In the minds of many is set the view that the forces of nuclear power are so horrible that its activities should immediately be ended. Surely, we all stand before the images of Hiroshima and Nagasaki in a stunned and fearful contemplation. This is the clear and underpinning manifestation of our horror; and even if we have not witnessed atom bombs as agents of war in our own lifetimes (these images are now old), the media representations of the disaster at Chernobyl, and of the human burden which is its aftermath, fill us with dread. We are warned there are many potential "Chernobyls" awaiting us in a rotting nuclear legacy.
The U.K. is a small island-land, buffeted by the winds and tides; surely we might use this good-fortune to supply our energy from "wind-power", from "wave-power", since we also have sufficient economic resources to implement the technology this would require. Failing this, the U.K. remains sufficient in coal and can access oil and gas reserves; again because we can pay for it. So we, at least, could do without nuclear power, if we so decided. But, there is, as I have documented in this series of postings, a limit upon which we can place precisely what could be substituted by "renewables" for the colossal energy use that sustains modern society, and which must surely cease, in an alternative adoption of localised and communal societies. We might indeed "make it" if we chose our course carefully, growing our own food, insulating our houses and constructing them from more thermally efficient materials, and living in smaller communities of an estimated size of a mile or so square (hence containing just a few thousand people; i.e a village settlement), and moreover in this estimate thus shoving all the SUV's and most of the other unnecessary petroleum impelled vehicles off the road . We have in the U.K. enough coal, certainly for 200 years or so, and for the next decade or so we can probably buy-in enough hydrocarbon fuel; but we may need to fund an appropriate university to research into the"sequestration" of the consequent CO2 emissions (as Gordon Brown has promised in the recent budget) which will use-up anywhere up to 40% (no-one knows, so this is an average) of the energy produced in burning it in that particular "nappy changing" process!
Many countries have less choice, lacking both funds and natural resources. In the former Soviet state of Armenia, nuclear power is considered the lesser of evils in providing sustainable energy for the future. Armenia is in a precarious position: political instabilities - notably its conflict with Azerbaijan - prevent a reliable oil supply into the country; Lake Sevan has been severely drained for hydroelectric power, which has impacted on its ecology; the forests have been devastated - cut for firewood. Even some environmentalists in Armenia agree that the nuclear power plant is the only feasible means to fulfill this country's energy needs. Neighbouring countries are concerned, however, since the Armenian nuclear power plant, near the town of Metsamor, is located on an earthquake fault-line; the plant was closed in 1988, amid fears for its safety, when a large part of northern Armenia was devastated by an earthquake.
The main problem with running nuclear power plants (NPP's) is the interception and disposal of the nuclear waste that they generate. If, in the absence of alternatives, NPP's will continue to operate into the foreseeable future, it is vital to ensure they do so as safely as possible, and their radioactive waste must be contained securely.
A nuclear power plant will not explode like an atomic bomb, as many believe and fear. It was not a nuclear explosion that occurred at Chernobyl, but a conventional explosion caused by the development of enormous steam pressure in one of the reactors. Therefore, while the worst outcome will never be the cataclysm of Hiroshima or Nagasaki, it may cause the wholesale contamination of large areas, and potentially of their human and animal populations, by radioactive material (an accidental "dirty bomb"). Such catastrophes are extravagant events whose likelihood is minimised by adherence to correct operational practices and maintenance procedures for NPP's. Should such a terrible event occur, it is necessary to clean-up the radioactive detritus, having evacuated those populations most at risk; generally in near proximity of the event, or those at the mercy of adventitious weather-plumes which deposit the contamination more widely.
Radioactive material is an inherent feature of NPP's, since they use an initial radioactive fuel, which produces further radioactive products of its "burning". While radiation can be employed in humanly useful applications, some of which are mentioned later, it is highly toxic to humans: radiation can modify the constituent molecules of living cells - DNA is especially vulnerable, and is very dangerous when damaged, since later biological events in living cells can propagate mutations and cancers. This is principally why we fear radiation.
In an NPP, the nuclear fuel is contained in the form of "fuel-rods" which are in contact with cooling water. Some of the radioactive material is leached-out into this water, and must be removed prior to its drainage into the environment.
