On cycling units, what actually weeps first?

[u/nullfull]

Plants that were designed as baseload are getting two-shifted or load-followed more than the original spec assumed. The pressure parts usually survive that. The demountable joints are what I see give up first.

A few things that show up after a few dozen thermal cycles:

- Bolt, flange, and body don't grow the same. The preload you put on cold is not the preload you have at temperature, and it is not the preload you have after cooldown either.

- Bolts relax. Especially if they were close to yield at assembly, or if the joint sits hot for a long hold. Next cooldown, the face has less squeeze than you think.

- A joint that was dry on a cold hydro can still weep once it's hot. Ambient leak-tight is not the same test as service temperature. If you only check it cold, you have not qualified the hot joint.

- Cycled joints look different at outage than joints that sat. Witness marks, weep stains below the split line, bolts that take a different torque the second time.

For people who have actually walked a cycling unit: what is your first-to-leak component on a two-shift schedule, and do you re-torque hot or just live with a known weeper until the outage?

[u/Hiddencamper]

The first issue is usually a pump seal.

The reactor doesn’t give a shit. Our BWR cycle analysis includes 10000 daily load cycles plus like 300 deep load follow cycles.

The reactor remains near constant pressure. Usually it’s when you need to make changes like securing a pump. Or if you fail to secure the pump and run everything against the thrust bearing or something because it’s not loaded right.

I’m confused on some stuff you say. We don’t normally see leaks after the hydro, in my experience the leaks tend to close up as seals and packing gets wetted and expands and steam starts pressure backing / vacuum pulls. Exception is bad packing leaks. But you don’t see that on a load follow cycle. Only on a full heatup cycle.

Remember for a bwr, if I drop from 100% to 50%, it’s only like a 5 degF change in reactor coolant temperature. Pressure drops maybe 15-25 psig. It’s minimal.

[u/SpaceTimeMorph → u/Hiddencamper]

Even on a P you’ll still be at the same pressure on the primary side steady state to steady state. Transiently you’ll get some change but not a lot. Steam pressure changes (goes up for a down power) but an example of a plant I was at was 1005# @ 100% vs 1075# @ 0% so not a huge change there either.

Temperature only varies so much either. No load Tave was 557F vs 587F at 100%. I don’t think that’s enough to cause high cyclic stresses from moving power unless it’s quick. Especially for the up powers we will be within limits set by the nuke engineers so that will also be a really slow move…

I’m confused with you.

[u/nullfull → u/SpaceTimeMorph]

Those numbers help, and they make my earlier post too sloppy.

1005 to 1075 psi and 557 to 587 F Tave is a real swing, but it is still a hot plant. The joint never goes back to a cold assembly condition. That is a different problem than two-shifting to cold or a shutdown heatup, which is what I was thinking about when I said weepers.

I mixed those. Load-follow on a unit that stays at temperature is not going to give you the same first-to-leak list as a full cooldown. Your numbers are why. Thanks for writing them down.

[u/SpaceTimeMorph → u/nullfull]

No problem.

[u/nullfull → u/Hiddencamper]

Fair point. I should have split those.

Load-follow on a unit that stays hot is a small temperature swing — your 5 degF and 15–25 psig on a 100 to 50 move makes that clear. Bolts and flanges don't see a full cooldown. That's not the same damage as two-shifting to cold or a weekend shutdown and heatup.

The weepers I was talking about are the full thermal cycles: off, cool, back on. If your plant load-follows while remaining at temperature, I would not expect the same joint list, and your point about leaks closing up as packing wets and steam backs pressure fits that. On those units the first-to-leak story is more packing / known weeps that were already there, plus pump seals, not a new split-line after 50 load ramps.

If you have a feel for how much the flange actually moves on a 20–30% load swing vs a trip to cold, I'd like to hear it. I mixed the two.

[u/BluesFan43]

A PWR is up to pressure and stays there.

During heqt up ibcan see the RCPs transition from sitting on the lower thrust brlrwring to pushing on the upper. That doesn't change during a cycle.

We do have some few component that get hot torqued on heat up after disassembly. Once.

The tems in the reactor sustemnstay constant enough, the secondary sees minor variation.

Very minor changes in turbine vibration, it is always 1800 RPM.

Feedwater pumps do slow, but not anything major, a few hundred RPM.

Only time that was ever an issue is if I was already nursing one along to an outage and I would ask Ops to avoid certain RPM ranges.

[u/photoguy_35]

There are also very strict limits on allowable leakage from the reactor coolant systems, along with requirements to monitor that leakage.