2980 - Dirk Morgna is born the son of a Nuclear Matrix power plant employee.
2994 - 14-year-old Dirk saves his father from a reactor meltdown by disregarding all safety protocols. He is somehow imbued with a stable Nuclear Matrix.
2995 - 15-year-old Dirk joins the Legion of Super Heroes in their third recruitment drive as Sun Boy, operating primarily in space.
3002 - 22-year-old Dirk falls under the control of the Dominators.
3005 - 25-year-old Dirk battles the reformed Legion of Super Heroes for the Dominators. They manage to defeat him without killing him, but he is left comatose, rehabilitating on Earth.
3007 - 27-year-old Dirk is used as a fuel source by Earth Man. He is brought out of his coma and helps defeat the Justice League of Earth with his nova blast, expending the last of his powers.
There are a lot of characters that came through the lineup of the Legion that were clearly designed powers-first. It was a pretty common bit in the early decades: to have a recruitment drive and suggest that a character would have some sort of new, exciting power on the cover, only to reveal it inside. This more than anything is probably to blame for the huge roster of legionnaires. Sometimes that led to characters that certainly had neat powers but didn't immediately tell a complete story.
Of course, no character can appear in comics for decades and not have some fun material to work with. Dirk Morgana's Sun Boy is mostly just a cool set of powers, but there's also some neat stuff involving his later career as an agent of Earthgov. Given the fact that earth is soon taken by the Dominators that leads to a cool story where he actually has to fight AGAINST the Legion. There's a little precident from the Geoff Johns reboot of the Legion of Sun Boy being used as a power source, so we're using that in the story involving the Justice League of Earth.
Also, in case you missed it, we deliberately built his origins around a 'nuclear matrix reactor'... the same technology responsible for Firestorm.