Zeolites are naturally occurring minerals with a tremendous capacity to absorb environmental pollution, both from radioactive and other kinds of contamination; indeed, thousands of tonnes of zeolites were transported to Chernobyl during the resulting clean-up process; some of which were mined in Armenia. The word "zeolite" means "stones that boil", a feature which provides a clue to their abilities overall: the structures of zeolites comprise tiny holes, comparable to the size of individual molecules (micropores), into which actual molecules may be sorbed. In consequence, natural zeolites usually contain a large quantity of water which, when they are heated, erupts as a froth of hot liquid and steam, appearing as though the stones themselves are boiling. Their facility of molecular sorption confers to zeolites a number of important applications, for example: "catalytic cracking" in the petroleum industry; as drying agents (molecular-sieves) in chemical synthesis; in the separation of oxygen from nitrogen in the air; in ion-exchange. Of this (highly incomplete) list, "ion-exchange" is the accomplishment of zeolites most central to the issue of radioactive decontamination. The structure of a zeolite may be viewed as an essential porous framework, which carries an overall negative electrical charge; this is neutralised by the presence of positively charged ions (cations), e.g. H+, Na+, K+, Ca2+, Mg2+, and many more (some cations have one and others two units of positive charge, depending on their type of atom). In consequence of some delicate interplay between the charges in zeolites and the size of their micropores, a given zeolite will have a different affinity for different cations. We most often encounter this effect in "water softeners", some of which use zeolites, and so the calcium (Ca2+) and magnesium (Mg2+) cations that make water "hard" are absorbed into the zeolite, by displacing into the water those H+ or Na+ cations originally present (H+ and Na+ cations do not cause water hardness). The process is called "ion-exchange".
The cooling-water from nuclear power plants is contaminated with strontium (Sr2+) and caesium (Cs+) cations, which are radioactive, and hence are a cause for concern. These contaminants are present at low levels in relatively large volumes of water (8,000 tonnes per year are produced from the Armenian NPP). Clearly, this water cannot simply be "dumped" into rivers or drained into the ground, while acknowledging unnerving tales from less-enlightened days when precisely this was done. (It is probably a fair judgement, that all parties involved in the early years of nuclear power each had their own share of calamity).
It is uneconomic and impractical to simply boil-off the water to obtain the Cs+ and Sr2+ as a residue for further use or disposal. The ion-exchange capacity of zeolites comes into its own right here; in a collaborative project involving U.K. and Armenian scientists, the NPP cooling water is filtered through barrels loaded with a natural zeolite called clinoptilolite. Each barrel contains about "half-a-ton" of zeolite. Clinoptilolite is especially abundant in Armenia, and may be mined for this and other applications. In its natural condition, the zeolite is rich in Na+, Ca2+, Mg2+ and K+ cations, which are displaced by Sr2+ and Cs+ cations ("water-hardness" is not an issue here!). Ultimately, the displacement is complete, and the zeolite is saturated with radioactive cations, which are now in a fairly concentrated form. The zeolite is then removed, either for recovery of the Cs+ and Sr2+ in the form of chemical compounds, such as CsCl and SrCl2, or for long-term disposal, as discussed later. Fresh charges of zeolite are next introduced, and the entire process is repeated.
Clinoptilolite does not occur as a pure material, but needs to be purified from accompanying rock and clay. The crude mineral is known as "tuff". This is first crushed into a fine gravel, and then treated with dilute acids and alkalis, to release the zeolite in almost pure form. Experiments are underway to improve the efficiency of clinoptilolite in ion-exchange, and to increase its selectivity for Cs+ and Sr2+ cations. The aim is to make the whole process of cleaning NPP cooling water more effective, so that the output water contains the minimum possible levels of these cations. Such an improved zeolite would also need to be replaced less often, but this is not a major consideration given the overall costs involved.
The simplest way to monitor the speed of an ion-exchange process and its efficiency, is to compare, over time, the cations in the input solution with those in the output water. This does not, however, reveal the inner-workings of the zeolite. Sorption into the zeolite may be investigated using a variety of molecules as probes. To investigate the diffusion of the cations themselves, radioactive ions are often employed, e.g. 137Cs+ and 90Sr2+ cations may deliberately be added to an input solution and monitored by measuring their radioactive decay. This is a very sensitive method. Since the solvent (water) plays a major part in the whole process, it is also necessary to determine its diffusion, for which a similar approach, using radioactive water, might be helpful. Alternatively, more complex cations can be introduced, responsive to the pervasive "eye" of spectroscopy, which reveals their movement within the zeolite in association with the solvent. This system provides by far the greater insight.
