The breakthrough would be more like saying, "the switch to heliocentric worldview was a major breakthrough for solar system modeling."

1) The reactors have been failing to generate sustained plasma due to specific failures.

2) The simulation models of the reactors did not correctly predict those failures.

3) The new model correctly predicts those failures.

4) Now you can iterate designs using the model with the expectation that these particular failures can be identified before building a full reactor.

5) Breakthrough.