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| Fig. 1: How Chernobyl looked after the explosion. (Source: Wikimedia Commons) |
The explosion of the Chernobyl nuclear power plant (CNPP) was one of the worst nuclear accidents in human history. Radioactive materials from the accident affected not only humans, either acute or chronic, but also plants and animals. What's more, it reshaped the nuclear industry: stricter rules were enforced, and much careful and thorough operations were required. Although almost 40 years have passed, this tragic event is still memorable and educational.
The CNPP is located in the north of Ukraine, and it is at the border of three countries: Ukraine, Belarus and Russia. [1] In the 1970s and 1980s, nuclear power plants were popular all over the world and Chernobyl was one of the biggest at that time. The Chernobyl power plant consisted of four RBMK-100-type reactors. [2] One of its four reactors, Unit 1, had already started to generate electricity in 1977. [2] The reactor at which the accident occurred, Unit 4, had begun generating electricity in 1983. [2] Two other Units (5 and 6) were scheduled to be built in 1986, but the construction was stopped after the accident occurred.
The failure of the plant happened during testing against failure of its main electricity supply. [2] During the testing, the power output was reduced to 1000 MW, only 25% of the maximum output. [2] The experimenters started testing at 1 pm on 25 Apr, 1986. But they received orders to continue power supply until 11 pm, so the testing was postponed. [2] At 11 pm, the testing finally started and the power output was 720 MW. [3] At about 11.30pm, the thermal power output had fallen sharply to 30 MW. [3] This led to an accumulation of Xe-135 poisoning on the fuel rods. [3] The control rods were removed accordingly in order to return the thermal power to safe conditions.[4] However, this caused a thermal-hydraulic instability in the reactor, and after removing the control rods, only 4 (out of 8) water pumps for cooling were still working. [4] The steam bubbles inside the cooling water increased reactivity. [3] Eventually, Xe-135 were burned and the thermal power kept increasing to up to 30000 MW . [3] At 1.22 am the following day, the reactor exploded. [2] Fig.1 shows what the CNPP looked like after the explosion.
Table 1 shows the area with Cs-137 deposition density range after the Chernobyl accident, with the first row demonstrating deposition density in unit of Ci/km2 and the first column demonstrating different countries. Three observations are shown in this table. First, more than one countries was contaminated. These countries include Russia, Belarus, Ukraine, Sweden, etc. Second, as the Cs-137 deposition density increases, the area generally decreases. Third, if the deposition density is above 5-15 Ci/km2, only Russia, Belarus and Ukraine have areas of contamination, meaning that those countries are the most contaminated ones. The reason is that they are closet to the CNPP.
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| Table 1: Area od Cs-137 deposition versus contamination range. (Measurements taken in 1996.) [5] |
The tragedy of Chernobyl had acute and chronic impacts on human, as well as on ecosystems. 134 emergency workers and firefighters suffered acute radiation illness and 28 died soon thereafter. [2] One study points out that it is reasonable that radioiodine from Chernobyl led to an increase in thyroid cancer. [6] For thyroid cancer itself, the same study also found that children were at a greater risk than adults. [6] Another study observes that most radiation-related cancers will continue to occur decades after exposure. [7] It is thus still too early to evaluate the full radiological impact of the Chernobyl accident. [7]
© Chunchen Hong. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] V. Saenko et al. "The Chernobyl Accident and Its Consequences," Clin. Oncol. 23, 234 (2011).
[2] J. Smith and N. A. Beresford, Chernobyl: Catastrophe and Consequences (Springer, 2005).
[3] K.Alnoaimi, "Xe-135 Reactor Poisoning," Physics 241, Stanford University, Winter 2014.
[4] Marc Joshua, "The Chernobyl Disaster," Physics 241, Stanford University, Winter 2018.
[5] UNSCEAR, Sources and Effects of Ionizing Radiation, Vol II, Annex J (United Nations Publications, 2000).
[6] E. Cardis and M. Hatch, "The Chernobyl Accident - An Epidemiological Perspective," Clin. Oncol. 23, 251 (2011).
[7] E. Cardis et al., "Cancer Consequences of the Chernobyl Accident: 20 Years On," J. Radiol. Prot. 26, 127 (2006).
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