OTTAWA - The tab for fixing the isotope-making reactor at Chalk River, Ont., is running at $11 million a month, including the cost of the repairs themselves and lost revenues.

That's on top of $72 million earmarked for the work in the 2009-10 fiscal year.

Hugh MacDiarmid, president of Atomic Energy of Canada Ltd., says the repair is a complicated job of a kind never done before.

It has caused delays and forced the company and its technicians to design one-of-a-kind tools and procedures, he told a House of Commons committee Tuesday.

The Rube Goldberg improvisations have worked like a charm so far, he said, but it's a slow, painstaking process. Technicians are essentially using remote control to weld patches on a spot three storeys below them, which they can't actually see or touch.

The basic problem is corrosion in the thick, aluminum walls of the 50-year-old NRU reactor. It sprang a leak last May and was shut down. Careful inspection with custom-designed TV scanners found eight more trouble spots, where the metal walls of the reactor vessel had corroded sharply.

The decision to patch the leak and reinforce the thin spots began an unprecedented process, MacDiarmid said.

"What we're doing has never been done in the history of the nuclear industry," he said. "We have invented our own tools."

The problem areas are deep down in the reactor vessel, in a radiation field that precludes direct human involvement.

The access ports are little bigger than a baseball. The solution to getting a welding torch inside is a long pole with an arm that holds the welding head and which can extend once it's through the narrow access.

Then a technician with a TV screen and a joystick starts the repair from a safe seat 10 metres above.

The company has 300 people working on the project, which runs 24 hours a day, seven days a week.

Each patch takes days of rehearsal and practice before welders actually start on the repair. They use a full-scale mock-up of the reactor to perfect their techniques, accounting for the various curves at the different repair sites.

"Once we're in the vessel, we need to get it right the first time," said Bill Pilkington, the company's chief nuclear officer. A mistake simply means another repair job.

All this eats up time and has forced the company to keep pushing back its deadline for restarting the reactor. It originally said the reactor might be down a month, then three. Then it would be back in the first quarter of this year. Then it was April.

MacDiarmid now says he hopes it will be back in the business of producing medical isotopes by the end of July. And, he says, they've built in extra time to allow for problems.

"Predictions are a challenge in an environment where you're dealing with first-of-a-kind complexities," he said.

The reactor is a combination research tool and medical isotope production facility. Before the shutdown, it was producing a major portion of the world supply of isotopes used in medical diagnostic tests.