There are good, factual descriptions of the Three Mile Island (TMI) accident in 1979 and of the Chernobyl accident in 1986.

Note that the design of the TMI reactor ensured that there were no deaths or injuries as a result of the accident. All Western reactors follow the same design philosophy as was used in TMI.  Modern reactors have improved safety as a result of lessons learnt over the years, incorporating sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations.

This was not the case in Chernobyl, whose design was defective. A Chernobyl-type accident is not possible at a reactor that follows Western design philosophy. Having said that, Chernobyl did happen and resulted in the only civilian deaths ever to occur in a nuclear power station. Estimates of the number of deaths vary, but the United Nations’ latest summary report on the accident put the estimate at a much lower figure than many people might expect.

An updated United Nations report, published in late 2011 has reconfirmed the earlier findings. It states that radiation doses to the general public in the three most affected countries were relatively low and most residents “need not live in fear of serious health consequences”.

A typical western reactor, such as that at Three Mile Island, is sealed in a 4- to 8-inch-thick high-tensile steel pressure vessel. About this is an additional 4ft-thick leaded-concrete enclosure. These, together with the radioactive coolant systems, are then enclosed in a further 1- to 2 inch-thick steel containment vessel, which in turn is enclosed in a 3ft-thick shield building.

The Chernobyl reactor lacked these vital layers of containment structures. As a result, when steam pressure caused the reactor vessel to rupture, the radioactive material that rushed outwards escaped immediately into the atmosphere. Neither the Three Mile Island nor the Chernobyl accident was caused by a nuclear explosion. In both cases the problem was caused by rupture of the nuclear reactor containment vessel as a result of steam pressure.

No death or injury occurred during the Three Mile Island accident, since radioactivity was contained within the reactor. The one at Chernobyl was a radically different matter.The reactor design was gravely defective and the Soviets ignored public safety by omitting the enclosures provided in all western reactors to prevent radiation leaking into the atmosphere.The graphite moderator went on fire, burned for nine days and the radioactive smoke particles were carried by the wind over large areas of the Soviet Union and Europe. The area within 30 km of the reactor was seriously contaminated. If Chernobyl had been enclosed in the same way as Three Mile Island, this would not have happened.