2 Design Basis Requirements: Thermal and Chemical Characteristics of Dissolution Reaction

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1 FY2011 PROGRESS REPORT IN DEVELOPING AMBIENT PRESSURE NITRIC-ACID-DISSOLUTION LEU FOILS James L. Jerden, James Bailey, Lohman Hafenrichter Argonne National Laboratory, 9700 South Cass Ave, Argonne, IL, 60439, USA 1 Introduction A nitric-acid-dissolver system was designed to allow the dissolution of up to 250 grams of irradiated uranium foil and associated fission recoil barrier metal (e.g., Ni) at ambient pressure. Components of the dissolver system are currently being tested so that the design can be optimized in preparation for a full-scale demonstration. The key design criteria that this dissolver system must incorporate are listed below. these topics as well as the ongoing performance testing will be discussed in this report. Each of All water vapor, reaction products, and fission gases must be contained within the dissolver system at a maximum temperature of 125 o C and 2 atmospheres of pressure (absolute) under both normal and off-normal (loss of cooling during reaction) conditions. The acid feed system must be designed so that the thermally hot LEU foil (hot from decay heat) can be immersed in nitric acid without losing solution due to instantaneous boiling. All dissolver system components must designed for remote operation in a hot cell facility. Gas-trap components must be designed to trap/neutralize all nitrogen oxide and acid gases (NO, NO 2, HNO 2, HNO 3 ) as well as trap iodine gas for possible extraction of economically important iodine isotopes (noble fission gases will be passively contained). 2 Design Basis Requirements: Thermal and Chemical Characteristics of Dissolution Reaction Stoichiometry of Dissolution Reaction and Reaction Off-gas The volume and concentration of nitric acid for a given experiment will depend on the mass of the metal being dissolved as well as the desired final acid concentration of the product solution (i.e. the solution produced by dissolution experiment). Controlling the final acid concentration is important for optimizing the Mo-99 extraction step that comes after dissolution. The volumes and concentrations of acid as well as the amount of nitrogen oxide gas (NO x : NO, NO 2, N 2 O 4 ) that will be produced are determined by the following general reactions: U + 4HNO 3 UO 2 (NO 3 ) 2 + 2H 2 O + 2NO (1)

2 Ni/Cu/Zn + 8/3HNO 3 Cu/Ni/Zn(NO 3 ) 2 + 4/3H 2 O + 2/3NO (2) In the presence of oxygen, the NO (g) produced in these dissolution reactions is rapidly converted to NO 2 (g): NO + 0.5O 2 NO2 (3) Using the kinetic rate law presented by Chilton, 1968 (page 29 of Chilton, 1968), one finds that the rate of reaction (3) is on the order of milliseconds to seconds even at relatively low O 2 partial pressures ( atm). When water vapor and oxygen are present, NO 2 can be converted to both nitrous and nitric acid vapors [HNO 2(g) and HNO 3(g) ]. An example calculation of how the acid concentrations, volumes, and amount of NO x are determined for a given experiment is summarized in Tables 1 and 2. This example is a bounding case (i.e., an experiment in which the maximum amount of metal is dissolved). We first calculate the moles of HNO 3 that will be consumed in the dissolution reaction based on reactions (1) and (2) [see 4 th column Table 1, 3 rd column Table 2]. Using the total HNO 3 consumed and the desired final acid concentration, the initial acid concentration can be calculated for a given volume of acid (4 th column Table 2). The moles of NO x produced are determined by the mass of the metal being dissolved. The example shows that for dissolving 250 grams of uranium metal and 10 grams of nickel metal (a maximum bound), the initial nitric acid concentration needed will vary from 6.6 to 13.3 molar depending on the desired final acid concentration and the dissolver solution (acid) volume (Table 2). Table 1. Example calculation for determining the initial nitric acid volume and concentration for a dissolution experiment, as well as the amount of NO x that will be produced. Metal grams of metal grams/mole of moles of moles HNO 3 sample metal metal sample consumed U Ni Table2. Example calculation for determining the initial nitric acid volume and concentration for a dissolution experiment, as well as the amount of NO x that will be produced. Dissolver solution Volume (L) Desired final acid con (mole/l) Moles HNO 3 consumed total Initial acid con. (mole/l) NO(g) produced (moles) NO(g) produced (L)* *Assume NO x behaves as ideal gas at 25 o C and 1 atm. In an effort to be more precise about the concentrations of off-gas species, relative amounts of the important nitrogen oxide gases produced by the dissolution of different amounts of LEU

