FY2011 PROGRESS REPORT IN DEVELOPINGAMBIENT PRESSURE NITRIC-ACID-DISSOLUTION LEU FOILS

James L. Jerden, James Bailey, Lohman HafenrichterArgonne 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 ofirradiated 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 beoptimized in preparation for a full-scale demonstration.

The key design criteria that this dissolver system must incorporate are listed below. Each ofthese topics as well as the ongoing performance testing will be discussed in this report.

All water vapor, reaction products, and fission gases must be contained within thedissolver system at a maximum temperature of 125oC 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 fromdecay heat) can be immersed in nitric acid without losing solution due toinstantaneous boiling.

All dissolver system components must designed for remote operation in a hot cellfacility.

Gas-trap components must be designed to trap/neutralize all nitrogen oxide and acidgases (NO, NO2, HNO2, HNO3) as well as trap iodine gas for possible extraction ofeconomically important iodine isotopes (noble fission gases will be passivelycontained).

2 Design Basis Requirements: Thermal and Chemical Characteristics ofDissolution 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 ofthe 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 isimportant for optimizing the Mo-99 extraction step that comes after dissolution. The volumesand concentrations of acid as well as the amount of nitrogen oxide gas (NOx: NO, NO2, N2O4)that will be produced are determined by the following general reactions:

U + 4HNO3 → UO2(NO3)2 + 2H2O + 2NO (1)

Ni/Cu/Zn + 8/3HNO3 → Cu/Ni/Zn(NO3)2 + 4/3H2O + 2/3NO (2)

In the presence of oxygen, the NO(g) produced in these dissolution reactions is rapidly convertedto NO2(g):

NO + 0.5O2 NO2 (3)

Using the kinetic rate law presented by Chilton, 1968 (page 29 of Chilton, 1968), one finds thatthe rate of reaction (3) is on the order of milliseconds to seconds even at relatively low O2 partialpressures (0.1 – 0.001 atm). When water vapor and oxygen are present, NO2 can be converted toboth nitrous and nitric acid vapors [HNO2(g) and HNO3(g)].

An example calculation of how the acid concentrations, volumes, and amount of NOx aredetermined for a given experiment is summarized in Tables 1 and 2. This example is a boundingcase (i.e., an experiment in which the maximum amount of metal is dissolved).

We first calculate the moles of HNO3 that will be consumed in the dissolution reaction based onreactions (1) and (2) [see 4th column Table 1, 3rd column Table 2]. Using the total HNO3

consumed and the desired final acid concentration, the initial acid concentration can becalculated for a given volume of acid (4th column Table 2). The moles of NOx produced aredetermined by the mass of the metal being dissolved. The example shows that for dissolving 250grams of uranium metal and 10 grams of nickel metal (a maximum bound), the initial nitric acidconcentration needed will vary from 6.6 to 13.3 molar depending on the desired final acidconcentration and the dissolver solution (acid) volume (Table 2).

Table 1. Example calculation for determining the initial nitric acid volume and concentration fora dissolution experiment, as well as the amount of NOx that will be produced.

Table2. Example calculation for determining the initial nitric acid volume and concentration fora dissolution experiment, as well as the amount of NOx that will be produced.DissolversolutionVolume (L)

Desired finalacid con(mole/L)

Moles HNO3

consumedtotal

Initial acidcon.(mole/L)

NO(g)produced(moles)

NO(g)produced(L)*

1.0 1 4.66 6.6 2.21 49.630.5 1 4.66 10.3 2.21 49.631.0 4 4.66 8.7 2.21 49.630.5 4 4.66 13.3 2.21 49.63

*Assume NOx behaves as ideal gas at 25oC and 1 atm.

In an effort to be more precise about the concentrations of off-gas species, relative amounts ofthe important nitrogen oxide gases produced by the dissolution of different amounts of LEU

Metalgrams of metalsample

grams/mole ofmetal

moles ofmetal sample

moles HNO3

consumedU 250 238.0289 1.05 4.20Ni 10 58.69 0.17 0.46

were calculated using the thermodynamic code OLI-ESP (Table 3). These calculations predictthat approximately 2.1 moles of NOx +H2O(g) will be present in the dissolver following thedissolution of 250 grams of LEU foil. These calculations agree with the generalizedstoichiometric calculations described above and provide a design basis for the off-gas treatmentcomponents of the dissolver system.

