One worker has been sent to hospital after concrete formwork collapsed at the Muskrat Falls hydroelectric construction site in central Labrador, raising new questions about the progress of the much-delayed and over-budget megaproject.

This before and after comparison of the Muskrat Falls project in central Labrador shows damage to the site's powerhouse. (CBC has agreed to withhold name of photographer)

Seven other workers also received first aid after the collapse, which happened at the project's powerhouse close to midnight Sunday.

Officials say the one worker who was taken to hospital did not sustain serious injuries.

"This is a very, very serious incident for us," said Gilbert Bennett, Nalcor Energy's vice-president responsible for the Lower Churchill project, confirmed that drying concrete fell when formwork — a mould — collapsed in the powerhouse.

Concrete was being poured to construct the lower draft tube, essentially a large pipe that will return water to the Churchill River.

"Any time that we have something that was unplanned like this event, and we see potential for worker injury, it's one that we take very seriously," said Bennett.

Seven workers received first aid and one was sent to hospital in Happy Valley-Goose Bay after formwork holding up newly poured concrete collapsed at the Muskrat Falls project site. (CBC has agreed to withhold name of photographer)

What workers say happened

Multiple sources told CBC News that workers had been pouring concrete for hours and that the formwork collapsed, coating a couple of workers in wet concrete.

"Shaken up is an understatement," one worker told CBC News.

"I could have lost some of my brothers tonight. We are very lucky to not have lost anyone."

The collapse happened during the night shift. Employees were reportedly on their lunch break when the form gave out.

Investigation launched

Muskrat's powerhouse is shown in this image taken last month. Around midnight on Sunday, formwork supporting newly poured concrete at the powerhouse gave way. (Gilbert Bennett)

In an interview with CBC News on Monday, Bennett said that the company has launched a full investigation to find out what happened at the powerhouse, as has Astaldi Canada, the contractor on site.

"We need to look at all aspects of the work and understand what the root cause was," he said.

"So we're interested, Astaldi is interested, provincial Occupational Health and Safety are interested."

Bennett said that the piece of failed of equipment was manufactured in the United States, but assembled on site by the project's main contractor, Astaldi.

Controversial project

The Muskrat Falls project is one of the most controversial issues in Newfoundland and Labrador, with skepticism among key members of the newly elected Liberal government about how Nalcor has managed the megaproject.

First power at Muskrat Falls was originally expected in December 2017. Last fall, Nalcor pushed the project's projected start date into 2018 and has since declined to comment on a potential start date.

Bennett said he would not speculate on how the incident will affect the project's timeline. Similar work is planned in other parts of the construction site, but it has been paused pending the investigation.

"First and foremost, we need to understand that the work can be undertaken safely and that we can continue with this activity," he said.

The collapse happened at the powerhouse, a major part of the Muskrat Falls hydroelectric dam project. (Nalcor)

Stan Marshall, Nalcor's new CEO, is reviewing the project, but said earlier this month that it is "very unlikely" that the megaproject will be cancelled.

The current cost forecast is $7.7 billion, but that number is expected to increase. It was originally estimated the project would cost $6.2 billion. At the end of 2015, $4.4 billion had been spent.

It's the most costly project in the history of Nalcor, the Crown-owned energy corporation.