The explosion occurred on a train which carries molten metals into the steelworks, South Wales Police said in a tweet.
The smoke from the blast did not present a hazard, the tweet said, and that the M4 highway, that links the Welsh capital Cardiff with London, remains open.
Two people suffered minor injuries, according to an earlier tweet from the South Wales police.
Tata spokesman Damien Brook told CNN that there were no major injuries and all personnel have been accounted for.
"All fires are now under control," he said.
The incident happened outside one of the buildings at Tata Steel, a major steelworks in the area, he added, and Tata is still trying to determine what led to the incident.
A resident told CNN that they heard the early morning explosion.
"There was a long compression and the sound like a jet was about to pass," Damian Healy said. "Then three shocks came and shook the doors and windows, then there was silence.
"In the distance I heard sirens begin. That's what happened. A friend has told me she was woken by a flash and (then something) like hail hit her window."
Healy said that the steelworks was central to the industrial town's identity.
"If you're not from an industrial town it's hard to explain what the steel works is. We talk about it being the heart of the town -- its not just that -- it's the rib cage too, and protects our way of life."