Decontamination & decommissioning | Hanford
Digging up Hanford24 August 2010
After 30 years of producing plutonium, the Hanford site in the southeastern Washington state desert is heavily contaminated with radioactive and chemical waste. Over the next few years, the first generation of interim groundwater remediation measures will give way to high-volume treatment systems to clean up the site for good.
At the Hanford site near Richland, Washington, CH2M HILL Plateau Remediation Company is the US Department of Energy’s prime contractor for the monitoring and remediation of groundwater (the water below the surface of the earth occupying spaces in soils or geologic strata).
It is developing techniques to prevent contamination in the ground from reaching groundwater, maximizing the amount of groundwater treated, and containing contaminated groundwater to minimize the potential for contamination reaching the Columbia River, which flows through the 1,520-square-kilometer (586-square-mile) site.
The quantity of contaminated waste is sufficient to bury the entire site a metre deep, according to Dyan Foss, CH2M HILL’s vice president of the soil and groundwater remediation project. “The Hanford groundwater problem is unique due to the amount of groundwater requiring cleanup, the location of the groundwater relative to the river and salmon spawning areas, and the aggressive schedule and standards we’re working toward to help the DOE meet cleanup milestones.”
Hanford was created in 1943 as part of the American Manhattan Project. The effort at Hanford focused on plutonium production for the country’s nuclear weapons programme. Nine nuclear reactors were constructed along the Columbia River and five chemical processing facilities were built in the centre of the Hanford Site to support the production of plutonium during the Cold War.
In 1989, production operations stopped and the mission shifted to clean up a site that is now contaminated with hazardous substances, including both radionuclides and chemical waste.
Weapons production at Hanford resulted in more than 33 million m3 (43 million cubic yards) of radioactive waste and over 100 million cubic meters (over 130 million cubic yards) of contaminated soil and debris. Approximately 1.8 trillion litres (475 billion gallons) of liquid was discharged to the soil, which is underlain by layers of gravel, sand and silt up to 800 ft thick.
Overall, environmental cleanup of the Hanford Site encompasses many technical and logistical challenges:
- 1,700 waste sites
- 270 billion gallons of groundwater contaminated above acceptable standards over an area of approximately 80 square miles
- 53 million gallons of radioactive liquid waste currently stored in 177 underground storage tanks
- 22,000 drums of mixed waste
- 2,300 tons of spent nuclear fuel
- 17.8 metric tons of plutonium-bearing material
- 500 contaminated facilities
- Nine nuclear reactors
The Department of Energy’s groundwater strategy focuses on cleanup along the Columbia River, treating groundwater in the central areas of the site and slowing further movement of contamination toward the river. The strategy is part of the greater DOE vision to shrink the Hanford Site cleanup footprint from 586 square miles to 75 square miles in the centre of the site by 2015.
In the 1990s, the Department of Energy and regulatory agencies the U.S. Environmental Protection Agency and the Washington State Department of Ecology made installing interim groundwater treatment systems a priority in Hanford cleanup.
Currently, there are several interim remediation systems in operation that treat more than 40 millions of gallons of groundwater every month. Since CH2M HILL’s contract started in October 2008, 11,800 kilograms of carbon tetrachloride have been remediated on the central plateau and 1,100 kilograms of hexavalent chromium have been removed along the river corridor. Those systems helped contain plumes of contamination and in some cases shrink them. Now, CH2M HILL is installing final treatment systems with much larger treatment capacities, either by expanding current facilities or by constructing new ones.
The DOE monitors groundwater at the Hanford Site to fulfill a variety of state and federal regulations, including the Atomic Energy Act of 1954 (AEA), the Resource Conservation and Recovery Act of 1976 (RCRA), the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA), and state regulations, the Washington Administrative Code. Many aspects of this cleanup project are subject to regulations and agreements made with the U.S. EPA, the Washington State Department of Ecology and other regulators. Key stakeholders and the general public provide feedback through various DOE forums.
The location and amount of contamination in the site groundwater carry challenges, but the effects on the Columbia River add an additional challenge for CH2M HILL. Groundwater cleanup standards for hexavalent chromium are based on thosefor aquatic life, which are ten times lower than drinking water standards (100 µg/L).
