Reuters International

A view of the Samarco mine, owned by Vale SA and BHP Billiton Ltd, in Mariana, Brazil, April 12, 2016. REUTERS/Washington Alves

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By Stephen Eisenhammer and Marta Nogueira

RIO DE JANEIRO (Reuters) - Brazilian regulators plan to tighten rules on dams used in the mining industry after a breach last year caused the nation's worst environmental disaster but the changes, while opposed by struggling companies, look unlikely to improve safety.

Environmental authorities say they will demand an increase in the number and focus of audits for hundreds of dams holding mining waste, known as tailings. They also want to limit the size of dams and how often their walls can be raised to store more waste.

But engineers, prosecutors and tailings dam experts interviewed by Reuters say the changes will do little to prevent another tragedy if Brazil's chronically under-resourced regulators are not themselves improved.

When the Fundao dam burst in November at the Samarco mine, owned by BHP Billiton and Vale SA, enough mud to fill 12,000 Olympic swimming pools flattened an entire village, killed 19 people and left hundreds homeless.

The sludge flooded the Rio Doce river, choking fish and spitting them lifeless to the surface.

"Fundao is the Chernobyl of the mining industry. There is a before and there is an after," said Geraldo Abreu, head of licensing at the Semad environmental agency for the state of Minas Gerais where the spill occurred.

Abreu and the task force he joined to revise state and national rules for the industry in the wake of the disaster, is focusing on dams built the same way as Fundao, a design known as upstream.

It costs about half the price of other dams but is regarded as having a greater risk of failure because its walls are built on a foundation of mining waste rather than external material or solid ground. It is also the most common, holding back waste at mines across the world.

"We now understand that this type of dam needs to be treated carefully," Abreu said.

It is still not known exactly why Fundao failed, but Abreu says it was probably the result of a loss of stability in the tailings foundation, a process known as liquefaction. This is usually caused by earthquakes but can result from other factors such as rapidly raising the dam's walls, which in an upstream design are built inwards on top of more dried tailings.

Under new rules to take effect this month or next, Abreu said miners will have to pay for an extra annual audit to check for liquefaction. Licensing will also set a height limit on the dams and require companies to specify from the outset how much waste the dam will hold and set a date for closure. Existing dams and mines that do not comply could be forced to close.

Abreu said he initially backed an outright ban on upstream dams but others on the task force persuaded him against it, saying it would be an over-reaction.

BAN UPSTREAM

A ban is exactly what some experts, like geophysicist David Chambers, say Brazil needs.

Chambers, who heads the Montana-based Center for Science in Public Participation and advises mining communities, says companies should be forced to build tailings dams using other more expensive designs: downstream and centerline. These do not use tailings for foundations.

"The unpredictability that works itself into the upstream system is just too high," Chambers said, referring to the way the consistency of tailings can vary depending on factors such as rainfall and the area being mined.

There is precedent for a ban. In 1965, a relatively small earthquake led to the collapse of upstream dams at Chile's El Cobre mine, killing over 200 people. Five years later a decree was passed banning upstream dams in all but exceptional circumstances. In 2007, this was revised to an outright ban.

Even after Chile's giant 2010 quake, downstream dams remained standing.

The mining industry has long argued Chile is an exception due to high seismic activity absent from countries like Brazil or Australia.

But Raul Espinace, an engineer and academic who for the past 40 years has been at the forefront of Chile's policy on tailings dams, said the rules implemented there would improve safety globally, especially in countries like Brazil that have weak regulators.

"This is precisely the right moment to make changes in Brazil," he said.

UNDER-STAFFED REGULATOR

The proposed regulatory changes appear to have irked Vale, Brazil's biggest miner and the world's largest producer of iron ore.

In a rare public audience, Vale warned stricter licensing could cost it 100 million tonnes of iron ore production per year, about 8 percent of the global sea-borne market. Thousands of jobs are at stake, it said, without specifically mentioning the dam collapse or regulatory changes.

The company has also said the upstream design is as safe as other types.

Experts who oppose banning upstream admit the dams require more careful operation.

Joaquim Pimenta da Avila, a consultant who designed the Samarco dam but says changes were later made against his advice, thinks upstream dams are safe as long as they are correctly monitored. "The key is ensuring regulation works," he said.

Brazil's regulator is chronically underfunded. The Department of National Mineral Production (DNPM) only has 20 people to monitor Brazil's mines, including 663 tailings dams.

Just one province in Canada, British Columbia, has eight specialist engineers for 68 tailings dams, and a recent audit said that still was not enough.

"Staffing is a serious issue. We can't do everything," said Walter Arcoverde, DNPM's director of regulation, adding that the government has not committed to improve funding or staffing. "It's being discussed."

(Editing by Christian Plumb and Kieran Murray)

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