Summary Paramagnetic of Uranium and Plutonium Application to Decontamination Projects By J. W. Voss, EurIng, FINucE, CEng Terra Verde Group of Companies March 2000 Copyright Terra Verde Environmental, Inc. 2000 All Rights Reserved Plutonium and uranium, in both elemental and oxide forms, are known to be paramagnetic. Within the past few years, commercial paramagnetic separation systems have been constructed that can generate sufficiently strong magnetic fields to isolate uranium and plutonium. In 1993 on the Nevada Test Site, the paramagnetic separation of uranium and plutonium was demonstrated to be applicable to the remediation of contaminated soils. This paper describes the technical basis for the application of the technology, the limitations, and the potential applications. Paramagnetism All elements and compounds exhibit one of three magnetic properties: Ferromagnetic Paramagnetic Diamagnetic (or nonmagnetic) The property is defined by the electron shell configuration. Consequently, one element may be para- or ferromagnetic in pure form, but diamagnetic in a particular compound. For example, some iron compounds are not magnetic at all. Figure 1 shows the periodic table, with an indication of the magnetic properties for each element when in its pure form.
Paramagnetic Page 2 Figure 1 Properties of All Elements in Pure Form H 1 Li 3 Na 11 K 19 Rb 37 Cs 55 Fr 87 Be 4 Mg 12 Ca 20 Sr 38 Ba 56 Ra 88 Sc 21 Y 39 La 57 Ac 89 Ti 22 Zr 40 Hf 72 V 23 Nb 41 Ta 73 Ce 58 Cr 24 Mo 42 W 74 Pr 59 Mn 25 Tc 43 Re 75 Nd 60 Fe 26 Ru 44 Os 76 Pm 61 Co 27 Rh 45 Ir 77 Sm 62 Ni 28 Pd 46 Pt 78 Eu 63 Cu 29 Ag 47 Au 79 Gd 64 Zn 30 Cd 48 Hg 80 Tb 65 B 5 Al 13 Ga 31 In 49 Tl 81 Dy 66 C 6 Si 14 Ge 32 Sn 50 Pb 82 Ho 67 N 7 P 15 As 33 Sb 51 Bi 83 Er 68 O 8 S 16 Se 34 Te 52 Po 84 Tm 69 F 9 Cl 17 Br 35 I 53 At 85 Yb 70 He 2 Ne 10 Ar 18 Kr 36 Xe 54 Rn 86 Lu 71 Th 90 Pa 91 U 92 Np 93 Pu 94 Am 95 Cm 96 Bk 97 Cf 98 Es 99 Fm 100 Md 101 No 102 Fe 26 Ferro U 92 Para Cm 96 Dia or non The difference between ferromagnetism and paramagnetism is shown in Figure 2.
Paramagnetic Page 3 Figure 2 Responses Response Ferromagnetism Paramagnetism Field The figure is best interpreted by considering iron dust and a magnet. When the magnetic field gets near the iron dust, the dust leaps to the magnet. In essence, increasing the strength of the magnet does little to increase the leap of the iron dust. This leap (a combination of the height that a particle would jump and the speed at which it would move) is the magnetic response. This behaviour describes ferromagnetism. Ferromagnetic species (iron, nickel, cobalt) all respond to magnetic fields with what can be considered for this discussion to be a step function. Increasing the magnetic field causes a slight increase in the response, but this increase is not material in magnitude in comparison to the initial step function. Paramagnetic species respond linearly to magnetic fields. Figure 2, while not quantitative, should be considered to be a log-log plot. Until a few years ago, it was not even practical to construct industrial magnets of sufficient field strength for any paramagnetic application. However, technological advances in this field have enabled the manufacture and application of industrial scale paramagnetic separation systems. The most advanced paramagnetic separators are manufactured by CARPCO in the UK and US. Figure 3 shows one of CARPCO s systems.