In addition, changing river levels, by as much as 16 feet at the 100 B/C Area, affect groundwater flow throughout the year, requiring the remediation techniques be dynamic. Although the average annual flow of the river through the site is 120,000 ft3/sec, over the year it varies hugely from 39,000 ft3/sec to 264,000 ft3/sec.
Aggressive regulatory targets were established in 2009 to stop hexavalent chromium from entering the Columbia River by December 2012. This target has inspired flexibility in the remediation systems that will not only target the source removal, but will also be capable of containing contamination and restricting its movement toward the river.
The groundwater treatment work is a key component of protecting the Columbia River, as is all cleanup work that is done to prevent contamination of the groundwater. This includes digging up waste sites and demolishing buildings, as well as developing technologies to reach contamination in the soil too deep for conventional cleanup techniques.
“We’re developing and deploying remediation technologies as needed and avoiding recontamination of the sites already cleaned, or those currently undergoing remediation,” said Foss.
The size of the problem
The Hanford Site is divided into groupings of similar waste units within a geographic area. These units help define the areas of concern at the Hanford Site and the types of contamination that were disposed of into the soil from the 1940s through most of the 1980s.
The northern section of the Hanford Site is referred to as the 100 Areas. The 100 Areas encompass 6,800 hectares (17,000 acres), which contain Hanford’s nine production reactors, more than 200 inactive support buildings, 36 former solid waste burial sites and more than 200 identified sites with surface or subsurface contamination.
Sodium dichromate, a chemical used as a corrosion inhibitor, was added to the river water used to cool Hanford’s older nuclear reactors while they were operating. Over time, the soil and groundwater became contaminated with chromium because of leaks in the dichromate transfer systems and piping, and because cooling water treated with dichromate was periodically discharged into the soil near the reactors.
Contaminants were also introduced into the environment when some of the basins and pipelines overflowed or leaked. In addition, over the years large quantities of sludge that settled in the basins were pumped out into disposal trenches near each basin. Further, each area had sites where solid waste generated during routine reactor operations (contaminated rags, filters, clothing, equipment, disposable supplies, etc.) were buried.
The centre of the Hanford site is known as the Central Plateau, which encompasses the 200 West and 200 East areas of the site. Plutonium was extracted from irradiated reactor fuel in massive chemical processing facilities in the central plateau. During the operation of the processing plants, liquids contaminated with high levels of radioactive waste were disposed in large underground waste tanks. More than 60 of those 177 waste tanks have leaked an estimated one million gallons of liquids in the past, with some of it reaching groundwater. Monitoring has shown that the leaked liquid hasn’t moved very far from the tank storage area, relative to much larger plumes of contamination from discharges of contaminated liquids from the processing facilities in the central plateau.
Over 350 billion gallons of liquids, ranging from cooling water to supernatant from single-shell tanks, were discharged directly into the soil through drain fields and wells.
In the past, the direct disposal of liquids was considered safe because the soil was thought to filter and trap a large portion of the radioactive contaminants in its upper layers. Unfortunately, not all radioactive contaminants were absorbed by the upper soil layers. Instead, they have migrated into the groundwater, along with chemical solvents (such as trichloroethylene or carbon tetrachloride) used by processing plants. Nine contaminants exist at levels exceeding current national drinking water standards.
Contaminant flow depends on several factors, including the amount of liquid discharge (driving force), the chemistry of the contaminant and the soil (some contaminants sorb to soil particles and don’t move much, whilst others move more freely), the chemistry of the waste solution (for example, acidic waste makes some contaminants more mobile) and the physical characteristics of the sediment (silty material slows downward movement compared to sands and gravels; all three are present on the site).
The southernmost 300 Area of the Hanford Site is a 46-hectare (115-acre) industrial area just north of the city of Richland. The facilities in this area have been used for fabrication of reactor fuel assemblies, reactor research and development, metalworking, chemical process development and research and development. The most significant contaminant of concern is uranium.
Some of the contamination found within the 300 Area, such as solvents, is similar to that found in many industrial areas in the United States including solvents. During fuel fabrication and materials processing research, pipeline leaks, spills, airborne releases from shops, burial of process waste and release of liquids into ponds introduced radioactive materials into the environment. The bulk of the contamination is concentrated in buildings and in approximately 20 hectares (50 acres) of soil within the main industrial area. Contaminants including uranium, nitrates, and trichloroethylene are present in groundwater beneath the 300 Area.
