This is the 16063rd most frequent Russian word.
радиоактивный
"Radioactive" – emitting radiation as a result of nuclear decay.
In this sentence, 'радиоактивный' is used as an adjective to describe the present condition of an area as having radioactive properties.
Этот район объявлен радиоактивным и опасным для жизни.
This area has been declared radioactive and hazardous to life.
Here, 'радиоактивный' qualifies 'материалы' to indicate the nature of the substances involved in the experiment.
Научный эксперимент показал радиоактивные материалы, образующиеся в процессе деления.
The scientific experiment revealed radioactive materials formed during the fission process.
In this example, 'радиоактивный' is used to apply a possible radioactive characteristic to the 'часы' being discussed.