In the sentence, "Sally jumped," "Sally" stands for the entity that causes or controls the jumping--in other words, an agent or agent-like argument.
In the sentence, "The statue fell," "the statue" stands for an entity that undergoes the falling--in other words, a patent or patient-like argument.
In the sentence, "The knife cut the paper," "the knife" stands for a secondary agent, which accomplishes the task but is controlled by a primary agent. Contrast secondary agents with tools, which help the agent accomplish a task. Hence, this contrast:
"He cut the paper with a knife" --> "The knife cut the paper." (knife = secondary agent)
"He ate the soup with a spoon" --> "*The spoon ate the soup." (spoon = tool).
With all this in mind, what is the thematic role of the subject in the sentences "The bomb exploded" or "The sun shone"? The bomb and the sun arguably cause the actions in question, but they have no control over them.
EDIT: I found this useful list of thematic roles. Still can't figure out the answer to the question: https://www.ling.upenn.edu/~beatrice/syntax-textbook/box-thematic.html#instrument