There's no talk of a bonus on paper until after you pass the interview. At this point, you're just an applicant that doesn't know if you'll even be invited to an interview.
A letter of intent is just as it is named...just a statement of your intentions. Signing that does not lock you into anything. After you make it in front of the Admiral, you'll shortly find out if he said yes or no. It's at that time you have a decision to make. You have the ability to accept or reject his offer to let you into the Nuclear Navy. If you accept, then you'll be processed and asked to sign enlistment documents. That's when you want to ask for everything in writing. It's at that time when signing the dotted line locks you in. You have no obligations to the governement until you signature ends up on enlisment documents. Even if you passed all the interviews, if you don't sign you're still a civilian free to go home with no military obligation if you choose. However, if you are fortunate enough to get sent to and interview, it's probably best to make up your mind before hand and not waste the Admiral's time.