The number of researchers exposed to low-level radiation in an accident at a nuclear laboratory in Japan last week has hit 30, officials said Monday, with human error likely worsening the problem.

The accident happened on Thursday as 55 people were working at a laboratory in Tokaimura, 120 kilometres (75 miles) northeast of Tokyo, the Japan Atomic Energy Agency (JAEA) said.

Japan's Nuclear Regulation Authority condemned site operator JAEA for lacking a "culture of safety" and said it was a "Level 1" event, the lowest of an international scale measuring nuclear incidents and accidents.

The disaster at Fukushima in the aftermath of the 2011 tsunami was a "Level 7", the most serious on the scale.

The researchers were carrying out an experiment that involved firing a proton beam at gold when the accident happened, it said.

The agency, which had initially said six researchers were exposed to radiation, announced late Sunday that 24 more people were affected.

There was no widespread radiation leak, although two researchers were exposed to up to 1.7 millisieverts of radiation, a dose roughly equivalent to a strong medical X-ray.

"None of them required medical attention," a JAEA spokesman said.

JAEA has come in for criticism, from a public already wary of nuclear energy after the Fukushima crisis, for reportedly continuing with the experiment despite an alarm warning of abnormality and for waiting more than 30 hours before reporting it.

According to the agency, radiation was accidentally released during the experiment "due to overheating, which we suspect was caused by some technical problems".

Radiation then leaked from the facility after workers used fans to reduce levels of radioactivity in the laboratory, it said.

The agency spokesman said the fans should not have been used, adding: "We don't know why they switched on the fan. We suspect some wrong decisions were made by workers concerned."

Nuclear safety is a particularly sensitive issue in Japan, which in 2011 experienced the world's worst atomic accident in 25 years when a tsunami crippled a nuclear power plant in Fukushima.

The Nuclear Regulation Authority criticised JAEA for releasing radioactive materials when it knew their facility was contaminated.

"It operated ventilation fans and released radioactive materials outside when it acknowledged contamination inside the building," the authority said.

"It has failed to appropriately control radioactive materials and shows the absence of a culture of safety."

Tokaimura is the site of Japan's previous worst nuclear accident in 1999, when workers at a uranium processing plant poured too much uranium into a precipitation tank, sparking a nuclear chain reaction.

Two of the workers who triggered the disaster later died from their injuries in hospital, while more than 600 people were exposed to radiation.