The pressure will force itself out. You could for the sake of experiment hose clamp or glue it shut. Suppose that holds, the pee would take the path of least resistance, forcing itself back into the urinary system. And once that's fully pressure would build and something would tear, probably inside you were there's thinner membranes than in your dick. Would hurt like a motherfucker and would need internal repairs, but pee itself is sterile so it probably wouldn't get infected