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Critical Path

Often used in the construction industry, the term "critical path" refers to work activities which are required to be completed.  The sequence of which these activities must be carried out and the duration sets the final completion date of the project. In a very complicated nuclear plant outage, there may be hundreds of simultaneous work activities; however, there is only one path of work through all the activities which determines the overall duration of the outage, this sequence of activities is known as the critical path.

For example, the main goal of a nuclear outage is to refuel the reactor, but before they can move fuel they have to remove the reactor head, and before they do they have to remove many other components. Because you can't move fuel before all this is done, and you can't put everything back together until you move fuel, this train of work is typically the critical path for the outage.

Here is a simplified three step summary of a critical path example.

Remove Head → Move Fuel → Replace Head

Now each of those three steps is comprised of hundreds of smaller steps, all part of the critical path. Now say a plant has some other type of time intensive maintenance, such as a steam generator replacement. Because work will take longer than the normal refueling outage, the work becomes critical path:

Remove Head → Remove Fuel → Install Temporary Reactor Cover → Replace Steam Generators → Replace Fuel → Replace Head

Typically before most construction work begins, a schedule is created to track progress and to keep a continuous flow of work activities. As work commences, there are commonly unforeseen delays and possible shortages of resources. As the work schedule slips and more tasks are delayed, the possibility of meeting the final deadline gets harder to accomplish. Critical Path Scheduling is typically a result of slipped work schedules. What it means for the workers and Field Supervisors is to work faster and longer to accomplish their task. At this point it is considered “Schedule as you go.” This can sometimes be good, but can also create an extremely inefficient schedule and poor productivity.