J. Bradley Mason, Thomas W. Oliver, Marty P. Carson, G. Mike Hill
Studsvik, Inc.
Columbia, SC

Studsvik has completed large-scale testing and has commenced construction of a commercial, Low-Level Radioactive Waste (LLRW) processing facility in Erwin, TN. The Studsvik Processing Facility (SPF) has the capability to safely and efficiently receive and process a wide variety of solid and liquid LLRW streams including: ion exchange resins (IER), charcoal, graphite, sludge, oils, solvents, and cleaning solutions with contact radiation levels of up to 100 R/hr. The licensed and heavily shielded SPF can receive and process liquid and solid LLRWs with high water and/or organic content.

The SPF will use the THermal Organic Reduction (THOR^sm) process, developed by Studsvik, which utilizes pyrolysis/steam reforming technology. THOR^sm reliably and safely processes a wide variety of LLRWs in a unique, moderate temperature, dual-stage pyrolysis/reforming, fluidized bed treatment system. The THOR^sm technology is suitable for processing hazardous, mixed and dry active LLRW (DAW) with appropriate licensing and waste feed modifications.

 Large-scale process system tests demonstrated consistent, reliable, robust operating characteristics with volume reductions up to 80:1 and weight reductions up to 85:1 when processing depleted, mixed bed, ion exchange resins with over 99% of all radionuclides in the waste feed incorporated in the final solid residue product. Final reformed residue comprises a dry, granular solid suitable for long-term storage or direct burial in a qualified container. THOR^sm effectively converts hexavalent chromium to non-hazardous trivalent chromium and can destroy nitrates with over 99 percent efficiency in a single pass. 

Studsvik Inc., with offices in Columbia, SC and Erwin, TN, is a subsidiary of Studsvik Holding AB located near Stockholm, Sweden. Since 1947 Studsvik has been actively involved as a research center for nuclear power in Sweden. Studsvik operates a research test reactor and hot cell facility for production of medical isotopes, commercial nuclear fuel testing, and materials irradiation. Studsvik operates a Dry Active Waste (DAW) incinerator which has been in commercial operation since the early 1970s. Full metal melting and recycling capabilities for carbon and stainless steels and aluminum have been in use for several years.  

Initial interest in development of an improved LLRW volume reduction process began in the early 1990's due to the difficulty European utilities have with handling and volume reducing radioactive ion exchange resins. A thorough review of available technologies was performed. The following design and performance criteria were selected: 

The process must: 

1. be easy to license and permit, therefore, incineration processes were not considered;
2. be simple and reliable to operate with no complex or high maintenance components in radiation areas;
3. be easily shielded due to high radiation levels of depleted ion exchange resins;
4. retain greater than 99 percent of all radionuclides in a stable, final waste form;
5. produce final waste residue that is inert and will not release gases (e.g. free of water and organic materials which can decompose through radiolysis or bacterial action thereby releasing hydrogen and other gases) inside the final, sealed waste package;
6. produce final waste form that is reduced in both mass and volume. A Volume Reduction (VR) goal of 15 to 50 and a weight reduction (WR) goal of 12 to 30 was desired depending upon the inorganic fraction on the resin; and
7. allow processing with segregation of one customer's waste from other customers' waste so final residues can be returned "atom-for-atom" to the waste generator. This requirement will be the only viable operating mode if burial sites close or generator chooses to store on-site until plant is decommissioned.

A test program was approved and implemented for development of such a process as discussed below.

Initial test efforts focused on evaluating methods that could be used to dry the IER, which contains 50 to 65 percent water, and decomposing the organic component of the resin (cross-linked polystyrene). A five phase test program was implemented which culminated in the decision to proceed with the licensing, design, and construction of a commercial LLRW processing facility. Testing phases one, two and four were performed in Studsvik's large research facility near Stockholm.  

The first phase of testing involved the construction of an unique, bench-scale, drop-tube pyrolysis gasifier which demonstrated the ability to pyrolyze the organic content of previously dried IER. It was found that the resulting high-carbon, metal oxide rich residue was inert and easy to handle. Additional testing with radioactive IER showed that essentially all radionuclides, including relatively volatile cesium, were retained in the final solid residue. 

For the second phase test program a much larger pilot-scale, multiple drop-tube pyrolysis gasifier was constructed. The reactor was electrically heated and had a full off-gas control system designed to condense any volatile organic components and oxidize non-condensable gases exiting the pyrolysis vessel. Testing proceeded with few problems and demonstrated throughputs of up to 15 kg/hr with dried IER feed. Significant weight reductions were obtained, however, the high content of fixed, activated carbon in the final residue yielded a waste VR of only 2 to 3. Methods were evaluated for gasifying the carbon in the final residue to increase the VR. It was also determined that a simplified IER feeding system would reduce maintenance in a subsequent commercial-scale system. The pilot-scale pyrolyzer is shown in Fig. 1. 