3 were calculated using the thermodynamic code OLI-ESP (Table 3). These calculations predict that approximately 2.1 moles of NO x +H 2 O (g) will be present in the dissolver following the dissolution of 250 grams of LEU foil. These calculations agree with the generalized stoichiometric calculations described above and provide a design basis for the off-gas treatment components of the dissolver system. Table 3 Thermodynamic modeling results simulating the off-gas composition for the dissolution of uranium metal after the dissolver has cooled to a temperature of 25 o C. Calculations were done in the absence of oxygen. Under oxygenated conditions NO 2 rather than NO will be the dominant species due to the relatively fast kinetics of Reaction 3. U metal (g) H 2 O (moles) NO (moles) NO 2 (moles) HNO 3 (moles) HNO 2 (moles) Total (moles) 5 1.2E E-05 3E E E E-04 2E E E E-04 2E E E E-03 2E E E E-03 5E E E-03 7E E E-03 9E E-03 1E E-03 1E E-03 2E E-03 2E E E Dissolver Design Basis: Reaction Heat The following equation was used to determine the energy released during the exothermic dissolution of the LEU foil. This calculation is conservative because it assumes adiabatic conditions. o o o ΔH R n ih fi(products ) n ih fi(reactants) i i Where n is the molar coefficient for each reactant and product from a balanced equation representing the reaction, and i identifies the compounds or ions in the reaction. The enthalpy of reaction for the oxidative dissolution LEU in nitric acid depends on what reaction products are formed: U + 4HNO 3 UO 2 (NO 3 ) 2 + 2H 2 O + 2NO (4) U + 4HNO 3 UO NO H 2 O + 2NO (5) U + 8HNO 3 (aq) 4H 2 O + UO 2 (NO 3 ) 2 + 6NO 2 (g) (6)

4 U + 8HNO 3 (aq) 4H 2 O + UO NO NO 2 (g) (7) The convention used in these calculations is that a negative (-) enthalpy of reaction indicates an exothermic reaction (heat is produced by dissolution). Table 4. Enthalpy and thermal output (in Watts) for the exothermic uranium dissolution reaction (this conservative estimate assumes adiabatic conditions and a 30- minute dissolution time). Reaction Thermal Thermal Watts H r H r 250g U metal (kj Watts for 1 g for 250 g (kj/mole) (kj/g) released) (J/sec.) (J/sec.) (4) (5) (6) (7) Thermodynamic data used is from the compilation by Genthe et al., As the results indicate (Table 4) the dissolution of 250 grams uranium metal (~1.05 moles) the total energy released may be up to 1600 kj, but will probably be closer to 1000 kj [reaction (5) is probably most representative]. Assuming adiabatic conditions and a 30 minute reaction time this energy would correspond to a maximum thermal power output of approximately 890 watts. For an hour reaction time (again assuming adiabatic conditions) the maximum thermal output would be around 445 watts. It has been noted in previous uranium metal dissolution experiments performed at Argonne (e.g., Jerden et al., 2010) that there can be a thermal spike during dissolution that involves the relatively sudden release of heat (within a few minutes). Dissolution experiments are planned to test if this sudden heat output also occurs in the larger, better mixed dissolver system that is being discussed in this report. Dissolver Design Basis: Decay Heat The composition (actinides and fission products) of the LEU foils that will be processed in the two front-end processes described in this report was calculated by Charlie Allen, University of Missouri, using ORIGEN2, Version 2.2. The assumptions used for these calculations were as follows (see Jerden et al., 2011, for more details on actinide and fission product yields for irradiated LEU foil). Irradiation of 1 gram of uranium foil enriched to 19.75% 235 U. Power = 1.9E-3 megawatts, Burnup = 1.59E-2 megawatt days, Flux = 2.1E14 N/cm 2 sec. Burnup is for 200 hours, foil composition is given for cooling times of 12, 24, 36 and 48 hours. The thermal wattage produced by decay heat of the actinides and fission products was calculated and is shown in Figure 1. The total thermal output for 250 g of irradiated LEU is around 1000 watts for a 12-hour cooling time and around 500 watts for a 48-hour cooling time. These thermal values have implications for the front-end processes because they must be designed to account for the initial elevated temperatures of the LEU foils. For example the nitric acid dissolver

