The amount of the radionuclides (fission products along with activation & actinides) that can be released (or gets released) from a nuclear facility in an accident is known as Source term. Evaluation of source term release from fuel is the first step for assessment for health effects and associated risk in public domain. The key factors that influence the source term release from fuel are postulated accident scenario and initial operating conditions viz. reactor power and fuel burn-up. Based on core condition, severe accidents, also known as design extension conditions (DEC) are divided into two groups viz. design extension condition without core melt (DECA) scenarios and design extension condition with core melt (DEC-B) scenarios. In DEC-B scenario, in addition to volatile and semi volatile radionuclides, release of low volatile radionuclide and actinides also takes place, hence two different sets of radionuclides are considered for DEC-A and DECB scenarios. This paper presents a method for estimation of fission product release from Pressurised Heavy Water Reactors (PHWRs) for severe accident conditions. PHWRs have horizontal fuel channels submerged in heavy water filled horizontally placed cylindrical vessel called Calandria. The gap release is the substantial contributor of fission product release during initial release phase of accident. As accident progresses, fuel heats up and subsequently core melt down on uncovery of fuel channels. This results rapid release of most of the volatile and some amount of semi volatile radionuclide inventory present inside the fuel matrix of uncovered channels. After complete core collapse, further release of fission products get ceased till fuel debris present at the bottom of calandria remain submerged in water. The release starts again after heat-up of debris after complete moderator boil off. Release fractions of low volatile radionuclides were found very low as compared to volatile and semi volatile radionuclides.