Four scrap metal dealers have been arrested following an investigation into the dumping of 25 tonnes of radioactive metal waste, thought to come from ships ballast blocks, police said on Wednesday.

The dealers, one from Drenthe, two from Friesland and one in Overijssel, are suspected of offering radioactive material for sale and deliberately exposing people to radiation.

The investigation began after a scrap metal dealer in Friesland told officials he had been given several kilos of waste which set off the alarms at his scrapyard. He warned the nuclear safety authority ANVS which then called in the police.

The waste appears to radioactive metal ballast blocks used on ships, and which probably came from a ship that was being demolished, police said.

The police say they suspect the demolition company was trying to get rid of the waste as cheaply as possible and mixed it in with ordinary metal.

The ANVS has now secured the radioactive metal at the different scrapyards and is arranging for its removal.

There is no danger to the public or to the environment from the metal waste, RTL Nieuws quoted the ANVS as saying. Nevertheless, the police report says that long-term exposure can be damaging to health.