5 system must be designed so that the acid feed component does not pressurize or leak when the hot fuel is initially immersed. Decay Heat (Watts thermal) 1.0E E E E+01 Fission Products 12hr Fission Products 48hr Actinides 12hr Actinides 48hr 1.0E Mass of LEU foil irradiated for 200 hours (grams) Figure 1. Cumulative decay heat for fission products and actinides for a range of LEU foil masses after 200 hour irradiation. Curves are for 12 and 48 hours of cooling (time out of reactor). Based on the enthalpy and decay heat calculations above the cooling system for the LEU nitric acid dissolver system must be able to sink out a maximum of 2000 watts (thermal). Therefore, if it is assumed the dissolution if 250 g of irradiated LEU foil takes 30 minutes, 2000 watts of thermal power will be generated and will need to be removed from the system to ensure that water vapor (acid) is not lost during the dissolution process. If acid is lost during dissolution, the reaction will be halted and cause a delay in processing and Mo-99 extraction. Thermal heat-flow calculations indicate that with cooling fins on the condenser section of the dissolver (see Section 3 for design description) and an ambient air cooling flow velocity of around 20 meters per second adequate heat will be removed from the dissolver to allow the reflux of the acid and a continuous dissolution reaction. Initial results from experiments that are being performed to confirm the results of these calculations indicate that the cooling system works properly and is adequate to dissipate the amount of heat generated by decay and the exothermic nature of the dissolution reaction. Composition of Off-Gas As shown by the calculations discussed above, most of the off gas from the dissolver will consist of the NO x and acid gas species shown in Table 3 (Jerden et al., 2011). The ORIGIN calculations indicate that the other off gases produced during dissolution are fission product gases. The amount of the major fission gases are shown in Tables 5 to 8 (see Jerden et al., 2011 for more detail on calculations).

6 Table 5 Iodine yields in Ci per 250 grams of irradiated LEU (200 hour irradiation at a flux of 2.1E14 N/cm 2 sec). Yields after five cooling times are shown. Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr) I E E E E E+02 I E E E E E-12 I E E E E E+03 I E E E E E+03 I E E E E E+03 I E E E E E-01 I E E E E E-06 I E E E E E+00 Total (Ci) 9.8E E E E E+04 Table 6 Iodine yields in moles per 250 grams of irradiated LEU (200 hour irradiation at a flux of 2.1E14 N/cm 2 sec). Yields after five cooling times are shown. Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr) I E E E E E-07 I E E E E E-21 I E E E E E-05 I E E E E E-06 I E E E E E-04 I E E E E E-09 I E E E E E-04 I E E E E E+00 Total 6.9E E E E E-04 Table 7 Fission gas yields in Ci per 250 grams of irradiated LEU (200 hour irradiation at a flux of 2.1E14 N/cm 2 sec). Yields after five cooling times are shown. Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr) H-3 5.6E E E E E-02 Kr E E E E E-01 Kr E E E E E-08 Kr-85M 4.9E E E E E+00 Kr E E E E E+00 Kr-83M 2.0E E E E E-03 Kr E E E E E-13 Total Kr 3.0E E E E E+00 Xe E E E E E+00 Xe-135M 4.3E E E E E+01 Xe E E E E E+03 Xe-133M 6.9E E E E E+02 Xe E E E E E+04 Xe-131M 1.4E E E E E+01 Xe-129M 1.2E E E E E-05 Total Xe 4.6E E E E E+04

