South Wales Police says the A465 is closed between Aberdulais and Resolven, after a serious crash.
Diversions are in place via the B4242.
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Philip Thomas was a 10-year-old pupil at Pantglas Junior School in Aberfan when a coal tip collapsed and slid downhill.
ITV News speaks to Jeff Edwards - who was eight years old when he became the last survivor to be pulled from the rubble.
Until 21st October 1966, Aberfan was a place that most of the world had never heard of.