Conditions for thermonuclear ignition are determined by three parameters: fuel density, temperature and hot-spot size. A simple three temperature model is developed to calculate the critical burn-up parameter or the minimum ρR product. Extensive results obtained are compared with earlier one temperature model for DT and DD fuels. While the two approaches are found to provide similar results for DT fuel except at low temperature regime (~10 keV), three temperature modeling is found to be necessary for DD fuel. This is argued to be due to the lower fusion reactivity and energy production in DD reactions.
