The oil sludge was sent to the Sysav waste disposal plant in Malmö by Indus Kjemisk Teknisk Fabrikk, a company based outside Bergen which specialises in the disposal of hazardous substances.
When the waste arrived at Sysav's plant, the high level of radioactivity was picked up by Geiger counters on the site, which detected the radiation from more than seven metres away, forcing Sysav staff to don full protective gear with hoods and breathing masks.
“It's clearly a concern that radioactive waste has been transported. How this could have happened needs to be investigated in the country of origin,” Gunilla Carlsson, press spokesperson at Sysav, the company that imported the waste told SVT.
According to SVT, Indus has been reported to the police on previous occasions for failing to handle hazardous waste safely.
Trond Andersen, the chairman and chief executive of Indus, said he has no idea how radioactive waste had come to be exported.
“We have never experienced anything like this before," he told Norwegian newspaper Bergens Tidende. "I am counting on this being low-level radiation. We are waiting for the analysis and we will do everything we can to make this right."
Andersen explained that the oil sludge had originated from a Bergen machine shop and that Indus had had no reason to suspect that it might be radioactive.
“I understand that the oil came from an oil separator, I have no information to suggest that we should check oil sludge from such a source. I think it is strange that they got a radioactive reading from the waste,” he said.
The waste was stored for almost a year in Malmö before being sent back to Norway in May this year.
If the Geiger counters had not picked up the radiation, the waste would have been burnt to heat and provide power for Swedish homes.
In 2014, Sweden imported 800,000 tonnes of waste from abroad, enough to heat 950,000 homes and provide electricity for 260,000.
Due to increased recycling, Sweden does not produce enough waste for its combined heat and power incinerator plants to work at full capacity.