Since this is the first mention of an accident, you use the indefinite article, 'a/an'.
However, now that we know about the accident, I will refer to it as "the accident" - since we both know which accident we are referring to.
No sooner had he arrived than the accident took place.
The accident took place as soon as he arrived. On his arrival, the accident took place.
He arrived just as the accident took place.
The accident took place just as he arrived.
The accident and his arriving took place simultaneously. The accident and his arrival formed a temporal synchronicity.