Nuclear fall-out

Until now, only small concentrations of tungsten have been evident in the atmosphere, and these are caused primarily by industrial emissions and nuclear fall-out. The ascertained air concentrations have been <1.5 tgm according to Cawse (1974). Rain water analyses performed in the UK by the same author showed that tungsten concentrations were mainly < 1 pgkg while seawater concentrations may be of the order of 0.1 pgkg (see also Bruland 1983). According to Sohrin etal. 

Both naturally occurring and artificial radioisotopes find application in medicine, industrial products, and consumer products. Some specific radioisotopes, called fall-out, are still found in the environment as a result of nuclear weapons use or testing. 

TU any of the less-understood phenomena leading to the observed fall-out distribution resulting from a nuclear explosion occur on a relatively short time scale (a few tens of seconds or less). These short term phenomena lead to an initial distribution of radioactive material referred to as the source term in a fallout study. Many predictive calculations are based on an assumed source term, which of necessity has been quite oversimplified. Two typical simplifications made for purposes of model development are (1) that the radiochemical composition of fallout is well defined and uniform (2) that the particles comprising the initial debris are uniform with respect to settling rate in the atmosphere. The latter assumption has received considerable attention elsewhere, notably in the work of Miller (2). However, the former assumption concerning the radiochemical uniformity of the debris has received far less systematic attention. 

During the 1950s, Patey became concerned about the health hazards of the testing of nuclear weapons in the atmosphere. In addition to speaking at meetings, in 1957 Patey edited the book, Fall Out Radiation Hazards from Nuclear Explosions, 108 a compilation of contributions from nine scientists across a range of disciplines, which became an authoritative source on the subject. Patey died on 11 October 1991 at Oxford. 

Pirie, A. (ed.) (1957). Fall Out Radiation Hazards from Nuclear Explosions. McGibbon Kee, London. 

Other activities of man have led to the distribution of appreciable amounts of radionuclides in the atmosphere and on the earth s surface. In the first place nuclear explosions and nuclear weapon tests have to be mentioned, by which Pu and fission products have been deposited on the earth, either directly or via the atmosphere in the form of fall-out. The amount of Pu released by nuclear weapon tests between 1958 and 1981 is estimated at 4.2 tons, of which 2.8 tons were dispersed in the atmosphere and 1.4 tons deposited locally. By underground nuclear explosions about 1.5 tons of Pu have been liberated. Radionuclides released into the air are mainly present in the form of aerosols. 

Artificial radionuclides released by nuclear explosions, weapon tests and accidents have been deposited from the air as fall-out on soil and vegetation. In 1963 values of up to 0.8 Bq/1 °Sr and up to 1.2 Bq/1 Cs were measured in precipitations in central Europe. In 1964, the concentration of Cs in beef reached values of about 36 Bq/kg. Consequently, the concentration of Cs in man went up to about 11 Bq/kg. 

Beck K. M. and Bennett B. G. (2002) Historical overview of atmospheric nuclear weapons testing and estimates of fall out in the continental United States. Health Phys. 82(5), 591-608. 

The primordial nuclear reactor is short-lived, quickly encountering an energy crisis. Because of the falling temperatures and the coulomb barriers, nuclear reactions cease rather abruptly when the temperature drops below roughly 30 keV, when the universe is about 20 minutes old. As a result there is nuclear freeze-out since no already existing nuclides are destroyed (except for those that are unstable and decay) and no new nuclides are created. In 1000 seconds BBN has run its course. 

The sun is not commonly considered a star and few would think of stars as nuclear reactors. Yet, that is the way it is, and even our own world is made out of the fall-out from stars that blew up and spewed radioactive debris into the nascent solar system. 

In this paragraph we briefly describe some of the largest anthropogenic sources causing far field effects, i.e. nuclear weapons tests and nuclear power plant accidents. The cause of the releases is discussed in Chapter 19. Chapter 22 discusses both near and far field effects in further detail, particulary with regard to chemical properties liquid releases from nuclear power plants, dissolution of solidified nuclear waste and of fall-out particles, migration in the environment, and possible consequences. 

Nuclear weapons were tested in the atmosphere up to 1990, with total releases up to 2 x 10 ° Bq fission products, as well as some lesser amoimts of Pu isotopes cf. Table 22.2. Most of the debris injected into the troposphere had a mean residence time of 30 d, causing fall-out mostly in the neighborhood of the test area. Some of the debris passed through the tropopause and ratered the stratosphere where it was carried by winds around the globe at approximately the latitude of the release. The residence time in the stratosphere... 

In order to relate the emissions of radioactivity from nuclear power installations or the accumulation of radioactivity in the body from fall-out to a resulting dose to the population, the ICRP has introduced the committed dose concept (equival t or effective), S... 

Strontium-90 was widely dispersed in the 1950s and 1960s in fall out from atmospheric testing of nuclear weapons. It has been slowly decaying since then so that current levels from these tests are very low. 

The Fixed Bed Nuclear Reactor (FBNR) concept assumes the use pressurized water reactor (PWR) technology, but incorporates hi temperature gas cooled reactor (HTGR) type fuel and the concept of a suspended fixed bed core. Spherical fuel elements are fixed in the suspended core by the flow of water coolant. Any accident signal will cut off the power to the coolant pump causing a stop in the flow. This would make the fuel elements fall out of the reactor core, driven by gravity, and enter a passively cooled fuel chamber where they would reside in a subcritical condition. The Fixed Bed Nuclear Reactor (FBNR) is a simplified version of the fluidized bed nuclear reactor concept [XII-1 to XII-9]. In the FBNR, spherical fuel elements are in a fixed position in the core therefore, there is no concern about the consequences of multiple collisions between them, an issue that may be raised about the fluidized bed concept. Relatively little work has been done for the fixed bed nuclear reactor so far, but the experiences gained from the development of a fluidized bed reactor can facilitate the development of the FBNR. 

Anthropogenic nuclides mainly Cs, from nuclear weapon and Chernobyl fall-out, and °Co from steel manufacture. In particular circumstances, there could be other fission product nuclides. 

Often considered chemical reactions in the QM region or other large-amplitude nuclear motions including, for instance, conformational transitions, fall out of the general consideration of the problem given above. The main reason is that in such cases the vibrational averaging like that performed in Eq. (13) is invalid. It comes out that in these cases the treatment of the nuclear motions in each of the two regions separately from the other one is invalid and a reformulation of the problem is required. 

---