The filmmaker made the revelation to Total Film magazine.

He said: "I think recreating the Trinity test [the first nuclear weapon detonation, in New Mexico] without the use of computer graphics, was a huge challenge to take on."

"Andrew Jackson – my visual effects supervisor, I got him on board early on – was looking at how we could do a lot of the visual elements of the film practically, from representing quantum dynamics and quantum physics to the Trinity test itself, to recreating, with my team, Los Alamos up on a mesa in New Mexico in extraordinary weather, a lot of which was needed for the film, in terms of the very harsh conditions out there – there were huge practical challenges," he told Total Film.

He said the story of Oppenheimer has 'immense scope and scale'.

"And one of the most challenging projects I’ve ever taken on in terms of the scale of it, and in terms of encountering the breadth of Oppenheimer’s story. There were big, logistical challenges, big practical challenges. But I had an extraordinary crew, and they really stepped up. It will be a while before we’re finished. But certainly as I watch the results come in, and as I’m putting the film together, I’m thrilled with what my team has been able to achieve," he said.

Oppenheimer is an upcoming American biographical film written and directed by Christopher Nolan and starring Cillian Murphy as J. Robert Oppenheimer, the American theoretical physicist credited with being the "father of the atomic bomb" for his role in the Manhattan Project—the World War II undertaking that developed the first nuclear weapons.

The film is based on American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, a biography written by Kai Bird and Martin J. Sherwin. Emily Blunt, Matt Damon, Robert Downey Jr., and Florence Pugh also star.

It is a co-production between Universal Pictures, Syncopy Inc. and Atlas Entertainment, with Nolan producing the film alongside Emma Thomas and Charles Roven.

Oppenheimer is scheduled to be released in the United States on July 21, 2023 by Universal Pictures.