Radiation sealed under sea

A radioactive device stuck down an oil bore off the Taranaki coast had to be sealed under 60 metres of concrete to stop radiation leaking out.

The incident, in March, was one of four potentially hazardous situations last year requiring emergency responses by the Health Ministry's National Radiation Laboratory. Manager Jim Turnbull said none caused harm to humans.

There were two accidents involving nuclear density meters, which use a radioactive isotope source to measure soil density and moisture content.

In one case, the machine fell off the back of a ute on to a road. The local health protection officer and police confirmed there was no damage and NRL did not need to attend. In another, a nuclear density meter was run over by a bulldozer on a building site and damaged. Inspection showed the radioactive component was undisturbed and the instrument was repaired.

In April, a gauge containing radioactive material melted to the outside of a factory furnace, exposing four staff to radiation. Dr Turnbull said the dose was not significant from either a health or environmental perspective.

"It was well below normal background radiation levels."

NRL staff packaged the radioactive material into a safe container and took it to the laboratory's secure storage, which contains all kinds of radioactive waste, including depleted uranium.

The Taranaki incident happened about 30 kilometres offshore, where a bore was being tested using a radioactive tracer. "It appeared the device became stuck in the bore, about 3km beneath the ocean floor, and had to be abandoned." The bore was sealed with 60 metres of concrete to contain any leak of the material, which could stay radioactive for hundreds of years.

Dr Turnbull said the specialist unit, which is based in Christchurch, deals with five to 10 incidents a year, most of them "relatively mundane". "There are about half a dozen packages of radioactive isotopes coming into the country every week for medical and scientific purposes.

"Occasionally the packaging gets damaged in transit and we'll be called in as a precautionary measure. We've never had a situation in which someone has done something that put the public at risk ... though we came close about 10 years ago."

After an industrial accident in Auckland radioactive material was sealed in 500kg of lead. Later, it was stolen. "A couple of people were chipping off the lead in their garage to use for fishing sinkers.

"Fortunately family members discovered what they were up to before they reached the radioactive core, otherwise we could have had a significant problem."

NRL staff had personal dose meters to monitor their exposure, but incidents rarely involved radiation levels higher than those found in daily life. "We are constantly being exposed to radiation from cosmic rays from space, not to mention X-rays, etc."