Cw-122300-Conf-001 Unrestricted a Small

CW-122300-CONF-001 UNRESTRICTED

A SMALL CLOSED-CYCLE COMBINED ELECTROLYSIS AND CATALYTIC EXCHANGE TEST SYSTEM FOR WATER DETRITIATION

H. Boniface*a, S. Suppiaha, K. Krishnaswamya, L. Rodrigoa, J. Robinsonb and P. Kwonb

aAtomic Energy of Canada Limited, Chalk River Laboratories, Chalk River, Ontario, Canada, K0J 1J0 bTyne Engineering Inc., Oakville, Ontario, Canada, L6L 6L4

AECL has been actively involved in exploring been made1, showing that very high detritiation factors advanced electrolysis technologies for its Combined are achievable. That demonstration highlighted that all Electrolysis and Catalytic Exchange (CECE) technology three major components—recombiner, Liquid Phase for water detritiation. A small-scale CECE system (mini- Catalyst Exchange (LPCE) column and electrolysis cell CECE) has been built and operated at AECL to explore (e-cell)—were able to perform at design conditions in a its operation as a closed-cycle system with a proton- high-tritium process for a reasonable length of time. That exchange membrane (PEM) type electrolysis cell. A CECE demonstration used an e-cell which was a unique similar mini-CECE system suitable for service with design, operated at low pressure, had caustic electrolyte tritium concentrations up to 1000 Ci/kg(water) has been and used asbestos in the separator. assembled, in collaboration with Tyne Engineering, for With the emergence of improved e-cell technologies, installation in a glovebox in AECL’s Tritium Facility. AECL has put in place a program to study the features of These systems were developed as test-beds for membranes each and the performance in high-tritium service. Initial that had been selected for their expected tritium work concentrated on caustic cells that used various resistance. The systems allowed the measurement of polymeric materials and ran at elevated pressure. More membrane performance over long periods at very high recently, we have begun to focus on e-cells using proton tritium concentrations, as well as the ability to monitor exchange membranes (PEM) because of their promise of any effects of membrane degradation products on the lower water inventories and benign chemistry. performance of exchange and recombiner catalysts. Preliminary work by us and others suggests that the Preliminary work has been done with Nafion-112 standard PEM material, Nafion®, is only capable of membrane samples by exposing them to gamma and beta resisting degradation effects up to very modest levels of radiation to determine their suitability for use in tritiated tritium2,3. Our program therefore is focused on finding CECE system. Doses of up to 1250 kGy of gamma or new and modified PEM materials that have tritium 200 kGy of beta were applied. Visual observations resistance significantly superior to that of standard showed that gamma irradiation at doses below 400 kGy Nafion. The program includes construction of systems to produced severe damage to the membrane. No significant expose these materials to completely realistic conditions physical damage was observed for samples exposed to and systems to fully examine the physical and chemical 200 kGy from tritiated water. However this level of effects of tritium and the actual performance of the exposure to either gamma or beta radiation was sufficient components in the CECE process. to significantly decrease membrane performance in fuel cell tests. II. E-CELLS FOR CECE DETRITIATION

I. INTRODUCTION Typical industrial e-cells are designed with the following features: All commercial nuclear power reactors produce • High electrical efficiency tritium from neutron absorption by deuterium in the core, • Low cost but CANDU® heavy water reactors are particularly prone • Hydrogen purity (electrolyte, oxygen, moisture) because of the large quantity of heavy water used as • Compactness moderator. Various technologies are available for • Simplicity and low maintenance removing tritium from water and AECL has been • Safety (hydrogen, oxygen and pressure) pursuing Combined Electrolysis and Catalytic Exchange When considering commercially available e-cells for (CECE) for this purpose over the past decades. At use with tritium, the above features are not necessarily as AECL’s Chalk River Laboratories a significant-scale important and the following become the major design demonstration of heavy water detritiation by CECE has features: CW-122300-CONF-001 UNRESTRICTED