Paramagnetic Page 4 Figure 3 CARPCO Cryofilter High Gradient Separator (5 8 tonne per hour throughput) The (orange) cylinder in the centre of the photo is the magnet. The centre of the magnet is hollow. The long (silver) cylinder that is inserted into the magnet contains a cartridge that is used to remove paramagnetic material. Figure 4 diagrams a cross section of this.
Paramagnetic Page 5 Figure 4 Cross Section of Paramagnetic Separator Magnet Cartridge Magnet With the magnet on, a slurry is introduced into the cartridge. The slurry may be on the order of 5% to 7% solids, by weight. Prior to introduction into the magnet, the slurry has been conditioned, mainly by size reduction, to ensure that larger particles do not become clogged in the cartridge. The cartridge can take many forms. At its simplest, it may be packed with fine steel wool. On the other extreme, it may be packed with layers of steel mesh, with each layer rotated slightly from the one next to it. The cartridge packing is ferrous. Its intent is to break up the magnetic field lines. In the absence of this, all of the magnetic field lines run parallel to the axis of the hold in the magnet. Any material that will respond to a magnetic field will simply ride the field lines and pass through the magnet, unless the field lines are broken. The cartridge material accomplishes this. particles are drawn to the end of the field lines in this case, to sites on the media in the cartridge. When it is judged that the cartridge is nearly full, it is removed from the magnetic field, and backflushed. This is shown in Figure 5.
Paramagnetic Page 6 Figure 5 Backflushing Cartridge Magnet Magnet with Contaminant In trial demonstrations conducted on the Nevada Test Site to remove environmental plutonium oxide from soils, the backflushing is sufficiently effective so that the cartridge can be released without restrictions. Once the cartridge is outside of the magnetic field, the plutonium oxide will not adhere to the cartridge media and can be flushed with water. Paramagnetic separator systems operate at full industrial scale. Throughputs run as high as 50 tonnes per hour of dry material (which is diluted into a slurry, as noted above). The two dominant applications are associated with kaolin and bauxite. In both cases, large deposits of the resource contain small fractions of contaminants that are para- or ferromagnetic. By applying the magnetic separation technology, the resource value is enhanced from a subgrade to the highest commercial grade. Application to Waste Stream Decontamination Plutonium Only This discussion will describe how a magnetic separation system can be applied to remediating a site contaminated with plutonium. The flow diagram is also applicable to liquid waste streams containing suspended plutonium. The first point is to ask why this technology should be considered at all. It seems logical that gravimetric separation approaches, such as soil washing and/or sonic separation, should be able to decontaminate soils and sediments, given the density difference between plutonium and soil particles. The problem is in the particle size of the contaminant. Most environmental plutonium and uranium is less than 10 microns in size. The reason for this is that during the manufacture of
Paramagnetic Page 7 nuclear reactor fuel, the feed material is reduced to this size. Gravimetric separation systems have diminished effectiveness when the contaminant sizes drop below 100 microns. Hence, if a large volume of soil is contaminated with uranium and plutonium, and the U and Pu have particle sizes of less than 10 microns, some other separation method is needed. It is generally accepted that in such cases, the only other approach is dissolution and subsequent precipitation or adsorption. In some situations, economics would dictate that this approach is valid. For example, if the plutonium in contaminated soils could easily be taken into solution, then the economics would likely favour this approach, coupled with precipitation or adsorption. However, if the volume is large, then economics would discount the feasibility of chemical separation, and would require that alternate approaches be developed. As shown in Figure 1, elemental uranium and plutonium are paramagnetic. Plutonium oxides and uranium oxides are also paramagnetic. In the simplest example, sediments from a pond are contaminated with plutonium. The plutonium, entirely Pu 239, is at a concentration of 50 Bq/g. Free release of the sediments is 0.4 Bq/g. The concentration of plutonium, in mass terms, is 22 ppb, and must be reduced to approximately 0.44 ppb. A decontamination factor of at least 125 must be achieved. Treatability characterisation shows that the plutonium is in an oxide form and that the average particle is less than 10 microns in size. It is also shown that the sediments do not contain any other materials that are para- or ferromagnetic. The simplified flow sheet for this problem is shown in Figure 6.
