Temps de lecture :4 minutes
In due course the process generates waste in the form of spent fuel elements and, whether these are then reprocessed and re-used or not, eventually they have to be disposed of. But first they must be allowed to cool off in ponds to allow the isotopes to decay to some extent, for between 10 and 100 years – sixty years may be taken as typical. The ponds need a reliable electricity supply to keep them stirred and topped up with water to stop the radioactive fuel elements drying out and catching fire. In due course, these wastes will need to be packed, using remotely-controlled robots, into very secure canisters lined with lead, steel and pure electrolytic copper, in which they must lie buried in giant geological repositories considered to be stable. It may turn out in due course that there is one best solution, but there will never be an ideal way to store waste which will be radioactive for a thousand centuries or more and, whatever option is chosen, it will require a lot of energy. For example, the energy needed over the lifetime of a reactor to manufacture the canisters (each weighing more than ten times as much as the waste they contain), and to make the electrolytic copper, has never been verified, but it is estimated to be about equal to the energy needed to build the reactor in the first place.
A second form of waste produced in the generation process consists of the routine release of very small amounts of radioactive isotopes such as hydrogen-3 (tritium), carbon-14, plutonium-239 and many others into the local air and water. The significance of this has only recently started to be recognised and investigated.
A third, less predictable, form of waste occurs in the form of emissions and catastrophic releases in the event of accident. The nuclear industry has good safety systems in place; it must, because the consequences of an accident are so extreme. However, it is not immune to accident. The work is routine, requiring workers to cope with long periods of tedium punctuated by the unexpected, along with “normality-creep” as anomalies become familiar. The hazards were noted in the mid-1990s by a senior nuclear engineer working for the U.S. Nuclear Regulatory Commission: “I believe in nuclear power but after seeing the NRC in action, I’m convinced a serious accident is not just likely, but inevitable… They’re asleep at the wheel.” Every technology has its accidents; indeed, the Nuclear Regulatory Commission estimates the probability of meltdown in the U.S. in a twenty-year period as between 15-45 percent. The risk never goes away; society bears the pain and carries on but, in the case of nuclear power, there is a difference: the consequences of a serious accident – another accident on the scale of Chernobyl, or greater, or much greater – would take nuclear power towards being an uninsurable risk, even with the help of government subsidies for the premiums.
And a by-product of this – “waste” in the fourth sense – is the plutonium itself which, when isolated and purified in a reprocessing plant, can be brought up to weapons-grade, making it the fuel needed for nuclear proliferation. This is one of three ways in which the industry is the platform from which the proliferation of nuclear weapons can be developed; the second one is by enriching the uranium-235 to around 90 percent, rather than the mere 3.5 percent required by a reactor. The third consists of providing a source of radioactive materials which can be dispersed using conventional explosive – a “dirty bomb”.
Every stage in the life-cycle of nuclear fission uses energy, and most of this energy is derived from fossil fuels. Nuclear power is therefore a substantial source of greenhouse gases. The delivery of electricity into the grid from nuclear power produces, at present, roughly one third as much carbon dioxide as the delivery of the same quantity of electricity from natural gas…
… or, rather, it would do so, if the full energy cost of producing electricity from uranium were counted in – including the energy cost of all the waste-disposal commitments. Unfortunately (in part because of the need to allow high-level waste to cool off) that is not the case. Nuclear waste-disposal is being postponed until a later date. This means that the carbon emissions associated with nuclear energy look rather good at the moment: at about 60 grams per kWh they are approximately 16 percent of the emissions produced by gas-powered electricity generation. The catch is that this figure roughly doubles when the energy-cost of waste-disposal is taken into account, and it grows relentlessly as the industry is forced to turn to lower-grade ores. What lies ahead is the prospect of the remaining ores being of such poor quality that the gas and other fossil fuels used in the nuclear life-cycle would produce less carbon dioxide per kilowatt-hour if they were used directly as fuels to generate electricity.
Carbon dioxide is not the only greenhouse gas released by the nuclear industry. The conversion of one tonne of uranium into an enriched form requires the addition of about half a tonne of fluorine, producing uranium hexafluoride gas (hex) to be used in the centrifuge process. At the end of the process, only the enriched fraction of the gas is actually used in the reactor: the remainder, depleted hex, is left as waste. Not all of this gas can by any means be prevented from escaping into the atmosphere, and most of it will eventually do so unless it is packed into secure containers and finally buried in deep repositories.
The Lean Guide to Nuclear Energy: A Life-Cycle in Trouble