It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that at noon on 28 April led to the first hint of a serious nuclear problem in the western Soviet Union.
Foi a busca por parte de Suecia da fonte de radioactividade, despois de descartar que houbese fugas na súa planta, a que na tarde do 28 de abril levou ao primeiro sinal dun problema nuclear serio no oeste da Unión Soviética.