Towards safer nuclear waste disposal
By Lawrie Delroy
ERINDALE, Australia (JP): Radioactive wastes from the production of arms and the operation of nuclear power stations have been accumulating in storage areas around the world since the first nuclear bomb was dropped on Japan 50 years ago.
The "high level" portion of these wastes will remain dangerous for tens of thousands of years and poses a threat to humanity. However there is now real hope that those wastes can be disposed of safely using new technology developed by Australian scientists.
The internationally recognized Australian Nuclear Science and Technology Organization (ANSTO) in Sydney and the Australian National University (ANU) in Canberra are close to perfecting their "Synroc Process" which permits high-level nuclear waste to be incorporated into a synthetic rock which, when deposited deep into the earth in stable geological stratas, renders the waste stable for millions of years. It will, in fact, offer no greater risk than that already existing from natural radioactive material occurring deep in the earth's strata.
In 1979, Ted Ringwood, a professor of geochemistry at the Research School of Earth Sciences at ANU, invented what may turn out to be one of the world's most important scientific discoveries -- the Synroc Process for immobilizing nuclear wastes. Professor Ringwood died in 1993, after a long and brilliant scientific career. He was Australia's preeminent earth scientist, receiving more than 30 medals and prizes and being elected to fellowship of the most prestigious of the world's scientific academies.
Ringwood led studies of the earth's mantle and the nature and origin of the solar system. He was one of the key scientists to study samples taken from the moon during the Apollo landings in the early 1970s.
He developed equipment capable of reaching pressures and temperatures equivalent to those 800 kilometers inside the earth. He was adamant that pursuit of excellence in basic research could provide the key to solving many of the practical problems facing industry. Awarded the prestigious Bowie and H.H. Hess medals by the American Geophysical Union and highly honored by the National Academy of Italy, he was offered many senior positions in the United States and Europe but remained committed to the concept of Australia as a center of independent scientific excellence. In spite of his wide involvement in science he may well be remembered best for his discovery of Synroc which was described by Dr. David Cook, former Director of the Australian Nuclear Science and Technology Organization, as "a stroke of genius".
Acceptance
Synroc is an advanced ceramic composed of titanate minerals which mimic nature and have the ability to immobilize radioactive waste by binding it in chemical compounds. The material is then compressed under pressure to produce a very dense synthetic rock which remains impenetrable to water. This characteristic, combined with its ability to withstand high temperatures without breaking down, allows Synroc to be buried far below the earth's surface.
Although it has been several years since the Synroc Process was developed to a stage where it was suitable for use by industry, its acceptance by industry is by no means assured. In 1989, with the backing of the Australian Government, ANU and ANSTO joined forces with four of Australia's largest resource companies, BHP, CRA Limited, Energy Resources of Australia Ltd. and Western Mining Corporation to examine the best means of achieving commercial acceptance. They formed a separate body, the Synroc Study Group, to evaluate the product from an industrial standing and to recommend how its use could be implemented.
In its first major report in 1991, the Group concluded that it was technically beyond doubt that Synroc was capable of immobilizing spent nuclear waste for millions of years if it were buried in regions of geological stability at a suitable depth -- as much as four kilometers below the surface of the earth. It was also concluded that its commercial implementation was dependent upon world recognition of its superior qualities and the willingness of overseas nuclear power instrumentalities to incorporate the technology into their fuel cycles. They considered that at that stage of its development Synroc was best suited to the treatment of the small quantities of highly radioactive waste which remained after traditional nuclear power plants had their spent fuels treated in a reprocessing plant to retrieve the useful fuel remaining in them.
However, since that time, there has been a world trend away from reprocessing spent fuel, because of the cheapness and ready availability of uranium. Some countries are opting for the once- through fuel cycle where spent fuel is simply held until the short-life, heat-producing elements have decayed sufficiently for them to be buried. That takes approximately 50 years and, at this stage, neither the spent fuel from either the once-through, or the high-level waste from the reprocessing cycle, have been disposed of from any nuclear generating plant in the world -- and there is no licensed repository for such wastes in existence at this time.
Attractive
"The delay in disposing of wastes for such a long period can be commercially attractive because the electricity generator is earning interest on the component of electricity price charged for eventual disposal," said Dr. Adam Jostsons, Director of the Advanced Materials Division at ANSTO. "As stockpiles of radioactive waste accumulate around the world at more than four hundred nuclear power stations, a variety of mechanisms are employed, including government controlled trust funds, to ensure that the electricity generators have provided adequate funds for the eventual disposal of wastes so that the public will not be left to bear the cost."
