A passively cooled Reactor is a Reactor that generates energy directly and that does not use an external medium to dissipate it's internal heat.
The first Reactor described in the early pages of this book is a Reactor operating in Passive Cooling mode.
Operational mode
Passive Cooling
Мод не переведён на русский — текст на английском, как в игре. Рецепты и предметы ниже — с нашими названиями.