A glass act29 October 1999
The successful vitrification of high level waste (HLW) at the West Valley Demonstration Project near Buffalo in New York State is a great achievement. The process developed by Westinghouse subsidiary West Valley Nuclear Services Company represents the state-of-the-art in waste treatment.
Dealing with nuclear waste remains one of the most controversial issues within the nuclear industry. It is one which still remains largely unresolved with construction of geologic repositories throughout the world stalled by political rather than technical problems.
There has however been significant progress in the development of technology aimed at stabilising fission products and other high level waste within a physical matrix. In the late 1970s the DoE established the Alternative Waste Form Peer Review Panel to consider technical options for dealing with wastes. The need for a solution was driven by the large quantities of wastes at the DoE’s sites, including Savannah River and Hanford. The West Valley problem was much smaller , but work at this site has emerged as a leading example of waste treatment.
The DoE waste panel published a report titled The Evaluation and Review of Alternative Waste Forms for Immobilisation of High Level Radioactive Wastes. In it the panel considered 11 different possible techniques for dealing with HLW, including immobilisation in concrete, ceramic material or lead. But the consensus opinion favoured vitrification within a borosilicate glass matrix.
“The final decision was based on rankings,” said Larry Hench, the panel chairman. “In particular we considered long-term leaching in water, the ability to maintain the waste form over the long term and the ability of the material to hold a wide variety of radionuclides as well as non radioactive materials within the structure. Borosilicate glass vitrification is a major triumph of materials science.” Hench and his colleagues needed a material which could hold as many as 40 different elements but remain homogeneous and not form discontinuities.
According to Hench, the Australian technology, Synrock, which is often cited as a possible alternative to borosilicate glass, is not suitable as discontinuities can form within it leading to cracks, greatly increasing the structure’s surface area and hence its leach rate should water penetrate the canisters and other layers of shielding around the waste. In considering a solution Hench’s panel had to assume the shielding would be penetrated.
“I don’t think the public appreciates the sheer volume of wastes we’re dealing with,” he said. “Defence wastes alone in the United States, Europe and Russia amount to 109 litres.”
Putting theory to practice
West Valley was the only commercial nuclear reprocessing site in the United States. Between 1966 and 1972 the plant reprocessed 640 t of spent fuel, producing 2200 m3 of high level liquid waste. The company that ran the plant at the time Nuclear Fuel Services (NFS), closed it in 1972 to carry out environmental improvements, but the costs became prohibitive and NFS decided not to renew its lease, returning the responsibility for managing the site to the State of New York. In 1980 then-president Jimmy Carter signed the West Valley Demonstration Project Act, authorising the DoE to deal with the waste at the site. The DoE contracted Westinghouse subsidiary West Valley Nuclear Services (WVNS) to carry out the work.
The waste was stored in an underground steel lined tank 8 m high and over 20 m in diameter. Over the years the waste had separated into solid and liquid phases. Within the liquid, or supernatent, phase the dominant radionuclide was caesium-137. Between 1988 and 1991 the company pre-treated the liquid phase to remove the caesium. The liquid was pumped through four zeolite columns placed in series. This process removed more than 99.9% of the radioactivity, reducing the activity of the liquid to a level where it qualified as low level waste. Water was boiled off to reduce the waste’s volume and it was then mixed with cement and solidified within steel drums. The zeolite material containing the caesium was mixed back into the solid phase.
Between 1991 and 1994 the solid phase was washed to remove salts. Then the wash water was solidified in cement. These pretreatment phases reduced the number of high-level glass canisters that would be needed from an estimated 1300 to less than 300.
At the same time as these pretreatment processes were taking place, WVNS was developing the vitrification process itself. The company used as much as possible the structures which already existed at the site; the storage area where sealed canisters are placed at the end of the process was formerly a chemical processing cell (CPC) where spent fuel elements were cut up and placed in an acid solution to separate out the uranium and plutonium. The area is still inaccessible due to its high radiation levels; WVNS cleared and prepared the area for canister storage entirely remotely. The contaminated equipment from the cell was put in steel containers and has been stored and monitored on site.
Between the CPC and the waste tank WVNS constructed a process line to vitrify the waste. Construction was completed in 1995 and the first transfer of HLW from the waste tank took place on 24 June 1996. Pumps within the tank had mixed the sludge to ensure a uniform mix before the transfer took place.