7 Total 7.6E E E E E+04 Table 8 Fission gas yields in moles per 250 grams of irradiated LEU (200 hour irradiation at a flux of 2.1E14 N/cm 2 sec). Yields after five cooling times are shown. Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr) H-3 1.9E E E E E-06 Kr E E E E E-05 Kr E E E E E-12 Kr-85M 1.4E E E E E-05 Kr E E E E E-05 Kr-83M 5.9E E E E E-08 Kr E E E E E-18 Total Kr 3.6E E E E E-04 Xe E E E E E+00 Xe-135M 1.0E E E E E-04 Xe E E E E E-02 Xe-133M 7.1E E E E E-03 Xe E E E E E-01 Xe-131M 1.1E E E E E-04 Xe-129M 1.3E E E E E-10 Total Xe 2.1E E E E E-01 Total 5.8E E E E E Dissolver Design and Performance Tests Dissolver Design Overview The nitric-acid process involves dissolving LEU foil and the fission-recoil barrier in a nonpressurized, steel dissolver at elevated temperature. The dissolver is designed to operate at pressures less than 2 atmospheres (absolute) and at temperatures less than 125 o C. A flow diagram of the component sections of the nitric acid dissolver and Mo-99 extraction process is shown in Figure 2. The design concept of the dissolver is shown in Figure 3and 4. The dissolver system consists of a 304 stainless steel vessel (approximately 2 liter volume) connected to an approximately 65 liter (30cm x 90cm) off-gas reservoir. The dissolver vessel is open to the off-gas reservoir during the dissolution process. The volume of the reservoir was chosen to provide passive containment of all gas reaction products at a pressure less than 2 atmospheres (absolute). The reservoir is designed to contain reaction-product gases, at low pressure, during both normal and off-normal (loss of cooling during reaction) conditions. In order to keep the temperature of the gas within the reservoir to below 100 o C during a potential loss of cooling it is wrapped with an aluminum heat sink (Al fin rings). The dissolution process is started by first lowering the uranium foil (contained within a steel mesh basket into the dissolver vessel and then sealing the vessel with a metal cap) (Figure 4). Pre-heated acid (~100 o C) is then added to the vessel using a two chamber acid feed system that is designed to avoid pressurization of the acid bottle in the event that the dissolution reaction begins instantaneously when the acid addition step is started.

8 The dissolver vessel is cooled by forced air blown from the base of the unit. The temperature of the dissolver solution is monitored by a thermocouple. The dissolver vessel is insulated so that the top of the vessel is cooled continuously during the reaction. Heat loss from the top of the vessel is optimized by the presence of steel cooling fins attached to the condenser part of the dissolver system (shown and discussed in Section 3 below). This design causes the water vapor to condense along the walls at the top of the vessel during the dissolution reaction (as acid is boiling). This process is shown schematically in Figure 2. Following dissolution of the LEU foil, any iodine remaining in the product will be volatilized using a standard chemical technique (e.g. adding hydrogen peroxide at elevated temperature: see Cathers et al., 1975). The product solution is then drained from the base of the vessel and run through a TiO 2 Mo-recovery column to recover the molybdenum. The product solution from the dissolver contains uranium, nickel, and non-volatile fission products in a 1 molar nitric acid. The key components of the dissolver system and the performance tests that are being done to optimize the design are discussed in more detail below. Figure 2. Schematic flow diagram showing components of the LEU nitric acid dissolver system and the Mo-99 extraction column.

9 Figure 3. Conceptual drawing of the nitric-acid dissolver system.

10 Figure 4. Conceptual drawing of the nitric-acid dissolver system showing the steps involved in starting the dissolution and gas flows during operation. Condenser/Cooling System Design and Testing As stated above, all water vapor, reaction products, and fission gases must be contained within the dissolver system at a maximum temperature of 125 o C and 2 atmospheres (absolute) under both normal and off-normal conditions. A key technical challenge for the dissolver system design was making sure that water vapor and acid gases did not escape from dissolver vessel condenser section. If these gases escaped during the dissolution reaction the reaction would halt, delaying the overall Mo-99 production process. To retain water vapor and acid gases within the dissolver, we designed a forced-air cooling system (located at the base of the dissolver vessel) and a condenser section that is attached to the top dissolver vessel (Figure 3 and 4). The condenser section consists of cooling fins, through which the cooling air is passed. A schematic diagram of the condensation process and actual pictures of the dissolver are shown in Figures 5 to 9 below. The cooling fins keep the outer wall of the top of the dissolver cool relative to the base. This causes water vapor to condense and run back down into the dissolver vessel. Nitrogen oxide and acid gases such as NO 2 and HNO 3 will dissolve in the condensed water ensuring that there is enough acid to complete the dissolution reaction. To test this process only the condenser section and reaction vessel are being used in our initial heat flow experiments. Figures 5 and 6 show the experimental set up schematically. Figures 4 to 9 show photographs of the experimental set up in the laboratory.

