Tarik Choho
Numatec Hanford Company

Carolina Pacheco
U.S. Department of Energy, Richland Operations Office

The Hanford Tank Farm Ventilation Upgrade Project (W-030) replaces the ventilation system serving four double-shell underground storage tanks, each containing highly radioactive liquid wastes. The existing ventilation system is operating beyond its design life and does not comply with current environmental regulations. It must be replaced without compromising the safety of the public or construction and operations personnel. Existing conduct-of-operations requirements must also be respected.

This paper addresses the application of recognized safety guidelines observed in the implementation of this upgrade project in all of its phases, and offers some lessons learned for the benefit of future similar projects.

Safe storage of radioactive wastes which result from reprocessing spent nuclear fuels requires the combined management of the following risks through appropriate design of ventilation systems and other storage features:

• heat generation
• direct radiation
• radiological contamination
• flammable gas accumulation

When increasing age and regulatory requirements dictate upgrade or replacement of operating systems, technical challenges are multiple throughout design, construction, and startup activities.

The Aging Waste Facilities (AWF), located in the 200-East area of the United States Department of Energy (USDOE) Hanford Site in Washington State is a complex of four 3800-m3 (1,000,000 gallon) underground double-shell tanks. These four tanks (designated as 241-AY-101, -102, and 241-AZ-101, -102) contain highly-radioactive wastes in the form of sludges and supernates; these wastes generate heat (from radioactive decay) and flammable gas, mainly hydrogen (from radiolysis, corrosion, and other chemical reactions).

The AWF ventilation system is operating beyond its design life and was not designed to comply with current DOE environmental regulations. Project W-030 replaces the aging ventilation system with a new system, identified as 702-AZ. In addition to current aging waste storage activities, the 702-AZ system will support waste retrieval and preparation operations for the delivery of waste feeds to future privatized pre-treatment and vitrification facilities. These tanks, commissioned circa 1971, are expected to be needed for many more years.

This paper presents the approach used by the project to apply "As Low as Reasonably Achievable" (ALARA) principles toward mitigating the aforementioned risks at all project stages. Maintaining ALARA conditions during construction activities was particularly challenging; the tanks contain highly radioactive aging waste and were required to be continuously operated during construction. In addition to the expected radiation hazards, the vapor space of these tanks had to be considered to have the potential to be flammable.

In order to properly and safely manage the aforementioned risks within ALARA guidelines, the 702-AZ design generally followed current ASME and regulatory codes, standards, and guidelines for nuclear ventilation systems; additionally, the design addressed the following specific requirements:

• A confinement vacuum (within the waste storage tanks) of 2.5-7.6 cm (1-3 inches) water had to be maintained during normal operation, for contamination control.
• A flow rate in the range of 0.235-0.047 m3/sec (500-100 scfm) of filtered inlet air had to be provided for flammable-gas dilution at each tank. This flow is monitored continuously.
• Tank cooling was provided (while minimizing discharges to the environment) by recirculation and vapor-condensation of approximately 0.235 m3/sec (500 scfm) of tank headspace air.
• Condensable vapors and tritium were removed from the radioactive offgas streams of the four tanks using two stages of condensers and a High-Efficiency Mist Eliminator (HEME).
• Aerosol particulates and iodine gas were removed from the offgas stream using High Efficiency Particulate Air (HEPA) filters and a High Efficiency Gas Adsorption (HEGA) filter.
• Sufficient redundancy at the component level was required to ensure continued operation of safety-related functions during and following design basis events, including natural phenomena.
• Provisions for immediate and automatic changeover to redundant filtration is provided in the event of measurable release of contamination within the exhaust stack.
• A National Emissions Standard for Hazardous Air Polutants (NESHAPS) compliant stack monitoring systen was installed.
• To reduce liquid effluents, closed-loop offgas cooling water is used rather than once-through water to the soil column.
• Cathodic corrosion protection was installed to ensure maximum life of buried piping and components.
• All underground piping was double-encased, with leak monitoring to ensure against leakage of underground contamination.

