Most of the so-called Fort de Vaujours is located in France's most financially destitute department Seine-Saint-Denis. The site is believed to contain enough high-end gypsum for Placoplatre's nearby factory to function for the coming 30 years. The company's other gypsum deposits will run out in three to four years, so Placoplatre, a subsidiary of French conglomerate Saint-Gobain, desperately needs the site.

"It's important to maintain our plant - we employ 400 people at the factory which generates 3,000 indirect jobs and an additional average 1,000 workers will be operating at the industrial site," Placoplatre Head of Mining Development Gilles Bouchet told DW.

Nuclear test site

But environmentalists say developing the Fort de Vaujours site is far too risky. The Fort was constructed in 1881 and initially used to defend Paris against the Prussian army. During World War II, German Nazis stored weapons throughout the roughly 300 buildings. After the war, the French government ran pyrotechnical tests in the area.

Most importantly though, France's Atomic Energy Commission carried out trials of core components of the country's nuclear bombs during the 1950s to the 1990s. There were no full nuclear detonations at Vaujours, but more than half a ton of uranium was blown up in about 2,000 detonations. At least 150 kg of the radioactive material was left behind when the government abandoned its tests in 1997. The area remains under the purview of the government and the information about the tests is classified, so it's difficult to know precisely what took place during those five decades.

"The Fort de Vaujours is like a black box - no-one knows exactly what went on there and what's hidden beneath the ground," Christophe Nedelec, President of environmental NGO Gagny-Les Abbesses-Chelles, told DW.

"I think it's crazy to undertake such a dangerous project on the outskirts of Paris without properly evaluating the risk for workers, residents or passers-by," he added.

Tests have shown that several areas at the Fort are still contaminated

'Zero risk for the population'

But Placoplatre's Bouchet reassures there's no need to worry. "We have had numerous impact and radiation studies carried out that have been submitted to France's nuclear safety body ASN, and it, too, agrees that there is zero risk for the region's population," he said.

"We are performing an environmental survey and specially air quality at the site and on the cities around and providing the necessary protective gear for workers," he added.

However, Nedelec is dubious of the company's studies. He says trust has completely broken down - especially since 2014 when radioactive measures put forward by Placoplatre turned out to be inaccurate.

The Commission for Independent Research and Information on Radioactivity (Criirad), an independent group campaigning for greater transparency around atomic sites, carried out studies on a sample-basis at Vaujours in 2001. The group found several highly contaminated spots inside some of the buildings and a highly polluted patch of land.

The Atomic Energy Commission then decontaminated the zones and declared the Fort risk-free. It was sold to Placoplatre in 2010.

But Nedelec wanted to see for himself. He broke into the Fort in 2011 with an amateur Geiger counter and found that three of the spots flagged up by the Criirad in 2001 were still showing high levels of radiation. For the next three years, Placoplatre disputed those results. Studies commissioned by the company and carried out by a private laboratory showed no contamination at the site. Further tests by the government's own Institute for Radioprotection and Nuclear Safety came to the same conclusion.

Wrong approach

In 2014, independent body Criirad was finally invited back to the Fort. New tests confirmed Nedelec's results. One area in particular was found to emit 70 times the normal radiation levels. Bruno Chareyron, head of Criirad, says he was shocked to see that not only private laboratories paid for by Placoplatre but also the highly respected Institute for Radioprotection and Nuclear Safety had not detected the contamination. "You can only wonder - were all these people incompetent or is there another reason?" he said.

One problem, the nuclear physicist states, was that the other researchers hadn't used the right material and measurement techniques to detect the radiation. "They were only testing for gamma radiation, but should also have looked into alpha and beta rays" he told DW adding the scientists had been employing the wrong meters.

Placoplatre says the area no longer poses a health hazard

Contamination

But Placoplatre says those contaminated areas have now been decontaminated. The gypsum producer started demolition works in 2014. Currently, the works have been halted.

A total of 47 of the buildings have so far been destroyed. Several areas of contamination and around 130 shells and grenades have been found - and neutralized according to Placoplatre. "We are today the only company capable of managing this long-term project that is important for the region and are obviously decontaminating the area and taking the necessary protective measures before each additional leg of demolition works," Bouchet said.

Watchdog ASN says it is supervising the company and making sure low pollution thresholds are not exceeded. "After the findings in 2014, Placoplatre adapted its work protocol," according to the ASN Head of Paris Division Bastien Poubeau.

"The company is putting in place the necessary protective measures and whenever new contaminated areas are discovered, they are being decontaminated," he told DW.

The nuclear safety body has assigned two university laboratories to control radiation levels at the site. Those laboratories are being paid by Placoplatre, but Poubeau maintains that's normal procedure in these types of cases. "Placoplatre doesn't know the details of the studies and we are doing everything to make sure they are being carried out independently," he said.

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Independent study

But environmentalist Nedelec says an independent body such as the Criirad should be mandated to conduct a comprehensive radiation study prior to any construction works. For him, that's the only way to restore trust.

Nuclear expert Chareyron underlines the studies in 2001 only covered a tiny fraction of the area. "We need to examine the remaining part of the site - including the huge network of underground pipes which were used to evacuate the radioactive water," he said. The pipes were not removed from the Fort after the nuclear tests ended, but just filled with concrete.

Nedelec's organization, jointly with other environmental NGOs, has launched an online petition also calling for a parliamentary inquiry and an impact study on the effects of 50 years of nuclear tests in the region. So far more than 130,000 people have signed the petition.

Those same groups are now pressing charges against Placoplatre for endangering the life of others.