Radioactive waste from a nuclear reactor in Norway has been wrongly fed into a town's sewage system for nine years, some of it ending up as farm fertiliser, officials revealed on Tuesday.

The nuclear waste from the research plant was pumped into the sewers in Halden, south-east Norway, after what amounted to a plumbing mistake.

The Norwegian Radiation Protection Authority (NRPA) said the waste water was wrongly linked to the sewage system when it should have been pumped straight into the sea.

But officials insist there has been no risk to human health - even though some of the sewage sludge was turned into fertiliser - as the nuclear material had only very low levels of radioactivity.

Ecologists are demanding radiation tests for local farmers.

"It is frightening that IFE has so little control over its emissions," said nuclear physicist Nils Boehmer of campaign group Bellona.

The mistake was made in 1991 and was not rectified until 1999.

NRPA scientist Sverre Hornkjoel said the contaminated water had been used to cool the 42-year-old reactor, operated by the Institute for Energy Technology (IFE).

He blamed the mistake on council officials, but said the nuclear industry took final responsibility.

"The municipality made the howler, but it is still IFE which is responsible," Mr Hornkjoel said. "In principle, this is a serious incident, but the emissions were very small."

The plant at Halden is part of an international project to test fuel rods, run by the Organisation for Economic Co-operation and Development.

IFE spokesman Viktor Wikstroem said the cooling water had undergone tests before leaving the reactor, and was below the safety limit.

"Our annual emissions are 4,000 times lower than what you and I and everyone are exposed to each year," Mr Wikstroem said.