The third phase test program involved two process operations: first, a production-scale drop-tube gasifier with single point IER feed, and second, a bench-scale steam reforming process to gasify the carbon in the resultant pyrolyzed final residue from the drop-tube gasifier. The drop-tube gasifier was constructed and assembled complete with: loss-in weight IER feeder with pneumatic transfer/injection system, thermal oxidizer and continuous emissions monitoring system (CEMS). Moist IER was processed at throughputs of up to 100 kg/hr. The IER feed system worked as planned except for a minor seal leak at the feeder. The indirect-heated, drop-tube, pyrolysis vessel and ceramic particle filters performed as designed. Simple, reliable operation was demonstrated. Various additives were tested to adjust sulfur retention. It was subsequently determined that additives were not needed to provide desired sulfur retention. The production-scale pyrolyzer is shown in Fig. 2.

A portion of the high-carbon residue from the pyrolysis reactor was then subjected to a steam reforming process in a bench-scale, fluid bed contactor. Various operating temperatures, fluidizing gases and bed media combinations were tested with excellent success. Steam reforming was utilized to gasify essentially all of the inert, fixed, activated carbon in the final pyrolyzed residue. Through selection of steam reforming operating conditions it is possible to produce an inert, inorganic final waste that consists of only the radioactive elements, metal oxides and inorganic calcium and silica compounds initially absorbed on the IER. 

The phase four test program was performed to verify the retention of radionuclides in the final residue when subjected to a range of steam reforming process operations. Actual radioactive IER residue was contacted with superheated steam in a bench-scale, fluid bed contactor. Radioactive cesium volatility was monitored through a range of reformer operating conditions. It was demonstrated that greater than 99.3 percent of cesium and overall greater than 99.8 percent of all radioactive materials were retained in the final, inert solid residue. 

The phase five test program demonstrated high throughput processing of depleted IER utilizing fluid bed systems. Two pilot-scale, fluid bed contactors were assembled complete with slurry feed system, electrical heaters, ceramic filters, and offgas control system with CEMS, thermal oxidizer, and scrubber. Depleted IER was mixed with water to make a slurry which was injected into the main fluid bed. The main (first stage pyrolysis) fluid bed served to evaporate all water from the resin slurry and pyrolyze the organic components through destructive distillation. Fluidizing gases, volatile organic vapors, and steam released in the stage 1 fluid bed comprise a synthesis gas which passes through the ceramic filter and to the gas handling system. The high-carbon, metal oxide-rich residue removed by the ceramic filter is then processed in the second stage steam reformer. Carbon is gasified in the reformer leaving a metal oxide-rich, inorganic residue as the final waste. High sodium nitrate slurry, oils, activated carbon, antifreeze solution, steam generator cleaning solvent and IER were all successfully processed by the two stage, pilot-scale, pyrolysis/reforming, fluid bed THOR^sm process. Photographs of mixed bed IER, pyrolyzed residue and reformed residue are shown in Fig. 3. 

Fig. 3. Photos of resin, pyrolyzed residue and reformed residue 

Successful completion of the five phase test program demonstrated the following;  

1. IER/water slurry, sodium nitrate slurry and other high-water content and/or high-organic content wastes can be efficiently processed by the two stage pyrolysis/reforming process;
2. the THOR^sm fluid bed process is simple to remotely operate and requires only minimal maintenance;
3. vertical, cylindrical fluid bed gasifiers are easily shielded;
4. commercial throughputs are attained with reasonable sized hardware; and
5. volume and weight reductions approach maximum theoretical values without addition of inorganic additives.

 For average, depleted, mixed-bed resin it is possible to achieve an VR of 15-80 with a corresponding WR of 12-85. Scaleup factors for a commercial facility were confirmed.

Based upon the success of the test programs, Studsvik obtained a radioactive materials license from the State of Tennessee for an LLRW and IER processing facility. Detailed design is nearing completion and construction of the commercial-scale THermal Organic Reduction (THOR^sm) facility commenced in mid-1997. Commercial operation of the Studsvik Processing Facility (SPF) is scheduled for late 1998. The SPF and THOR^sm process systems are described below. The SPF is designed to meet all laws, codes, and standards related to processing LLRW. A rendering of the SPF is shown in Fig. 4.