Table 3 Thermodynamic modeling results simulating the off-gas compositionfor the dissolution of uranium metal after the dissolver has cooled to atemperature of 25oC. Calculations were done in the absence of oxygen.Under oxygenated conditions NO2 rather than NO will be the dominantspecies due to the relatively fast kinetics of Reaction 3.U metal(g)

H2O(moles)

NO(moles)

NO2

(moles)HNO3

(moles)HNO2

(moles)Total(moles)

5 1.2E-03 0.0404 3.5E-05 3E-08 2.2E-04 0.0418

10 2.3E-03 0.0816 1.4E-04 2E-07 6.0E-04 0.0847

20 3.8E-03 0.1632 9.8E-04 2E-06 2.0E-03 0.17

25 4.7E-03 0.2041 1.2E-03 2E-06 2.5E-03 0.2125

50 9.4E-03 0.4083 2.4E-03 5E-06 5.0E-03 0.4252

75 0.0141 0.6124 3.7E-03 7E-06 7.6E-03 0.6377

100 0.0188 0.8164 4.9E-03 9E-06 0.0101 0.8502

125 0.0235 1.0205 6.1E-03 1E-05 0.0126 1.0628

150 0.0283 1.2248 7.3E-03 1E-05 0.0151 1.2755

175 0.033 1.4288 8.5E-03 2E-05 0.0176 1.488

200 0.0377 1.6329 9.8E-03 2E-05 0.0201 1.7005

225 0.0424 1.8372 0.011 2E-05 0.0227 1.9132

250 0.0471 2.0412 0.0122 2E-05 0.0252 2.1257

Dissolver Design Basis: Reaction Heat

The following equation was used to determine the energy released during the exothermicdissolution of the LEU foil. This calculation is conservative because it assumes adiabaticconditions.

i i

ofii

ofii

oR )reactants(Hn)products(HnΔH

Where n is the molar coefficient for each reactant and product from a balanced equationrepresenting 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 whatreaction products are formed:

U + 4HNO3 → UO2(NO3)2 + 2H2O + 2NO (4)

U + 4HNO3 → UO2++ + 2NO3

- + 2H2O + 2NO (5)

U + 8HNO3(aq) → 4H2O + UO2(NO3)2 + 6NO2(g) (6)

U + 8HNO3(aq) → 4H2O + UO2++ + 2NO3

- + 6NO2(g) (7)

The convention used in these calculations is that a negative (-) enthalpy of reaction indicates anexothermic reaction (heat is produced by dissolution).

Table 4. Enthalpy and thermal output (in Watts) for the exothermic uraniumdissolution reaction (this conservative estimate assumes adiabatic conditions and a 30-minute dissolution time).

ReactionHr

(kJ/mole)Hr

(kJ/g)250g Umetal (kJ

released)

ThermalWatts for 1 g

(J/sec.)

Thermal Wattsfor 250 g(J/sec.)

(4) -1521.9 -6.4 1598.4 3.56 888.06(5) -982.0 -4.1 1031.4 2.30 573.0(6) -1227.1 -5.2 1288.8 2.9 716.04(7) -687.2 -2.9 721.8 1.60 401.0

Thermodynamic data used is from the compilation by Genthe et al., 1992.

As the results indicate (Table 4) the dissolution of 250 grams uranium metal (~1.05 moles) thetotal energy released may be up to 1600 kJ, but will probably be closer to 1000 kJ [reaction (5) isprobably most representative]. Assuming adiabatic conditions and a 30 minute reaction time thisenergy would correspond to a maximum thermal power output of approximately 890 watts. Foran hour reaction time (again assuming adiabatic conditions) the maximum thermal output wouldbe around 445 watts. It has been noted in previous uranium metal dissolution experimentsperformed at Argonne (e.g., Jerden et al., 2010) that there can be a thermal spike duringdissolution 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 thetwo front-end processes described in this report was calculated by Charlie Allen, University ofMissouri, using ORIGEN2, Version 2.2. The assumptions used for these calculations were asfollows (see Jerden et al., 2011, for more details on actinide and fission product yields forirradiated LEU foil).

