Complex sentence: About 14 seconds later valves were opened to drain the heavy water from the calandria, which however took some time; power increased for 5 more seconds, peaked at 80 MW, then went down as the moderator level decreased and was at zero 25 seconds later.
Split into simpler sentences: About 14 seconds later valves were opened to drain the heavy water from the calandria. As this took some time, power increased for 5 more seconds, peaked at 80 MW, then went down as the moderator level decreased and was at zero 25 seconds later.