• Safety (tritium, hydrogen, oxygen and pressure) warm gas was returned to the LPCE through a small • Compatibility with tritium metal-bellows compressor (Senior MB158). Oxygen was • Leak-tightness generated in the flooded anode compartment of the e-cell • Low inventory of water and flowed out into a gas-liquid separator and then to the • Low maintenance and ease of maintenance recombiner inlet. The water was recycled to the bottom of • Hydrogen purity (electrolyte, catalyst poisons) the anode compartment, driven by the generation of Current caustic-based e-cells, such as the IMET® cell oxygen gas in the e-cell—effectively forming a “gas-lift” from Hydrogenics, are relatively well suited to these pump. A portion of this circulating water was drawn off requirements. The main drawbacks of this technology are before it entered the e-cell and pumped (Cole-Parmer RK- the presence of caustic that will damage LPCE catalyst 07002-25magnetically coupled gear pump) to the top of and the relatively large inventory of water. The other the LPCE column. It passed down the column over the required features for tritium compatibility can all be catalyst, allowing isotope exchange to occur at conditions addressed, though this adds to the cost. In particular, we approximately optimal for this catalyst. At the bottom of have found that typical materials such as EPDM, the LPCE, the water flowed out to join the main cell polysulfone and IMET membranes meet the requirements anode water circulation loop. for tritium-compatibility. Water Argon PEM cell technology looks attractive as an alternative SP H2 to the caustic-based technology for two reasons: 1) Low P A water inventory and 2) complete avoidance of caustic Oxygen issues. The overriding issue for PEM e-cells is the SP T stability of the membrane material. Legend LPCE SP - Sample Point A - Analyzer T F - Flow III. MINI-CECE DESIGN AND OPERATION I - Current Separ- L - Liquid level Recombiner ator P - Pressure The most effective way to determine the stability and SP L T - Temperature performance of e-cell membranes is to construct a V - Voltage

complete CECE system and operate it at relevant Water Water conditions. Because of the complexity of building SP facilities to operate with high tritium, we began by building an equivalent system to be operated without F F SP T tritium. This system (mini-CECE #1) was designed and Pump Compressor

Oxygen+water built and operating experience was gained before Hydrogen embarking on the design and construction of the tritiated I V system (mini-CECE #2). E-cell

