W. J. Crooks III
Westinghouse Savannah River Company
Aiken, SC 29808

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The thermal behavior and reactivity of irradiated Reillex™ HPQ in 64% nitric acid was evaluated to address an accident scenario in which 64% nitric acid solution is inadvertently added to an HB-Line ion exchange column. The Advanced Reactive System Screening Tool was used to screen the reactivity of irradiated Reillex™ HPQ resin in nitric acid solutions, to age resin in 64% nitric acid at 50°C, and to evaluate the effects of aging on the reactivity of the resin. Irradiated Reillex™ HPQ showed similar reactivity in 64% and 35% nitric acid, with the initiation of self-heating for both reaction mixtures occurring at about 80°C. Aging tests at 50°C did not cause the runaway reaction to occur, however, a gradual increase in pressure with time was evidence that nitric acid oxidized and degraded some of the resin. Reillex™ HPQ that had been pretreated by irradiation and/or aging in nitric acid solution demonstrated lower reactivity than the unirradiated, unaged resin. These pretreatments remove or minimize the low- temperature, low- intensity exotherm that had been observed for unirradiated, unaged resin in an earlier study. For unirradiated Reillex™ HPQ and 64% nitric acid, self-heating initiated at about 50°C. In summary, this work showed that the reactivity of irradiated Reillex™ HPQ in 64% and 35% nitric acid is similar and that long-term exposure to nitric acid at 50°C did not initiate a runway reaction. However, for unirradiated resin, self-heating initiates at about 50°C due to the low- temperature exotherm.

The purpose of this work was to evaluate the reactivity of Reillex™ HPQ in 64% nitric acid.

The reason for this work is to address an accident scenario in which 64% nitric acid solution is inadvertently added to an HB-Line ion exchange column containing Reillex™ HPQ anion exchange resin (chemical structure shown below).

Reillex™ HPQ is a strong-base macroporous anion exchange resin, and is sold by Reilly Industries Incorporated. It is composed of a copolymer backbone of 1-methyl-4-vinylpyridine (70%) and a divinylbenzene mixture (30%). Approximately 63% of the amine sites are methylated, forming pyridinium cations, which function as the principal anionic exchange sites. Unquaternarized amines protonate in concentrated nitric acid solutions, also resulting in anionic exchange sites. During the polymeric resin synthesis, the "divinylbenzene mixture" reactant is actually composed of 80% divinylbenzene positional isomers and 20% ethylvinylbenzene positional isomers.

Our earlier work¹ characterized the reactivity of Reillex™ HPQ under at 8 and 12M nitric acid concentrations and in the presence of cerium(IV), as a simulant for plutonium(IV). With an imposed heating rate of 0.5°C/min, a low- temperature, low- intensity exotherm was observed for the unirradiated Reillex™ HPQ. However, the exotherm was not present in resin that had been previously exposed to 100MRad of Co-60 irradiation (a gamma source). The reactivity of model compounds that simulated subunits of the polymeric resin suggests that the low- temperature exotherm is due to the oxidation of ethylbenzene pendant groups, which form benzoic acid pendant groups. In addition, a pretreatment method was developed that eliminated the exotherm. That pretreatment method involved heating resin in 8M nitric acid solution for 45 minutes at 85°C. The chemical change associated with pretreatment was minimal, as evidenced by a plutonium loading capacity that was comparable to that of untreated resin.

The Advanced Reactive System Screening Tool (ARSST) was used to characterize the thermal effects of the runaway reactions between solid organic resins and nitric acid solutions. The ARSST is a nearly-adiabatic calorimeter that heats an open, well-insulated, 10-mL test cell within a sealed 350-mL Parr bomb. In a typical test, the Parr bomb is pressurized to minimize the endothermic process of evaporation of solvent (i.e. loss of oxidant), and the test cell mixture is stirred and heated at a constant rate. The test cell mixture and the Parr bomb are monitored for temperature and pressure, respectively. Long-term isothermal (50°C) experiments were performed to evaluate the effect of resin aging in nitric acid solution. Thermal scans were performed from ambient temperature up to the observation of the runaway reaction.

The ARSST is a modified version of the RSST. The ARSST has an improved PID control mechanism, more reliable heater design, and a Microsoft™ Windows-based operating system.

All chemicals used in this study were reagent grade and used without additional purification except where noted. Nitric acid solutions [35% (8M) and 64% (14.4M)] were prepared from dilutions of stock nitric acid (69.7%, purchased from Fisher Scientific, Inc.).

