I was told that I must work at least 18 hours "because we are a union" any validity in this?
Depending on your contract, Kroger is required to give you a certain number of hours. In my Division, it's 15 hours, but it could be 18 for your Division/KMA. HOWEVER, you can ask either your Union Rep, Union Steward, or someone at the Customer Service Desk for a waiver form if you wish to work less than 18 hours. Read your contract for more info (you already should have read the contract, because why else are you paying $10 a week?!?).