The design achieved both environmental-discharge minimization and cost effectiveness by combining one exhaust system, common to the four tanks, and one dedicated recirculation cooling system for each tank. This concept also provides for operational flexibility:

• Exhaust flow rate from each tank can be adjusted remotely above the minimal value of 0.047 m3/sec (100 scfm), within the available total flow rate of the common redundant exhaust fans.
• Each recirculation system can be operated independently depending on specific cooling needs of each tank.

A special feature of the 702-AZ system is the HEME, which uses "off-the-shelf" equipment qualified by pilot testing [1]. Based on reported test results, the combined efficiency of the exhaust system condenser and HEME for tritium removal is 98%. Figure1 shows a simplified functional diagram of the system.

Fig. 1. System schematic diagram.

There are always difficulties working inside an older nuclear waste facility due to both known and unexpected hazards, especially where excavation in the soil directly adjacent to the tanks is involved. There is a continuous need to manage the radiation, contamination, and flammable gas accumulation risks. But the tank tie-in activities were particularly challenging because of the need to expose, open, and cut into known high radiation structures and components having potential flammable gas concentrations. Figure-2 shows the typical configuration of the tie-in to the four AWF tanks.

Fig. 2. Typical tie-in configuration

The 702-AZ system connects to three existing pipe risers located on the domespace of each waste tank. The air-inlet stations connect to an unused six-inch diameter riser, the suction lines are connected to 20-inch risers used by the previous ventilation system, and the return lines are connected to unused 42-inch risers.

In order to minimize the contamination and radiation risks to the construction workers, a sequenced approach was used:

1. The system was fully assembled in a non-radiation zone adjacent to the AWF prior to hot tie-in. Shop fabrication and weld testing were maximized.
2. Construction mock-up practice sessions were used for training and to minimize durations of worker exposures. These sessions were recorded on video and critiqued, as was each actual hot tie-in operation.
3. Tie-in activities were sequenced with the least-hazardous operations being worked first, in order to gain low-risk experience.

The establishment of a broad-based Project Safety Team for continuous review of field work conditions was critical to the success of the project. The team included representatives from Radiation Control, engineering, operations, and crafts and had full management support and participation. In addition to inplementation of the project safety team the tie-in process observed the following primary ALARA practices which served to minimize recordable exposure incidents and to reduce the total estimated combined cumulative personnel doses by more than 50% from the original estimate of about 300 rem:

• Periodic, routine radiation and hazard assessment surveys of job sites were conducted by the official site ALARA review board.
• All field work was performed under approved procedures and work control packages.
• Extensive daily pre-job work/safety briefing was conducted with all involved personnel, first as a combined team, then smaller crews.
• Use of was made of perimeter access control (e.g., fences, locked gates, anti-contamination change capability and step-off pads).
• Dedicated temporary and/or permanent storage provisions were made for contaminated equipment;
• Temporary jobsite decontamination provisions were employed to reduce equipment radiation and toxic hazard levels
• Temporary work enclosures (glovebags, windbreaks) were used.
• Pre-job radiation/contamination surveys were performed. Personnel exposure evaluations were developed. Individual exposure records were maintained.
• Project radiological exposure data was used for comparison with goals and for trend analysis;
• Active confinement ventilation was maintained at all times and routine radiological and flammable-gas monitoring were conducted within the waste-tank environment, especially before intrusive operations such as cutting.
• Radio communication was used between workers inside a hazardous work area and supervising and radiological control/safety personnel outside the area.
• Innovative engineered ALARA features, designed and fabricated in the field to suit actual needs were used. Examples include:

• a unique temporary riser shield plug that allowed riser tie-in work without respiratory protection, while minimizing deflagration potential (maintained a layer of water between the domespace and construction activities involving potential ignition sources);
• Closed-circuit video was used for work and inspection in direct radiation zones;
• Unique non-spark duct-cutting equipment (TRITOOL, developed by the natural-gas industry) was used in order to comply with ignition controls applied to the facility for flammable-gas risk mitigation;
• Extensive excavation permitting was used, including underground scans and composite underground maps of all potential interferences.

