On March 31, a little-known part of Canada's nuclear history will go dark for the last time.

The National Research Universal Reactor — or NRU — at Chalk River, Ontario will be turned off for good Saturday evening. It first came online in 1957.

Retired Atomic Energy of Canada Limited engineer Fred Blackstein was there almost from the start. He got his first job in the lab when he was just 19 years old.

"I sort of think I grew up here. I hope I grew up," he said.

Blackstein would go on to work in a number of jobs at the reactor, from 1961 to 1985. He said that during those decades, Chalk River was the "place to be if you were a scientist anywhere in the free world."

Fred Blackstein, retired AECL engineer, chats with the CBC's Susan Lunn in the building housing the NRU. Blackstein worked in the Chalk River labs from 1961 to 1985. (Sarah Sears/CBC News)

"A number of people I'm pleased to have said I knew, and in some cases worked with, have received Nobel Prizes for the work here on the NRU," Blackstein told CBC.

Walking into the NRU's control room for the first time in 20 years earlier this month, Blackstein felt like he was stepping back into his own past.

Many of the dials, alarms and sensors lining the NRU control room walls are artifacts of his time at the NRU. He pointed to his old station, at the central control desk, which is now braced against a bank of newer computers.

CBC visited the NRU at the Chalk River Labs in 1961 0:50

The NRU wasn't quite Canada's first foray into nuclear science. As was the case with researchers in the U.K. and the United States, Canadian scientists made their first efforts to probe the secrets of the atom before the Cold War began.

During the Second World War, the British government was looking for a safe place to relocate its Cambridge-based nuclear laboratory.

By 1942, Canada had built the secret Montreal Laboratories to help develop nuclear weapons through the U.S.-led Manhattan Project.

The war, the Bomb and the beginning

Canada was already helping the United States with its work on the atomic bomb by supplying uranium-bearing ore from a mine in the Northwest Territories, which was refined in Port Hope, Ont.

But a new joint effort with the U.S. and the U.K. as the war was winding down took Canada fully into the nuclear age.

American, British, French and Canadian scientists collaborated on the design of the Zero Energy Experimental Pile (ZEEP) reactor, housed at Chalk River.

​When ZEEP went online in September 1945, it was the first operational nuclear reactor outside of the United States.

A small, prototype reactor, it was built to demonstrate that uranium and heavy water could be used for nuclear fission and that plutonium could be produced and extracted from the process for military applications.

A young Fred Blackstein (left), AECL engineer, and an unidentified colleague stand in front of a valve test rig in this 1961 photo. The experimental apparatus was designed to test nuclear reactor valves, to ensure there were no leaks. (Fred Blackstein)

ZEEP was also the basis for the National Research Experimental reactor, or NRX, which is still being decommissioned at the Chalk River site.

But by the time these reactors were online, the war was over and Canada was starting to explore peaceful uses for nuclear fission.

"I think that's unique," Blackstein said. "Every other nuclear power who had nuclear reactors also had a military context. And I guess it's in keeping with Canada's reputation as peacekeepers [for] our role in atomic energies to only be for peaceful applications."

Peaceful, yes — but as far as the federal government was concerned, still highly sensitive. In this Google age of instantly-accessible satellite maps, the two red brick buildings housing the NRU and NRX two hours northwest of the capital, along the shore of the Ottawa River, are impossible to hide.

But when the reactors were built, their location was chosen for its strategic value. Tucked away in a remote area where few could find it, they ran around the clock, producing isotopes and quietly conducting cutting-edge research.

A reactor lattice light panel for the National Research Universal Reactor (NRU), which first came online in 1957. (Sarah Sears/CBC News)

And so they continued for decades, with few Canadians even aware of their existence. Until 2007.

The NRU was once responsible for producing about 40 per cent of the world's supply of the medical isotopes used for diagnosis and cancer therapy — starting with cobalt-60 and later extending to other isotopes, such as molybdenum-99.

