The paper investigates the dependence of the heat production/heat loss ratio and mantle viscosity on heat source decay rate by varying the relative and absolute concentrations of the heat-producing isotopes of K, U, and Th in the mantle. Examined are three models with progressively increasing effective half-lives, respectively characterized by K/U ratios of 6 multiplied by 10**4 ('chondritic'), 1 multiplied by 10**4 ('terrestrial'), and 0. 25 multiplied by 10**4 ('low K/U'). Each of these models can be constrained to yield a common present-day heat flow, mantle temperature, and mantle viscosity, thus demonstrating the inability of those present-day constraints to differentiate between widely varying blends of the earth's nuclear fuel. The present-day heat production/heat loss ratio ranges from about 73% for the chondritic model to about 86% for the low K/U model. Other possible criteria diagnostic of the earth's radioisotope mix are the present-day isotopic abundances. The low K/U model requires U and Th greater by a factor of 2. 5 over the chondritic model and K less by a factor of 10. However, measurements of concentrations in candidate mantle rocks reveal a range that far exceeds the relatively small variations that distinguish the different compositional models. Paleothermal conditions also may be indicative of the earth's radioisotope blend.