11 Figure 5. Conceptual diagram showing a 3-D representation of the reaction vessel and condenser sections that are being used in the initial performance tests.

12 Figure 6. Schematic diagram showing the key features involved in the performance testing of the dissolver system. Two types of experiments are being performed to test the performance of the dissolver design. The cold tests (currently underway) are being run with water and nitric acid to confirm and quantify the performance of the condenser section of the dissolver system. An array of thermocouples is being used in the initial testing to measure all relevant thermal gradients during the dissolution reaction process (inside and outside the dissolver vessel and condenser section) (Figure 6 right). The hot tests involving the dissolution of depleted uranium and irradiated uranium foil from relevant targets are planned for FY The two chamber acid feed system was also tested as part of the cold tests. The laboratory set up for the acid feed system is shown in Figure 10 and observations from the tests are described in Section 3.5. Cold tests boiling water or nitric acid with no U/Ni present: Purpose: Determine the heat exchange capacity between the coolant air flow on the outside of the condenser section and the condensing vapor to liquid on the inside of the vessel. Heating coils attached to the base of the dissolver vessel (e.g. Figures 6, 7, 8) are used to simulate the exothermic heat from the LEU dissolution reaction as well as the decay heat from the irradiated foil (discussed below).

13 A measured amount of water or nitric acid (< 1 liter at ambient temperature) is poured into the bottom of the vessel. The dissolver is sealed. At this point the heaters are off and vessel is at ambient temperature. The blower is turned on and temperatures and air flow velocity are measured. temperatures are allowed to stabilize before initiating the next step. The The heaters are turned on at low power and the transient temperature increase is measured. Once the system reaches a steady state [water or acid is boiling (< 125 o C) and the electrical power supplied to the heaters is constant] temperatures within and outside the dissolver are continuously monitored using a computer data-logger. The cooling-air flow velocity and the electrical power to the heaters are measured. The experiment is run at this steady-state condition for an hour (or less). The vent tube is constantly monitored to see if vapor is escaping from the condenser section. If the temperature inside the vessel increases above 125 o C (indicating that all of the liquid has been vaporized) the experiment is terminated immediately. After approximately 1 hour of running the experiment at steady state, the heaters are turned off and the blower is kept on until the vessel returns to ambient temperature. When the vessel is at ambient temperature the water or acid is drained out and it mass/volume measured to determine how much vapor was lost (vented into the fume hood) during the experiment. Hot tests dissolution of uranium +/- nickel at ~1 atm: Purpose: Measure the dissolution rate of metal foils in nitric acid (at ambient pressure) for a number of different starting conditions (temperature, mass of metal, initial acid concentration). The amount of heat produced from the exothermic dissolution reactions will also be measured as well as the efficiency of the sodium hydroxide NO x trap. Up to 260 grams of uranium +/- nickel metal is loaded into the basket that will be lowered into the acid at the base of the dissolver vessel (Figure 4). A measured amount (< 1 liter) of ambient temperature nitric acid (< 14 molar) fed into the bottom of the vessel and the metal foil samples are immersed. The dissolver vessel is sealed. At this point the heaters off and vessel is at ambient temperature. The blower is turned on and temperatures and air flow velocity are measured. temperatures are allowed to stabilize before initiating the next step. The The heaters are turned on at low power and the transient temperature increase is measured.

14 The temperature of the acid will be measured continuously during the exothermic dissolution reaction. If the temperature increases above 125oC (indicating loss of solution) the experiment will terminated and the coolant system optimized before the next experiment is performed (e.g., increase cooling air flow rate, better insulate dissolution vessel). Once the acid is boiling the heaters will be turned off and the blower will be kept on. The temperature within the dissolver vessel will be monitored continuously until it reaches ambient temperature. The dissolver solution (U +/- Ni, in 0.1 to 5 molar nitric acid) will be drained into a sealable, chemically compatible vessel and saved for chemical analyses or prepared for Mo-99 extraction tests. The cap is removed from the dissolver so that the interior can be visually inspected for any undissolved metal. If metal is still present the dissolution procedure will be repeated until all of the metal is dissolved and drained from the dissolver. Figure 7 Shown on the left is a picture of the dissolver including the condensing fins on the condenser section and the heating coils on the dissolver reaction section. On the right is another picture of the dissolver showing the dissolver reaction vessel without heating coils.