3.2 Experimental Procedures

3.2.1.1. Standard Method for Resin Conversion from Chloride Form to Nitrate Form

The resins used in this study are commercially available as the chloride salts. Typically, approximately, 25 mL of resin (chloride form) were added to a 150-mL beaker. The resin was slurried with about 50 mL of 1 molar sodium nitrate and loaded into a glass, 30-mL ion exchange column. The chloride form resin was converted to the nitrate form resin by washing with ten bed volumes of 1 molar sodium nitrate. Excess sodium nitrate was removed by washing with ten bed volumes of water. The resin was collected by filtration under house vacuum, and air-dried overnight to give free-flowing beads. The converted product was stored in a plastic screw cap bottle until use.

3.2.2.1. Charging the test cell

First, 2.5 A 0.1 grams of the dry nitrate form resin were added to a tared 14-mL ARSST test cell containing a small Teflon™ stir bar. Second, 9.9 A 0.2 grams of a nitric acid solution were added dropwise to the dry resin in the test cell.

3.2.2.2. Loading the test cell into the calorimeter

A dual-bottom heater was wrapped around the test cell and secured with a heater belt. The test cell assembly was wrapped with an aluminum foil square. The wrapped test cell was placed into the insulation jacket, and sealed into the insulation sheath, with the four heater wires exiting through the grommet. The four heater wires were connected to the corresponding wires of the heater glands, and the insulation sheath assembly was lowered into the bottom of the Parr bomb. The Parr bomb was attached to the nitrogen line and was placed on a stir plate. Stirring was confirmed by visual inspection. A thermocouple was threaded through an extension tube and secured onto the top of the test cell. Temperature and pressure sensors were connected, the Parr bomb was sealed, and charged with nitrogen.

3.2.2.3. ARSST operation

The test began with an initial imposed heating rate of 1°C/min using a heater calibration polynomial for a mixture of resin in water. This imposed heating rate is twice the rate used in the previous experimental work¹ with 8M and 12M nitric acid solutions. The current 64% (14.4M) nitric acid-resin tests were run from room temperature to 245°C with an imposed heating rate of 1°C/min under 400 psig (nitrogen gas). Preliminary testing using the RSST of resin/8M nitric acid mixtures indicated that some vapor saturation ("boiling") occurred under 300 psig but could be avoided by running the tests at 400 psig. Vapor saturation increased the endothermic stripping of the water/nitric acid solvent and interfered with observation of exotherms.

Additional experimental details that document that the mass of resin and nitric acid were essentially constant for each test are given in the Table below.

The general reaction between an anion exchange resin and nitric acid is an exothermic (energetically downhill) process and is heavily favored to proceed. Resin degradation products form and gas evolves. However, the "resistance" to reaction results from an energy shortage to break bonds (the activation energy) to initiate the reaction. Bond-breaking energy can be supplied by increasing the temperature of the reaction mixture, and exceeding the activation energy. As the exothermic reaction proceeds, heat is evolved.

With adequate venting, heat is dissipated from the system due to evaporative cooling, and the temperature is moderated. Without adequate venting, the heat evolution may increase the system temperature and accelerate the rate of chemical reaction until the reaction rate is self-accelerating and uncontrollably runs away to completion.

Addition of nitric acid solution to dry resin (which contains absorbed water) results in a heat of dilution. Before ARSST testing began, the heat evolved during dilution was evaluated. Ten grams of 64% nitric acid solution were added to 2.5 grams of irradiated resin in a well-insulated test cell at ambient temperature, about 21°C. Upon addition, the temperature quickly rose about 20°C reaching a maximum at about 40°C. No gases were observed during this addition, evidence that the heat evolved did not cause the resin to degrade.

Using the ARSST, Reillex™ HPQ resin samples were heated in 64% nitric acid solution to determine how aging and irradiation affect the thermally induced run away behavior. Isothermal aging tests were performed at 50°C, as a bounding condition. Testing at this bounding temperature increased the probability of reaction between nitric acid and resin. Although the activation energy of the resin-nitric acid reaction is not known, the probability of reaction is expected to be lower at the lower processing temperatures, which are typically less than 40°C.