Paramagnetic Page 8 Figure 6 Simplified Flow Sheet Plutonium from Dredge Size < 100 > 100 Pretreatment Fines & Gravimetric & Washing Additives Contaminant Waste The flow balance for this process is shown below in Table 1. Table 1 Flow Balance Simple of Plutonium from Dry Mass Pu Mass Pu Activity Pu Concentration kg (microgram) (Bq) (Bq/g) Dredged 1.00 21.74 50000 50.0 > 100 micron fraction 0.40 2.17 5000 12.5 Fines 0.05 2.14 4930 98.6 0.35 0.03 70 0.2 < 100 micron fraction 0.60 19.57 45000 75.0 <100 micron + fines 0.65 21.71 49930 76.8 Waste 0.10 21.59 49654 509.3 0.55 0.12 276 0.50 Total 0.90 0.15 346 0.38
Paramagnetic Page 9 The flow balance shown in Table 1 is, of course, hypothetical. There are a number of factors that can alter these figures. These include the presence of paramagnetic minerals in significant mass fractions in the sediments, chemical complexing of plutonium with sediment materials, and aggregation of plutonium in clay. Each of these factors leads to alterations of the flow sheet, and possible inclusion of other processing steps. The presence of uranium and americium as contaminants, with no other complications, should not alter significantly, the flow sheet and mass balance shown above. The key to making this (or any other) treatment feasible is the treatability test phase of work. In nearly every situation of environmental plutonium, the environmental managers have characterised the site to determine the nature and extent of contamination. This characterisation, while providing valuable information, does not provide the data that are necessary for design of a proper treatment system. It is an essential feasibility step. Application to Radioactive Site Decontamination Plutonium and Fission Products This situation can be quite complex and challenging to the site managers. One goal in remediating such a site is to minimise the quantity of total waste and of alpha-bearing waste. This creates a challenge the plutonium is mixed with the fission products. Generally speaking, the soils and sediments in such situations could simply be exhumed and packed for disposal. This would result in massive quantities of waste. The radioactive constituents can be separated and concentrated, but this results in all waste being alpha-bearing. The real challenge is to separate the plutonium from the other radioactive constituents and to minimise the volume of both. Figure 7 is a simplified flow diagram for this problem.
Paramagnetic Page 10 Figure 7 Simplified Flow Diagram: Decontamination of Soils with Plutonium and Fission Products Contaminated Soil Containing Pu & U plus Fission Products Physical of > 100 micron Fraction > 100 u Fraction: Wet Decon with and/or EDTA "Dirty Stream": Adsorb <100 u Fraction: Dilute Stream to ~ 5% Solids Soils Treat for Storage and/or Disposal Non-Pu Fraction: Wet Decon with EDTA "Dirty Stream": Adsorb Pu Fraction: Treat for Storage and/or Disposal In this system, the first separation is a physical split of soil particles that are smaller or larger than 100 microns. This split may involve a treatment such as grinding (eg, ball or rod mill). The physical separation may involve a sonic system to enhance the movement of plutonium into the <100 micron stream. The mass separation is a critical step, because if too much plutonium moves with the >100 micron stream, then the resulting waste may be considered to be alpha bearing, depending on the final treatment process. The >100 micron stream is then washed. may be sufficient, but an agent such as EDTA may also be used. If the EDTA (or equivalent) is used, then the resulting dirty stream can be readily adsorbed, and the resulting waste stream be treated for disposal as LLW (non-alphabearing).