The SPF is designed to meet the following criteria:

The SPF consists of a heavily shielded Process Building, unshielded Ancillary Building, and an Administration Building. The Process and Ancillary Buildings are licensed for receipt, handling, processing, and packaging of LLRW.

The Process Building contains all radioactive processing, handling, and packaging systems for volume and weight reduction of incoming LLRW. Major areas include: Truckbays, LLRW Input Holding Tank Vault, Pyrolysis/Reforming Vault, Gas Handling Vault, Salt Dryer Room, Final Residue Packaging Vault, and Auxiliary Equipment Rooms.

LLRW is shipped to the SPF in DOT or NRC qualified non-shielded containers and/or shielded casks. Most LLRW is received in the Truckbay where containers and casks are surveyed, opened and the waste transferred to shielded Waste Input Holding Tanks located in shielded vaults. Cask maintenance activities are performed in the Truckbay where an overhead bridge crane provides lifting capability.

Three large stainless steel Slurry Holding tanks are provided for receipt and holdup of incoming liquid and slurry wastes. A separate Liquid Waste tank is used to receive more volatile organic solvents, cleaning solutions, and oils. A Lockhopper Feeder is used to receive and feed granular and powdered LLRW, such as charcoal. A separate Waste Feed tank with Injection Pumps is used to meter slurry and liquid wastes from the Slurry Holding tanks into the stage one Pyrolysis vessel.

The Pyrolysis/Reforming THOR^sm system comprises: stage one Pyrolysis contactor, stage two Reformer contactor and associated Filters. The Pyrolyzer is a vertical, cylindrical fluid bed gasifier designed to operate at up to 800 ^oC. LLRW is injected into the electrically heated, fluidized Pyrolyzer where: 1) water is instantly vaporized and superheated, and 2) organic compounds are destroyed as organic bonds are broken and resulting synthesis gas (principally carbon dioxide, carbon monoxide, hydrogen, and steam) exits the Pyrolyzer. Residual solids from the pyrolysis of the LLRW (including fixed carbon, >99.8 percent of radionuclides, metal oxides and other inorganics and debris present in the LLRW feed) are removed from the Pyrolysis vessel and collected in the stage one Ceramic Filter vessel. The Pyrolyzer is fluidized with superheated steam and additive gas. Figure 5 shows the simplified Flow Diagram for the THOR^sm process.

The stage two Reforming contactor is a vertical, cylindrical fluid bed designed to operate at up to 800 ^oC. Pyrolyzed solid residues are transferred to the Reformer which is an electrically heated, fluidized bed where the fixed carbon is gasified to carbon monoxide and carbon dioxide by contact with the superheated fluidizing gases. The reformed, low-carbon, final residue is collected in the stage two Ceramic Filter vessel. The Reformer is fluidized with superheated steam and additive gas.

The Gas Handling system comprises an Energy Recovery Heater, Submerged Bed Evaporator, Scrubber/Mist Eliminator, Condenser, CEMS, Process Blower, HEPA filter, Vent Blower and Radiation Monitor. The purpose of the Gas Handling system is to convert synthesis gas constituents to carbon dioxide and water, recover energy from the synthesis gas, convert acid gases to stable salts, control water content of exiting process gas, and control negative pressure levels throughout the THOR^sm pyrolysis/reformer system.

Synthesis gases from the Pyrolyzer and Reformer are filtered and then oxidized in the Energy Recovery Heater to carbon dioxide and water. The Heater recovers energy from the synthesis gas and provides heat needed to evaporate excess scrubber water. The Heater is a vertical, refractory lined vessel that operates at up to 1200^oC.

The Submerged Bed Evaporator is an energy recovery system that channels the hot Heater outlet gases through a volume of scrubber water, thereby evaporating excess water. The Evaporator concentrates scrubber solution to 10 to 20 percent salts. The wet evaporator gases pass through the packed bed Scrubber where sulfur and halogen gases are efficiently converted to salts. Sodium hydroxide is metered into the Scrubber to neutralize sulfur and halogen gases that are absorbed by the scrubber solution. The Scrubber is fitted with a high efficiency Mist Eliminator that removes >99.9 percent of particulates and mists from the Scrubber outlet. 

The clean, moisture-laden gases exit the scrubber and excess moisture is condensed for recycle/reuse in the process. The Condenser serves as the process heat sink and serves to control water balance in the SPF. The cool gases are then compressed to atmospheric pressure by the Process Blower. A Continuous Emissions Monitoring system (CEMs) is provided on the Process Blower outlet to monitor and record the release of any traces of carbon monoxide, acid gases, total hydrocarbons, and NOx. 

