Sciences, "Management and Disposition of Excess Weapons Plutonium," National Academy Press, Washington DC, 1994.
2. Department of Energy, "Record of Decision (ROD) for the Storage and Disposition of Weapons-usable Fissile Materials," U. S. Department of Energy, Washington, DC, January 1997.
3. Delegard, C.H. and D.G. Bouse, "Characterization, Particle Size, and Dissolution Tests of Three Rocky Flats Oxides," Internal Letter #65454-85-120, Rockwell International, Hanford, Washington, April 30, 1985.
4. Delegard, C.H and D.G. Bouse, "Thermal, Chemical and X-ray Analysis of Rocky Flats Oxide Feed," Internal Letter #65454-85-056, Rockwell International, Hanford, WA, April 2, 1985.
5. Long, J.L., "Analysis of Incinerator Ash and Ash Heel", Job Report No. PER-01-74, January 4, 1974.
6. Draft Environmental Impact Statement on Management of Certain Plutonium residues and Scrub alloy Stored at the Rocky Flats Environmental Technology Site, vol. 2, DOE/EIS - 0277D, U. S. Department of Energy, Washington DC, November 1997
7. Vienna, J. Personal Communications, December 1997.
8. Bronson, M. and D. Riley, "Plutonium Pyrochemical Baseline Report" L-13857, Lawrence Livermore National Laboratory, Livermore, CA, September 1991, UNCI.

Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

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