Irradiation of 1 gram of uranium foil enriched to 19.75% 235U.Power = 1.9E-3 megawatts, Burnup = 1.59E-2 megawatt days, Flux = 2.1E14 N/cm2 sec.Burnup is for 200 hours, foil composition is given for cooling times of 12, 24, 36 and 48hours.

The thermal wattage produced by decay heat of the actinides and fission products was calculatedand is shown in Figure 1. The total thermal output for 250 g of irradiated LEU is around 1000watts for a 12-hour cooling time and around 500 watts for a 48-hour cooling time. These thermalvalues have implications for the front-end processes because they must be designed to accountfor the initial elevated temperatures of the LEU foils. For example the nitric acid dissolver

system must be designed so that the acid feed component does not pressurize or leak when thehot fuel is initially immersed.

Figure 1. Cumulative decay heat for fission products andactinides for a range of LEU foil masses after 200 hourirradiation. Curves are for 12 and 48 hours of cooling (timeout of reactor).

Based on the enthalpy and decay heat calculations above the cooling system for the LEU nitricacid dissolver system must be able to sink out a maximum of 2000 watts (thermal). Therefore, ifit is assumed the dissolution if 250 g of irradiated LEU foil takes 30 minutes, 2000 watts ofthermal power will be generated and will need to be removed from the system to ensure thatwater vapor (acid) is not lost during the dissolution process. If acid is lost during dissolution, thereaction 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 thedissolver (see Section 3 for design description) and an ambient air cooling flow velocity ofaround 20 meters per second adequate heat will be removed from the dissolver to allow thereflux of the acid and a continuous dissolution reaction. Initial results from experiments that arebeing performed to confirm the results of these calculations indicate that the cooling systemworks properly and is adequate to dissipate the amount of heat generated by decay and theexothermic 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 consistof the NOx and acid gas species shown in Table 3 (Jerden et al., 2011). The ORIGINcalculations indicate that the other off gases produced during dissolution are fission productgases. The amount of the major fission gases are shown in Tables 5 to 8 (see Jerden et al., 2011for more detail on calculations).

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

10 50 90 130 170 210 250 290

De

ca

yH

ea

t(W

att

sth

erm

al)

Mass of LEU foil irradiated for 200 hours (grams)

Fission Products 12hr

Fission Products 48hr

Actinides 12hr

Actinides 48hr

Table 5 Iodine yields in Ci per 250 grams of irradiated LEU (200hour irradiation at a flux of 2.1E14 N/cm2 sec). Yields after fivecooling times are shown.Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr)I-135 2.4E+04 6.9E+03 2.0E+03 5.5E+02 1.6E+02I-134 2.9E+04 9.1E+00 7.4E-04 5.6E-08 4.3E-12I-133 2.6E+04 1.8E+04 1.2E+04 8.0E+03 5.4E+03I-132 1.4E+04 1.3E+04 1.2E+04 1.0E+04 9.3E+03I-131 5.6E+03 5.5E+03 5.3E+03 5.0E+03 4.9E+03I-130 8.9E+00 4.5E+00 2.4E+00 1.2E+00 6.0E-01I-129 2.3E-06 2.3E-06 2.3E-06 2.3E-06 2.3E-06I-128 2.5E-01 5.3E-10 1.1E-18 2.4E-27 0.0E+00Total (Ci) 9.8E+04 4.2E+04 3.1E+04 2.4E+04 2.0E+04

Table 6 Iodine yields in moles per 250 grams of irradiatedLEU (200 hour irradiation at a flux of 2.1E14 N/cm2 sec).Yields after five cooling times are shown.

Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr)

I-135 5.1E-05 1.5E-05 4.1E-06 1.2E-06 3.3E-07I-134 8.1E-06 2.5E-09 2.0E-13 1.5E-17 1.2E-21I-133 1.8E-04 1.2E-04 7.9E-05 5.4E-05 3.6E-05I-132 1.0E-05 9.4E-06 8.4E-06 7.6E-06 6.8E-06I-131 3.5E-04 3.4E-04 3.3E-04 3.1E-04 3.0E-04I-130 3.5E-08 1.8E-08 9.1E-09 4.6E-09 2.4E-09I-129 9.8E-05 1.0E-04 1.0E-04 1.0E-04 1.0E-04I-128 3.3E-11 7.0E-20 1.5E-28 3.1E-37 0.0E+00

