googlefans said:
Hello,everyone
What is the reason for a reactor shutdown to refuel at the time when k is approximately equal to 1 rather than 1?
many thanks
I believe some plants get to the point where k is slightly less than 1 at full power, but with reduced moderator temperature via reduced feedwater temperature, operators can 'stretch' the cycle and even get k
eff back to 1, or slightly above if necessary. In BWRs, this is after 'all rods out' (ARO); PWRs operate with all control rods out, except for the small set the use axial shaping (grey) rods.
At steady-state, k = 1, during operation. Power decrease (descension) means k < 1, while power increase (ascension) means k > 1. Control of k is accomplished in a variety of ways according to reactor type and design, and reactivity management strategy.
Now, many, perhaps most, cycles do end with some residual (excess) positive reactivity, partly by design (to accommodate unforseen circumstances), partly because of uncertainties in the codes and methods, and partly due to operation not going according to plan. Ideally, a plant would operate at full power for as long as possible, but that doesn't usually happen. The plant may have periods of reduced power operation, or it might shutdown (unplanned outage or trip) due to weather or some plant condition.
Furthermore, some plants must do end-of-cycle testing, so they need to be able to maneuver the plant which requires some positive reactivity to enable k > 1.
Core design is done with a target energy production to a certain calendar date. That date is determined by a number of factors related to the grid demand and capacity, availability of personnel, type of work to be done during the outage, and others. There are uncertainties, so it's entirely possible cycles end with some excess reactivity, and that can be addressed in the subsequent core design. Over the years/decades, codes and methods have improved.
