Do you remember Chernobyl? Nuclear disaster contamination: apparently worse than previously thought
On 26 April 1986, reactor # 4 at the Chernobyl (Chornobyl) Nuclear Power Station, 100 km north from Kiev, blew up during a routine daily operation. Nearly nine tons of radioactive material – 90 times as much as the Hiroshima bomb – were hurled into the sky. Winds over the following days, mostly blowing north and west, carried, fallout into Belarus, as well as Russia, Poland and the Baltic region.
On 25 April, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested.
A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. A peculiarity of the design of the control rods caused a dramatic power surge as they were inserted into the reactor
The interaction of very hot fuel with the cooling water led to fuel fragmentation along with rapid steam production and an increase in pressure. The design characteristics of the reactor were such that substantial damage to even three or four fuel assemblies can – and did – result in the destruction of the reactor. The overpressure caused the 1000 t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down. Intense steam generation then spread throughout the whole core (fed by water dumped into the core due to the rupture of the emergency cooling circuit) causing a steam explosion and releasing fission products to the atmosphere. About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite.
There is some dispute among experts about the character of this second explosion, but it is likely to have been caused by the production of hydrogen from zirconium-steam reactions.
Two workers died as a result of these explosions. The graphite (about a quarter of the 1200 tonnes of it was estimated to have been ejected) and fuel became incandescent and started a number of firesf, causing the main release of radioactivity into the environment. A total of about 14 EBq (14 x 1018 Bq) of radioactivity was released, over half of it being from biologically-inert noble gases.
About 200-300 tonnes of water per hour was injected into the intact half of the reactor using the auxiliary feedwater pumps but this was stopped after half a day owing to the danger of it flowing into and flooding units 1 and 2. From the second to tenth day after the accident, some 5000 tonnes of boron, dolomite, sand, clay and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles.
Environmental and health effects of the Chernobyl accident
Several organisations have reported on the impacts of the Chernobyl accident, but all have had problems assessing the significance of their observations because of the lack of reliable public health information before 1986.
In 1989, the World Health Organization (WHO) first raised concerns that local medical scientists had incorrectly attributed various biological and health effects to radiation exposureg. Following this, the Government of the USSR requested the International Atomic Energy Agency (IAEA) to coordinate an international experts’ assessment of accident’s radiological, environmental and health consequences in selected towns of the most heavily contaminated areas in Belarus, Russia, and Ukraine. Between March 1990 and June 1991, a total of 50 field missions were conducted by 200 experts from 25 countries (including the USSR), seven organisations, and 11 laboratories3 . In the absence of pre-1986 data, it compared a control population with those exposed to radiation. Significant health disorders were evident in both control and exposed groups, but, at that stage, none was radiation related
Radiation units and biological effects
We meet radiation everywhere in nature. Naturally occurring radioactive elements in the ground emit radiation which gives an external dose. Some of these elements enter food chains and can enter the human body, resulting in an internal dose. In addition, there is constant exposure to radiation from space (cosmic radiation). This is collectively described as “background radiation”.
A number of units are used to express quantities of radiation absorbed by the body or amounts of radioactivity deposited on the ground or existing in a cloud. These units have been expressed differently over the years and each has its own precise physical description. For the purposes of this booklet, the most recently agreed international units have been employed although their detailed scientific descriptions are not addressed.
The most useful in this context is the unit of effective dose — the sievert. (The older unit was the rem. 1 sievert (Sv) = 100 rem.) In the present context, the millisievert (mSv) is often employed. That equals 1/l000th (1 x 10– 3) of a sievert. The following notes put the size of this unit in perspective.
The total natural background dose varies a great deal. The average is about 1 to 2 mSv per year, but values several times higher are not uncommon. In some areas, the natural background dose is more than 60 times the global average. It has not been possible to find a relation between natural radiation dose and adverse health effects. Natural radiation background and its variations therefore may serve as an indicator of what doses may be considered to be acceptable.
The naturally occurring radioactive radon gas from the ground is sometimes trapped and concentrated in buildings. Where houses are well insulated, doses from radon in homes may exceed the natural background dose many times. Doses of radon of more than 10 mSv per year are not uncommon, and they are levels at which controls must be exercised.
Pictuers of the disaster
Chernobyl disaster: First aerial video
Chernobyl Uncensored – Documentary
The Chernobyl disaster (Ukrainian: Чорнобильська катастрофа, Chornobylska Katastrofa — Chornobyl Catastrophe) was a catastrophic nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine (then officially Ukrainian SSR), which was under the direct jurisdiction of the central authorities of the Soviet Union. An explosion and fire released large quantities of radioactive particles into the atmosphere, which spread over much of Western USSR and Europe.
The Chernobyl disaster is widely considered to have been the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the International Nuclear Event Scale (the other being the Fukushima Daiichi nuclear disaster in 2011). The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles. The official Soviet casualty count of 31 deaths has been disputed, and long-term effects such as cancers and deformities are still being accounted for.