Fallout is the residual radiation hazard from a nuclear explosion, so called because it "falls out" of the atmosphere after the explosion. It commonly refers to the radioactive dust created when a nuclear weapon explodes. This radioactive dust, consisting of hot particles, is a kind of radioactive contamination. It can lead to the contamination of ground and the animal food chain.
Fallout can also refer to nuclear accidents, although a nuclear reactor does not explode like a nuclear weapon. The isotopic signature of bomb fallout is very different from the fallout from a serious power reactor accident (such as Chernobyl). The key differences are in volatility and half-life.
U.S. Fallout Exposure: Per capita thyroid doses in the continental United States of Iodine-131 resulting from all exposure routes from all atmospheric nuclear tests conducted at the Nevada Test Site.
Credit: National Cancer Institute as part of a Congressionally funded study
The following figure indicates that the world may have not yet returned to normal for atmospheric atom-bomb tests in the 1950s and 1660s,
Atmospheric 14CO2 in the second half of the 20th century. The figure shows the 14C / 12C ratio relative to the natural level in the atmospheric CO2 as a function of time in the second half of the 20th century. The plot was generated with gnuplot from data of atmospheric radiocarbon in Wellington, New Zealand 1954AD to 1993AD and in Vermunt, Austria 1959AD to 1983AD.
The data sources provide the data as 14CO2. For the plot the absolute percent modern value has been calculated assuming that each measurement was done in the same year as the respective sample was taken.
According to the World Health Organization, the actions proposed by the Government of Japan are in line with the existing recommendations based on public health expertise. The government is asking people living within 20 km of the Fukushima Daiichi nuclear power plant to evacuate and those between 20 km and 30 km away from the plant are asked to stay indoors in unventilated rooms. People living farther away are at lower risk than those who live nearby.
This assessment can change if there are further incidents at these plants and WHO is following the situation closely. However, radiation-related health consequences will depend on exposure. Exposure in turn is dependent on the amount of radiation released from the reactor, weather conditions such as wind and rain at the time of the explosion, the distance someone is from the plant, and the amount of time someone is in irradiated areas.
Fat Man, An Early Atomic Bomb
Credit: Wikipedia
Credit: National Cancer Institute as part of a Congressionally funded study
Natural sources of radiation are very prevalent in the environment, and come from cosmic rays, food sources (bananas have a particular high source), radon gas, granite and other dense rocks, and others. The collective radiation background dose for natural sources in Europe is about 500,000 man Sieverts per year. The total dose from Chernobyl is estimated at 80,000 man Sieverts, or roughly 1/6 as much. However, some individuals, particular in areas adjacent the reactor, received significantly higher doses.
Chernobyl's radiation was detectable across Western Europe. Average doses received ranged from 0.02 mrem (Portugal) to 38 mrem (portions of Germany). For comparison, the dose received from eating one banana per day for one year is roughly 3.3 mrem.
Far fewer people died as an immediate result of the Chernobyl event than died of radiation at Hiroshima, and the eventual total is also significantly less when including those predicted by the WHO to die in the future. Due to the differences in half-life the different radioactive fission products undergo exponential decay at different rates. Hence the isotopic signature of an event where more than one radioisotope is involved will change with time.
Some comments have been made in which the radioactive release of the Chernobyl event is claimed to be 300 or 400 times that of the bomb dropped on Hiroshima. The work of SCOPE suggests that the two events cannot be simply compared with a number suggesting that one was XX times larger than the other.
The radioactivity released at Chernobyl tended to be more long lived than that released by a bomb detonation hence it is not possible to draw a simple comparison between the two events. Also, a dose of radiation spread over many years (as is the case with Chernobyl) is much less harmful than the same dose received over a short period.
The relative size of the Chernobyl release when compared with the release due to a hypothetical ground burst of a bomb similar to the Fat Man device dropped on Nagasaki. A ground burst creates considerably more fallout than the air bursts used at Hiroshima or Nagasaki.
| Isotope | Ratio between the release due to the bomb and the Chernobyl accident |
| 90Sr | 1:87 |
| 137Cs | 1:890 |
| 131I | 1:25 |
| 133Xe | 1:31 |
The graph below was drawn using data calculated using a method similar to that of T. Imanaka, S. Fukutani, M. Yamamoto, A. Sakaguchi and M. Hoshi, J. Radiation Research, 2006, 47, Suppl A121-A127 for calculating the dose due to the bomb fall out which calculated for a surface burst with no separation of the different isotopes.
131I, 133I, 132Te, 133I, 135I, 140Ba, 95Zr,97Zr, 99Mo, 99mTc, 103Ru, 105Ru, 106Ru, 142La, 143Ce, 137Cs, 91Y, 91Sr, 92Sr, 128Sb and 129Sb are the gamma emitters which have been considered for the bomb fallout. The fission was assumed to have been caused by 1 MeV neutrons and 20% occurred in the 238U tamper of the bomb, the dose does not include the effect of any shielding. The data was from the Korean table of the isotopes.
For the Chernobyl fallout
The contributions made by the different isotopes to the dose (in air) caused in the contaminated area in the time shortly after the accident. Note that this image was drawn using data from the OECD report, the Korean table of the isotopes and the second edition of 'The radiochemical manual'.
Data on the release was taken from the following two sources. It was assumed that the whole release occurred on the first day in an instant.
A comparison of the gamma dose rates from Chernobyl and bomb fallout, to 10,000 days. These have been normalised to the same Cs-137 level.
Gamma dose rates from Chernobyl and bomb fallout, to 1,000 days.
Gamma dose rates from Chernobyl and bomb fallout, to 100 days.
The following figure indicates that the world may have not yet returned to normal for atmospheric atom-bomb tests in the 1950s and 1660s,
Atmospheric 14CO2 in the second half of the 20th century. The figure shows the 14C / 12C ratio relative to the natural level in the atmospheric CO2 as a function of time in the second half of the 20th century. The plot was generated with gnuplot from data of atmospheric radiocarbon in Wellington, New Zealand 1954AD to 1993AD and in Vermunt, Austria 1959AD to 1983AD.
The data sources provide the data as 14CO2. For the plot the absolute percent modern value has been calculated assuming that each measurement was done in the same year as the respective sample was taken.
According to the World Health Organization, the actions proposed by the Government of Japan are in line with the existing recommendations based on public health expertise. The government is asking people living within 20 km of the Fukushima Daiichi nuclear power plant to evacuate and those between 20 km and 30 km away from the plant are asked to stay indoors in unventilated rooms. People living farther away are at lower risk than those who live nearby.
This assessment can change if there are further incidents at these plants and WHO is following the situation closely. However, radiation-related health consequences will depend on exposure. Exposure in turn is dependent on the amount of radiation released from the reactor, weather conditions such as wind and rain at the time of the explosion, the distance someone is from the plant, and the amount of time someone is in irradiated areas.
Sources: Wikipedia, World Health Organization
Not a single life should be lost or disabled by nuclear accident or radiation. Money cannot replace a life.
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