Integral closure of a PID is torsion-free

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Can anyone explain me why the integral closure of a PID $A$ in a separable finite extension of its fraction field is a torsion-free $A$-module?

I know that it is a finitely generated $A$-module, but how can i prove that? Thanks in advance...

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Let $L$ be a field, and $B$ subring of $L$ containing $A$ as a subring. Then $B$ is a torsion-free $A$-module.

Pick $b\in B$ and $a\in A$ such that $ab=0$ and see this in $L$. What do you get?