Total 6.9E-04 5.8E-04 5.2E-04 4.8E-04 4.4E-04

Table 7 Fission gas yields in Ci per 250 grams of irradiated LEU (200hour irradiation at a flux of 2.1E14 N/cm2 sec). Yields after five coolingtimes are shown.Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr)H-3 5.6E-02 5.6E-02 5.6E-02 5.6E-02 5.6E-02Kr-88 1.4E+04 7.4E+02 3.9E+01 2.1E+00 1.1E-01Kr-87 9.8E+03 1.4E+01 2.0E-02 3.0E-05 4.3E-08Kr-85M 4.9E+03 7.6E+02 1.2E+02 1.9E+01 2.9E+00Kr-85 1.5E+00 1.6E+00 1.6E+00 1.6E+00 1.6E+00Kr-83M 2.0E+03 2.1E+02 8.0E+00 2.6E-01 8.3E-03Kr-81 8.1E-13 8.1E-13 8.1E-13 8.1E-13 8.1E-13Total Kr 3.0E+04 1.7E+03 1.7E+02 2.3E+01 4.6E+00Xe-138 2.4E+04 1.2E-11 6.1E-27 0.0E+00 0.0E+00Xe-135M 4.3E+03 1.1E+03 3.1E+02 8.9E+01 2.5E+01Xe-135 9.1E+02 7.9E+03 5.3E+03 2.8E+03 1.3E+03Xe-133M 6.9E+02 6.8E+02 6.4E+02 5.9E+02 5.3E+02Xe-133 1.6E+04 1.6E+04 1.6E+04 1.6E+04 1.5E+04Xe-131M 1.4E+01 1.6E+01 1.8E+01 1.8E+01 1.9E+01Xe-129M 1.2E-05 1.2E-05 1.1E-05 1.1E-05 1.0E-05Total Xe 4.6E+04 2.6E+04 2.2E+04 2.0E+04 1.7E+04

Total 7.6E+04 2.8E+04 2.3E+04 2.0E+04 1.7E+04

Table 8 Fission gas yields in moles per 250 grams of irradiated LEU (200hour irradiation at a flux of 2.1E14 N/cm2 sec). Yields after five coolingtimes are shown.Isotope 0 (hr) 12 (hr) 24 (hr) 36 (hr) 48 (hr)H-3 1.9E-06 1.9E-06 1.9E-06 1.9E-06 1.9E-06Kr-88 2.1E+00 1.2E-01 6.1E-03 3.3E-04 1.8E-05Kr-87 1.4E+00 2.1E-03 3.0E-06 4.3E-09 6.3E-12Kr-85M 1.4E-01 2.3E-02 3.4E-03 5.4E-04 8.4E-05Kr-85 2.8E-05 2.8E-05 2.8E-05 2.8E-05 2.8E-05Kr-83M 5.9E-03 6.0E-04 2.3E-05 7.6E-07 2.4E-08Kr-81 1.2E-18 1.2E-18 1.2E-18 1.2E-18 1.2E-18Total Kr 3.6E+00 1.4E-01 9.6E-03 8.9E-04 1.3E-04Xe-138 1.9E+00 9.4E-16 4.8E-31 0.0E+00 0.0E+00Xe-135M 1.0E-01 2.5E-02 7.3E-03 2.0E-03 5.8E-04Xe-135 2.3E-02 2.0E-01 1.3E-01 6.8E-02 3.1E-02Xe-133M 7.1E-03 7.0E-03 6.6E-03 6.0E-03 5.5E-03Xe-133 1.3E-01 1.3E-01 1.3E-01 1.3E-01 1.2E-01Xe-131M 1.1E-04 1.2E-04 1.2E-04 1.4E-04 1.4E-04Xe-129M 1.3E-10 1.3E-10 1.2E-10 1.2E-10 1.1E-10Total Xe 2.1E+00 3.6E-01 2.6E-01 2.0E-01 1.6E-01Total 5.8E+00 5.0E-01 2.7E-01 2.0E-01 1.6E-01

