MEGAWATTS AND MEGATONS

Technologies tapping the enormous energies churning within the nucleus of the atom have had world-changing impacts. The list includes nuclear medicine, radiography, isotope-dating of pre-historical artifacts, commercial nuclear-power reactors, and of course the atomic bombs unleashed at the end of World War 2 and the subsequent nuclear arms race. But here I am primarily concerned with the impact of radiation on human health. Assessing that impact is a complex task. Typically it requires a graduate degree in health physiology with a specialization in radiation physics. The authors devote several chapters to the topic. I present some aspects of their work here.

They begin by discussing the various units used to measure radioactivity. I summarize these in Table 1. There are many sources of natural radiation, and modern life throws additional exposures at us in forms such as dental x-rays, radiation therapy for cancer, lingering fallout from open-air atom-bomb tests in the 1960s, high-altitude air travel, and more. I attempt to tally these in Table 2.

The natural sources include radioactive forms of potassium and carbon in our own bodies, uranium and thorium found in rocks, and radon gas from the decay of uranium. The thing to remember is that we have grown up amid these exposures, not just as individuals, but as a species. We therefore have a certain amount of radiation tolerance built in. Averaged over the world's population, annual exposure from natural sources is 2.4 millisievert. Keep this baseline level in mind when comparing the exposures given in Table 2. The danger from artificial radiation such as fallout comes when that exposure substantially exceeds the natural radiation levels.

While acknowledging that there is ample reason for public wariness toward nuclear technology, the authors offer this sound advice about radioactive contamination (pages 79-80):

The following table shows the principal radioisotopes found in spent fuel rods from a 1 Gigawatt-electric light-water-moderated nuclear power plant. Amounts are expressed as kilograms produced in one year of plant operation. Equivalent in Natural Doses is calculated by dividing the average worldwide annual exposure of 2.4mSv, cited above, into the given values of "Dose per Gram Ingested (Sv)". Some of the half-life values given in the book were corrected as noted.

One of the best features about this book is its frequent reminders to keep the dangers presented by nuclear power in proper perspective. Examples are Table 7.2, which compares the radiation exposures due to nuclear and coal-fired power plants, and Table 7.3, which gives the 1994 death rates from familiar causes like motor vehicle accidents. To this second table, the authors added the expected deaths due to the nuclear fuel cycle and the Chernobyl and Three Mile Island reactor accidents. (The sound-bite version: Chernobyl 1, coronaries 876.) Noteworthy, too, is this comment from page 200:

This last table compares the radiation emission from 1-GWe nuclear power plants using both the once-through (i.e. direct disposal) and recycling fuel cycles against that from a coal-fired plant of equivalent electrical output. According to the authors, exposure values in the table represent "Collective effective dose to the public from effluents of the nuclear fuel cycle. Dose commitment in person-Sv per GWe-yr of operation." I have some concern over whether the numbers are accurate. I also corrected two addition mistakes found in the authors' Table 7.2.

The thing to note is that the concept of nuclear plants exposing the public to radiation while coal-burning plants do not is just simplistic. Note too that such a coal-fired plant burns 2 million tons of coal per year, and releases quantities of carbon dioxide as well as some level of nitrogen oxides (contributors to photochemical smog), sulfur oxide (contributor to acid rain), and mercury (a toxic metal).

References

Finding a definitive source for half-life values on the Web proved more difficult than I expected. I initially searched for a list that gave all isotopes in order by name and mass-number, together with the corresponding half-life values. I failed to find that sort of table. I did find some that gave a subset of isotopes, but no one of those had all the isotopes I wanted to know about, and the half-life values they gave for a given isotope were not consistent.

So I turned to the table of nuclides. Quite a few Web-based tables of nuclides exist, but they represent the isotopes as a matrix of tiny squares colored according to activity level. The size of the squares makes it hard to pick the isotope you want; thus these tables are somewhat unwieldy to use. Eventually I discovered Brookhaven Labs' "NUDAT 2.1 Table of Nuclides". This follows a similar format; but when you hover the mouse cursor over any square, a "tooltip" pops up to tell you the element symbol and isotope mass number. With a little practice, I could home in on the ones I wanted very quickly. Also, Brookhaven seems to be the ultimate source for such data. I based my corrections on its table.

For those interested, here's the link, in Brookhaven-approved citation format:

National Nuclear Data Center, information extracted from the NuDat database, http://www.nndc.bnl.gov/nudat2.

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