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This article compares the radioactivity release and decay from the Chernobyl disaster to various other events which involved a release of uncontrolled radioactivity.

Contents 1 Chernobyl compared to Hiroshima 2 Chernobyl compared to Tomsk 3 Chernobyl compared with the Goiânia accident 4 Chernobyl compared with criticality accidents 4.1 Process accidents 4.2 Reactor accidents 5 References

Far fewer people died as an immediate result of the Chernobyl event than died 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.

However, the radioactivity released at Chernobyl tended to be more long lived than that released by a bomb detonation. Chernobyl released 890 times as much caesium-137 as the Hiroshima bomb, released 87 times as much strontium-90 as the Hiroshima bomb and when the iodine-131 release is compared between the events (decay corrected to three days after the event) then Chernobyl released 25 times as much as the Hiroshima bomb. When the xenon-133 release is compared between the events (decay corrected to three days after the event) then Chernobyl released 31 times as much as the Hiroshima bomb. Hence it is not possible to draw a simple comparison between the two events. Sources of environmental radioactivity

A comparison of the gamma dose rates due to Chernobyl accident and Hiroshima bombing.

Normalized to the same Cs-137 level. (logarithmic scale).

Normalized to the same dose rate for day one.

Normalized to the same Cs-137 level (dose rate on day 10000).

The graph of dose rate as a function of time for the bomb fallout was done 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. Our graph exhibits the same shape as that obtained in the paper. The bomb fallout graph is for a ground burst of an implosion-based plutonium bomb which has a depleted uranium tamper. The fission was assumed to have been caused by 1 MeV neutrons and 20% occurred in the ²³⁸U tamper of the bomb. It is assumed that no separation of the isotopes occurred between the detonation and the deposit of radioactivity. The following gamma-emitting isotopes are modeled ¹³¹I, ¹³³I, ¹³²Te, ¹³³I, ¹³⁵I, ¹⁴⁰Ba, ⁹⁵Zr, ⁹⁷Zr, ⁹⁹Mo, ^99mTc, ¹⁰³Ru, ¹⁰⁵Ru, ¹⁰⁶Ru, ¹⁴²La, ¹⁴³Ce, ¹³⁷Cs, ⁹¹Y, ⁹¹Sr, ⁹²Sr, ¹²⁸Sb and ¹²⁹Sb. The graph ignores the effects of beta emission and shielding. The data for the isotopes was obtained from the Korean table of the isotopes. The graphs for the Chernobyl accident were computed by an analogus method.

The release of radioactivity which occurred at Tomsk (While the nuclear centre is known as "Tomsk" it is at Seversk) is a better comparison to the Chernobyl release. During the reprocessing some of the feed for the second cycle (medium active) part of the PUREX process escaped in an accident involving red oil. According to the IAEA it was estimated that the following isotopes were released from the reaction vessel.^[1]

• ¹⁰⁶Ru 7.9 TBq
• ¹⁰³Ru 340 GBq
• ⁹⁵Nb 11.2 TBq
• ⁹⁵Zr 5.1 TBq
• ¹³⁷Cs 505 GBq (estimated from the IAEA data)
• ¹⁴¹Ce 370 GBq
• ¹⁴⁴Ce 240 GBq
• ¹²⁵Sb 100 GBq
• ²³⁹Pu 5.2 GBq

It is important to note that the very short lived isotopes such as ¹⁴⁰Ba and ¹³¹I were absent from this mixture, also the long lived ¹³⁷Cs was only in a small concentration. This is because it is not able to enter the tributyl phosphate/hydrocarbon organic phase used in the first liquid-liquid extraction cycle of the PUREX process. The second cycle is normally to clean up the uranium and plutonium product. In the PUREX process some zirconium, Tc and some other elements are extracted by the tributyl phosphate. Due to the radiation induced degradation of tributyl phosphate the first cycle organic phase is always contaminated with ruthenium (This is extracted by the dibutyl hydrogen phosphate). Because the very short lived radioisotopes and the relatively long lived cesium isotopes are either absent or in low concentrations the shape of the dose rate vs. time graph is different to Chernobyl both for short times and long times after the accident.

The size of the radioactive release at Tomsk was much smaller, and while it caused moderate environmental contamination it did not cause any early deaths.

While both events released ¹³⁷Cs, the isotopic signature for the Goiânia accident was much simpler.^[2] It was a single isotope which has a half life of about 30 years. To show how the activity vs. time graph for a single isotope differs from the dose rate due to Chernobyl (in the open air) the following chart is shown with calculated data for a hypothetical release of ¹⁰⁶Ru.

During the time between the start of the Manhattan project and the present day, a series of accidents have occurred in which nuclear criticality has played a central role. The criticality accidents may be divided into two classes. For more details see nuclear and radiation accidents. A good review of the topic was published in 2000, "A Review of Criticality Accidents" by Los Alamos National Laboratory (Report LA-13638), May 2000. Coverage includes United States, Russia, United Kingdom, and Japan. Also available at this page, which also tries to track down documents referenced in the report.

• Press release on a report on criticality accidents from Los Alamos National Laboratory
• List of radiation accidents
• U.S. report from 1971 on criticality accidents to date

In the first class (process accidents) during the processing of fissile material, accidents have occurred when a critical mass has been created by accident. For instance at Charlestown, Rhode Island, United States on July 24, 1964 one death occurred and at Tokaimura nuclear fuel reprocessing plant, on September 30, 1999^[3] two deaths and one non fatal overexposure occurred as result of accidents where too much fissile matter was placed in a vessel. These accidents tend to lead to very high doses due to direct irradiation of the workers within the site, but due to the inverse square law the dose suffered by members of the general public tends to be very small. Also very little environmental contamination normally occurs as a result of these accidents, a trival release of radioactivity occurred as a result of the Tokaimura event even while the building in which the accident occurred was not designed as a containment building the building was able to retard the spread of radioactivity. Because the temperature rise which occurred in the vessel where the nuclear reaction occurred was small the majority of the fission products remained in the vessel.

In this type of accident a reactor or other critical assembly releases far more fission power than was expected, or at the wrong moment in time it becomes critical. The series of examples of such events include one in an experimental facility in Buenos Aires, Argentina, on September 23, 1983 (one death)^[4] and during the Manhattan Project several people were irradiated (two, Harry K. Daghlian and Louis Slotin, fatally) during "tickling the dragon's tail" experiments. These accidents tend to lead to very high doses due to direct irradiation of the workers within the site, but due to the inverse square law the dose suffered by members of the general public tends to be very small. Also very little environmental contamination normally occurs as a result of these accidents. For instance at Sarov according to the IAEA report (2001)^[5] the radioactivity remained confined to within the actinide metal objects which were part of the experimental system. Even the SL-1 accident failed to release much radioactivity outside the building in which it occurred.