3. Dissolver Design and Performance Tests

Dissolver Design Overview

The nitric-acid process involves dissolving LEU foil and the fission-recoil barrier in a non-pressurized, steel dissolver at elevated temperature. The dissolver is designed to operate atpressures less than 2 atmospheres (absolute) and at temperatures less than 125oC. A flowdiagram of the component sections of the nitric acid dissolver and Mo-99 extraction process isshown 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 isopen to the off-gas reservoir during the dissolution process. The volume of the reservoir waschosen to provide passive containment of all gas reaction products at a pressure less than 2atmospheres (absolute). The reservoir is designed to contain reaction-product gases, at lowpressure, during both normal and off-normal (loss of cooling during reaction) conditions. Inorder to keep the temperature of the gas within the reservoir to below 100oC during a potentialloss 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 steelmesh basket into the dissolver vessel and then sealing the vessel with a metal cap) (Figure 4).Pre-heated acid (~100oC) is then added to the vessel using a two chamber acid feed system thatis designed to avoid pressurization of the acid bottle in the event that the dissolution reactionbegins instantaneously when the acid addition step is started.

The dissolver vessel is cooled by forced air blown from the base of the unit. The temperature ofthe dissolver solution is monitored by a thermocouple. The dissolver vessel is insulated so thatthe top of the vessel is cooled continuously during the reaction. Heat loss from the top of thevessel is optimized by the presence of steel cooling fins attached to the condenser part of thedissolver system (shown and discussed in Section 3 below). This design causes the water vaporto condense along the walls at the top of the vessel during the dissolution reaction (as acid isboiling). This process is shown schematically in Figure 2.

Following dissolution of the LEU foil, any iodine remaining in the product will be volatilizedusing a standard chemical technique (e.g. adding hydrogen peroxide at elevated temperature: seeCathers et al., 1975). The product solution is then drained from the base of the vessel and runthrough a TiO2 Mo-recovery column to recover the molybdenum. The product solution from thedissolver 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 tooptimize the design are discussed in more detail below.

Figure 2. Schematic flow diagram showing components of the LEU nitric acid dissolver systemand the Mo-99 extraction column.

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

Figure 4. Conceptual drawing of the nitric-acid dissolver system showing the steps involved instarting 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 withinthe dissolver system at a maximum temperature of 125oC and 2 atmospheres (absolute) underboth normal and off-normal conditions. A key technical challenge for the dissolver systemdesign was making sure that water vapor and acid gases did not escape from dissolver vesselcondenser 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 coolingsystem (located at the base of the dissolver vessel) and a condenser section that is attached to thetop dissolver vessel (Figure 3 and 4). The condenser section consists of cooling fins, throughwhich the cooling air is passed. A schematic diagram of the condensation process and actualpictures of the dissolver are shown in Figures 5 to 9 below. The cooling fins keep the outer wallof the top of the dissolver cool relative to the base. This causes water vapor to condense and runback down into the dissolver vessel. Nitrogen oxide and acid gases such as NO2 and HNO3 willdissolve in the condensed water ensuring that there is enough acid to complete the dissolutionreaction.

To test this process only the condenser section and reaction vessel are being used in our initialheat flow experiments. Figures 5 and 6 show the experimental set up schematically. Figures 4to 9 show photographs of the experimental set up in the laboratory.

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

Figure 6. Schematic diagram showing the key features involved in the performance testing ofthe 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 andquantify the performance of the condenser section of the dissolver system. An array ofthermocouples is being used in the initial testing to measure all relevant thermal gradients duringthe 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 irradiateduranium foil from relevant targets are planned for FY 2012.

The two chamber acid feed system was also tested as part of the “cold” tests. The laboratory setup for the acid feed system is shown in Figure 10 and observations from the tests are described inSection 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 ofthe condenser section and the condensing vapor to liquid on the inside of the vessel. Heatingcoils attached to the base of the dissolver vessel (e.g. Figures 6, 7, 8) are used to simulate theexothermic heat from the LEU dissolution reaction as well as the decay heat from the irradiatedfoil (discussed below).

• A measured amount of water or nitric acid (< 1 liter at ambient temperature) is poured into thebottom of the vessel. The dissolver is sealed. At this point the heaters are off and vessel is atambient temperature.

• The blower is turned on and temperatures and air flow velocity are measured. Thetemperatures are allowed to stabilize before initiating the next step.

• 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 (< 125oC) and the electricalpower supplied to the heaters is constant] temperatures within and outside the dissolver arecontinuously monitored using a computer data-logger. The cooling-air flow velocity and theelectrical power to the heaters are measured.

