I can't figure out what you're saying here. You're assuming that the control rods aren't in, days later, for the purpose of calculating heat days after shutdown?
If you're going to apply "passive safety" globally, sure.
But what we're talking about here is passive cooling system safety, not that the entire reactor is passively safe. The multiply-redundant shutdown systems suffice to end the chain reaction.
If the chain reaction ends, you're pretty much immediately at 7% of decay heat -- so sure, a 1.5GW reactor will put out 100MW of decay heat, still. But this will rapidly fall off. After about an hour, it's more like 15MW; after a day, 6MW.
Your statement of "200MW of decay heat" days later assumes either a ridiculous initial condition (an implausibly large reactor) or assumes you still have an operating reactor, which... isn't decay heat anymore.
There have been multiple cases where reactors haven’t fully shut down safely. Assuming a scram will 100% work every time in an emergency simply isn’t appropriate or realistic.
Also, Palo Verde Nuclear Generating Station has 3 different 4000MW thermal reactors, 7% of that is 280MW, though sure if everything shuts down properly it should hit ~15MW. Mitsubishi APWR is aiming for 4.5GW thermal in normal operation though some safety margin needs to be considered on top of that.
> There have been multiple cases where reactors haven’t fully shut down safely
There's Chernobyl, and a few cases where a scram was delayed by 15 minutes or less. How can this produce hundreds of megawatts days later?
> There have been multiple cases where reactors haven’t fully shut down safely. Assuming a scram will 100% work every time in an emergency simply isn’t appropriate or realistic.
Assuming that every system fully fails is unrealistic, too.
> 7% of that is 280MW,
You said hundreds of megawatts days later.
> Mitsubishi APWR is aiming for 4.5GW thermal in normal operation though some safety margin needs to be considered on top of that.
What, they're going to run it over nameplate for days straight? A small excursion over 4.5GW won't appreciably change the amount of power output days later. Now you're just being silly.
Your boiling a fixed pool of water. 15 minutes of nameplate capacity is boiling over 5 days of reserve at an expected 0.2% thermal output. The margins are often measure in minutes not days which is a long way from anything that could be called passively safe.
> Assuming that every system fully fails is unrealistic, too.
Not if you want to say your system is passively safe. I fully believe nuclear can be operated safety, but a huge part of that is acknowledging every possible failure mode rather than just saying unlikely means impossible.
I think you're trying really hard to salvage a point talking about hundreds of megawatts of "decay heat" days later.
An operating reactor isn't making "decay heat".
The claim made is that the cooling system is passively safe in shutdown. Fudging the amount of decay heat by a couple orders of magnitude, and then arguing about "what if it doesn't shut down" is a bogus argument.
Obviously if you cannot reduce a reactor below nameplate power indefinitely, you have a big problem. Thankfully, we have multiply-redundant protections against this in modern designs: redundant control rod assemblies, neutron poisoning, positive stability, etc. Other than Chernobyl (a clearly bad design), all cases of delayed shutdown experienced so far have been innocuous and we've learned a lot from them.
I can only assume my original point wasn’t clear. The normal amount of decay heat is the best case possibility and should be handed just fine by any reasonable design. I don’t think there’s any reason to assume a design has that kind of fatal flaws. “quickly lose multiple GW of heat in an emergency and as much as 200+MW of heat for days after a shutdown.” Was in reference to something compounding the issue of which their’s two main issues either it didn’t shutdown quickly or it didn’t shutdown completely.
I am objecting to is the assumption that safety systems should assume things are fine in an emergency. Chernobyl had multiple compounding issues, many other accidents where less serious because X and Y happened but Z didn’t happen. Depending on such trends continuing results in a false sense of security.
A passively safe system doesn’t mean there isn’t damage. It’s perfectly reasonable for a design to say in the event of X, Y, and Z stuffs going to break. Causing a billion dollars in damage is a perfectly reasonable trade off, losing containment isn’t.
PS: Part of that is acknowledging bad designs are going to happen, we engineers are going to make mistakes. Which means not all assumptions hold.
This would be the only possible explanation, and it is directly contradicted by calling it "decay heat".
It's pretty tricky to think of a scenario where you'd have 5-10% of nameplate days after attempted shutdown.
The worst incident where there was a failed shutdown-- other than Chernobyl-- that I'm aware of was a 1980 BWR incident.
* The reactor was at nearly no power except decay power for the entire duration of the incident: half the rods fully inserted.
* Manual remediation got all the rods in within 15 minutes.
* Last-ditch shutdown procedures, e.g. SLCS, were unnecessary because there was still sufficient control and rapid rampdown of reactor output.
* This is an old BWR design and...
* Procedures were updated and improved, and even with these old BWR designs we've had no subsequent incidents in 40 years.
Failure to shut down is indeed something really, really bad-- but insisting that cooling be designed to withstand this is a bit silly. Instead, we'd best design to be sure to avoid failures to shutdown, excursions in power far over nameplate, etc... rather than insist cooling systems survive fundamentally unsurvivable events without any intervention. E.g. we don't criticize SL-1's cooling design for not surviving the excursion to 10,000x nameplate.
Decay heat is below a half percent of operating after about a day. So 200 MW decay heat days later would mean a 40GW (thermal) reactor.
That's about ten times larger than the largest reactors in existence today.
Also, watts measure power, not heat.