If anything is going to be exploding, it is going to be my eyes after trying to read all of the posts in this thread in a single sitting.
I have a few things I don't think were really mentioned.
1: Pebble Bed reactors.
Using nuclear fuel in layers of special materials, it blocks the majority of radiation, converting it into extra heat. The size of the pellets, and being 'self contained', means that they seem to be technically impossible of generating any sort of reaction that could cause a melt down.
On a side note, it gives me images of people replacing fuel pellets using oven mits and metal tongs...
Such things would be excellent for portable nuclear power sources, as even if containment was breached, the shells around the fuel pellets would add an additional layer of radiation protection. In such cases, only the 'cheap' nuclear power sources are likely to be vulnerable to rather dangerous radiation leaks if damaged/breached.
Such things would be very helpful for PA, since it means less shielding would be needed, since the fuel itself is effectively shielded.
2: Iridium clad plutonium fuel pellet.
Upwards of a 30 year half-life, in a package roughly 4x4x2in and only about 3 pounds. (iirc)
And that is today's stuff, so in 9 decades, compact fission based power should be easily compact just in terms of the heat pile.
Perhaps MDC materials (50 times stronger than modern steel) used to make equally small steam turbines (rated in thousands of PSI?), in a system basically impervious to normal wear and tear (polymer based lubricants fused to materials make motor oil look like molasses in winter), and generate power much better and more efficiently than a solid state system or Stirling Engine could hope to achieve in an equally sized space with an equal amount of heat.
3: Particle Beam Weapons, etc, vs. fissionable materials.
Okay, so fission reactors cannot go boom, but can cause explosions through secondary events (water turned to steam that 'explodes' due to the pressure, or water superheated and broken down into Oxygen and Hydrogen, which combusts, etc)
What would happen if some sort of high energy particle beam were to strike a chunk of fissile material?
What if a very high velocity rail gun were to slam a U-round into uranium heat pile of a nuclear fission power source? Would it have any chance to create a reaction similar to one of the earliest nuclear bombs, which basically slammed a chunk of fissionable material into a larger chunk of fissionable material of larger size?
I have also been thinking about some apparent small bit of comparing nuclear power sources to stuff like combustion engines.
Four basic things come to mind.
Availability, reliability, ease of use, and costs.
In terms of availability and cost, combustion engines (and electrical vehicles) have the advantage. Heck, fuel for combustion vehicles (and electric generators), can technically and literally be grown on trees... (veggie/nut type oil, stuff distilled to alcohol, etc)
Ease of use, just need to know how to use whatever vehicle is being powered by the power source, be it combustion engine or nuke power plant.
In terms of reliability...
A nuke power source will have to be a self contained system, for protection of proprietary technologies, and keeping the nuclear fuel safe (and keep people safe from the nuclear fuel).
Toss in high grade SDC or even cheap MDC materials, and modern day polymer based lubricants, and chances are, even a steam turbine running 24/7 will have minimal signs of wear and tear after 20 years of regular use.
Also, natural selection.
Lack of EPA and means to take legal actions against a maker of shoddy nuclear power plants means 'wilderness justice' comes into play if shoddy nuke power sources cause problems.
Be it an ill person, their next of kin, an entire settlement/town, a merc company they work for/with, or a merc company they hired... well, it is in the best interest of a company to make high quality, durable, safe, reliable and otherwise idiot proof nuclear power sources, to prevent disgruntled customers (or their next of kin, or 'hired mercenary representatives') from literally putting the makers 6ft under via any numbers of methods. Heck, giving a bad rap to nuclear power could cause other makers of nuke power sources, even the big users of nuke powered stuff, to take action against a maker of a shoddy maker of nuke power sources, in order to protect their own assets and customer base.
And now my little boredom induced rant about that stuff is done...
And now, something relating to the basic subject... exploding nuke power sources...
I can think of one surefire way to get a nuclear fission power source to explode...
it requires a long range weapon, such as a sniper rifle, using a special bullet housing 1 milligram of anti-matter. The equivilent of 41 tons of TNT going off on top of whatever energy and radiation is released by the blast, should have some sort of effect on the nuclear fuel of the nuclear fission power source... or maybe not... either way, there won't be a nuclear power source (or vehicle/PA) left, due to a really big explosion...