15 Figure 8. Reaction vessel and condenser section of the dissolver system showing the placement of thermocouples for the performance tests that are currently being performed.

16 Figure 9. Condenser section of dissolver and the reaction vessel wrapped in insulation in preparation for initial reflux condenser performance tests.

17 Figure 10. Two chamber acid feed component for the dissolver. This design ensures that acid is not lost to instantaneous vaporization when introduced to the thermally (from decay heat) LEU foil in the dissolver. Initial Results from Condenser Section and Acid Feed Tests The performance tests of the performance of the nitric acid LEU dissolver system are currently underway. Both the cooling air blower and the heating coils used in the cooling performance experiments have been wired and successfully tested. The thermocouples have been monitored while the heating coils and blower were on and the initial observations suggest that the cooling air flow and insulation of the dissolver vessel will be sufficient for the condenser section of the dissolver to work properly. However these experiments are not complete. More work is needed using nitric acid and uranium metal to map out the thermal gradients and thermal transient features during the dissolution reaction. As part of the cold tests the two chamber nitric acid feed component was tested alone (not attached to the dissolver) to make sure its valve set up worked properly. The laboratory set up for the acid delivery test is shown in Figure 10. For these tests the steel acid bottle (top chamber) was filled with water and the valve opened to the bottom chamber (acid delivery chamber). It was confirmed that the water had all been drained into the secondary chamber using a watch glass attached to the system. The top valve was closed and the water bottom valve (acid delivery valve was opened to a heated (100oC) and sealed steel vessel. Some of the water converted to

18 steam and reentered the secondary (bottom) chamber but quickly condensed and ran readily into the heated vessel. The acid feed system was not pressurized (due to its volume relative to the volume of water) and none of the valves on the system leaked. Summary of preliminary results: Thermocouple measurements indicate that the cooling-air flow calculated for efficient condensing of water/acid vapor within the dissolver is adequate as long as the reaction vessel/heating coils are well insulated. More tests are needed to confirm and fully quantify these observations. The acid feed components have been successfully tested and appear to be amenable for remote operation with a hot cell facility. Planed tests within a hot cell mock-up facility are planned to confirm this later conclusion. 3.6 Design and Performance Testing of Off-gas Traps Iodine Trap The sequestration of iodine gas by copper metal has been demonstrated (e.g., Metalidi et al., 2009). However, the efficiency of this process has not been studied in enough detail to design an efficient iodine trap for the type of irradiated LEU foil dissolution process we are investigating. We have performed scoping tests to confirm that a copper metal based iodine trap is feasible for the nitric acid dissolver system being designed at Argonne. It has been shown that iodine fission gas produced by LEU dissolution can be selectively removed from the other fission gases using a copper-based trap or scrubber (Metalidi et al., 2009). A generalized reaction for how such an iodine trap would work is as follows: Cu + 0.5I2 CuI (8) This simplistic reaction can be used to estimate the mass of copper that would be required to trap the amount of iodine produced during the LEU foil dissolution process. As The maximum amount of I2(g) that will be processed in the iodine trap is around 6.9E-4 moles for a 12 hour cooling period and 4.4E-4 moles for a 48 hour cooling period (Table 6). Therefore, the amount of copper needed for the iodine trap is less than one gram. As a conservative estimate 10 grams of copper metal (as a higher surface area mesh or beads) is assumed for the iodine trap per 250 grams of irradiated LEU. The iodine can be recovered by either reducing the CuI back to Cu metal using hydrogen gas or oxidizing the CuI to release Cu++ and 2I- in solution. The recovery of iodine from this type of trap needs is poorly understood and will be a potential focus of future work. The redox chemistry that makes the sequestration and recovery of iodine using copper metal possible is summarized in Figure 11. The redox speciation shown in Figure 11 is illustrated as Eh-pH or Pourbaix diagrams, where Eh is defined as the oxidation/reduction potential (in volts)