The clean, cool process gases commingle with the building ventilation air flow. The combined gases flow through a HEPA filter bank, Vent Blower and are then released through a monitored vent stack. A complete Radiation Monitor system measures and documents any trace radionuclides that may pass through the stack. The Radiation Monitor system includes gamma, beta, alpha, iodine, carbon¹⁴, and tritium samplers and detectors.  

The salts that are formed in the scrubber and concentrated in the evaporator are transferred to the Salt Handling system which comprises a filter, an ion exchange system and Salt Dryer. The concentrated salt solution is filtered to remove any trace particulates that may pass through the pyrolyzer and reformer filters. Any trace radioactive species are removed from the solution by a high-efficiency, metals selective ion exchange medium. The Salt Dryer dries the purified salt solution to form a salt cake suitable for direct disposal at a licensed landfill. 

The reformed, low-carbon residue from the reformer is transferred to the High Integrity Container (HIC) packaging vault. Qualified HICs are filled with the solid, inert residue. Filled HICs are transferred from the packaging vault to a shipping cask be means of a shielded transfer bell. Dual containment and seals are provided on residue handling components. The packaging vault is provided with separate HEPA filtered ventilation system and water washdown capability. 

The HIC packaged residue is suitable for direct burial at either the licensed Barnwell or Hanford LLRW burial sites. The packaged residue is also suitable for long-term storage due to it's dry, inert, all inorganic nature. The packaged residue is not subject to common problems with long-term storage of wet wastes including: bacterial activity and hydrolysis of water based and organic compounds which form flammable gases in the waste container. 

It is possible to package the low-volume residue in any of the following forms: 

• stabilized in High Integrity Containers (HIC);
• compacted, cold-sintered, high-density, metal oxide monolith;
• solidified monolith using polymer sulfur cement, portland cement, thermoplastics, or polymers;
• vitrified monolith using borosilicate or phosphate glass; or
• melted metal monolith.

All interior surfaces of the SPF are provided with durable, easy-to-decon coatings. The interior wall and roof panels are of interlocking and sealed construction to eliminate leakage paths from the inside of the SPF to the outdoors. Interior concrete and steel surfaces have a special multi-layer coating to prevent migration of spills or contaminants from the SPF to the environment. The HVAC system also maintains the inside of the SPF at a slight negative pressure relative to the ambient outdoors effectively eliminating potential airborne releases. Dikes, berms and sumps are located so as to prevent tank leaks and even large fire water events from escaping to the outdoor environment.

The Process Building contains all auxiliary and utility subsystems required to support SPF operations and THOR^sm operations including:

 The Ancillary Building is designed for storage of spare parts, empty waste shipping containers and equipment for use at customers' locations. Full salt containers are accumulated for shipment for disposal. Low activity LLRW can also be received and offloaded in the Ancillary Building. Maintenance of plant equipment is also performed in a controlled area. A modular, skid-mounted, pilot-scale THOR^sm system can be located in the Ancillary or Process Building to perform testing on surrogate and low activity wastes.  

The Administration Building has: offices for plant staff and management, control room, switchgear and UPS, health physics and personnel contamination monitoring areas, and count room. The THOR^sm control room provides remote readout of all process parameters. Trained operations personnel utilize the fully automated SCADA system to monitor and control all system operations. The SCADA provides a comprehensive man-machine-interface that monitors the PLC panels, instruments, and equipment located in the Process Building. Automated safety systems, alarms, and interlocks are provided together with real-time data acquisition and trending. The SCADA provides the operators automated flow diagram windows to monitor and control the process through graphical interfaces.

  The THOR^sm process, as implemented in the SPF, will have the following features and provide the following significant advantages over other current LLRW processing technologies: 

• VR of 15 to 80 for IER wastes;
• WR of 12 to 85 for IER wastes;
• Atom-for-atom processing mode is possible;
• Inert, inorganic, homogeneous, final waste form;
• Direct disposal in HIC;
• Accept IER with contact dose rates up to 100 R/hr;
• Accept LLRW including: IER, charcoal, SGOG solvents, antifreeze, oils, sludge, high-water content wastes, and high-organic content wastes;
• Packaged final waste form suitable for long-term storage with no risk of gas generation due to bacterial or radiolysis action (residue has only trace water and essentially no organic content);
• Final waste form is reprocessable to alternative waste forms including: vitrification, solidification, encapsulation, cold-sintering, and melting.

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