UPPER SIXTH
Case study for Natural Hazards - CHERNOBYL NUCLEAR ACCIDENT BY D Preece

CHERNOBYL NUCLEAR ACCIDENT
Chernobyl is a small town in the Soviet Union, standing on the banks of the Prypiat River near its confluence with the Rivers Dnieper and Uzh in northern Ukraine, about 150km north of Kiev. The power station in Chernobyl is a nuclear power plant, with four reactors. In the early hours of the morning of the 26th April 1986, operators at the complex lost control of the Unit 4 reactor whilst conducting experiments at low power, with the purpose of improving safety in the plant.

       The reactor core exploded, blowing a 1000 ton shield from the roof.
       Much of the hall was destroyed, and clouds of radioactive gas were emitted in to the atmosphere.
       An atomic fire burned for several days. The explosion caused just one death directly.
       The world knew nothing, until abnormal levels of radioactivity were detected in Sweden. The subsequent cloud was dispersed by unusual weather patterns.
       It moved over the European Soviet Union. Initially the wind blew it towards Scandinavia, Belgium, the Netherlands and the UK. As the plum shifted south, Central Europe and the Mediterranean received radioactive deposits.

       The severity depended on the height of the plume, wind speed and direction, local relief and amount of precipitation received.
       Radioactivity was greatest in northern Ukraine. Everyone within a 2800 sq km area around the station was evacuated in the aftermath of the explosion. 135 000 people and 35 000 people left the exclusion zone. Human inhabitation of this area is still forbidden.
       The majority of the effects and deaths were long term, from radiation linked causes. Since the accident, more than 500 cases of thyroid cancer have been reported in Belarus, compared to the average of two per year previously.
       Due to the uptake of relatively mobile caesium from the soil, restrictions were placed on the movement and slaughter of 4.25 million UK sheep. By April 2000, some 89 500 sheep were still restricted on 344 farms.
       Scientists claim that fish in Lake Kojanovskoe in Russia have radiation levels 60 times the recommended norms. Reservoirs downstream from Chernobyl provide drinking water for nine million Ukrainians, as well as irrigation and fish for another 23 million.

       Nine million people (2.5m in Belarus, 3m in Russia, and 3.5 in Ukraine) still live in contaminated areas, still eat radioactive food, drink radioactive liquids and breathe radioactive air. Official Ukrainian statistics say 3 048 318 citizens of Ukraine bear the status of “Chernobyl disaster victim”, including 644 249 children and teenagers.
       Initial medical focus was on the iodine-131 isotope, which decays quickly. Attention then shifted to focus on the more dangerously long lived atoms, like caesium-134 and -137 (30 year half life), strontium-90 (29 years) and plutonium-239 (24,000 years).
       Restrictions are still in place on some foodstuffs up to 3000 km away, because of the caesium fallout. A report for the scientific journal Nature in May 2000 found more radioactivity than expected; which will last for 50 more years - 100 times more than expected.
       Research showed that the amount of caesium declined by a factor of ten within the first 5 years following the accident, but then has remained relatively stable. The environment is not cleaning itself as fast as previously thought that it would.

What has been learned?

   Safety regulations exist not to be broken by safety engineers. Human error makes risk assessment very difficult, since it assumes that people will not do exactly what the operating instructions tell them precisely not to do.
   An accident in a relatively small plant has international repercussions. The member states held a summary inquiry in to their own reactor safety and recommended changes in operating practices for all power stations. The reactor type used in Chernobyl is a Soviet design, not used anywhere else.

       Following the accident, the radioactive material was collected and buried in the hole where the reactor core used to be. It was sealed in concrete tens of metres thick. The other reactors were then put back in to operation.
       The economic cost is enormous. $900 million is needed to decommission the reactors, another $770m is needed to build a new concrete sarcophagus over Reactor 4. A further $90-100 million will be needed to pay for conventional fuel to replace Chernobyl’s generating capacity.
       The process of decommissioning the station began in late 2000. It will take around 50 years to complete.