Abstract: The structure and model of oxidation module MIDAC-OX were introduced. Taking AP1000 as an example, the break of fuel cladding, the oxidation of fuel cladding and the production of hydrogen were calculated, and then the DNBR during coolant pump inert rotating, the fuel cladding integrity under natural circulation and the delay of fuel cladding break due to oxidation were analyzed. The validity of this code would be checked after the code development fully finished. The results of this module have already partly compared with existing results. The comparison shows the results of this module are good.