Nuclear Waste: Storage and Disposal Methods

"In the half century of the nuclear age, the U.S. has accumulated some 30,000 metric tons of spent fuel rods from power reactors and another 380,000 cubic meters of high-level radioactive waste, a by-product of producing plutonium for nuclear weapons.  None of these materials have found anything more than interim accomadation, despite decades of study and expenditures in the billions of dollars on research, development and storage,"  Chris G. Whipple, Can Nuclear Waste Be Stored at Yucca Mountain?  Scientific American, June, 1996
 

Nuclear Waste:  The Dilemma

For over 40 years nuclear technology has spread into many areas of modern society, enabling advances in energy production, defense and medicine.  But along with the use of nuclear technology comes an added burden -- nuclear waste.  Nuclear waste is the type of waste that results from the use and production of nuclear materials.  As nuclear materials are produced and used up, one by-product of the process is a large amount of dangerous chemical elements.  Plutonium is the most dangerous of these and will therefore be the elemental topic of this discussion.  Plutonium is highly radioactive and has a half-life of 25,000 years (Bullen and McCormick 682).  This means that plutonium takes approximately 25,000 years to decay to half of its original potency.  The immediate and long-term threats of radioactivity include causing cancer or genetic damage in humans and animals; large amounts lead directly to radiation sickness and death ("Nuclear" 588).  Also, any form of plutonium may be fashioned into a very potent nuclear weapon; this poses a threat to the safety of humanity (if this nuclear waste were to fall into the wrong hands).  The stockpile of nuclear-grade plutonium continues to grow as the use of nuclear energy in its various forms is proliferated.  Hence, what is being done to stave off the possible negative effects of this valuable metal -- either the material for energy in the future or the stuff of nuclear weapons and potential environmental danger?

The Sources

At least four sources exist which contribute to the growing plutonium inventory: (1) the increasing amount of excess military plutonium worldwide from arms reduction agreements (As nuclear warheads are dismantled, plutonium is a leftover); (2) the increasing amount of civilian plutonium from nuclear power reactors (which transform energy for civilian use); (3) the increasing amount of separated plutonium from the reprocessing of spent nuclear fuel; and (4) the small amount of plutonium from nuclear research reactors (Bullen and McCormick 684).  And all of this surplus must be discarded somewhere, in a safe and effective manner.  Therefore, the major processes explained her are how the plutonium waste is stored and disposed of.

Current Storage Methods

Currently, only temporary storage areas exist for the disposal of radioactive waste.  The U.S. government is working to devise a plan for the safe storage and permanent disposal of nuclear wastes.  Plutonium waste from nuclear fuel used in nuclear power plants and similar reactors is highly radioactive and needs to be shielded and handled very carefully at remote distances.  At this time the waste from these spent fuel rods is stored in specially-designed, water-filled basins or dry casks at commercial power reactor sites or at one away-from-reactor storage facility.  Most of the weapons grade plutonium pits (pits are the nuclear triggers for a hydrogen bomb) from the dismantlement of nuclear weapons are being dry-stored in casks at a facility outside of Amarillo, Texas (Bullen and McCormick 683).  But as the plutonium is broken  down, plutonium dust is created which presents a relevant safety concern.  If this dust were dispersed into the atmosphere, radioactive plutonium could spread quickly and easily through the environment, creating a very dangerous hazard to humans and the environment.
 

Projected Disposal Methods

 A few options are being considered for the permanent disposal of nuclear waste, including: the mixed oxide (MOX) fuel burning method, the vitrification method and the subductive waste disposal method.

 The MOX fuel burning option is carried out by mixing plutonium with uranium and producing a slightly different fuel than is now used for civilian reactors.  The object of this is to burn up the plutonium by nuclear fission (the splitting of atomic nuclei which releases large amounts of energy).  This means more of the plutonium can be used as an energy source in a relatively quick fashion.  The burning of the fuel does produce a product of excess plutonium, but this product is far less dangerous to the environment and the global community than the initial plutonium is.  So the amount of plutonium is actually reduced as well as leaving the remaining plutonium much less usable for weapons and easier to dispose of (Bullen and McCormick 692).  However, the product would still need to be disposed of in some effective manner in order protect the environment and to keep the excess plutonium safeguarded from use in nuclear weapons manufacturing.

  The vitrification method involves the mixing of weapons-grade plutonium with radioactive waste from civilian reactors and placing this mixture in borosilicate glass logs.  The logs would then be buried in a deep borehole that is at least 4 kilometers deep (Bullen and McCormick 690).  The idea here is that the plutonium could be suitably encased and isolated to the extent that its decay process may occur without polluting the environment, or being utilized in the manufacture of nuclear weapons.  But unfortunately, the only way to know for sure if the encasement will not leak is to try it.  This means that it is possible for leakage to pollute the water table also the plutonium could still be mined in the future and used for the manufacture of nuclear weapons.

  The Subductive Waste Disposal Method is the most viable means of disposing of radioactive waste.  The idea is that the waste is removed from the biosphere faster than it can return.  Subduction refers to a process in which one tectonic plate slides beneath another at rate of about 6 cm annually, while being reabsorbed into the Earth's mantle.
 

The Subductive Waste Disposal Method involves the formation of a radioactive waste repository in a subducting plate.  As the plate is reabsorbed, the waste will be absorbed along with the plate where it will be dispersed through the mantle.  Subducting plates are naturally structured for absorption in the Earth's mantle.  And the plate is constantly renewed at its originating oceanic ridge.  The plate moves slowly so that any fractures over a repository would be sealed at the contact point between the overriding plate and the subducting plate.  Therefore, this method would obviously need to be implemented in a geographically active region.  The most accessible site would then be on the ocean floor at a point above where subducting plates meet several thousand feet below the water's surface and then another 2,000 meters beneath ocean sediment.  The repositories would be virtually inaccessible once filled and sealed.  Once the waste is carried into the interior of the Earth, it would take many millions of years "for the waste to circulate through the Earth's mantle before it could re-emerge in a diluted, chemically and physically altered form at an oceanic ridge (Baird)."  The Subductive Waste Disposal Method would prevent radioactive waste from mixing with the water table, provide inaccessibility to eliminated weapons material, remove radioactive waste completely from its threatening position, and be completely safe for marine life.

Conclusion

 As the search for a definitive method for the disposal of nuclear waste drags on, the drawbacks associated with merely storing the waste drag on as well.  The mixed oxide fuel method would provide a relatively quick solution, but would not completely provide for the waste's disposal.  The vitrification method would also provide a more attractive means for dealing with nuclear waste than simply storage, however the long-term problems with vitrification are uncertain.  Subductive Waste Disposal would require extensive research and development to implement, but this is a small price to pay for an effective solution to the as-of-yet unanswered question -- what to do with nuclear waste?

Bibliography

Bullen, Daniel B. and James M. McCormick.  "Disposing of the world's excess plutonium." Policy Studies Journal.  26.4  (1998):  682-703.

"Nuclear Energy." World Book Encyclopedia.  1989 ed.

Baird, Jim.  "The Subductive Waste Disposal Method."  Online.  17 October 1999. www.Radwaste.org