Thai authorities are scrambling to find a cylinder containing dangerous radioactive material that went missing from a power station, warning of serious health risks from direct exposure.

Staff at the coal-fired plant in Prachinburi province, about 160 kilometres east of Bangkok, noticed the steel tube, 30 centimetres long and 13 centimetres wide, was missing during routine checks on Friday.

A weekend search failed to locate the 25-kilogram tube containing highly radioactive isotope Caesium-137, said Kittiphan Chitpentham of the National Power Supply Public Company, which owns the plant.

The company believes it may have fallen from a wall mount, about 18 metres high, days earlier.

Radiation tests at the plant show it has been taken off the premises.

"We are asking people in the area to help find it," Prachinburi governor Narong Nakornjinda said.

The missing cylinder is part of a device used to measure steam pressure at the plant. Officials have not said how much Caesium-137 is inside it.

The Bangkok Post, an English-language daily newspaper in the Thai capital, posted two photos that reportedly show the cylinder before it disappeared.

The Office of Atoms for Peace — the Thai government agency responsible for nuclear research — said the plant was using surveillance footage to try to identify who took the cylinder, and warned against opening it.

"If someone breaks the cylinder, when you are directly exposed to it, you could be exposed to a high risk of cancer and serious illness, so please don't break the cylinder," said Office of Atoms for Peace secretary Permsuk Sutchaphiwat.

The revelation of the missing cylinder follows a similar incident in Western Australia, where authorities recovered a tiny radioactive capsule that had fallen off a truck on a remote outback highway in mid-January.

AFP