The concept “limiting condition for operation” can be a bit tricky to understand at first. In the industry, we simply call them “LCOs”. This example will help you get it straight . . . .
An example LCO is this: a nuclear power plant must have two external sources of power available and two emergency diesel generators in working condition at all times. If the plant discovers that one of the two emergency diesel generators is out of service (“inoperable”) for some reason, then it’s required to check the offsite power sources every 8 hours and confirm that they remain available until the LCO is restored. It also has to fix the broken EDG within 3 days or start shutting the reactor down (which is achieved by shifting plant modes modes).
If both EDGs become inoperable at the same time, the consequences for violating this LCO are more severe. It’s typically a requirement at that point that the plant must restore one of them immediately, within two hours, or else it must start shutting down.
This example is taken from Westinghouse’s Standard Tech Specs, Section 3.8.1, Condition E under Actions on page 3.8.1-3, which is available on the NRC’s website as NUREG 1431, Volume 1, Revision 4. The requirement in th standard TS (for pressurized water reactors designed by Westinghouse) is to shift the reactor to hot standby if one of the EDGs has not been restored to operability within two hours. If the condition prevails, then the plant must be in cold shutdown within 36 hours. Here’s the link so you can check it out for yourself:
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1431/