Since zeolites are not fully transparent to light their contents remain largely invisible, unless they are squeezed artificially into very thin wafers; alternatively, a magnetic "eye" can be used. A magnetic field can penetrate large samples, even a human body in magnetic resonance imaging (MRI), used to scan for cancers and other diseases; therefore a small zeolite sample presents no obstacle. A selectively designed cation probe is introduced to the input water and allowed to be absorbed into the zeolite micropores. The entire sample is then placed in a strong magnetic field. Since the cation probe is a tiny molecular magnet, we could measure the magnetic strength of the collection of these magnets in the zeolite, which would be futile as it would only tell us what we knew already - i.e. how many probe molecules we had introduced. There is no need for disappointment, since a greatly sophisticated adaptation of the strategy is possible, based on the fact that the magnetic molecules are sensitised by the applied magnetic field to absorb very weak microwaves, which can also penetrate zeolites easily. Since the detail of the microwave absorption depends on how the cations and the solvent water molecules coexist and diffuse within the zeolite structure, the influence of the electrical charges in differently treated zeolites can be determined. Information is thereby provided, on the molecular level, about the relative efficiency of different zeolites in ion-exchange.
The aim of this work is to provide an immediate evaluation of batches of clinoptilolite, before they are introduced directly to clean the NPP cooling water output. This will help to minimise overall operational costs.
Disposal of nuclear waste.
One question stands-out from the many - what ultimately is to be done with thousands of tonnes of zeolites, heavily laden with concentrated radioactive material? Since there are beneficial uses for radiation, it may sometimes be advantageous to recover Cs+ and Sr2+ cations, by re-ion-exchanging the zeolite with Ca2+ cations. In the highly concentrated form of their compounds, e.g. CsCl and SrCl2, 137Cs+ and 90Sr2+ may be used to irradiate other materials for a variety of purposes: radiation kills bacteria, so food and medical supplies (bandages and surgical instruments) can be sterilised; radiation can be used in cancer-treatment; plastic materials with specific properties can be created with radiation; there are many such applications. Alternatively, and ultimately, radioactive material must be disposed-of. Clearly, this must be done carefully, with respect for the longer-term. In isolation, zeolites store radioactive cations very securely; they are also highly robust materials, and resist the effects of radiation so their integrity is preserved over many hundreds or thousands of years. In this respect alone, zeolites are far superior to ion-exchange resins, which sustain severe damage from radioactive cations, and can no longer contain them after even only a few years. The security may be further supplemented by melting the zeolite into glass "bricks", which are then stored.
It is essential to choose a final storage site well-away from groundwater, otherwise ion-exchange and erosion forces will slowly release the radioactive charge into the environment. One example is Yucca Mountain, in Nevada. Here, the storage channels are situated at least 200 metres both below the surface, and above the water-table; the annual rainfall is also very low. The channels are sealed in clay, and are further reinforced by rich veins of zeolites present at lower levels, to guard against the long-term leaching of radioactive material into the environment. It is intended to store the entire radioactive output of the U.S. nuclear industry over the next 100 years at Yucca Mountain; there it will remain for millennia. High-level nuclear waste from fuel reprocessing will also lie here. Other such sites will need to be found for other centuries and other countries; an on-going effort will be required, while nuclear power continues.
A nuclear future?
It is for politicians to decide on the future of nuclear power, and I am not a politician. In any event, it is clear that existing NPP's will continue to operate for decades to come; indeed, the former USSR has for some time considered that a realistic sustainable energy programme requires their proliferation; an opinion now shared by the U.K. government. Political decisions are driven by economic will and priority, and both the decommissioning of NPP's and their substitution by alternative technologies, will prove extremely costly (estimated at £85 billion in the U.K., alone). It is arguable that money is not the only cost involved, but that long-term radioactive pollution (including acts of terrorism) is the ultimately high price. These are all complicated issues, which are the responsibility of our politicians, but while they are debated, and weak nuclear empowered economies have little choice of alternatives, there will be nuclear power. It is my own hope that the present time reflects a temporary and interstitial period, pending the ultimate resolution of economic and social dilemmas. Meanwhile, science can offer some practical consolation, both in reducing future contamination from NPP's and in mitigating past legacies. We can at least try to "cleanse" the nuclear earth, and so minimise our own legacies to it.
There is a rider to all such considerations: namely that uranium is not an unlimited resource. The final day for nuclear power must surely be reached when its vital fuel has run-out, and this may occur in only 50 years, unless we go down the route of fast-breeder reactors which the French are focussed upon in the long-term and that strategy would eke-out the uranium [by "burning" the 99+% isotope of uranium (238) via its conversion into plutonium (239)] perhaps to several hundred years, but by flooding our planet with a mightily toxic material that terrorists would rub their hands with joy in handling it (though carefully!).