Originally there was only one setting, where you had to have the 15 minute shift timer, and the limited oxygen (though that was less of a problem to me).

Fans spoke however, and the devs introduced a new mode that does away with the shift timer but, and this is the important part, keeps progression intact. It's not a creative mode, you still have the 1B debt, you still have to upgrade and rent your gear, you still have the certification system, you even have the option of disabling the limited air supply. The only thing that goes away is the 15 minute shift timer.

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Also fun is the idea that the larger reactors are very complex. They are perfectly fine sitting there as long as their fuel and cooling supply are intact, disable both and the reactor immediately begins melting down.

This prompts you to move fast to remove the reactor and store it, but the large reactor is made out of parts (unlike the smaller ones you've been removing for quite some time) so just grabbing it with your tether and yanking it will end up with you getting a single panel door instead of the still-melting, reactor. This can be quite panic inducing, even if you were expecting it.