I know hangul was originally written as Kanji.
불안 should be written as
부 is "不",
안 is "安".
부안 written as
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First, Hangeul was not originally written as Kanji. Kanji is the term for Chinese characters used in the Japanese language; Hanja is the term for these characters used in Korean language. Hangeul is another writing system for Korean.
不 actually has 2 pronunciations in Korean: 불 and 부. This can be traced back to Middle Chinese, where the pronunciation included /pɨu/, /pɨuX/, /pɨut̚/. The last one ends in a /t/; final /t/ in Middle Chinese characters normally correspond to a ㄹ 받침 in Korean.
The actual pronunciation of 不 in Korean somewhat depends on the following syllable, although there are many exceptions. But in general: