An analytical model was developed to study the prompt escape of gaseous and volatile fission products present in the fuel-cladding gap of a light water reactor fuel rod following a cladding breach. Key questions include how fast fission gas is released and what mass fraction of the gap gas remains in the failed rod. Analysis shows that only a small mass fraction (∼0.17) of the total gas inventory initially present in the fuel-cladding gap escapes promptly following cladding rupture, regardless of the size of the rupture, when the rod is surrounded by liquid coolant at high pressure (∼6 to 15 MPa). However, during a loss-of-coolant type of accident, where the fuel rod is surrounded mainly by low pressure steam (∼0.1 MPa), a large mass fraction (∼0.95) of the gap gas is found to escape the rod shortly after cladding rupture due to the greater ratio of the initial rod pressure to the coolant pressure.