Helion said it raised $465 million in a funding round led by Thrive Capital, giving the nuclear fusion startup a $15.5 billion post-money valuation. The new valuation is nearly three times its previous $5.43 billion level. Backed by OpenAI co-founder Sam Altman, the company plans to use the capital to expand power generation and testing facilities, validate key technologies, and advance long-term power supply agreements for its first commercial fusion plants.