Изображения страниц
PDF
EPUB
[blocks in formation]
[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][merged small]
[graphic][subsumed][subsumed][subsumed][merged small][merged small][subsumed]
[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

IV RADIOACTIVITY

A. General

All nuclear explosions produce some quantities of radioactive material. The main factors that determine the amount and kind of radionuclides produced in an underground nuclear explosion are: the type of nuclear explosive employed, the chemical elements present in and associated with the explosive, and the elements present in the surrounding rock.

There are two types of nuclear reaction mechanisms which can be employed to release nuclear energy, namely fission and fusion. In a fission explosion heavy nuclei are split into lighter ones to release energy, whereas in a fusion explosion heavier nuclei are formed from lighter ones to cause the release of energy. Starting a fusion reaction, however, requires temperatures of millions of degrees. Temperatures this high are available from a fission reaction; thus the energy from a fission reaction can be used to initiate fusion.

Major gaseous radionuclides resulting from an underground detonation and produced by the fission reaction are xenon 133, iodine 131, and krypton 85. Carbon 14 from neutron activation of nitrogen in the explosive environment may be present in gaseous compounds such as carbon monoxide, carbon dioxide and methane. In a fusion explosion tritium will be produced from the explosive, in addition to the fission products. Radionuclides which are solid at chimney temperatures, such as zirconium 95 and cerium 144, are also present. The majority of these refractory radionuclides are trapped in the solidified melt at the bottom of the cavity shortly after the detonation. The gaseous species, however, remain largely in the void spaces of the chimney rubble or are absorbed on the rock surfaces.

B. Radioactive Decay

Each radionuclide distintegrates at a specific rate. The term half-life is used as the unit of measurement to denote the length of time during which one-half of the atoms in a given quantity of a radionuclide disintegrate. For example, if a radionuclide has a half-life of 12 hours, the decay after 12 hours would reduce the quantity of that radionuclide to half its original amount. During the second 12-hour period, decay of half of the remaining amount would occur. In this example, in 2 half-lives or 24 hours, the quantity of the radionuclide would be reduced by 75% of its original amount. Apply

ing this concept further, in 3 half-lives the original quantity of the radionuclide would be reduced 87% and in 4 half-lives 93%, etc.

C. Federal Regulations

The Federal Radiation Council (FRC) was established by the Federal Government in 1959 to recommend to the President guides that should be established for control of human radiation exposure. The Atomic Energy Commission uses these guides as well as the recommendations of the National Committee on Radiation Protection and Measurement (NCRP), the International Commission on Radiological Protection (ICRP) and the National Academy of Sciences Natural Research Council (NAS-NRC) to set standards.

The Maximum Permissible Concentration (MPC) for any radionuclide is the average concentration which is not expected to result in a radioactive dose greater than the Maximum Permissible Dose (MPD) recommended by the ICRP and the NCRP for a radiation worker. The FRC has developed Radiation Protection Guides (RPG's) for suitable samples of the public at large which are generally equivalent to 1/30 of the Maximum Permissible Dose, recommended by the ICRP and the NCRP. These recommendations apply to the total exposure that may be received from all sources except natural background and medical uses.

D. Radioactivity in Storage Gas

At an early post-detonation time, a nuclear chimney would contain significant amounts of radionuclides. After a period of six to nine months, however, the total amount of radioactivity would be substantially reduced through radioactive decay. Some nuclides, for example, xenon 133 (5.3 day half-life), and iodine 131 (8.1 day half-life), would have decayed to the point where only insignificant amounts would remain. Others, such as Krypton 85 (10.8 year half-life), and tritium (12.3 year half-life), because of their longer half-lives, would be more persistent and might require flushing from the chimney prior to the injection of natural gas. If natural gas were stored in a chimney without prior flushing it would pick up small amounts of radionuclides. In the normal process of gas transmission and distribution, the gas would be diluted in the pipeline with gas from other sources, mixed with air for combustion, and the combustion products further diluted with ambient air. The resulting

« ПредыдущаяПродолжить »