Iodine pills, which help reduce radiation build-up in the thyroid gland, had previously only been issued to people living within 20km (14 miles) of the Tihange and Doel nuclear plants.
Maggie De Block, the Health Minister, said that would be extended to 100km, covering the whole country of 11 million people, following advice from an expert council.
The pills will be sent to pharmacies, and the public would be ordered to collect their ration in the event of a meltdown. Children, pregnant women and those breast-feeding would be given priority.
It emerged following last month’s terrorist attacks that an Isil cell may have been plotting to kidnap a nuclear expert in order to build a “dirty bomb”. Eleven nuclear workers had their passes revoked.
A senior Belgian nuclear industry official was secretly filmed by jihadists late last year, according to the country’s nuclear authority, and the brothers were linked to the surveillance.
Last week Germany asked that the 40-year-old Tihange 2 and Doel 3 reactors be turned off “until the resolution of outstanding security issues”.
The reactor pressure vessels at both sites have shown signs of metal degradation, raising fears about their safety. They were temporarily closed but resumed service last December.
Belgium’s official nuclear safety agency (AFCN) rejected the German request, saying the two plants “respond to the strictest possible safety requirements.”
The key figure in the suspected dirty bomb plot is Mohammed Bakkali, 28, from Brussels, who was arrested in November on suspicion of helping to plan the Paris massacre. Police raided his wife’s flat and found a ten-hour video taken by a camera hidden opposite the home of an executive at the Centre for the Study of Nuclear Energy in Mol, northern Belgium. The executive had access to radioactive isotopes at the country’s national nuclear research centre.