This study investigated a low-temperature pyrolysis system for the spent cationic resins that guarantees the retention of semi-volatile cesium in a pyrolysis furnace while a substantial oxidation of pyrolysis gas in the following thermal oxidizer. Cationic ion-exchange resins doped with radioactive metal surrogates (Co, Sr and Cs) were used as a model spent cationic waste. Sr and Co did not escape from the pyrolysis residue at the tested temperature range (500-600℃). The retention of cesium was greatly influenced by the pyrolysis temperature. Nearly 2% of cesium escaped at 600℃. The highest pyrolysis temperature with cesium retention of 99% was 560℃, which was established as the optimum pyrolysis temperature. Both modelling and experimental investigations were performed to establish optimum conditions of a plug-flow reactor (PFR) type thermal oxidizer. Optimum conditions of a PFR-type thermal oxidizer with 5-cm ID and 30-cm length for the destruction of hydrocarbons generated from the pyrolysis furnace were determined as an air equivalence ratio (φ) of ≥0.3 (equals ≥230% excess air) and temperature of ≥750℃.