Disposal of Nuclear Wastes
From Thermal-FluidsPedia
Waste management is one of the most controversial and pressing issues tarnishing the image of the nuclear industry. Nuclear waste has not only presented challenges for the present generation, but has also endangered the welfare of future generations for many tens of thousands of years.
Example: The concentration of strontium-90 in high-level liquid wastes is about 1 curie (Ci) per gallon. If the concentration is to be dropped to 10-9 curie per gallon for it to become safe, how long do we need to wait?
Solution: Stronium-90 has a half-life of 30 years. In 10 half-lives (300 years), the concentration drops by 1000; in 20 half-lives (600 years), the concentration drops by a million, and in 30 half-lives (900 years), the concentration drops by a billion to an acceptable level. Examples like this help us understand the concern environmentalists have for the storage of nuclear waste.
As of yet, there is no completely reliable method of permanently disposing of nuclear waste. All current methods are only interim measures. In addition, there is no reliable information on the total amount of nuclear waste we have produced so far. No matter how small or large a role nuclear energy plays in meeting future energy needs, developing technologies that can safely dispose of the intensely radioactive byproducts remains a top priority. Here are some promising ideas.
Permanent subterranean storage – The most commonly favored method for disposal is the placement of waste into deep geological repositories. Factors affecting this determination are soil stability, proximity to large water reservoirs and run-offs, seismic activity, and the local population. An ideal site must be completely dry, with no possibility of moisture percolating through the cracks and corroding the alloys. Salt has been shown to be an effective barrier to radiation; therefore lands with large salt deposits located in geographically stable landmasses are believed to be the best choice.
The US government has considered many sites and has decided on a repository near Yucca Mountain. This Nevada desert site, with its thick section of porous volcanic rock, will serve as the US’s first permanent underground repository for more than 40,000 metric tons of nuclear waste that has already been accumulated. The plan includes processing the spent fuel in steel canisters and inserting them into holes drilled in the rock floor of caverns hundreds of meters below the surface.
Residents of nearby communities have vehemently opposed this construction (Figure 1). Some jokingly refer to Yucca Mountain as “Yucky Mountain” or “Yuck-a-Mountain.” Other opponents charge that the government has overlooked seismic and volcanic activity and other potential dangers in the area in the rush to find a location. Still others claim that since the Department of Energy has set its mind on Yucca as the permanent repository and has already spent billions of dollars on the Yucca project, they are unwilling to consider other sites or other storage alternatives. As a result, incentives for seeking more innovative solutions and better options have been gradually diminishing.
In the meantime, nuclear wastes are piling up across the nation. Waste levels are getting so high that even if the Nevada facility were constructed, it would not be large enough to house all of the waste. Originally planned to open by 1998, the date has now been pushed back to 2010, although many experts doubt that even this date is realistic.
Entombment under the seabed – This plan is similar to the plan outlined above, but in this case, canisters are dropped from ships and allowed to come to rest on the deep ocean floor far below. The advantage of this plan is the use of seafloors, which are far away from the shore and from people. However, the potential leakage could be catastrophic, and there are international treaties that bar the disposal of radioactive wastes at sea.
Nuclear transmutation – This method converts (transmutes) the radioactive waste materials with very long half-lives to short-lived or non-radioactive products by bombarding them with elementary particles such as neutrons. For example, by absorbing a neutron, technetium-99 (T1/2= 210,000 years) becomes technetium-100, which then rapidly decays to rubidium-100. The Ru-100 isotope is non-radioactive, with a half-life of only 17 seconds. Another example is the transmutation of iodine-129 (T1/2 = 17 million years) into the stable element xenon-130. An intense beam of fast-moving protons produced by a linear accelerator hits a lead or tungsten target and knocks out a large number of neutrons which then attacks the radioactive waste. In the process, additional energy is released, a portion of which is used to run the accelerator; the remainder would be fed into the regional power grid.
In addition to these, there have been other proposals that are considered impractical. Among the less serious proposals are:
Taking nuclear waste into space or to the moon – The major concern in regards to this method is an explosion during the launch of the rocket containing nuclear waste. In light of the explosions of the space shuttles Challenger in 1986 (Figure 2) and Columbia in 2003, this fear seems ever more relevant. As space launch technology becomes more and more reliable, it may become possible to find a nearly fool-proof method to send the waste products into orbit around the sun or bury them on the moon and other planets.
Storage under polar icecaps – The concern surrounding this proposal is that a large amount of heat generation would result in melting of major icecaps with irreversible environmental consequences.
References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005