SimBiology is primarily designed to model reactions of species in compartments. It can be used to model other problems that are represented mathematically as differential aglebraic equations. However, you may find that some features are harder to use when using SimBiology in other domains. For example, SimBiology supports automatical unit conversion and can check dimensional consistency of mathematical expressions. However, the units for species are limited to amount (for example, mole), mass (for example, milligram), or amount/mass concentrations (for example, mole/liter or milligram/liter). Parameters can have arbitrary units, but SimBiology does not consider parameters to be located in compartments. So if you want to study the distribution of radiation in compartments, you may find that you can't take advantage of all of the feature of SimBiology.
If you'd like to build your model using equations, you can do this in SimBiology using rules. Differential equations are represented with rate rules. There are other rule types to support various kinds of algebraic constraints.
If you have additional quesitions about how to build certain kinds of models in SimBiology, I suggest sharing some example models or sample equations that you would like to solve.