Central to the vitrification is the slurry-fed ceramic melter (SFCM). It is within this melter that the borosilicate molten glass is formed. HLW is pumped from the waste tank into the vitrification cell and is initially held within a concentrator feed make-up tank. There it is sampled, as is the molten glass former held within the melter feed hold tank.
Once the chemistry is satisfactory the glass formers and HLW are mixed in the ceramic melter. The SFCM operates at a temperature of 1150°C, controlled by three immersed electrodes. The molten glass mix is poured from the melter into a stainless steel canister which is on a four canister turntable directly under the pour spout.
Before any waste was treated this way, WVNS carried out five years work testing the melting and solidification qualities of the glass. This extensive testing period gave the company solid evidence that the glass solidified in a uniform manner.
“We don’t actually sample the glass melt at all,” said Ron Palmer, a principal engineer at West Valley, whom Larry Hench supervised during his PhD into vitrification technology. “We sample the waste and the glass formers and make sure that the chemistry of both is under control. We take a solid sample from each canister one to two weeks after they’ve been filled with glass melt, just before the canister lid is sealed.” “Once you prove your mix is of the same composition, you’re pretty safe.” said John Chamberlain, public communications manager at West Valley.
The canisters are 3 m high and 60 cm in diameter. Within the canister the glass solidifies and a sample is taken from the top. The canister top is then welded on and the outside surface is decontaminated. The sealed canister is then finally placed in the old CPC, now an interim storage facility.
“One of the specifications we must meet requires that the glass stays below about 400°C after the initial cooling,” says Palmer. “One possible source of re-heating is of course from the radioactive elements in the glass. We have done calculations to show that we cannot have enough of these elements in the glass to cause that much heating. In fact, our glass should be more or less at ambient temperatures for all time.” It is important that the glass does not rise above 400°C because it would then crystallise into another phase which would not be as durable.
The vitrification of the HLW at West Valley is now almost complete. To date 242 canisters have been filled and are now stored in the CPC. All that remains in the waste tank is residual material around the walls and floor which is being cleaned out using water jets. The focus is now shifting to the second phase of the West Valley project, the decommissioning of the site itself.
The DoE and New York State Energy Research and Development Authority have published a draft environmental impact statement (EIS) which examines possible options for the plant’s closure. The statement describes five options, from complete removal of all evidence that the plant ever existed, allowing the land to be returned to unrestricted use, to locking the gates and walking away.
In between these extremes, three other options are on the agenda. The most complete scheme would encompass removing the existing facilities, including waste that has been buried in two disposal sites within the West Valley land area. The low and high level wastes within canisters would remain on site. Another option would involve removing the HLW canisters; their ultimate destination would be Yucca Mountain, assuming the site does eventually accept waste. LLW would remain on site, and contaminated structures and buried wastes would be stabilised and remain on site. The third intermediate option involves managing the site in its present condition with long-term monitoring and maintenance operations ensuring no contamination occurs.
The options within the EIS are now being considered by the various actors with an interest in the final decision. These include a citizen task force, regulatory agencies, the state of New York and the DoE. The DoE and New York State Energy Research and Development Authority will then declare a preferred option which will go to a second public review process. A final EIS will then be published, preceding the final decision. The timetable remains vague, although WVNS hopes a decision on a preferred option will be made within a year. As with all these decisions, a balance between cost and safety has to be established based on the principle that risk must be reduced as low as reasonably achievable (ALARA).
Beyond the decommissioning of the contaminated areas of the plant the most difficult problems involve tackling the buried wastes and dealing with a contamination plume containing strontium-90 which was discovered on the site during detailed environmental monitoring in the early 1990s. The discovery of this plume raised concerns that contamination might be reaching the groundwater and entering Buttermill Creek into which groundwater from the site flows. Buttermill Creek flows into Cattaraugus Creek which flows into Lake Erie, a vital source of drinking water for a large proportion of the United States’ population.
While there is no indication that the West Valley plume poses any threat; West Valley Nuclear Services has made great efforts to restrict its movement. A zeolite barrier has been placed below the plume, designed to prevent any Sr-90 reaching Buttermill Creek. Intensive monitoring of the site, which has taken place since the early 1990s has shown no evidence that the plume is threatening the creek. In 30 years it has shifted less than 1000 metres.
WVNS is proud of its achievement. The decommissioning challenges that remain, while substantial, appear less daunting in the context of the successful HLW treatment. West Valley may become a benchmark for waste treatment throughout the world and if its success can be translated into a shift in public opinion in the local region, may offer some hope that life remains in the US nuclear industry.