• The experiment is run at this steady-state condition for an hour (or less). The vent tube isconstantly monitored to see if vapor is escaping from the condenser section. If the temperatureinside the vessel increases above 125oC (indicating that all of the liquid has been vaporized) theexperiment is terminated immediately.

• After approximately 1 hour of running the experiment at steady state, the heaters are turned offand 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/volumemeasured to determine how much vapor was lost (vented into the fume hood) during theexperiment.

“Hot” tests – dissolution of uranium +/- nickel at ~1 atm:

Purpose: Measure the dissolution rate of metal foils in nitric acid (at ambient pressure) for anumber 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 aswell as the efficiency of the sodium hydroxide NOx trap.

• Up to 260 grams of uranium +/- nickel metal is loaded into the basket that will be lowered intothe acid at the base of the dissolver vessel (Figure 4).

• A measured amount (< 1 liter) of ambient temperature nitric acid (< 14 molar) fed into thebottom of the vessel and the metal foil samples are immersed. The dissolver vessel is sealed. Atthis point the heaters off and vessel is at ambient temperature.

• The blower is turned on and temperatures and air flow velocity are measured. Thetemperatures are allowed to stabilize before initiating the next step.

• The heaters are turned on at low power and the transient temperature increase is measured.

• The temperature of the acid will be measured continuously during the exothermic dissolutionreaction. If the temperature increases above 125oC (indicating loss of solution) the experimentwill 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. Thetemperature within the dissolver vessel will be monitored continuously until it reaches ambienttemperature.

• 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 extractiontests.

• The cap is removed from the dissolver so that the interior can be visually inspected for anyundissolved metal. If metal is still present the dissolution procedure will be repeated until all ofthe 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 thecondenser section and the heating coils on the dissolver reaction section. On the right is anotherpicture of the dissolver showing the dissolver reaction vessel without heating coils.

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

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

Figure 10. Two chamber acid feed component for the dissolver. This design ensures that acid isnot lost to instantaneous vaporization when introduced to the thermally (from decay heat) LEUfoil 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 currentlyunderway. Both the cooling air blower and the heating coils used in the cooling performanceexperiments have been wired and successfully tested. The thermocouples have been monitoredwhile the heating coils and blower were on and the initial observations suggest that the coolingair flow and insulation of the dissolver vessel will be sufficient for the condenser section of thedissolver to work properly. However these experiments are not complete. More work is neededusing nitric acid and uranium metal to “map” out the thermal gradients and thermal transientfeatures during the dissolution reaction.

As part of the “cold tests” the two chamber nitric acid feed component was tested alone (notattached to the dissolver) to make sure its valve set up worked properly. The laboratory set upfor 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). Itwas confirmed that the water had all been drained into the secondary chamber using a watchglass attached to the system. The top valve was closed and the water bottom valve (acid deliveryvalve was opened to a heated (100oC) and sealed steel vessel. Some of the water converted to

steam and reentered the secondary (bottom) chamber but quickly condensed and ran readily intothe heated vessel. The acid feed system was not pressurized (due to its volume relative to thevolume 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 efficientcondensing of water/acid vapor within the dissolver is adequate as long as the reactionvessel/heating coils are well insulated. More tests are needed to confirm and fully quantify theseobservations.

• The acid feed components have been successfully tested and appear to be amenable for remoteoperation with a hot cell facility. Planed tests within a hot cell mock-up facility are planned toconfirm this later conclusion.

3.6 Design and Performance Testing of Off-gas Traps

3.6.1 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 anefficient 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 forthe nitric acid dissolver system being designed at Argonne.

It has been shown that iodine fission gas produced by LEU dissolution can be selectivelyremoved 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 trapthe amount of iodine produced during the LEU foil dissolution process. As The maximumamount of I2(g) that will be processed in the iodine trap is around 6.9E-4 moles for a 12 hourcooling period and 4.4E-4 moles for a 48 hour cooling period (Table 6). Therefore, the amountof copper needed for the iodine trap is less than one gram. As a conservative estimate 10 gramsof copper metal (as a higher surface area mesh or beads) is assumed for the iodine trap per 250grams of irradiated LEU. The iodine can be recovered by either reducing the CuI back to Cumetal using hydrogen gas or oxidizing the CuI to release Cu++ and 2I- in solution. The recoveryof iodine from this type of trap needs is poorly understood and will be a potential focus of futurework.

