When Yuasa arrived in March 1940, Paris was under martial law, and in June was overtaken by Germany. Yuasa survived these difficulties by devoting herself to research. Under Professor Joliot-Curie (Madame Curie’s son-in-law) at the Nuclear Chemistry Laboratory of Collége de France, Yuasa used a Wilson chamber to analyze the tracks of alpha and beta rays generated by artificial radioactive nuclei, and began research to discover the mechanism of decay and energy structure. Her research that covered analysis of the energy spectrum of beta rays and the form of interaction that causes decay was highly acclaimed, and her paper, "Contribution à l'étude du spectre continu des rayons β- émis par les corps radioactifs artificiels (Continuous beta-ray spectrum generated by artificial radioactivity)" earned her a French doctorate degree in science in 1943.
In the summer of 1944, just before Paris was freed by the Allied forces, she was forced to transfer to Berlin. She temporarily lost freedom to conduct research, but then created a beta-ray spectrometer throughout the air raids and returned to Japan just before the war ended with the fall of Berlin, bringing her instruments with her. Yuasa returned to Tokyo Women’s Higher Normal School as a professor, but was prevented from continuing research due to the US occupation forces’ ban on nuclear research. Three and a half years later, in 1949, she returned to France on Professor Joliot’s invitation and this time stayed. She went on to establish a solid position as a researcher at the French National Center for Scientific Research.
Around 1960 she transferred to the field of nuclear reaction, which uses synchrocyclotrons. From around 1967 she began to work on Japanese-French exchange, inviting young researchers from Japan, while also taking on innovative issues herself. She achieved results that attracted global attention. She proposed a Japan-France joint project for research on few-nucleon systems, and made every effort to realize this even through illness and death.
Yuasa is also known for her contributions to cultural exchange, by introducing French culture to Japan and taking care of Japanese visitors to France. She earnestly considered issues such as "women and science" and "peace." Many people have been moved by her collection of reflections on science and life, scholarship and art, and science and religion in the three-part book series, Pari zuiso (Paris essays), as well as other books and contributions. Ochanomizu University currently offers the "Toshiko Yuasa Memorial Scholarship Fund for Special Researchers," in memory of her contributions to nuclear physics research and French-Japanese relations, and to encourage the international activities of young scientists today.