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The time for reactor shutdown to refuel

  1. Sep 29, 2012 #1

    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
  2. jcsd
  3. Sep 29, 2012 #2


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    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 keff 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.
    Last edited: Sep 29, 2012
  4. Sep 29, 2012 #3
    k isnt exactly 1 during reactor operation. Natural phenomena and feed back mechanisms keep it very close to one, and it fluctuates slightly above and below such that if you did a steady state average of k it would be exactly 1.

    When cores are designed, as Astronuc said, they are designed with a certain number of Effective Full Power Days (EFPDs) in mind, which if the core analysis is correct, is the number of days the reactor will maintain full power at rated conditions. After all EFPDs are used, the core will gradually decrease fractions of a percent per day.

    Time to shutdown is estimated during the previous cycle, and the outage dates are set in stone by the time the reactor comes back on line, regardless of how much coastdown is occurring.

    Common reasons for coast down are, not purchasing enough EFPDs (sometimes the cost both in thermal limits to the core, and in dollars, are not worth it), leaking fuel rods which must be suppressed (causing up to 12 bundles to become suppressed for the fuel cycle), and stuck or inoperable control rods which must be fully inserted disarmed. The plant I work at has a control rod which is disarmed full in right now, and its in between 4 new fuel bundles, so we ended up losing something like 2.5 weeks of EFPDs because of it.
  5. Sep 30, 2012 #4


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    Rodding failed fuel assemblies? Operating with stuck rods? What is this craziness! Must be a BWR! :rofl:
  6. Sep 30, 2012 #5
    Yes lol.
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