III.A. Mini-CECE #1 System Description Fig. 1: Flow Diagram of mini-CECE #1

The simplified system flow diagram for mini-CECE The mini-CECE was specifically designed to allow #1 is given in Figure 1. It was designed as a completely the performance of the three main components to be closed system, capable of operating unattended for long reliably measured over a long period of closed-cycle periods, but allowing continuous monitoring of the main operation. To do this, the system needed to be controlled system performance parameters. The e-cell was a GES at constant conditions that were accurately measured and model G2-300 cell with Nafion 110 membrane running at recorded (Keithley model 2700). With appropriate choice 8.6 amps (60 mL/min hydrogen). The system was set up of conditions, regular deuterium analyses (at the sample with an inventory of water containing about 30% D2O, points shown) to determine the deuterium distribution that enabled easy monitoring of the separation of D/H in through the system were used to follow the performances various parts of the system. of the e-cell, LPCE and recombiner. Hydrogen from the e-cell was added to a circulating stream of argon that flowed up through an LPCE column III.B. Mini-CECE #1 Performance Analysis with a small quantity of AECL’s Type 86-93 wetproofed exchange catalyst. At the outlet of this column, the Performance of the e-cell was measured by two oxygen from the e-cell was added to the argon and this parameters: cell voltage at constant current and e-cell mixture fed to the top of a recombiner vessel containing isotope separation factor. Over the course of about six AECL’s Type 99-11 wetproofed recombiner catalyst. months of operation, the e-cell voltage remained at 1.69 V With hydrogen maintained at slight excess, all the oxygen and the cell D/H separation factor was approximately 3.5 was converted to water vapor in the recombiner and the (close to the hydrogen-water equilibrium at e-cell CW-122300-CONF-001 UNRESTRICTED temperature). The constant long-term voltage suggests from that in the LPCE. This was achieved by setting that at these conditions (low current density), this e-cell the temperature of the LPCE at 10°C and allowing has not shown any change in internal resistance (as the e-cell to operate at its typical self-heating expected). temperature of about 40°C. Performance of the recombiner was determined by the oxygen analyses and the temperature profile in the IV. DESIGN OF TRITIUM-COMPATIBLE MINI- catalyst bed. Over the course of about 6 months operation, CECE neither of these showed any changes Performance of the LPCE catalyst required the most The design of mini-CECE #2 was based very closely difficult measurement and analysis. The experimental on the experience gained in building and operating design was aimed at creating different gas-liquid D/H mini-CECE #1, along with a good understanding of equilibria between the e-cell and the LPCE and operating tritium system safety. The goal was a system able to the LPCE with a very large liquid/gas flow ratio (approx. reliably contain very high tritium concentrations for long 100). This would ensure that the water stays at the same periods. In this second CECE system, isotope separation isotope concentration, but the gas in the LPCE would performance will be measured by the separation of tritium always have an isotope concentration change from bottom from deuterium. A significant advantage of this is that it to top. As long as the amount of catalyst was not too makes in-line measurement feasible (using ion chambers). great, the gas would not reach equilibrium and the Mini-CECE #2 was designed and built by Tyne approach to equilibrium would be a sensitive test of the Engineering Inc. The system, which is currently being catalyst performance. commissioned, is larger and more complex than To date, the focus has remained on determining the mini-CECE #1 (2.7m long, 1.2m high and 0.76m deep), important design features required for mini-CECE #2. but still small enough to be enclosed in an inert Indeed, the experience gave us many useful guidelines for atmosphere (argon) glove box that is kept free of tritium the design of mini-CECE #2. The observations were as with its own atmosphere clean-up system. The extra follows: features added to the CECE process included a small • The e-cell required a minimum flow of water through system to convert tritium gas to tritiated water up to the the anode compartment of about 10 mL/min. The required concentration (1000 Ci/kg) and equipment to gas-lift pump was arranged to work well and safely transfer tritiated water around the system. eliminated the need for another gear pump. Equipment, valves, materials and fittings were • Mixing of all the liquid in the system was critical to carefully chosen and manufactured to ensure precise achieving a steady-state isotope concentration profile control of operating conditions, while ensuring a helium in a reasonable time. All of the liquid should be in the leak tightness of better than 1x10-8 atm-cc/sec to protect two main circulation paths. Many small operators. Process conditions and the inert atmosphere modifications were made and remain to be made to purification system will be maintained by Programmable eliminate or minimize isolated liquid volumes. Logic Controllers (PLC) that will enable the system to • Although low water inventory is desirable in tritium operate continuously with minimal attention over systems, reduced water inventory increased the extended periods. The PLCs include an automated trip relative volume of isolated pockets of liquid. system that monitors escaped tritium and hydrogen. A • Condensation of vapor created many problems, such computer-based Human-Machine Interface (HMI) as plugging instrument lines, damaging the bellows program written by Tyne will enable the operator to set pump and acting as isolated volumes. In particular, process conditions and will record all information the cathode compartment of the e-cell was mostly retrieved from the