The trilogy "Chernobyl: 20 Years On" discusses the Chernobyl accident that happened on 26 April 1986. The first part is "The Incident" and second part is "Incident Consequence". This is the last part of the trilogy, summarizing the radiation contamination and lessons the world learnt from the accident.

The reactor released radioactive materials continuously in the next ten days after the explosion at Chernobyl on 26 April 1986. Materials released to the atmosphere were widely dispersed and eventually deposited onto the earth's surface. Open surfaces, such as roads, lawns and roofs, were most heavily contaminated. Residents near the nuclear power stations were evacuated immediately. Wind, rain and remedial efforts contributed to reduce surface contamination, but led to secondary contamination of underground water. At present, the radiation in air has returned to the background level in most of the settlements.

Following the accident, animals and vegetation in forest and mountain areas had taken up high levels of radioactive Caesium. To prevent the dispersion of radionuclides by forest fires, surface layer of soil was removed and dead trees were cut down and buried. Re-forestation and sowing of grasses were also undertaken to prevent the spread of soil contamination. The radiation levels in mushrooms, berries and hunted animals were persistently high. Transfer of radioactive Caesium from lichen to reindeer meat and in turn to human was observed in the Arctic and sub-Arctic areas. Some restrictions on hunting were put in force to minimize ingestion of contaminated food.

Through deposition, the radioactivity polluted the surface water in the river, lake and reservoirs, affecting fishery and drinking water. Contamination of surface water bodies in most places declined quickly through dilution, physical decay, and uptake of radioactivity in bed sediments and catchment soils. The contamination of surface water had not posed a public health problem since the accident. However, in view of the large quantities of radioactivity deposited in the catchment areas of water bodies in the contaminated regions around Chernobyl, the local authority is continuing the monitoring to ensure that washout from the catchment areas will not contaminate drinking water. In those areas where the radiation levels remain high, such as the exclusion zone within a 30 kilometres radius from the nuclear power station and in some 'closed' lakes with no outflowing streams, restriction on fishing and water use will be maintained.

The releases during the Chernobyl accident contaminated more than 120,000 sq. km of land in Russia, Belarus and Ukraine, of which more than 50,000 sq. km. were agricultural lands. Some mandatory countermeasures were implemented shortly after the accident, including the boundary control of personnel, the cessation of field work, harvesting, pasturing and slaughtering of carnivores. These measures caused huge losses in agriculture. Some remedial measures were implemented later, including the removal of surface layer of soil and use of special fodder to reduce contamination, so that agriculture could continue. Radioiodine, which was rapidly absorbed from grass and introduced into milk through animal uptake, was an early concern after the accident. Fortunately, because of the short half life of the nuclide, the concern abated quickly. Radioactive Caesium, however, could remain for years because of its half life of 30 years. Nowadays, with the exception of a few areas, the concentration of radioactivity has fallen to within safe levels.

When the accident occurred in 1986, there were totally 4 reactors in Chernobyl Nuclear Power Station. While Unit 4 plant was destroyed in the accident, the Ukrainian government continued the operation of the other 3 plants because of energy shortage. A fire started in the Unit 2 plant in 1991, and the authority subsequently announced that Unit 2 was beyond repair and stopped its operation. In November 1996, Unit 1 stopped operation under an agreement between the Ukraine Ukrainian government and International Atom Energy Agency. The remaining Unit 3 plant drew heightened concern from the international community, for fear of a repeat of the accident. After numerous negotiations and with the European Union agreeing to a financial assistance of 2,300 million US dollars, the authority closed down Unit 3 in December 2000, marking the permanent shut-down of the Chernobyl Nuclear Power Station.

Though the nuclear power station was permanently closed down in 2000, potential risk of radiation still remains. The sarcophagus, erected quickly after the accident, was constructed with 300 thousand tons of concrete and steel plates to cover the reactor to prevent further radiation leakage. Put in place in November 1998, the sacrophagus had undergone reinforcement and waterproofing several times. Currently, many cracks appear on the ceiling and some structures are showing signs of corrosion. The possibility of the sarcophagus' ceiling collapsing, leading to a release of radioactive dust, poses a potential risk. Moreover, as the original design of the sarcophagus did not cater for such effects as severe vibration or flooding, it may collapse in the event of natural disasters. The authority has decided to build a new confinement enclosing the existing sacrophagus that would last a hundred years. It would be possible to carry out work inside the new confinement, including dismantling of the sacrophagus and removal of radioactive fuel from the damaged reactor. The radioactive wastes and contaminated equipment left behind in the accident are being kept temporarily in non-standard trenches and landfills. A comprehensive plan to tackle the wastes is required to avoid the spread of contamination.

The Chernobyl accident is the worst accident in the history of nuclear power industry. The lesson is painful. On the brighter side, the accident has promoted the development of nuclear safety in the world. It has forced countries to critically review and enhance their work in nuclear safety, including the design, emergency management, implementation of strict operational rules, strengthening of staff training, and reinforcement of international cooperation and exchange on nuclear safety. The accident prompted authorities to seriously consider the importance of emergency preparedness and to strengthen professional training and co-ordination in emergency plans and drills. Rules and guidelines have been established to enhance communication among organizations, standardize the mechanism for information exchange, keep stock for protective equipment and medicines, provide psychological counselling and promote relevant scientific research.

The security level of nuclear power stations has improved significantly in recent years. More and more people understand and accept the development of nuclear power, and realize that nuclear power is one of the most important alternative energy sources in this century.