This dissertation entails the calculation of the release of gaseous fission products and iodine from the Pebble Bed modular reactor (PBMR) 400MW core under a Depressurised Loss of Forced Coolant (DLOFC), where the temperature history of fuel sphere is taken into account. The release is calculated by developing a computer code (REALESE) based on a simple coated particle failure and a gas diffusion model. The computer code calculates failure fraction, kernel release fraction, layer release, Iodine decay correction, core release fraction and other parameters pertaining to a PBMR. This source code is developed in the FORTRAN 77 programming language on a Compaq Virtual FORTRAN Development Platform. Verification of the results is done in Mathcad and the Getter code. RELEASE calculations compare very well with the MathCAD calculations with deviation from 0-5%. The effects of a number of parameters on fission product release are investigated by means of sensitivity analysis involving parameters like temperature, failure fraction and diffusion coefficient. Sensitivity analysis showed that the variation of failure fraction has the greatest effect on release fission products in a PMBR. The RELEASE results are compared with GETTER calculations in addition to the MathCAD verification. RELEASE calculations showed an expected value of 2.00x10 12 Bq whereas GETTER calculations produced an expected value of 2.13x10 12 Bq, representing a percentage deviation of 6%. / Thesis (MSc. ARST) North-West University, Mafikeng campus, 2003