19 of an aqueous solution, relative to the standard hydrogen electrode. The figure shows that it is thermodynamically possible for copper metal to reduce I2 gas at ph values less than around 6.0 (arrow on top two diagrams). Figure 11 also shows that the maximum stability field of CuI (the desired reaction product) spans from around 0.5 to 0 Volts at ph values less than a ph of approximately 5. For more basic conditions the copper oxides CuO (tenorite) and Cu2O (cuprite) become more stable. This is an important observation for the design of the iodine trap because the formation of copper oxides could limit its efficiency. This should not be a problem for the LEU nitric acid dissolver system because the off-gas that will be passed through the iodine trap will contain acid gases dissolved in water vapor (ph of condensate in iodine trap should be from -1 to 1. The redox speciation diagrams (Figures 11) were calculated using the thermodynamic code The Geochemist s Workbench Release 8.0 (GWB) using an adapted version of the thermodynamic database thermo.com.v8.r6.full (Wolery and Daveler, 1992) HIO3(aq) 1.8 I2.8 Tenorite.4 (CuO).2 Cuprite (Cu 2O) 0 Cu C Eh (volts).6 Diagram Cu++, T = 25 C, P = bars, a [main] = 10 3, a [H 2O] = 1 I Reduction of I2 in presence of Cu metal C ph ph HIO3(aq) Tenorite.4 (CuO) CuI.2 Cuprite 0.2 Cu.4 25 C ph I2.8 Eh (volts) Cu Diagram Cu++, T = 25 C, P = bars, a [main] = 10 3, a [H 2O] = 1, a [I-] = 10 3; Suppressed: I2(aq) ++.8 Eh (volts) I- IO3- - I3.6 I Diagram I-, T = 25 C, P = bars, a [main] = 10 3, a [H2O] = 1, a [Cu++] = 10 3; Suppressed: I2(aq) Eh (volts) Cu IO Diagram I-, T = 25 C, P = bars, a [main] = 10 3, a [H2O] = 1; Suppressed: I2(aq) CuI C ph Figure 11. Eh - ph plots showing the redox chemistry of copper and iodine in aqueous solution over a range of ph. The top two diagrams show the stable oxidation states of copper and iodine separately (10-3 molar dissolver Cu or I). The bottom diagrams show the oxidation states for copper and iodine in a combined Cu-I-aqueous system (10-3 molar for Cu and I).

20 4 Iodine Trap: CuI Feasibility Tests Scoping tests were performed investigating the kinetics of the sequestration of iodine gas by copper metal. The tests involved contacting approximately 1mm diameter copper beads with iodine gas (sublimated from pure I2 crystals) at three temperatures (25oC, 70oC, and 150oC). One milliliter of dilute (ph 1) nitric acid was placed in each vessel to obtain a humid environment. The tests were performed as batch experiments in Teflon lined steel PARR vessels. The vessels containing copper beads and iodine were placed in ovens at three different temperatures for time periods ranging from 10 minutes to 1 hour. Microscopic examination of the reaction products (see Figure 12) indicate that the copper beads are readily converted to CuI (white/gray material) in the presence of I2(g) as long as the temperature is high enough (so that the iodine does not condense to form a solid on the sides of the vessel (or within tubing) and enough reaction time is given. Some copper oxide (probably Cu2O) appears to have formed as well (Figure 12). Further chemical analyses of the reaction products will be performed to confirm the efficiency CuI formation. Even after an hour of reaction the 25oC sample had undergone little if any reaction. The 70oC sample showed signs of copper corrosion (CuI formation) after approximately 20 minutes (solids from this test are shown in Figure 12). After an hour the copper beads were completely replaced. Complete replacement of the copper beads in the 150oC sample occurred within approximately 10 minutes (or less). Our results generally agree with Metalidi et al., 2009 in terms of the general kinetics of the reaction; however, a more accurate surface area measurement of the copper beads used in our experiments is needed before our results can be used to quantify iodine sequestration by copper metal.

