60 © The Containment of Underground Nuclear Explosions Figure 4-1—The Typical Bimodal Curvefor Fission-Product Yieid Table 4-1—Common Radionuclides Invoived ina Nuclear Explosion Radionucide Half-Life Uranium-238 ..............-0 00.0005 4,500,000.000 years Plutonium-239 .......-. 00.0... 0000000 cee 24,300 years Cardon-14 200.0 eee . 5.800 years Radium-226 20000. ec nee 1,620 years Caesium-137 ...00 16. eee .... .. 30 years Strontium-90 20... ce ae 28 years Percent fission yleid Tritum oo. cee ... 12.3 years Krypton-€§ 6000 ee eee .... 10.9 years lodine-131 20... eee. eo. 2... , 8 days K@NON-133 2. ee eee . 5.2 days lodine-132 2... eee ee . 2.4 hours 1 | | oO FO x oO ho Ls | '| | 2395, — I er jot ey a I The type of release is also importantin predicting what radionuclides will be present. For example. atmospheric tests release al] radionuclides created. Prompt, massive ventings have released a nonnegligible fraction of the radionuclides created. Latetime, minorseeps,like those since 1970, release only the most volatile radionuclides. In an underground explosion, radionuclides also separate (called *' fractionation”’) according to their chemical or physical characteristics. Refractory particles (particles that do not vaporize during the nuclear explosion) settle out fast underground, while more volatile elements that vaporize easily condenselater. This has a strong 16 «118 30 142 94 ! 166 Mass number Products of a nuclear explosion occur predominantly in two major groups of nuclides. SOURCE: Modified from Lapp and Andrews, Prentice-Hall, Inc., 1972. An individual radioactive species follows the half-life rule of decay—that is. half of the nuclei disintegrate in a characteristic time. called a ‘*halflife.’” However, a mixture of fission products has a more complicated decay pattern. The general rule of thumb for a nuclear explosion is that the total activity decreases by a factor of 10 for every sevenfold increase in time. In other words, if the gammaradiation | hour after an explosion has an intensity of 100 units, then 7 hourslater it will have an intensity of 10. Consequently. the time after the explosion has a dramatic effect on the amount of radioactivity. A 1 kiloton explosion in the atmosphere will produce 41 billion curies | minute after determination, but this will decrease to 10 million cunes in just 12 hours. effect on radioactive gases that seep slowly through the soil from an underground explosion. In an underground explosion, nearly all the reactive materials are filtered out through the soil column. and the only elements that come up through the soil to the atmosphere are the noble gases. primarily krypton and xenon. CRITERIA FOR CONDUCTING A TEST Although every attempt is made to prevent the accidental release of radioactive material to the atmosphere, several safety programs are carmed out for each test. These programs are designed to minimize the likelihood and extent of radiation exposure offsite and to reducerisks to people should an accidental release of radioactive material occur. The Environmental Protection Agency monitors the population around the test site and has established plans to protect people should an accident occur. EPA's preparations are aimed toward reducing the whole-body exposure of the off-site populace and to minimizing thyroid doseto offsite residents, particu-