Finally, a sitewide plume approximately 80 square miles in area has contaminated groundwater on the site – with varying contaminants. The wide dispersal of contamination poses logistical challenges and the need for numerous technologies. Characterization of the waste sites and the groundwater must recognize differences between localized contaminants in the soil column. The more widespread, mingled contamination in groundwater plumes are defined using groundwater wells and data analysis. Monitoring wells are installed, drilled, and sampled to define the nature and extent of the contaminant plumes. Groundwater also is monitored by the wells to assess the effectiveness of groundwater remediation over time.
Pump and treat
CH2M HILL is working toward aggressive groundwater cleanup milestones established by the DOE and regulatory agencies. The overall strategy is based on a holistic approach toward cleanup that integrates building decommissioning and demolition, removal of sources from the soil column and groundwater remediation. Supplementing standard field activities with site-specific treatability testing ensures selection of the appropriate remediation technology and optimizes the implementation and effectiveness of the selected technology.
A variety of technologies have been used, and new technologies are being tested and fielded to treat contaminated soil and groundwater.
In the western portion of Hanford’s Central Plateau, known as the 200 West Area, the DOE and CH2M HILL broke ground on a $110 million groundwater treatment facility in July 2009. Funded with $80 million in American Recovery and Reinvestment (Recovery Act) government funds, the facility will be a key part of the strategy to treat groundwater contamination and prevent it from moving from the centre of the site to the Columbia River, a few miles away.
CH2M HILL designed the facility and began installing several wells that will be part of a treatment system that will pump more than 85 million gallons of contaminated groundwater per month (gpm) from a large area in the central plateau, once it begins operating in 2012. Skanska USA Building won the $50 million construction contract in May 2010.
The 200 West groundwater treatment system will be the largest system of its kind among former DOE nuclear weapons production sites, with a capacity to handle approximately 2,500 gpm (7.6m3/min).
The new treatment system will pump contaminated water from the ground to remove nitrates and metals, as well as radioactive and organic contaminants.
The facility will bring together a variety of techniques (resin for the radionuclides, air stripper for the volatiles, and bioreactor for the nitrates).
CH2M HILL is also utilizing pump and treat systems in the 100 Areas, to remove hexavalent chromium, the primary contaminant of concern in the area. DOE recently agreed to new regulatory milestones to contain all chromium in groundwater by 2012 and reduce chromium in groundwater near the river to drinking water standards by 2020. The agency expects to meet those milestones through the work of CH2M HILL to expand current treatment systems and build new ones.
Recent upgrades and expansions were made at the 100K Area groundwater treatment system, including adding a treatment facility. Operation of the newest groundwater treatment system began in May 2009, and within one month, the overall treatment capacity for the K Area Project reached over 35 million gpm, three times its previous capability. CH2M HILL is also expanding other treatment facilities in the 100 Area including H Area, near Hanford’s H reactor.
Construction of a new treatment system, the 100-DX pump and treat system, is under way near the D and DR Reactors next to the Columbia River. The $20 million project is funded by the Recovery Act and is needed to treat hexavalent chromium in the groundwater from reactor operations in the past. Additionally, CH2M HILL is designing a second facility that will provide an additional 700 gpm of treatment capacity and serve an area of groundwater between the D and H reactors.
The systems will replace five interim pump and treat systems installed since the 1990s to contain contaminants. Once all systems along the river are operational, CH2M HILL expects the 100 Area groundwater treatment systems to pump and treat at a rate of more than 90 million gpm – over three times the capacity available along the Columbia River before the treatment system expansions began.
In addition to pump-and-treat systems, in-ground treatment media have been installed and CH2M HILL is testing methods for expanding those systems or installing new ones to prevent contamination in groundwater from reaching the nearby Columbia River.
In the 1990s, a chemical was injected into the ground to slow the spread of chromium contamination in groundwater near the D reactor as it moved toward the Columbia River. As water passes through it, the barrier converts harmful hexavalent chromium in the dichromate ion (Cr2O72-) into a non-harmful form. The barrier is constructed by injecting a chemical compound (sodium dithionite) into the subsurface aquifer through a series of injection wells to develop a reducing environment, resulting in a permeable groundwater barrier preventing chromium contamination from discharging to the river environment. The passive system treats about six gallons of groundwater per minute. Unused chemical compounds and reactive byproduct waste (sulfates) are withdrawn from the aquifer during barrier construction.
