Researching Nuclear Fusion and Mechas in the Apocalypse

[Single female protagonist + apocalyptic science fiction + he + multiple male and female protagonists fighting side by side + male and female double strong + double cleanliness] In the year 2044 of Blue Star, the Bear Country has lost half of its territory in the war with the Eagle Country, and its capital is about to be breached. The leader of the Bear Country ordered the launch of all nuclear bombs, striking almost all land except the Dragon Kingdom and a few countries in Feizhou. In order to eliminate the land forces of the Eagle Country Allies that invaded the country, the Bear Country even launched a nuclear attack on its own territory. Due to the excessive number of nuclear bombs manufactured by the Bear Country during the war, the Eagle Country's anti-missile system only successfully intercepted some of the nuclear bombs. The Bear Country's plan was almost successful. The other landmass except the Dragon Country and Feizhou were almost completely destroyed. The Eagle Country and its alliance were almost completely destroyed. However, the Eagle Country still had several small nuclear test bases hidden in a certain country and a small island in Feizhou. On the vast ocean, the Eagle Country still had several large fleets. In a tense atmosphere, the top brass of the Eagle Kingdom's remaining army held an emergency secret meeting. They knew that they were in a desperate situation, but they were still determined to take the last resort to fight against the powerful Dragon Kingdom. After careful consideration, they decided to use the remaining nuclear forces to launch a devastating attack on important facilities such as Dragon Kingdom's military bases and major power plants. Wang Yu, the research backbone of the controllable nuclear fusion experimental base, woke up from the explosion and found that he was the only one to survive in the entire experimental base. Wang Yu also discovered that his body had undergone some changes under the radiation...