In this case, "safety" is commonly referred to as "reducing operator error" or "inadvertent user error"
Having an easy to use UI does not contradict having a mission-critical UI that minimizes operator error. For the classic nuke silo control panel, having two people turn keys to launch the missile is very easy in and of itself. However, your UI may require the user to expend more effort and time to reduce errors.
In most cases, reducing operator error requires additional steps in the interface, so the user has to expend more effort and/or re-confirm their intent. The easiest example of this is the classic "Are you sure you want to __?" dialogue box.
In mission-critical UIs, this trade-off of effort vs reducing operator error is typically based on two criteria:
- The severity of the error
- The likelihood the user will realize they made an error
In some cases, like accidentally launching a missile, the error itself is bad. Some famous plane crashes are due to pilots not realizing they have turned on the auto-pilot when it shouldn't be enabled. Turning the auto-pilot on isn't a sever error- it can easily be turned off. However, if the pilots don't realize they have the auto-pilot on, that could be bad.
A few resources on this are:
Nancy Leveson - Famous for her analysis of the Therac-25 accidents
2011 Catalogue of Human Factors, Safety and Risk books - Several of the books mentioned are on the topic you're interested in.
FAA Human Factors Design Standard - I find them a bit dry and tedious, but it may have concrete examples for you.