Rationale means “reasons explaining a particular decision” and is pronounced /ˌræʃəˈnɑːl/ (ræ-shə-naal) in the UK resp. /ˌræʃəˈnæl/ (ræ-shə-næl) in the US.
The “ale” at the end is misleading. There are only three English words in which “ale” at the end is pronounced in this way; the other two are morale /məˈrɑːl/ (mə-raal) UK, /məˈræl/ (mə-ræl) US which means “the amount of confidence a person or a group of people has” and chorale /kɒˈrɑːl/ (kor-aal) UK, /kəˈræl/ (kə-ræl) US, “a piece of church music”.