1. Effect of Nitric Acid Concentration on the reaction of Irradiated Reillex™ HPQ. Irradiated Reillex™ HPQ was reacted in the ARSST with 64% nitric acid (14.4M) and 35% (8M). Self-heating is observed for both nitric acid-resin mixtures at about 80°C (Figures 1 and 2). The more concentrated nitric acid solution (64%) reached maximum temperature and pressure in 67 minutes whereas the less concentrated nitric solution (35%) reacted in 92 minutes as shown in Figure 1. While the rate of reaction is expected to increase with nitric acid concentration, the difference could also be attributed to differences of the heat capacities of the reaction mixtures. Although the time to maximum temperature differed notably, the intensity of the runaway reactions were almost indistinguishable in the thermal scans as shown in Figures 2 and 3. The temperatures at which the runaway reactions initiated did not differ significantly. The rate of pressure increase as a function of temperature in Figure 3 provides evidence that gas-evolution is coupled with the onset of self-heating. The pressure rates are a little higher for the 64% acid than the 35% acid, consistent with a slightly higher reaction rate for the more concentrated solution.
   
   
2. Aging of Irradiated Reillex™ HPQ in 64% Nitric Acid Solution. To evaluate the effect of long term exposure of resin to process conditions, three samples of irradiated Reillex™ HPQ were prepared:

1. Unaged
2. Aged 2.7 days at 50°C in 64% nitric acid at 400 psig (initial pressure)
3. Aged 7 days 50°C in 64% nitric acid at 400 psig (initial pressure)

3. Evaluation of the Effect of Aging and Irradiation on Reillex™ HPQ. The effect of aging and irradiation is evaluated in Figures 6-8. The temperature/pressure profile as a function of time (Figure 6) shows that aging and irradiation increase the time to maximum temperature and pressure. The unirradiated, unaged resin reached maximum temperature at 64 minutes whereas under the same conditions, longer times were required for the irradiated and aged resin to reach maximum temperature: 66 minutes (unaged), 77 minutes (aged 2.7 days), and 100 minutes (aged 7 days). Interestingly, the time to maximum temperature/pressure increased with the duration of treatment/aging. Although the resistance to reaction is relatively high at 50°C, the resin has more potential energy early in its lifecycle, when it is unirradiated and unaged.

1. Irradiated Reillex™ HPQ showed similar reactivity towards 64% and 35% nitric acid, with the initiation of self-heating for both reactions occurring at about 80°C.
2. No runaway reaction occurred from long term exposure of irradiated Reillex™ HPQ to 64% nitric acid at 50°C, although the observed gradual increase in pressure is evidence that some resin-nitric acid reaction is occurring.
3. Reillex™ HPQ that had been pretreated by irradiation and/or aging in nitric acid solution demonstrated lower reactivity than the unirradiated, unaged resin. These pretreatments remove or minimize the low- temperature, low- intensity exotherm that had been observed for unirradiated, unaged resin in an earlier study.
   
   

Figure 1. Thermal Scan of Irradiated Reillexu HPQ (NO₃^- form) in Nitric
Acid Solutions: Temperature and Pressure vs. Time

Figure 2. Thermal Scan of Irradiated Reillexu HPQ (NO₃^- form) in Nitric
Acid Solutions: Self-heating Rate vs. Temperature

Figure 3. Thermal Scan of Irradiated Reillexu HPQ (NO₃^- form) in Nitric Acid
Solutions: Rate of Pressure Increase vs. Temperature

Figure 4. Aging of Irradiated Reillexu HPQ (NO₃^- form) in 64% Nitric Acid
Solution: Temperature and Pressure vs. Time (2.7 days)

Figure 5. Aging of Irradiated Reillexu HPQ (NO₃^- form) in 64% Nitric Acid
Solution: Temperature and Pressure vs. Time (7 days)

Figure 6. Thermal Scan of Unaged and Aged Irradiated Reillexu HPQ (NO₃^- form) in
Nitric Acid Solutions: Temperature and Pressure vs. Time

Figure 7. Thermal Scan of Unaged and Aged Irradiated Reillexu HPQ (NO₃^- form) in 64%
Nitric Acid: Self-heating Rate (°C/min) vs. Temperature (°C)

Figure 8. Thermal Scan of Unaged and Aged Irradiated Reillexu HPQ (NO₃^- form) in
64% Nitric Acid: Pressure Rate (psi/min) vs. Temperature (°C)

Reference

1. W.J. Crooks III, W.A. Kyser III, and S.R. Walter, Qualification of Reillex™ HPQ for Use in SRS Processes, WSRC-TR-99-00317, March 10, 2000.