system
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human
How do modern nuclear reactors avoid service interruptions due to slagging/poisoning?

Was reminded of a discussion I had with my grandfather (~WW2 era nuclear science engineer) about how problematic reactor poisoning was in the past and especially slagging.  

I believe more than a few of the US fleet of commercial reactors are at or are already surpassing 60 year total runtime licenses, was it just better designs or something else?
gpt
There are two cases a reactor or core designer has to deal with, steady state operation and transient operation. 

In general, poisoning is dealt with by having a large amount of hot excess reactivity so that you have xenon overrode capability. This means having sufficient hot excess reactivity to overcome any poisoning effects. Large light water power reactors are designed to run for up to 2 years continuously so they are loaded with a large amount of excess fuel to support that. 

In steady state, the whole poison process balances out with excess reactivity and isn’t an issue. For transient conditions, like sudden drops in power or a scram, unit’s have xenon override capability based on their design. 

For bwr plants, at full power you have a large amount of voids in the core producing negative reactivity. After a scram these steam voids collapse as the reactor shuts down, returning that reactivity and making it available for restart. Xenon may complicate ramping the unit back to 100% after the restart due to thermal limits, however bwrs always have xenon override capability and can start up in peak xenon at virtually any point in the cycle. 

PWRs have xenon override until they are in the end of their cycle in coastdown with most of all of their boron diluted. For a pwr, you reclaim a small amount of reactivity as the reactor cools down to hot standby after a trip, but you rely on the hot excess reactivity to overcome peak xenon. 

For CANDU plants, these units have very little hot excess reactivity, and can easily be poisoned out. The plant is designed to only do a full reactor trip when absolutely necessary, and in other cases runback to 60% or 2% based on he situation. At 60%, you have enough neutron production to prevent poisoning from shutting the core down. At 2% you have a time window where if the problem condition clears you can ramp back up to 60% before xenon catches up to prevent a xenon peak from poisoning the core out. 

Hope this helps.