21 Figure 12. Light microscope pictures (same illumination level for all photos) of copper beads and reaction products from iodine sequestration tests (70oC, for 1 hour in humid air). The extraction of the iodide from the copper trap can be accomplished by oxidizing the CuI and trapping the iodide in a solution form. The following general reaction indicates the basic approach to the iodine extraction processes; however, the kinetics and optimal conditions for this extraction have not yet been quantified. CuI + H O2(aq) = I H2O + Cu2+ (9) Nitrogen Oxide and Acid Gas Trap The conversion of NOx to sodium nitrite and nitrate can be described by the following general reactions: NO2(g) + NaOH O2(aq) NaNO H2O (10) NO2(g) + NaOH NaNO H2O O2(aq) (11) which combine to give:

22 NO2(g) + NaOH 0.5NaNO NaNO H2O (12) Assuming that 2.21 moles of nitrogen oxide gas need to be neutralized (Table 3), 2.21 moles of NaOH will be needed. This corresponds to approximately 90 grams of NaOH. This will produce approximately 75 grams of NaNO grams of NaNO3 as waste. Based on these calculations, the NOx trap will consist of a 500mL, 5 molar NaOH solution in a 1L flask or beaker with an in-line hose connected to the dissolver vent and out-line hose that vents to the to the off-gas reservoir. Summary and Future Work The key design criteria were addressed experimentally to optimize the components of the LEUfoil nitric-acid dissolver. Results from ongoing and future tests will be used to finalize the design and fabricate all parts in preparation for a full scale demonstration. The design criteria that have been investigated by ongoing experiments are as follows: Preliminary shakedown tests of the dissolver vessel, condenser section, and cooling air blower suggest that all water vapor, reaction products and fission gases will be contained within the dissolver system at a maximum temperature of 125oC and 2 atmospheres (absolute) under both normal and off normal (loss of cooling during reaction) conditions. However, more experimental work is needed to confirm and quantify this observation. A two-chamber acid delivery system was tested, and the initial results indicate that the design is capable of delivering nitric acid to thermally hot LEU foils (hot due to decay heat) without losing acid due to sudden boiling. The acid delivery component is also designed for remote operation in a hot cell facility. Preliminary feasibility tests show that the copper metal trap for iodine sequestration and recovery has promise. The NaOH NOx trap is a proven technology; however, we continue to work on dissolver system designs that most efficiently incorporate the NOx trap into the overall design. Future work on this project will involve the following: Continuation of the heat-flow testing of the dissolver cooling components. Ongoing tests will map out the thermal gradients both inside and outside the dissolver so that the condenser section design can be optimized. Dissolution experiments on both non-irradiated and irradiated uranium foils will be performed to test the cooling system/condenser performance in the presence of different amounts of uranium. This will allow us to quantify how the exothermic heat output from the dissolving uranium foil affects the cooling system performance. These tests will also allow testing of the gas-traps and off-gas reservoir.

23 All components will be tested in a manipulator mock-up facility to ensure that the dissolver system can be used at a production scale in a hot cell facility. 5 References Cathers, G. I., Shipman, C. J., Volatilization of iodine from nitric acid using peroxide, US Patent 3,914,388, October, Chilton, T. H., Strong Water. MIT, Cambridge, MA (1968). Grenthe I. et al. (1992) Chemical Thermodynamics of Uranium. Elsevier. Jerden Jr., J.L, Stepinski, D.C., Gelis, A., and Vandegrift, G.F., "Front-End Processes for Conversion of Current HEU-Based Alkaline Processes to LEU Foil Targets: Volumes and Compositions of All Waste, Product, and Off-Gas Streams from Both Front-End Options." Argonne National Laboratory (2011). Available at: J. Jerden Jr., S. Chemerisov, A. Hebden, S. Wiedmeyer, G. Vandegrift, 2010, Development of a Production-Scale Dissolver for Nitric-Acid Dissolution of LEU Foils, Conference Paper for the Reduced Enrichment for Research and Test Reactors Meeting in Lisbon, Portugal, Oct , 2010 Wolery, T.J. and Daveler, S.A., 1992, EQ6, A computer program for reaction path modeling of aqueous geochemical systems: Theoretical manual, user s guide, and related documentation (Version 7.0): Lawrence Livermore National Laboratory Report UCRL-MA PT IV