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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Nuclear Power Part III: Safety and Conclusion

By Jeff Danner Posted September 30, 2012 at 9:05 pm

This is the third and final installment of my series on nuclear power.  I covered the basic science in Part I and the challenges of waste management in Part II. This week I will discuss nuclear power plant safety and share my thoughts on whether we should continue to produce electricity by splitting atoms.
 
Here is a quick review of how a nuclear power plant generates electricity.
 

  • Uranium-containing ore is mined and purified to a level of 4% uranium-235 (U-235), the isotope which is used for nuclear power.
  • Within the reactor the U-235 nuclei are broken (fission) by hitting them with high-energy neutrons.
  • Nuclear fission generates heat which is used to boil water.
  • The resulting steam is used to spin a turbine.
  • The energy of the spinning turbine is converted to electricity through a process I covered in “Electricity Generation 101” which you don’t need to review for the purposes of this column. (Although it does make for a great read.)

 
While the world’s 400+ nuclear reactors have a wide variety of designs, there are several safety systems that all have in common.  (The graphic at the top of the page shows the basic design features of a nuclear reactor.)  By a significant, margin the most important safety feature of a nuclear power plant is limiting the U-235 content of the fuel roads to 4%, a concentration too low to reach the critical mass required for a nuclear explosion. 
 
Each time a U-235 nucleus splits, two or three neutrons are released, each of which have the potential to split an additional nucleus.  If left unmoderated this process generates heat far too quickly to be managed.  Therefore, all nuclear power plants utilize control rods (represented in the graphic with green lines) to moderate the rate of fission.  Control rods are made from metals, such as cadmium or cobalt, which can absorb neutrons without undergoing fission.  If for some reason the control rods are not successful in controlling the rate of fission and, thus, the reactor temperature, the system is equipped with an emergency cooling system (not shown on the graphic) in which water or molten salt is injected into the core.
 
The nuclear power plant safety incident of greatest concern is a reactor core meltdown.  If the emergency cooling system does not function when called upon, the fuel rods can reach temperatures of over 5000 oF.  A temperature increase of this magnitude can rupture the reactor core, either from a pressure buildup from superheated water and steam, or from the fuel rods becoming so hot that they turn into molten metal and melt their way through both the reactor core and the concrete shielding.
 
If there is a reactor core breach, a series of radioactive materials, the fission products of the U-235 fuel, are released into the atmosphere, including gases like radon, and radioactive forms of iodine, caesium, and tellurium.  Exposure to these radioactive materials causes cancers in humans and animals.  The highest concentration of radioactivity fallout is in the vicinity of the incident, but the radioactive materials can be transported hundreds or thousands of miles.  Bear in mind as you read this that the Shearon Harris Nuclear plant in New Hill, NC is about 25 miles from Chapel Hill.
 
The safety systems for nuclear reactors have been demonstrated to be sufficiently robust to manage day-to-day plant operations.  As you might expect, all three of the world’s most well-known nuclear accidents, Three Mile Island, Pennsylvania in 1979, Chernobyl, Ukraine in 1986 and Fukashima, Japan in 2011 occurred during non-standard conditions.  This is a common feature of all types of industrial accidents, not just nuclear power plants.
 
At Three Mile Island, the operators where refueling one of the two reactors when a relief valve on the other, still operating reactor, opened, allowing coolant to drain out of the reactor core.  At first the operators misunderstood the situation, thus their initial actions served to exacerbate the situation.  The reactor did not experience a full meltdown, but as the operators worked to understand and correct the situation, radioactive gases and iodine were released to the atmosphere.
 
The Chernobyl disaster in 1986 began, ironically, with a malfunction during a test of an emergency shutdown system designed to prevent just the sort of incident which occurred.  When the operators initiated the test temperature, spiked rapidly, causing a dramatic increase in water pressure which ruptured the reactor core.  Similar to Three Mile Island, the Chernobyl operators’ initial misunderstanding of what was happening led them to take actions which made the situation worse.  The Chernobyl reactor experienced a full core meltdown releasing more radioactive material into the atmosphere than the atom bomb in Japan from World War II.  The Soviet government initially attempted to conceal knowledge of the incident from the rest of the world and, tragically, did not inform nearby citizens of the danger.  The world learned of the truth two days later when workers in a Swedish Nuclear Plant detected radioactive particles on their clothing.
 
The earthquake and subsequent tsunami in Fukashima in 2011 knocked out the electricity supply for the plant.  When a nuclear plant loses power from the electric grid, it is designed to switch over to emergency backup power to shut down the reactor and to inject emergency coolant into the reactor core if needed.  The emergency backup power for Fukashima was designed to come from diesel-powered generators.  (These are just larger versions of the type of gas or diesel powered generator you can buy for your house from Lowes or Home Depot.)  Fukashima’s diesel generators where located in the basement of the plant and were, thus, destroyed by sea water when the plant was overrun by the tsunami.  By the time the situation was brought back into control, the release of radioactive materials from the Fukashima incident was worse than Three Mile Island but not as bad as Chernobyl.
 
Given that the world currently hosts a fleet of a little over 400 nuclear power plants, how frequently should we expect reactor core meltdowns to occur in the future?  Organizations ranging from the Massachusetts Institute of Technology and the Max Planck Institute in Germany have estimated that we can expect a serious nuclear accident every 8 to 20 years.  As the average age of nuclear plants around the world grows, I would expect the rate of incidents to increase.
 
Conclusion
 
Many thanks to all of you who stuck with me for all three installments on nuclear power, and for the insightful comments on chapelboro.com and your e-mails.  Now that we’ve covered the basic science, waste disposal challenges, and safety issues, let me share my thoughts on the place for nuclear power in our future.
 
Given the significant hazards associated with nuclear power, many have called for an immediate shut down of all facilities.  While I sympathize with this position, I cannot support it.  Nuclear power currently provides 14% of the electricity for the world and 20% for the United States.  An immediate removal of this amount of electric power capacity would result in a global economic collapse, the human costs of which would be more catastrophic than even a series of reactor core meltdowns.   Further, a dramatic near-term decrease in nuclear power would result in a large increase in the rate of green house gas emissions due to the corresponding dramatic increase in the number of coal-burning power plants which would be built to replace the lost supply from the nuclear plants.
 
The only practical and ethical pathway to a sustainable future that I consider to be plausible is a massive national effort to construct solar electric plants and corresponding upgrades to the power grid.  The efforts and expenses on this monumental project need to be on the equivalent scale of what we have invested in the military over the last several generations.  The growth of solar power capacity must be tied to firm targets to reduce per capita electricity use and an enforceable schedule for decommissioning both fossil fuel and nuclear power plants.  I plan a future series of columns on how this transition to solar power can take place.
 
Unfortunately, as I have bemoaned before, our current social and political culture is not at all prepared to grapple with issues of with import and complexity of nuclear power.  Discussions of energy resources and long-term sustainability during the current presidential election campaign have been shallow and inconsequential at best.  So for now, we’ll just have to hope the next melt down is far away.
 
Have a comment or question?  Want to disagree?  Use the comment interface below or send me an e-mail at commonscience@chapelboro.com.

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