But a month-long shutdown in 2007, and a leak detected in May 2009 that forced another year-long shutdown, pulled Chalk River into the international spotlight.

NRU's unexpected problems created a worldwide shortage of medical isotopes. It was back online in August 2010 and, by 2011, AECL was reporting that the NRU's medical isotope supply was helping more than 76,000 people daily, in more than 80 countries.

​Since then, newer reactors have come online in other countries and the NRU hasn't made molybdenum-99 isotopes since the fall of 2016.

A look inside the National Research Universal reactor's control room at Chalk River Labs 0:48

But isotopes were only ever a part of NRU's work. It played a key role in neutron physics research, and helped to develop the CANDU model reactors generating commercial electricity in Ontario, New Brunswick and around the world.

In 2011, the Harper government sold off the part of AECL that made CANDU reactors to SNC Lavalin. The remaining part, including Chalk River Laboratories, would be turned into a public-private partnership.

Its current CEO, Mark Lesinski, said that when he took on the job, many at Chalk River feared their work would end when the NRU shut down.

"There was a bit of a mood like that a couple of years ago, that with the NRU closing there wouldn't be anything else. But that's far from the truth," he said.

Lesinski said reinvestment by the Liberal government pointed the way to possible new directions for the Chalk River facility.

"When that commitment came through from the government ... to say it's not just about cleaning it up and stopping, that we do believe there's a place for Chalk River Nuclear Labs in the world again," he said, "we got into kind of thinking about a new chapter."

A nuclear reactor that fits in your basement?

Chalk River's new chapter is taking its research in several promising directions: finding the next generation of medical isotopes, researching hydrogen as a clean source of energy and the development of small, safe modular reactors.

Nuclear physicist Bronwyn Hyland talks with CBC about her program to develop small modular reactors. (Sarah Sears/CBC News)

Bronwyn Hyland is the program manager for the small modular reactor program. "Chalk River is the history of nuclear in Canada," she said. "We have a long history of many nuclear successes that we will be building on in our small modular reactor program."

These next-generation reactors, she said, will be easy to transport and assemble. Some, she said, will be small enough to "fit in my basement" but still powerful enough to supply electricity to any remote location — a mine, for example, or a northern community cut off from the grid.

"There are many areas of Canada that today don't employ clean energy, particularly remote locations that today use diesel power," she said. "So these small, modular reactors are potentially really great solutions for those locations."

Other countries are working on small reactor designs of their own, but Hyland said she believes Canada — with its strong regulatory system, domestic fuel supply chain and background with the CANDU reactors — is uniquely placed to corner the market.

"Canada has an enormous opportunity here to establish itself — to re-establish itself — as the world leader in nuclear technologies."

CNL employees watch over the control room of the National Research Universal reactor (NRU) at Chalk River, days before was scheduled to go offline for good. (Sarah Sears/CBC News)

Will such basement-sized reactors become a thing? That depends in part on whether the project finds a commercial partner. Hyland said she hopes to see that happen within the next decade, and will involve a demonstration reactor at Chalk River.

But before the future can get started, the past has to be decommissioned. More than 60 years of nuclear research at Chalk River have left behind a legacy of low-level radioactive waste that now has to be contained at a near-surface facility.

Atomic Energy of Canada Limited estimates the cost of dealing with waste at all of its federally regulated sites, including Chalk River, could be as high as $7.6 billion.

A small ceremony will be held the evening of March 31 for current and former staff to mark the NRU's retirement. Lesinski admits the mood might be a bit melancholy.

Fred Blackstein isn't shedding tears. For decades, the NRU was at the forefront of its field, where all good scientists want to be. But the world has moved on, and he's happy to see his old employer turn the page.

"The cutting edge work we did when I was here has already seen fruition. We've saved hundreds of millions of lives through cancer therapy, cancer diagnosis, safer aircraft ... the list just goes on and on.

"So it doesn't end. It's only the beginning."