Paramagnetic Page 11 The <100 micron stream is diluted and routed through the paramagnetic separation system for plutonium removal. The resulting alpha-waste stream should be extremely small in volume but should contain the majority of the plutonium. What Can Go Wrong? The first point here is that no technology is the universal fix for any waste treatment problem. Paramagnetic separation is simply one of a broad mix of gravimetric, physical, chemical, and thermal treatment technologies that must be considered when approaching a waste problem. Paramagnetic separation is somewhat unique amongst the group of treatment technologies available in that it preferentially separations plutonium, uranium and americium (as well as other species). This gives the environmental manager a tool that enables him to separate alpha and non-alpha wastes. In the absence of such a tool, the site manager must choose between generating large volumes of alpha wastes or taking no action other than minimising the off-site flow of radioactive material. In treating wastes containing plutonium, there are two technical objectives maximise the decontamination factor and the volume reduction. Virtually all of the things that can go wrong negatively affect one or both of these factors. In the initial attempts to demonstrate the applicability of paramagnetic separation on plutonium, the main problem was the presence of other magnetic material in the soils. This caused a waste volume to be generated that was unacceptably large. This problem can be anticipated and tested during the treatability characterisation phase of work. It can be technically managed by passing soils through the magnet at a lower field strength (assuming that the magnetic materials in the soils have a higher response to magnetic fields than the plutonium does), hence separating the plutonium from the other magnetic material. The plutonium-contaminated stream can then be passed through the magnet at a higher field strength, when the plutonium separation is accomplished. This is shown in Figure 8.
Paramagnetic Page 12 Figure 8 Two Pass Flow Diagram Dredge Size < 100 > 100 Pretreatment Fines & Gravimetric & Washing Additives Low-Field High-Field Contaminant Waste Another problem in the initial demonstration efforts was the physical bonding of plutonium in clay particles. This allowed the clay particles to pass through the magnet, and caused the resulting clean stream to have plutonium concentrations that were too high. This problem can be avoided in the initial physical treatment step. If the soils are crushed (eg, ball or rod mill) and then separated using a sonic separator, then the plutonium particles are released from the soil particles and are free to be removed by the magnet, as shown in Figure 9. This step can also be applied to the wastes coming from the magnet which contain plutonium, as shown in Figure 10.
Paramagnetic Page 13 Figure 9 Simplified Flow Diagram with Ball/Rod Mill and Sonic Dredge Ball or Rod Mill Sonic < 100 > 100 Pretreatment Fines & Gravimetric & Washing Additives Contaminant Waste
Paramagnetic Page 14 Figure 10 Simplified Flow Diagram: Two-Pass Treatment of Plutonium Stream with Ball/Rod Mill Treatment Dredge Ball or Rod Mill Sonic < 100 > 100 Pretreatment Fines & Gravimetric & Washing Additives Ball or Rod Mill Sonic Contaminant Waste There are, of course, situations in which magnetic separation is not the most efficient treatment approach for soils and sediments containing plutonium. Highest amongst such situations is where the plutonium is easily brought into solution. It can then be readily adsorbed on products such as KEECO s MetaLock adsorbent.
Paramagnetic Page 15 Safety The application of paramagnetic separation involves industrial hazards that are found in almost all treatment systems. Industrial safety is paramount and strict procedures must be established and adhered to. One unique hazard is the presence of the magnetic field. In the near proximity of the magnet, when at power, workers cannot be allowed to enter the field. This can be achieved by the erection of physical barriers. In terms of radiological safety, the paramagnetic system is entirely closed, and can be operated without any airborne effluent. If crushing is utilised on the feed material, then this crushing is done in a closed system, and is sprayed with water to minimise dust generation. Normal radiological safety procedures must also be established and adhered to, particularly if the resulting waste streams generate high radiation fields. Economics Paramagnetic separation systems can be constructed and applied for as little as US$300 per cubic metre of soils or sediments treated. This value can be higher if mixed fission products are present, or if more sophisticated physical separation systems are needed. The cost of this approach must always be compared with the costs avoided. If a site manager is required to remove radioactively-contaminated soils containing plutonium, then costs of removal, treatment, storage and disposal (if available) must all be considered. In most cases, no disposal system is available for alpha wastes, so the economic considerations must include long-term storage, possible repackaging of wastes, as well as ultimate disposal costs.