Selasa, 15 Maret 2011
Dangers of Nuclear Energy
While nuclear energy offers a source of greenhouse emissions-free electricity, it also poses dangers that are unique to it as an industry, for it is the only source of power that produces radioactive waste and has catastrophic consequences in the event of a massive safety failure. While the hazards posed by the waste products of nuclear energy are similar in some limited respects to those posed by other toxic industrial processes, radioactive spills have a the potential to poison the landscape for centuries, not years. Meanwhile, the perils of a reactor accident are incomparable to those of any other industry.
Nuclear energy is the process which converts heat produced by a controlled nuclear fission reaction, first, into kinetic energy by means of a turbine, and then into electricity by means of a generator. In this respect, it is no different in its general aspects from other thermal power plants that derive their heat from the burning of fossil fuels, or from geothermal sources. The main source of danger from nuclear energy stems from the nuclear fission reaction and the wastes produced.
There are two types of dangers involving nuclear energy. The first would be an accident resulting in loss of control over the fission chain reaction. The danger here is that the heat produced would outstrip the ability of the reactor coolant to cope, causing the nuclear reaction. This could cause system failures which would release radioactivity into the environment. In the case of an extreme failure, the result would be a nuclear meltdown, where the reacting nuclear material burns or melts its way through its containment vessel, into the ground, and then into the water table. This would throw a huge cloud of radioactive steam and debris into the atmosphere. Accidents of this type have the potential to release radioactivity over an immense area. A small, well-contained accident might just contaminate the power plant, while a major one could result in fallout being spread worldwide. The second danger stems from the disposal of waste from the reactor. Spent fuels from a nuclear power plant are radioactive and highly toxic.They also pose security risks, as a terrorist who acquired a substantial amount of nuclear waste could construct a so-called "dirty bomb," with the purpose of spreading radioactive materials over a large area. An accident or attack involving radioactive waste would likely contaminate a strictly local area.
The most significant aspect of a nuclear accidents is its incredible potential duration. Radioactive materials remain toxic for centuries or millennia, and have the potential to render contaminated areas dangerous or uninhabitable for vast lengths of time.
As nuclear power plant design has developed, it has become intrinsically safer. Systems are now designed with multiple redundant back-ups, with the entire design operaing around a defense-in-depth approach to prevent a complete failure and meltdown. Also, modern designs make the possibility of a critical loss of control less likely. For example, the light water design uses water as the reaction moderator. The hotter the water becomes, the less dense it becomes and therefore the greater moderation it places on the reaction, creating a negative feedback loop.
On the other hand, little of substance has been done to address the long-term storage and disposal of radioactive waste in the United States. The long-standing Yucca Mountain Repository project remains hotly contested, shows little prospect of ever opening its doors, and there is no substantial proposal for replacing it with another capable waste depository. In the meantime, radioactive waste remains scattered around the country in storage facilities that are, strictly speaking, only meant for temporary use.
It is often thought that a severe nuclear accident at a reactor would result in a nuclear explosion. That is impossible. An atom bomb requires a level of uranium enrichment that is simply never found in commercial nuclear power plants, and even breeder reactors containing plutonium are incapable of producing the conditions for the sudden, runaway nuclear fission required for an explosion. Any "explosion" in a nuclear accident is going to be from the massive eruption caused by the steam eruption from the hot nuclear pile hitting the local groundwater.
There have been two nuclear power plant accidents of note. The first was the 1979 Three Mile Island accident in Pennsylvania. The second (and far more substantial of the two) was the Chernobyl Disaster of 1986, which took place in the Ukraine (then the Soviet Union). Accidents involving the mishandling or theft of radioactive waste have been far more common, but not were comparable to either Three Mile Island or Chernobyl in terms of the amount of radioactivity released or area effected.