At the time of the Arkansas creation trial in December 1981 I had been at the Laboratory as a guest scientist for twelve and a half years. It was a most cordial and productive arrangement. During this period I had published research papers in collaboration with my colleagues at ORNL, and had undertaken cooperative research projects with scientists at other laboratories and universities, some of them overseas. Yet by the summer of 1981, my main purpose of coming to the Laboratory—finding superheavy elements—still had not been accomplished. One final year was given me to do this. My superheavy search involved several different experimental approaches; all of them were quite time-consuming.
Along with these investigations I turned some of my attention to a new research project: the long-term storage of nuclear wastes in granite. Two months prior to the Arkansas trial some colleagues and I had already obtained definitive experimental results concerning waste storage in those rocks. I hoped that the discovery of these new data might provide a basis for the Laboratory to extend my stay beyond June of 1982, irrespective of my results on superheavy elements.
Conventional Nuclear Waste Containment
It is well known that many individuals within and without government circles perceive the long-term storage of nuclear wastes to be one of the more important technological problems of our time. The goal of nuclear waste research is to determine (1) what type of storage container will best withstand nuclear radiation effects so as to prevent leakage during a several-thousand-year storage period, and (2) the geological site best suited to minimize nuclear [p. 162] waste leakage into the environment in case of accidental rupture of the primary containers. This involves a prediction of the long-term geological stability of the site based on both present-day geological assessment and an estimate of the geological age of the formation.
The standard approach to the problem of site selection assumes that the geological formations best suited for storage are those thought to have remained stable over long geological periods. The U.S. Department of Energy estimates geological age by reference to the presumed geological development of the earth. Site selection procedures thus depend partly on the assumption of uniformitarian geology. If uniformitarian geology does not provide a correct timetable for the earth's geological history, then one of the basic criteria for nuclear waste site selection is called into question. We have already discussed (in Chapter 4) how the results on the coalified wood from the Colorado Plateau provide evidence that those formations are only several thousands of years old instead of several hundred million. Professor Kazmann's article (Kazmann 1978) focused attention on the nuclear waste implications of these results.
Although it is possible to fill metal containers with radioactive wastes and bury them in some underground cavity, common sense tells us we must take additional precautions. There is always the possibility that container rupture might occur, due either to corrosion or to some disaster such as an earthquake. Thus it would be unwise to select burial sites near the earth's surface, with its higher risk of waste leakage into the environment.
The leakage hazard can be reduced by burial in granite. Granite formations, extending far below the earth's surface, would obviously permit waste storage at much greater depths. However, at greater depths the temperature rises sharply, again raising the possibility of waste-container rupture. One additional precaution would be to first encapsulate nuclear wastes within some type of impervious matrix, which would resist leakage even at higher temperatures. A most important goal of nuclear waste research is to identify what type of matrix would safely retain radioactive elements under high-temperature conditions.
In recent years nuclear waste specialists have investigated a variety of substances which could serve as the primary encapsulation medium. Certain types of glass have been investigated, and initially some of them seemed to hold great promise. The radioactive material was incorporated into the molten glass mixture and then allowed to cool in the form of a cylinder. Subsequent studies have shown, though, that after a few years the radioactive emissions had damaged the glass structure, making it more susceptible [p. 163] to corrosion. This raises questions about the long-term stability of nuclear wastes in this matrix.
An alternative approach is to investigate various types of synthetic minerals whose natural forms contain significant amounts of the radioactive elements uranium and thorium. By ascertaining which natural radioactive minerals have retained these elements over the course of the earth's history, we can identify the most suitable synthetic counterparts for long-term nuclear waste encapsulation.
There was also the question of where the waste containers themselves would be placed. One plan was to bury the waste containers in deep granite holes. The rationale was: even if the primary container did rupture, the radioactive hazard to the environment would be reduced. Prior to our studies, scientists had only investigated the retention of radioactive minerals taken from granite-rock formations near the earth's surface. But if nuclear waste containers were to be encased in granite, they would need to be buried in 15,000-foot-deep granite holes, where temperatures would be quite high. How much these higher temperatures would affect leakage of radioactivity from the minerals was a crucial question. The only solution was to analyze natural radioactive minerals from deep granite cores. But where were such specimens to be found? Holes deeper than 15,000 feet had been drilled in search of oil but always through sedimentary rocks such as limestones and sandstones.
An Innovative Approach to the Nuclear Waste Problem
In mid-1981 I learned of a 15,000-foot-deep hole in a granite formation drilled by the Department of Energy in New Mexico in the late 1970's. The purpose was to explore the possibility of using high-temperature rock at the bottom of the hole as a heat exchanger to generate steam energy. In this hot-dry-rock experiment (as it was called), water injected into one drill hole at the top would cascade to the bottom and be heated to steam. The steam would then return through a separate hole to a power-generating station on the surface.
Core sections were taken at five different depths from about 3000 feet down to about 15,000 feet during the drilling operation. Fortuitously, each of these granite cores contained many small crystals of the radioactive mineral zircon. These cores were exactly the samples needed to determine how well the radioactive zircons had resisted leakage under the increasing temperatures (up to 313° C at the bottom of the hole). My affiliation with the Laboratory [p. 164] proved invaluable in obtaining pieces of each one of these priceless cores.
The advantage of analyzing the zircons from these cores was clear: they had already experienced the exact environmental conditions anticipated for nuclear waste storage in granite. By determining the amount of diffusion or leakage of radioactivity out of these zircons, we could accurately determine whether it would be safe to encapsulate nuclear wastes in synthetic zircons of the same type. These experiments also had the potential of providing critical information about the age of the granites.
Remember that the element lead is the end product of uranium and thorium decay chains (and hence is known as radiogenic lead). Since zircon crystals contain small amounts of both uranium and thorium, there will be a constant accumulation of this element in zircons located on the earth's surface. That is, lead diffuses out of zircons very slowly at surface temperatures. With increasing depth, however, the temperature rises considerably, and the lead diffuses out of the zircons far more rapidly.
Now the age question enters the picture. If the granites in New Mexico are over a billion and a half years old, as uniformitarian geology supposes, this would be time for considerable amounts of lead to be lost from the zircons taken from the deepest (highest temperature) sections of the drill hole. In fact, in this scenario the lead should steadily diminish with increasing depth (due to steadily increasing temperatures). However, if the earth is only several thousand years old, only negligible lead loss is expected. In this case the amount of radiogenic lead in the zircons should be about the same regardless of depth. Here was a clear-cut test.
Earth Science Associates