One of the first things a visitor sees on entering Ontario Power Generation’s $35 million training facility at the Darlington nuclear station, sitting in the middle of the floor, is what looks like a doorframe.

And that’s exactly what it is.

Because when you’re about to spend billions overhauling four nuclear reactors, you’d better make sure that all your equipment fits through the door.

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It’s a small but vital detail in the mammoth project, and just one small element in a painstakingly constructed reactor mock-up that OPG will use to train its workers.

The mock-up reactor is part of a 350,000-square-foot training centre and warehouse that OPG has built at Darlington in preparation for the reactor overhaul, due to get under way in October, 2016.

The objective of the mock-up, says OPG senior vice president Bill Robinson, is to give workers a full-scale practice facility, before they first try their hand at rebuilding billion-dollar reactors.

“It’s as near to real-life as you can get,” says Robinson. “They’ll know what it looks like when they get in there.”

OPG’s chief executive Tom Mitchell has vowed to bring in the project on time and on budget.

If it happens, it will be a departure from history. The overhaul of the four-unit Pickering A nuclear station was supposed to cost $1.3 billion. In the end, it cost $2.6 billion to return just two reactors to service.

And privately operated Bruce Power ended up paying $4.8 billion to overhaul two mothballed reactors, after an original estimate of $2.75 billion.

The reasons for the over-runs were many. But part of the problem came from workers having to learn the complexities of doing unfamiliar, precision work in a cramped space, sometimes while wearing protective suits.

OPG acknowledges the pressure is on to perform.

In a speech last year, chief executive Tom Mitchell called the Darlington project “our biggest test.”

“We feel the eyes on us, and we know we must execute the project on time and on budget,” he said. “If we don’t, we think the whole industry in Canada will be affected.”

The current price tag is $10 billion in 2013 dollars, which OPG says “translates into a completion cost (of) $12.9 billion, including capitalized interest and contingency, by the end of the project.”

Coincidentally, OPG is applying to the Ontario Energy Board for permission to increase the rates it gets from its nuclear plants by 30 per cent. Although that decision is likely to be rendered long before the Darlington project gets into full swing, it focuses attention all the more on nuclear performance.

Hence the company’s anxiety to get all its ducks in a row from the start of the overhaul.

OPG’s skeletal mock-up will give workers a chance to hone their skills on a dummy, before they try their hand at the real thing.

In fact, in addition to the full-scale mock-up, models of smaller reactor components have been constructed, so that trainees can get a feel for the job working in their shirtsleeves, with plenty of elbow room, before they move up to the full scale model and don bulky suits.

It’s a bit like a pianist practicing scales before playing a Bach fugue.

The mock-ups themselves are precisely constructed. Technicians did a laser scan of a reactor face to get the dimensions just right.

“Every millimeter is going to be important here,” says Robinson.

Even the lighting in the access areas leading to the mock reactor is hung at the precise height as the lights inside the reactor area, so that workers bringing in equipment know what obstacles they have to deal with.

There are 480 pressure tubes to be extracted and inserted in each reactor. These 8-metre tubes hold the uranium fuel, and run from end to end of the calandria – in effect a big zirconium barrel, lying on its side and filled with the heavy water needed to nurture an atomic reaction.

(The mock-up doesn’t include the outer barrel; workers will be able to see the new tubes sliding into position.)

The pressure tubes – each encased in an outer tube called the calandria tube, which must also be replaced – have stretched and sagged about 10 centimetres after decades in the reactor. They must be loosened, then delicately withdrawn and replaced.

They’re highly radioactive after decades in the reactor, so all workers in this phase have to wear bulky radiation suits, hooked up to breathing tubes.

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Practicing in a non-radioactive environment gives reassurance to workers who’ve never plied their trade at a hot reactor.

“Some of them get overwhelmed when they’re first in there, in a plastic suit at the reactor face,” says senior vice president Dietmar Reiner.

Also due for replacement are the 960 feeder tubes – one at each end of the fuel channel assembly. These precisely curved steel tubes carry heavy water into the pressure tubes – where it is super-heated by the nuclear reaction – and out again.

The hot heavy water travels to a steam generator, which turns conventional water into the steam that powers the electric generators.

The old feeder tubes have to be cut off the fittings that cap the pressure tubes, leaving a precisely measured and contoured stub that will be welded to the new tube.

With a nest of 480 feeder tubes snaking out of each end of the reactor – and not allowed to touch each other – the removal and replacement must be done precisely, but in cramped conditions.

The point of the mock-up is to have the work teams figuring out the best working techniques before they start for real.

“We’re aiming to get into the reactor and hit high productivity rates right from the start,” says Reiner.

Still, is that worth spending $35 million for the mock-up?

Robinson and Reiner say yes.

At peak activity, with about 2,000 workers on the job, the project will be costing $2 million a day, they say. At that price, shaving days off the work schedule pays off in a hurry.

Similarly, they note, each day that one of Darlington’s reactors is off-line means $1 million in lost revenue, so getting reactors back on line faster quickly turns into extra cash.

One question mark in the project is the availability of skilled workers and suppliers who deliver highly specialized components and services for the nuclear industry.

While Darlington works on its four reactors – sometimes two at a time – Bruce Power is also embarking on a program to overhaul six of its reactors near Kincardine.

According to the current schedule, at certain times, work could be proceeding simultaneously on three reactors at Bruce and Darlington.

Robinson figures working on two reactors at once should be feasible. Three at a time could be “more challenging,” but that won’t happen before 2020, when everyone concerned will have more experience under their belts.

OPG remains confident of its schedule, which calls for all four Darlington reactors to be overhauled and back in service by the end of 2025.

Robinson even hazards a note of optimism:

“There is a good chance we could come in early.”

If it doesn’t, there’ll be no shortage of critics.