The redox chemistry that makes the sequestration and recovery of iodine using copper metalpossible is summarized in Figure 11. The redox speciation shown in Figure 11 is illustrated asEh-pH or Pourbaix diagrams, where Eh is defined as the oxidation/reduction potential (in volts)

of an aqueous solution, relative to the standard hydrogen electrode. The figure shows that it isthermodynamically 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 (thedesired reaction product) spans from around 0.5 to 0 Volts at pH values less than a pH ofapproximately 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 trapbecause the formation of copper oxides could limit its efficiency. This should not be a problemfor the LEU nitric acid dissolver system because the off-gas that will be passed through theiodine trap will contain acid gases dissolved in water vapor (pH of condensate in iodine trapshould be from -1 to 1.

The redox speciation diagrams (Figures 11) were calculated using the thermodynamic code “TheGeochemist’s Workbench®” Release 8.0 (GWB) using an adapted version of the thermodynamicdatabase “thermo.com.V8.R6.full” (Wolery and Daveler, 1992).

Figure 11. Eh - pH plots showing the redox chemistry of copper and iodine in aqueous solutionover a range of pH. The top two diagrams show the stable oxidation states of copper and iodineseparately (10-3 molar dissolver Cu or I). The bottom diagrams show the oxidation states forcopper and iodine in a combined Cu-I-aqueous system (10-3 molar for Cu and I).

–1 0 1 2 3 4 5 6 7 8 9 10–.6

–.4

–.2

0

.2

.4

.6

.8

1

1.2

pH

Eh

(volts)

I-

I3-

IO3-

HIO3(aq)

I2

25°C

Dia

gram

I- ,T=

25°C

,P=

1.01

3bar

s,a

[mai

n]=

10–

3 ,a[H

2O]

=1;

Sup

pres

sed:I

2(aq

)

–1 0 1 2 3 4 5 6 7 8 9 10–.6

–.4

–.2

0

.2

.4

.6

.8

1

1.2

pH

Eh

(volts) I

-I3-

IO3-

HIO3(aq)

CuI

I2

25°C

Dia

gram

I- ,T=

25°C

,P=

1.01

3bar

s,a

[mai

n]=

10–

3 ,a[H

2O]

=1,

a[C

u++]

=10

–3 ;S

uppr

esse

d:I 2

(aq)

–1 0 1 2 3 4 5 6 7 8 9 10–.6

–.4

–.2

0

.2

.4

.6

.8

1

1.2

pH

Eh

(volts)

Cu++

Cu

Cuprite

Tenorite

25°C

Dia

gram

Cu+

+,T

=25

°C,P

=1.

013

bars

,a[m

ain]

=10

–3 ,a

[H2O

]=

1

–1 0 1 2 3 4 5 6 7 8 9 10–.6

–.4

–.2

0

.2

.4

.6

.8

1

1.2

pH

Eh

(volts)

Cu++

CuI

Cu

Cuprite

Tenorite

25°C

Dia

gram

Cu+

+,T

=25

°C,P

=1.

013

bars

,a[m

ain]

=10

–3 ,a

[H2O

]=

1,a

[I- ]

=10

–3 ;S

uppr

esse

d:I 2

(aq)

(CuO)

(Cu2O)

(CuO)

Reduction of I2 inpresence of Cu metal

4 Iodine Trap: CuI Feasibility Tests

Scoping tests were performed investigating the kinetics of the sequestration of iodine gas bycopper metal. The tests involved contacting approximately 1mm diameter copper beads withiodine gas (sublimated from pure I2 crystals) at three temperatures (25oC, 70oC, and 150oC). Onemilliliter 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 vesselscontaining copper beads and iodine were placed in ovens at three different temperatures for timeperiods ranging from 10 minutes to 1 hour.

Microscopic examination of the reaction products (see Figure 12) indicate that the copper beadsare readily converted to CuI (white/gray material) in the presence of I2(g) as long as thetemperature is high enough (so that the iodine does not condense to form a solid on the sides ofthe vessel (or within tubing) and enough reaction time is given. Some copper oxide (probablyCu2O) appears to have formed as well (Figure 12). Further chemical analyses of the reactionproducts 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 70oCsample showed signs of copper corrosion (CuI formation) after approximately 20 minutes (solidsfrom 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 approximately10 minutes (or less).