An injection technique was also used in the 100N area near Hanford’s N reactor to construct another type of barrier that sequesters radioactive strontium-90 in the groundwater, an apatite barrier.
Apatite [Ca10(PO4)6(OH)2] is a calcium phosphate mineral occurring primarily in the Earth’s crust as phosphate rock (and is also a primary component in the teeth and bones of animals). Apatite minerals sequester elements into their molecular structures through substitution reactions that replace the calcium, phosphate, or hydroxide in its hexagonal crystal structure. The substitution of strontium for calcium in the crystal structure is thermodynamically favourable. As a result, apatite can remove soluble strontium and 90Sr from groundwater both during and after its formation.
CH2M HILL is expanding this 300-foot apatite barrier by 2,200 feet. With Recovery Act funding, it will install 171 wells. The rest of the work, including injection, monitoring and reporting will come from base funds.
As groundwater moves toward the river, apatite absorbs strontium-90 and prevents it from reaching the river. Strontium has a radioactive half-life of about 29 years, and the apatite binds the contaminant in place until it decays to harmless levels.
CH2M HILL is assessing jet injection drilling and alternative methods for creating an apatite barrier in the soil. Jet injection involves driving a long, thin drilling tool to a depth of 25 feet below the surface and injecting chemicals into the soil under high pressure. It is expected to perform well in the upper vadose zone–the soil above the water table–because it would be unaffected by the river levels. The method currently used to build an apatite barrier, however, is effective only when the river is at a high level. As part of its pilot project, CH2M HILL will evaluate three chemicals for injection: pre-formed apatite, a phosphate solution, and a phosphate solution followed by pre-formed apatite.
Polyphosphate is being tested to act as a sequestration barrier for uranium contamination in the soil and groundwater at the 300 Area next to the Columbia River north of the city of Richland. Operation of 300 Area facilities resulted in the discharge of contaminated liquids used in fabrication and research processes to soil disposal sites. There is extensive uranium contamination in the area’s soil and groundwater.
The use of polyphosphate technology for source treatment in the vadose zone and the partially and/or intermittently saturated area above, the so-called capillary fringe, is expected to accelerate the natural attenuation of uranium to stable uranium-phosphate minerals. The polyphosphate technology immobilizes dissolved uranium in the groundwater through a chemical reaction in which phosphate combines with uranium to form autunite, which has a very low solubility in Hanford-type groundwater chemistry (which is an aerobic, oxidizing, calcium bicarbonate environment).
The chemical reaction is 2Na+ + 2PO43- + 2UO22+ +xH2O ? Na2(UO2)2(PO4)2 . xH20)
This polyphosphate technology is preferred over a chemical reduction approach (such as what is being done for chromium) because the phosphate reaction binds uranium in the oxidized form. Unlike chromium, reduced uranium (U4+) is more easily remobilized when oxidizing conditions –which are natural at Hanford–return to the subsurface, so reducing conditions would have to be maintained indefinitely to keep uranium immobile.
The use of polyphosphate over simple phosphates allows the formation of the phosphate anion for uranium sequestration over an extended period. The longer the polyphosphate chain, the longer it takes to hydrolyze (break down) and make the phosphate available for reaction. This simplifies the requirements on a delivery system and increases the radius of influence.
Deep vadose zone tests
Two of the key ways to prevent further contamination of groundwater are rather obvious but sometimes underappreciated. Approximately $2 billion a year is spent on cleanup of the Hanford Site, with the majority of the funding going toward accomplishing cleanup that ultimately protects groundwater.
The first key way to prevent further contamination of groundwater is keeping waste in underground tanks and contaminated materials in surplus facilities from entering the soil. This is accomplished by cleaning out and demolishing excess facilities. There are a total of 1,433 facilities that have been designated as excess: 463 for fuel fabrication and reactor operations along the Columbia river, and 970 supporting fuel reprocessing in the centre of the Hanford site. As of the end of March 2010, 208 facilities along the river and 271 facilities in the centre have been demolished.
The DOE, through its Office of River Protection, had also removed all of the pumpable liquid from older, single-walled underground tanks by 2004. Bechtel is building a massive waste treatment plant to treat the remaining waste in the tanks.
