Past exam of the MPhil in nuclear energy course of the University of Cambridge / 2021 / Module NE1 / 1 / d / i / Solution

After three half-lives, $P(T)=P_0/8$. Therefore the minimum initial thermal power isEquality gives the stated terminal power exactly in the idealized exponential decay model. The associated duration is $415.2$ days. No thermal-to-electrical efficiency is needed because both specified powers are thermal.