In the email sent to US National Park Service workers, safety manager Elston Stephenson warned of possible health risks from three 19-litre containers filled with uranium ore that sat next to an exhibit popular with tourists, including visiting children.

Mr Stephenson claimed the buckets of radioactive material sat there between the years 2000 and 2018, USA Today reports.

And while federal officials removed the buckets after learning of them last year, he said nothing was done to warn workers or the public of the health consequences, describing the alleged cover-up as a “top management failure”.

“If you were in the Museum Collections Building (2C) between the year 2000 and June 18, 2018 you were ‘exposed’ to uranium by OSHA’s (US Occupational Safety and Health Administration) definition,” Mr Stephenson wrote in the February 4 email.

“The radiation readings, at first blush, exceeds (sic) the Nuclear Regulatory Commission’s safe limits. … Identifying who was exposed, and your exposure level, gets tricky and is our next important task.”

In another email dated February 11, which was sent to acting Interior Secretary David Bernhardt and Deputy Inspector General Mary Kendall, Mr Stephenson said his repeated requests for national park executives to inform the public were knocked back.

“Respectfully, it was not only immoral not to let our people know, but I could no longer risk my (health and safety) certification by letting this go any longer,” he said.

The Museum Collections Building is in Grand Canyon Village and attracts up to 1000 visitors each year with its artefacts and tours.

Mr Stephenson said the buckets had been stored next to a taxidermy exhibit — where children sit for presentations up to 30 minutes long — since around the time the museum building was opened in 2000.

He said one of the buckets was so packed with uranium specimens the lid couldn’t close. He estimated adults might have been exposed to radiation levels 400 times what is considered safe and children 4000 times the safe amount.

The National Park Service that oversees US national parks is investigating the allegations with state health and safety authorities, USA Today reports.

Grand Canyon spokeswoman Emily Davis said a recent review of the building found only “background radiation” in the area, which is considered safe and natural.

“There is no current risk to the park employees or public,” Ms Davis told USA Today.

“The building is open. … The information I have is that the rocks were removed, and there’s no danger.”

More than five million people visit the Grand Canyon each year, but Mr Stephenson’s claims relate only to the Museums Collections Building.