However, at this stage, no fully satisfactory system has been devised for the disposal of wastes from the once-through cycle.
One method under consideration involves encapsulating them in metal canisters, and then burying them in mined repositories five hundred metres below ground in geologically stable bedrock. The canisters would be surrounded by bentonite clay to further slow the dispersal of the radioactive elements. It is contended that the multiplicity of barriers involved would prevent any significant release of radioactivity into the surrounding countryside. However, as the metal canisters have a life of about a thousand years, and some elements have a half-life of hundreds of thousands of years, there would eventually be some release of radioactivity into the environment in spite of the barriers.
In the case of the reprocessed wastes, it is proposed to first condition (dissolve) them in borosilicate glass, and after thirty years in storage to encapsulate and bury them in the same way as is proposed for spent fuels from the once-through process. The conditioning in borosilicate glass would delay the leaching process compared to the once-through process by up to 30,000 years. But, once again, some radioactive material would leach into the environment, although that would only occur after a very long period (although natural disasters such as earthquakes, or human interference, could markedly shorten that time).
Fortunately, the eventual release of any radioactive material can be overcome where spent fuels are reprocessed and the wastes conditioned with Synroc. Synroc bonds chemically with the elements being disposed of, and, because of this, it strongly resists leaching. It can also be buried at great depth because of its resistance to heat. Moreover this resistance to heat enables it to be disposed of at a much earlier stage in the cooling cycle. Synroc is 1000 times more resistant to leaching that borosilicate glass below 100 degrees Celsius and is little affected by heat even in excess of 300 degrees Celsius, unlike borosilicate glass which disintegrates in groundwater above 200 degrees Celsius. This makes borosilicate glass unsuitable for deep burial as earth temperatures rise naturally by 25 degrees Celsius for every kilometer of depth, and heat which is still being generated within the waste from radioactive decay increases that temperature further.
Risk
Unfortunately, there are some risks involved in using Synroc. The reprocessing process itself has certain inbuilt dangers, although those risks have been greatly reduced in recent years. Nevertheless, the extraction of plutonium from the reprocessed fuels poses a problem as it is a difficult fuel to handle in nuclear generating plants. In spite of monitoring by the International Atomic Energy Agency, there is always a remote possibility of diversion to military purposes. Moreover, concerns have been heightened by the announcement of the Japanese government that it intends to make plutonium the basis of its fuel economy in the 21st century, using a fast breeder type reactor which it is developing. Although nuclear power plants in the vicinity of Kobe operated without problems during the recent earthquake, Makiko Tanaka, Director General of the Science and Technology Agency, is reported to have urged a review of all Japanese nuclear plants because "anything beyond imagination can happen". Ringwood was so concerned at the risks involved in using plutonium that, in a report made some time before his death, he suggested that it might be better to dispose of it completely, along with the other wastes being immobilized by the Synroc Process. He estimated that this course of action would only increase the price of electricity by 5 to 10 percent, a small price to pay for a dramatic increase in world security. Although his proposal has received little support to date, that attitude could change if terrorist blackmail becomes a reality, or if the dangers in using plutonium cannot be contained effectively.
Research
Efforts are being made to widen the application of Synroc in the nuclear industry. Research is being undertaken in Australia, in partnership with a number of overseas nuclear organizations. They are studying the partitioning of wastes into longer and shorter lasting radioactive elements so that Synroc can be used to immobilize the longer lasting ones and borosilicate glass the shorter lasting ones. If successful, this technology may also enable the large stockpiles of military wastes which have accumulated in the U.S., and elsewhere, to be treated.
In spite of nuclear power's inherent risks, it may well prove to be the safest form of energy available to man in the foreseeable future. However, if industry is to meet the standards demanded of it, it is imperative that it uses only the best technology available from all sources. Bob Foster of the Synroc Study Group considers that: "Initially, the biggest problem facing industry is the need to establish secure above-ground repositories to store, for the next few decades on an interim basis, the spent fuel which is accumulating in many countries."
However the actual disposal of those wastes will need to follow, and there seems little doubt that the Synroc Process will play a major role in that task.
Lawrie Delroy is a freelance scientific journalist based in Erindale, Australia.