Our results generally agree with Metalidi et al., 2009 in terms of the general kinetics of thereaction; however, a more accurate surface area measurement of the copper beads used in ourexperiments is needed before our results can be used to quantify iodine sequestration by coppermetal.

Figure 12. Light microscope pictures (same illumination level for all photos) of copper beadsand 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 andtrapping the iodide in a solution form. The following general reaction indicates the basicapproach to the iodine extraction processes; however, the kinetics and optimal conditions for thisextraction have not yet been quantified.

CuI + H+ + 0.25O2(aq) = I- + 0.5 H2O + Cu2+ (9)

Nitrogen Oxide and Acid Gas Trap

The conversion of NOx to sodium nitrite and nitrate can be described by the following generalreactions:

NO2(g) + NaOH + 0.25O2(aq) ↔ NaNO3 + 0.5H2O (10)

NO2(g) + NaOH ↔ NaNO2 + 0.5H2O + 0.25O2(aq) (11)

which combine to give:

NO2(g) + NaOH ↔ 0.5NaNO2 + 0.5NaNO3 + 0.5H2O (12)

Assuming that 2.21 moles of nitrogen oxide gas need to be neutralized (Table 3), 2.21 moles ofNaOH will be needed. This corresponds to approximately 90 grams of NaOH. This willproduce approximately 75 grams of NaNO2 + 85 grams of NaNO3 as waste. Based on thesecalculations, the NOx trap will consist of a 500mL, 5 molar NaOH solution in a 1L flask orbeaker with an “in-line” hose connected to the dissolver vent and “out-line” hose that vents to theto the off-gas reservoir.

Summary and Future Work

The key design criteria were addressed experimentally to optimize the components of the LEU-foil nitric-acid dissolver. Results from ongoing and future tests will be used to finalize thedesign and fabricate all parts in preparation for a full scale demonstration. The design criteriathat have been investigated by ongoing experiments are as follows:

Preliminary “shakedown” tests of the dissolver vessel, condenser section, and cooling airblower suggest that all water vapor, reaction products and fission gases will be containedwithin 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 designis capable of delivering nitric acid to thermally hot LEU foils (hot due to decay heat) withoutlosing acid due to sudden boiling. The acid delivery component is also designed for remoteoperation in a hot cell facility.

Preliminary feasibility tests show that the copper metal trap for iodine sequestration andrecovery has promise. The NaOH NOx trap is a proven technology; however, we continue towork on dissolver system designs that most efficiently incorporate the NOx trap into theoverall design.

Future work on this project will involve the following:

Continuation of the heat-flow testing of the dissolver cooling components. Ongoing testswill “map” out the thermal gradients both inside and outside the dissolver so that thecondenser section design can be optimized.

Dissolution experiments on both non-irradiated and irradiated uranium foils will beperformed to test the cooling system/condenser performance in the presence of differentamounts of uranium. This will allow us to quantify how the exothermic heat output from thedissolving uranium foil affects the cooling system performance. These tests will also allowtesting of the gas-traps and off-gas reservoir.

All components will be tested in a manipulator mock-up facility to ensure that the dissolversystem 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 Patent3,914,388, October, 21 1975

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 forConversion of Current HEU-Based Alkaline Processes to LEU Foil Targets: Volumes andCompositions of All Waste, Product, and Off-Gas Streams from Both Front-End Options."Argonne National Laboratory (2011). Available at:http://www.iaea.org/OurWork/ST/NE/NEFW/Technical_Areas/RRS/mo99-production-iwg.html

J. Jerden Jr., S. Chemerisov, A. Hebden, S. Wiedmeyer, G. Vandegrift, 2010, Development of aProduction-Scale Dissolver for Nitric-Acid Dissolution of LEU Foils, Conference Paper for theReduced Enrichment for Research and Test Reactors Meeting in Lisbon, Portugal, Oct. 10-14,2010

Wolery, T.J. and Daveler, S.A., 1992, EQ6, A computer program for reaction path modeling ofaqueous geochemical systems: Theoretical manual, user’s guide, and related documentation(Version 7.0): Lawrence Livermore National Laboratory Report UCRL-MA- 110662 PT IV