Fire Disaster, Conflagration
noun
A fire disaster is a fire disaster, or a conflagration. 火災 is a more formal version of 火事. It usually refers to a serious and significant fire that causes extensive damage or loss of life.
かさい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
明け方、あのビルで火災が発生したそうです。
I heard that a fire broke out in that building at dawn.
原子力発電所で火災が起きたと聞いたんですが、本当ですか？
I heard there was a fire at a nuclear power plant, is it true?
私の父はオフィスの火災でずっと意識不明でしたが、今日ついに目を覚ましました！
My father was unconscious for a long time after the fire in his office, but he finally woke up today!