There are multiple barriers between radioactive materials and the environment. There are systems backed up by emergency systems with backups to the backups. One of those holes is the reactor vessel. Hopefully not in the reactor vessel because there is only one. No nuclear plant has a backup for the reactor vessel.
Nuclear plants operate on the assumption that the reactor vessels never fail.
Another essential difference between a conventional reactor core and one with MOX fuel is the delayed neutron fraction. The power from a nuclear plant is produced by the splitting or fissioning of uranium and plutonium atoms. Splitting atoms releases energy and it also releases atomic particles called neutrons that are needed to sustain the nuclear chain reaction. These neutrons are released within fractions of a second after a uranium atom splits. When a plutonium atom splits, neutrons are released even sooner.