Volumetric heat generation in fluid?
 

I am trying to model volumetric heat generation within the fluid in Fluent. I have no idea how to approach this. Everything I have been able to find is for a chemical reaction. What I am modeling is a nuclear reactor in which the fuel is suspended in a molten salt. I want to assume constant composition since so little of the fuel is actually changing composition. Once leaving the section I am modeling a chemical process is taking place in which impurities etc are removed from the fluid. Basically what I want to do is get a reaction rate from another program, then calculate by hand a heat generation rate in the axial direction and apply this to the fluid as a profile. I assume this will be a UDF, but I have never written one and was wondering if anyone could point me to a source in which something similar has been done or give me some advice to get me started. Thanks, Justin