What did this character hit to cause the massive explosion?

Question

In the movie X-Men Apocalypse we see,

When Apocalypse infiltrates the Mansion to abduct Prof. Xavier, Alex tries to save him with his laser blasts. But Apocalypse teleports away, causing Alex to hit something which causes a huge explosion destroying the building.

This sequence was quite fast, so I couldn't understand what did he hit. Was it the Jet or some secret stash of explosives ?

Answer 1

It looked like the beam hit the engine of the jet that Beast was showing off earlier (as seen in the trailers, and shown below).

MoviePilot's spoiler-filled review/plot synopsis says that the X-Jet is nuclear powered, and confirms it is what fuelled the explosion.