[chimera]

Could be xenon poisoning. Remember that when you're running at full power you're producing a heckuva lot of xenon in the fuel. Run long enough and you reach equilibrium so it's not a problem, but if you runback power the equilibrium is disturbed in the direction of poisoning the core. Eventually you re-establish equilibrium but it takes time. You could compensate for the xenon poisoning at reduced power if you have enough excess reactivity, but it might require significant changes in the control rod positions, which in some cases is not good if you have to withdraw those rods close to their full extent (recall that this was one of the exacerbating conditions at Chernobil). The operators may have judged that simply shutting down for a bit was the best option rather than run at low power with a xenon-poisoned core.

Power demand in the TVA service area peaks during summer months so it behooves them to upgrade the cooling capacity of these plants. Since it is a BWR using low-profile cooling towers, perhaps simply cascading some additional cooling units would do it?

[OCCASparky]

Yeah, but even then you’re only looking at about 60-70 hours on a down-power until you’re at the new equilibrium reactivity.

And less steam flow means less condensate heating means lower outlet temps back to the river. Unless they had some maintenance which would have had relatively short tech specs (<72 hours) or this is as one of the Ops guys discussed with me this morning, a case of heat load requirements for train-related loads that hadn’t been analyzed for, it might have been easier just to stay at self-sustaining and then up-power later.

Then again, if long-term forecasts don’t show much rain or temps coming down much, you’re probably better off shutting down and waiting it out.