Creating a random password based off of a prime number
If you just need random numbers, there's no point in generating random primes.  Just make sure that you're using a cryptographically secure random number generator, properly seeded from a secure entropy source.

Also, passwords made up of random letters and numbers are very hard to remember (and type).  For a password meant to be memorized by a human, it's much more efficient (in terms of security vs. effort) to generate a passphrase made out of random words instead.  See diceware and this xkcd strip for some well known examples.

Also, 100 random base-32 characters equals 500 bits of entropy.  That's ridiculous for a password; 128 bits of entropy is plenty for anything, and you're probably OK with 80 bits or even less, depending on what kind of threats you're expecting.  If your key derivation function does proper key stretching, that'll add some 10 to 30 bits of effective strength to your password (depending on the iteration count), allowing you to reduce its length even further.

A typical randomly chosen word has about 10 to 14 bits of entropy, depending on the size of the word list (a diceware passphrase has just under 13 bits of entropy per word), so generating a passphrase of five or six random words and using a key-stretching KDF should be safe  enough.