The equation describes the transport of a radioactive gas called thoron an isotope of radon with a half-life of t1/2= 55.6 s. As seen from the equation all the other terms you mentioned are taken care of except for the extra term which accounts for the radioactive decay of the thoron. The λ is the decay constant of the thoron and is defined as λ= ln 2/t1/2= ln 2/(55.6 s) =1.25x10^-2 s^-1. I could probably include that term in the source term but I don't know how to edit the source term in the general transport equation in ansys cause I'm new to ansys. Please help