In spite of the unusual ALARA protective measures employed, many individual worker cumulative radiological doses approached 1000 m/rem. During excavation and rework of existing components, general area exposures of 20 mrem/hour and contact levels of 1500 mrem/hour were routinely encountered; on occasion, fields as high as 65 rem/hr were experienced. Beta/gamma contamination levels were routinely as high as 10,000 dpm/100 cm2.

To minimize exposure risks during system startup testing, a comprehensive pre-operational test program was performed on the new system while temporarily connected to burial boxes simulating the waste tank volume. Final tank tie-ins were delayed until correct system operation was fully demonstrated. The temporary test configuration allowed ease of system troubleshooting and corrective actions and also permitted extensive operator training under low-risk conditions. All testing was conducted under approved plans and procedures. Figures 3 and 4show unique ALARA-related construction applications used on this project.

To minimize potential system or personnel operational failures, the previous system was maintained in a standby mode until satisfactory hot operation and test of the new system was demonstrated. Hot startup was dependent on successful completion of the following events:

• in-depth safety analysis of design and installation;
• preoperational testing and release of approved test reports;
• preparation and release of approved operating, maintenance, and operational-test procedures;
• procurement of all necessary critical spares;
• comprehensive training of operating staff crews;
• formal contractor and owner operational readiness reviews, performed in accordance with USDOE requirements.

Following startup, operational testing included a performance test for flammable gas control in addition to in-service evaluation of tank-waste confinement and cooling controls.

The W-030 upgrade project was funded by the USDOE, owner of the Hanford Site. The site is a federal government reservation covering approximately 1450 square kilometers (560 square miles) and located in an arid region of Southeastern Washington state. The site, established in 1943 for the purpose of wartime production of nuclear weapons materials, is now engaged in a massive cleanup, the largest ever attempted anywhere [2]. The four tanks affected by this project are among a total of 177 on the site containing approximately 208 million liters (155 million gallons).

The work was planned and performed by the various Hanford operating contractors, including Fluor Daniel Hanford (Site Management), Fluor Daniel Northwest (Engineering/Construction), Numatec Hanford (Project Management), and Lockheed Martin Hanford (Tank Operations Management). The project was initiated in the late 1980s and evolved through several concepts; detailed design of the final concept was completed in early 1994. Phase-1 construction (pre-tie-in) began immediately and was declared complete in early 1996. Preoperational system testing was performed during 1997, while Phase-2 construction (tie-in) began in mid 1997 and completed in January 1998. The system became operational in February 1998 and is currently undergoing hot operational testing. The final project capital budget is $32 MM.

Implementation of the W-030 Project could have been made more effective if certain activities had been planned and performed somewhat differently. Below is a list of some of the major lessons learned from this work.

• Detailed job execution must be fully planned ahead and taken into account during engineering design.
• Maximizing proven technologies from other industries is cost effective and can improve safety.
• Mock-up testing for ALARA and other essential safety concerns is valuable and essential.
• Pre-operational testing, performed in a non-contaminated mode, should be maximized so as to reduce personnel risk and impacts on safe site operations.
• Effective early integration of all involved organizational elements can create a skilled and motivated safety-conscious project team which will result in efficient conduct of operations.

From conceptual design through startup activities, diversity of skills, good task planning, and integrated management are key to the success of government-funded environmental cleanup construction projects.

1. Rice, Paul D. Radioactive Waste Tank Ventilation System Incorporating Tritium Control; 24th DOE/NRC Nuclear Air Cleaning and Treatment Conference.
2. Westinghouse Hanford Company. Historical Research in the Hanford Site Waste Cleanup; WM'92 Conference, University of Arizona Tucson, March 1992 (www.hanford.gov/history).