A storage tank ruptured and caught fire at Stora Enso's Kaukopää industrial site in Imatra, southeastern Finland, early on August 18, with around 50 cubic metres of tall oil spilling onto the site and some entering nearby Lake Saimaa.

The incident occurred at about 2 am, when the plant's control room detected what appeared to be an explosion and alerted emergency services.

Toni Jaako, a duty fire officer with the South Karelia Rescue Department, said the tank had ruptured in an explosion-like event. The fire was brought under control after more than two hours of firefighting.

No one was injured, and no other tanks were damaged. About 15 rescue service units responded to the incident.

Some of the spilled tall oil reached Lake Saimaa through a stormwater drain. Booms were placed around the affected area as a precaution, while Stora Enso staff began removing the substance from the water using a suction tanker.

Janne Kallio, director of Stora Enso's Imatra mills, said the situation in the lake was under control and the company expected to complete the removal operation during Tuesday morning.

Tall oil is a by-product of the pulp-making process. According to the company, it is now used to produce process chemicals for the pulp industry. Jaako described it as a sticky, bitumen-like substance that moves slowly through pipelines and is biodegradable.

The cause of the tank rupture remains unknown. Jaako said Stora Enso was investigating why pressure had increased inside the tank and caused it to rupture.

The company said production at the Imatra mills had been shut down temporarily to allow the damage to be assessed and repair work to begin. It was not immediately clear how long the shutdown would last.