The second way to prevent further contamination from reaching groundwater is keeping contaminants already in the soil from moving deeper, and into groundwater. This is achieved near the surface by excavating the soil and moving it to a lined, engineered disposal facility. More than half (475) of the 814 waste sites in a 200 sq mile area along the Columbia River have been excavated, with approximately nine million tons of soil and debris placed in the Hanford’s disposal facility for low-level, radioactive waste, the Environmental Restoration Disposal Facility.
Cleanup of contaminants that are much deeper in the soil, as is the case near former processing facilities in the centre of the site is much more difficult. It is considered by some as one of the final frontiers of cleanup at the Hanford Site.
The central plateau at the Hanford site rises to about 700 feet above sea level, or a about 300 feet higher than the river. The plateau’s surface is also about 300 feet higher than the water table, since its elevation does not change.
Most source remediation is at or very near the surface. However, at Hanford there are several areas across the central plateau with pockets of contamination deep in the soil (100 to 150 feet below the surface) that are current or potential threats to the groundwater. Contaminants have reached so deep because of the depth of the water table.
Deeper parts of the vadose zone above the water table are out of reach of conventional approaches, such as excavation. In many cases, deep contamination plumes are located beneath facilities, such as large underground tanks and large processing plants, that cannot be removed prior to remediation of the deep vadose zone.
For these areas, DOE and its contractors have produced a so-called treatability test plan to demonstrate deep vadose remediation methods. CH2M HILL has also begun fielding treatability tests for technicium-99 and uranium. There will also be a focused evaluation of surface barriers, soil flushing and grouting. Surface barriers minimize or eliminate infiltration of rainwater to reduce further mobilization of contaminants. Soil flushing is the use of liquid to push the contaminant to the groundwater, where it could be remediated with another technology, such as a pump and treat system. Grout fills the pore spaces within the vadose zone, fixing the contamination in place.
A soil desiccation pilot test, which is focused on technicium-99, is currently being installed at the BC Cribs and Trenches Area. The test is scheduled to start in autumn 2010. It will evaluate using air to dry the soil in the vadose zone so the contaminant adheres to the soil, slowing transport of technicium-99 toward the aquifer. A cap on top of the soil preventing further moisture would immobilise the contaminant. The desiccation process involves the installation of a cover/impermeable barrier over the affected area and a series of wells. Injection wells are used with 0% humidity nitrogen, which is then pulled through the vadose zone by the extraction process using a suction or vacuum type process.
On the central plateau, one unit of an interim soil vapour extraction remediation system has been installed. It aims to remove the carbon tetrachloride vapours by filtering air pulled through a series of wells. The air is extracted through suction and run through carbon canisters to remove carbon tetrachloride vapors.
Work to design another treatability test focused on uranium sequestration is also under way. Laboratory tests have shown that ammonia gas injection into the vadose zone should significantly reduce the fraction of uranium that is subject to transport. The ammonia increases the pH of the sediment pore water so much (to approximately 11.9) that silicate minerals in the soil partially dissolve. When the ammonia injection is stopped, pore water returns to its normal value near neutral (7.0), and those previously-dissolved minerals precipitate. The precipitation process entrains and coats molecules of contaminants such as uranium to render them essentially immobile. Because silicate minerals are ubiquitous, this technology should be applicable throughout the Hanford Site. This uranium sequestration pilot test is scheduled to begin in autumn 2011. Both treatability tests are intended to collect data to facilitate feasibility evaluation. In addition to these planned field tests, laboratory scale work is currently under way to evaluate soil flushing and grouting as a mechanism for treatment of deep vadose contaminants.
Recognition of the distinct challenges represented by deep vadose zone contamination has led to a decision to create a separate deep vadose zone unit, which allows surface-based remedial actions to proceed while the treatability tests and other evaluations described above proceed. Solving the deep vadose zone problem may represent the last element in the site cleanup toolbox, as other technologies such as removal actions, and pump and treat are well established.
The Department of Energy and CH2M HILL continue to search for new technologies to address all of Hanford’s contaminants. The main challenge is developing methods that are cost effective and able to meet regulatory standards.
“We’re making progress in cleaning up the contaminants along the Columbia River,” said Foss at CH2M HILL. “We’re working closely with the DOE and using our experience at other projects to meet this exciting challenge.”
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