Anyway, if being attacked is a possibility, physical security for such plants isn't a hard problem.
Between blowing up an SMR and blowing up any number of modern chemical plants, I’d rather be downwind of the former. We don’t keep huge amounts of fissile material in modern reactor cores.
For instance, if someone dropped a sufficiently-sized bomb on a well-designed, modern nuclear power plant, would it melt down?
No. It would just shut down. A big enough bomb could potentially spread some radioactive material around the plant area, but any adversary with access to a bomb that big and the ability to deliver it anywhere they chose could do much, much more damage by targeting other places.
I mean, I may be totally misunderstanding this. I thought this is what contaminated Europe's farmland more than the meltdown itself. Am I wrong?
I'm not sure it's appropriate to just hand-wave it away. Pretty much all the farmland bombed across EU since WW2 could be used for agriculture shortly after each war ended. Whereas plenty of farmland still shouldn't be used today after being contaminated by "some radioactive material" that was spread around by Chernobyl.
There's been some renewed analysis of this with the ongoing war in Ukraine which points to situations where more modern nuclear plants could indeed still be significantly worse than wind/solar:
> Nuclear plants use a number of auxiliary safety systems, such as diesel generators and external grid connections, to keep reactors cool. Zaporizhzhia also uses a spray pond, a reservoir in which hot water from inside the plant is cooled. If those systems failed, then the nuclear reactor would heat up swiftly, triggering a nuclear meltdown.
> “The main danger here is damage to the systems needed to keep the fuel in the reactor cool – external power lines, emergency diesel generators, equipment to dissipate heat from the reactor core,” Acton said. “In a war, repairing this equipment or implementing countermeasures could be impossible. In the worst case, the fuel could melt and spread large amounts of radioactivity into the environment.”
https://www.cnn.com/2022/08/18/europe/zaporizhzhia-nuclear-p...
What contaminated the surrounding farmland was the release of gaseous fission products from the reactor due to a hydrogen gas explosion caused by the completely insane and clueless way that the plant operators were running an experimental procedure, which went out of control. That happened even before the reactor itself melted down.
As just noted, the explosion was not a nuclear explosion and did not involve a nuclear reaction running out of control; it was an explosion of hydrogen gas released inside the reactor. However, because the plant was built without a secondary containment structure (which is one key design flaw that no other country has ever built into a nuclear reactor), the explosion, since it disrupted the structure of the reactor core, released various gaseous fission products that were dangerously radioactive, over a wide area. If there had been secondary containment, the explosion and the radioactive materials would have been contained inside it and would not have contaminated the surrounding environment.
In short, there are at least two key factors that were necessary to create the Chernobyl situation that do not apply to any commercial reactor that any other country has ever built: running clueless experimental procedures on a reactor, and not having any secondary containment.
As for potential dangers from reactors in Ukraine, that's because Ukraine is a war zone. I don't think we should restrict commercial reactor construction in the US because Ukraine is a war zone. Certainly no other country in the world has restricted the building of nuclear reactors based on that kind of logic. France was invaded in WW II, but still makes the majority of its electricity using nuclear reactors today.
No nuclear plant other than Chernobyl has ever had the possibility of a meltdown like the one Chernobyl had. As I posted upthread, Chernobyl is not a good proxy for any commercial reactor that has ever been built.
Older plants do have the same decay heat removal failure mode that Fukushima has, but avoiding it is easy: don't put the backup power and switchgear where it might get flooded. (I am still surprised that the designers of that particular reactor at Fukushima missed this--particularly since other similar reactors on the same site did not.) Newer designs avoid that failure mode by having the coolant circulate by natural convection, so even with all power off, cooling flow still continues and decay heat still gets removed. This has been well understood for several decades (and US nuclear submarines have been using this capability for that long--they do it to remain quiet by avoiding the noise of coolant pumps, but the decay heat removal protection remains the same).