自発 じはつ かく分裂 ぶんれつじゃなかったです
jihatsukakubunretsujanakattadesu

Noun
Desu Copula

The polite, past negative form of 自発核分裂

1. Wasn't spontaneous fission.; Wasn't a spontaneous split.

Past/Perfective
Negative
Polite
Casual Register