The writer borrows a technique from many prose writers of commencing his narrative part way through the full story. i.e. there's initially no indication of what the poem is about (i.e. the Chernobyl disaster), thus firing the reader's curiosity and inviting him to read further.
He then proceeds (in his third paragraph)to contrast the harsh realities of the situation with the natural beauty of the surrounding landscape, helping to emphasise the sense of unreality which those on the bus must have experienced.
The fourth paragraph serves almost solely to provide an explanation of what was actually going on.
The final two paragraphs relate to the lasting psychological effects of the experience, suggesting that while he has in some ways been able to 'move on' in his life, he will always be haunted by what occurred decades earlier.