CECE instrumentation. filled with condensate and acted as an isolated liquid The mini-CECE #2 will be put into service in the volume. Heat tracing was required in certain areas to near future initially with 1000 Ci/kg tritiated water and eliminate condensation. Nafion 110 cell membrane. As soon as candidate • Removal of samples changed the inventory of water membrane materials are available, the Nafion will be and gas. These amounts were replaced periodically replaced and long-term performance of each CECE and the analysis techniques were adjusted to component will be monitored. minimize this, but in-line analysis was seen as a requirement for mini-CECE #2 to eliminate the V. CURRENT AND FUTURE MATERIALS difficulties. • The e-cell produced hydrogen with a deuterium Some baseline characterization of PEM cell content in thermodynamic equilibrium with the membrane materials has been done at CRL using mainly system water inventory. Developing a concentration Nafion 112. The intention has been to develop an driving force in the LPCE required conditions at understanding of the physical and chemical changes and which the separation factor in the e-cell was different performance effects for a typical PEM material after CW-122300-CONF-001 UNRESTRICTED doses of gamma radiation as well as beta radiation from Table 1: Loss of Fluoride and Sulfate from Nafion tritium. Dose Fluoride loss Sulfate loss Nafion samples placed in water with hydrogen or mg/g* mg/g* oxygen purge gas were irradiated in a 60Co Gammacell. 200 kGy (beta) 4.0 2.0 The doses to the samples ranged from 140 to 1250 kGy. 140 kGy (gamma) 2.6 1.4 The physical integrity was characterized by visual ~900 kGy (gamma) 13 8 inspection before and after irradiation. When possible, 1250 kGy (gamma) 19 15 irradiated membrane samples were used to prepare *mg of anion per g of Nafion membrane-electrode assemblies (MEA) and tested in a single fuel cell to determine any change in the proton The exposed samples were decontaminated and made exchange capacity of the membrane due to irradiation. into MEAs for testing in a fuel cell. The loss in proton Total organic carbon (TOC), fluoride, sulfate and sulfur conducting performance was 20-25% for 200 kGy dose— analyses of the water samples from the irradiation vessel showing somewhat less effect than was observed for were done to determine impurities originating from gamma irradiation. Nafion due to irradiation. Beds of isotope exchange With the aim of developing a PEM cell that can catalyst were exposed to the purge gases from the operate for long periods on high concentrations of tritium, irradiation vessel to investigate the potential deactivation most of the focus in future will be on new membranes, not characteristics of irradiation products. (It should be noted necessarily based on sulfonated fluorocarbons. A that AECL’s LPCE and recombiner catalysts have potential candidate that is currently being studied is previously been shown to retain their activities on modified polysulfone, the base polymer being well known exposure to very high doses of gamma and beta for its excellent resistance to radiation degradation. radiation—in excess of 2 MGy. Thus, in these and the planned mini-CECE testing, any loss in activity is VI. CONCLUSION assumed to be a consequence of chemical reactions with breakdown products in the exposed cell materials.) A well-designed and functionally simple system has Visual observations showed that gamma irradiation at been developed at CRL to allow long-term testing of the doses below 400 kGy (equivalent to four years of performance of CECE components in the presence of high -1 exposure to tritiated water at 180 Ci·kg ) produced severe concentrations of tritium using PEM electrolysis damage to the membrane. Increasing the radiation dose technology. Current PEM materials lose their proton increased the damage to the membrane. Since the conduction and physical stability on exposure to high membrane samples were very fragile, only the samples doses of radiation and new, more resistant materials are that received doses less than 400 kGy could be formed being sought. These will be tested in the new CECE with into MEAs for fuel cell testing. These irradiated very high concentrations of tritium. If successful, caustic- membranes had lost 29 to 46% of their proton exchange based cell technology can be avoided and the CECE capacity due to irradiation, which will undoubtedly have a process for any tritium application will be able to be significant effect on the performance of the e-cell. designed with reduced tritium inventory. Significant concentrations of TOC, fluoride and sulfate originating from the membrane were detected in the REFERENCES water, confirming the detrimental effects of radiation on the membrane. Exposure of these irradiation products 1. J.M. MILLER et.al., “Design and Operational with hydrogen purge stream to LPCE catalyst produced Experience with a Pilot-scale CECE Detritiation deactivation of the catalyst, but heating to 120°C with Process”, Fusion Science and Technology, 41, 1077 oxygen regenerated the catalyst almost completely. (2002). To study the effect of beta radiation, a few samples of 2. R. MICHLING et.al., “Behavior of Solid Polymer Nafion were placed in tritiated heavy water and stored for Electrolyzers in Use with Highly Tritiated Water”, over three years. In that time, they were exposed to about Fusion Science and Technology, 54, 470 (2008). 200 kGy of beta radiation. Samples in light water 3. Y. IWAI et.al., “Radiation Deterioration in followed the same treatment without tritium. No visible Mechanical Properties and Ion-exchange Capacity of changes in any of the samples were noted. However, Nafion N117 Swelling in Water”, Journal of analysis of the water for anions and organic carbon Membrane Science, 322, 249 (2008). showed similar degradation products as found in previous gamma exposures and by other groups2. The following table summarizes the measurements of released fluoride and sulfate for gamma and beta exposures: