As the world's leaders head home following United States President Barack Obama's Nuclear Security Summit in Washington DC, KATE CHAPMAN of NZPA looks at the use of radioactive sources in New Zealand.

Wellington, April 15 NZPA - New Zealand is proud of its nuclear-free status. Indeed the moniker earned Prime Minister John Key a seat at Mr Obama's summit alongside much larger, and more influential, nations.

Despite being nuclear-free New Zealand has a similar level of radioactive use as other developed countries -- a fact that might surprise some.

Tony Cotterill of the National Radiation Laboratory (NRL) said radioactive sources were used in New Zealand in medicine, agriculture, industry, research and education.

They were "useful and in some cases indispensable tools".

Mr Cotterill said the use of ionising radiation in New Zealand was controlled by the Radiation Protection Act 1965 and Radiation Protection Regulations 1982.

The NRL, a specialist unit of the Ministry of Health based in Christchurch, administers the New Zealand radiation protection legislation.

Anyone using ionising radiation is required to be licensed.

Radioactive sources can kill cells by emitting ionising radiation. Radio waves and lasers were non-ionising radiation and were not energetic enough to kill cells, Mr Cotterill told NZPA.

Radioactive sources could either be sealed, to prevent its spread, or unsealed, often in a liquid form.

Low strength sealed sources were used in industry, including to monitor the fill level of bottles, Mr Cotterill said.

Unsealed sources were mainly used in medicine.

"In medical therapy very high strengths of radioactive material which can damage tissue are administered to patients to localise in and kill cancer cells."

Ionising radiation was also produced by electrically powered equipment, including medical x-rays, he said.

According to a report on the NRL website some domestic smoke alarms sold in New Zealand contain radioactive source.

Any health risk associated with the use of the small americium-241 sources in the smoke detectors was negligible, the NRL said.

The external radiation from the detector made a very small addition to the naturally occurring background level and there was no credible risk from ingestion because the radioactive source was securely sealed and could not fall out or be accidentally removed.

Natural background radiation was the largest contributor to radiation levels in New Zealand, the NRL said.

Medical use of radiation for diagnostic x-rays, radiotherapy and radiopharmaceutical procedures was the next largest category.

The contribution from air travel added to the cosmic radiation from aircraft cruising altitudes.

Levels of radioactive fallout from atmospheric nuclear weapons tests have been declining since 1965, and were now lower than the levels from air travel.

In its national statement to the Nuclear Security Summit, New Zealand told the 46 other nations while it did not have a nuclear industry it had radiological material use similar to other developed nations.

The national radiation authority, the NRL, was in "close contact" with major radiation users to ensure all material was accounted for.