Environmental groups have voiced concern that Russia is again accepting shipments of uranium tails, a byproduct formed when uranium is enriched, from a German nuclear fuel firm, reigniting a debate over whether the substance meets the definition of nuclear waste.

The shipments of the toxic compound – also called uranium hexafluoride – were halted in 2009 over revelations that Russia was accepting it from foreign customers and storing it in the open. At that time, Rosatom, Russia’s nuclear corporation, bowed to environmental pressure and promised to no longer import the radioactive substance.

But German government documents revealed last week by Greenpeace and the Russian environmental group Ecodefense show that the German-based enrichment company Urenco resumed the uranium tail shipments as long ago as last May.

According to Urenco’s contract with the Russian nuclear-fuel giant Teksnabeksport (Tenex), a subsidiary of Rosatom, some 12,000 tons of uranium tails are set to be delivered to Novouralsk, near Yekaterinburg by 2022. Four thousands tons have been sent so far, with another 600 departing the Urenco facility earlier this week.

Were Russia to actually reprocess the uranium tails for further use as fuel, environmental groups say that the contract must stipulate that waste arising from this process be shipped back to Germany.

But according to Alexander Nikitin, who heads Bellona’s St Petersburg office, it is still unclear whether Tenex intends to do this.

Ecodefense and Greenpeace have argued that by exporting uranium tails, Urenco is merely ridding itself of the need to store radioactive waste in Germany, which is more expensive than it is in Russia.

Indeed, Urenco opened an enrichment facility of its own in 2005 – calling into question the purpose of transporting uranium tails thousands of kilometers to Russia for enrichment there.

Uranium hexafluoride, also called depleted uranium, is a colorless radioactive powder that is produced as a byproduct of enriching uranium for use as fuel in nuclear power plants. Urenco, which is a partnership involving German, British and Dutch energy firms, has operated an enrichment facility in Gronau, Germany since 1985.

This facility also stores depleted uranium in the open air. In the early 1990s, Russian opened its doors to reprocessing depleted uranium from foreign customers. A previous contract between Tenex and Urenco envisioned the import of 100,000 tons of uranium tails between 1996 and 2009.

The issue of whether uranium tails in fact constitute nuclear waste depends on whom you ask. Both Rosatom and Germany’s nuclear industry classify uranium hexafluoride as a recyclable material. The US Nuclear Regulatory Commission, however, has long held that uranium tails should be classified as nuclear waste – a view that Bellona, Ecodefense and Greenpeace share.

But while Rosatom asserts that uranium tails are valuable raw material, the motive for importing them is unclear. By most estimates, Russia already holds nearly 1 million tons of uranium tails from its own fuel production – making the need for another 12,000 tons from abroad questionable.

The issue has galvanized environmental protest in Germany and Russia for decades. Critics of the practice have called attention to the dangers of transporting the substance, as uranium hexafluoride can turn into gas when heated. Should an accident that brought the substance into contact with water occur, it could produce highly toxic hydrofluoric acid.

But the main argument made by environmentalists against the import of uranium tails is that only a small quantity of uranium is returned to Germany after reprocessing. The rest remains in Russia – as radioactive waste.

“The truth is that uranium from URENCO will remain on the premises of Russian companies, in addition to almost 1 million tons of nuclear waste from Russia’s own production,” Alexandra Koroleva, one of Ecodefense’s founders, told the Deutsche Welle German news agency. “Experience shows that more than 90 percent of the uranium remains in Russia.”

Past studies by Russia’s own industrial oversight agency Rostekhnadzor, bear Koroleva’s comments out. In 2007, for instance – when anti uranium tail protests were at their height – Rostekhnadzor reported that only 1,700 tons of uranium tails were reprocessed and returned to Germany of the 21,000 tons Russia received that year.

The agency also found that uranium hexafluoride containers are stored outdoors “in conditions of inadequate regulatory justification and at a significant degree of risk of loss of seal.”

Two years after those findings, Sergei Kiriyenko, then the head of Rosatom, told the non-governmental representatives making up the company’s public council that the import project would be stopped.

Bellona’s Nikitin criticized Rosatom’s apparent backpedalling on that commitment, saying, “These promises must be kept. If the plans have changed, then it must be explained why and for what and with what goals there are for the depleted uranium, and so on.”

“We need to have this information,” Nikitin continued. “If it’s all being quietly and secretly so no one finds out about it, then that’s a bad thing.”

Koreleva – who has applied for asylum in Germany as a result of pressure from Russian authorities – asserts that they secrecy surrounding the process is intentional, saying that waste imports are concealed behind terms like “strategic material” and “reprocessing.”