[Movie][Spoilers] Pacific Rim / Pacific Rim 2 / & 3!! - Pa…

physically impossible

no, reactors cannot explode like a bomb no matter what happens. The geometry makes it physically impossible.

just having weapons grade material doesn't set off a nuclear bomb. It requires very specific design and has to be assembled in a single mass
http://wiki.answers.com/Q/Why_can't_a_nuclear_power_reactor_explode_like_a_fission_bomb
https://en.wikipedia.org/wiki/Prompt_criticality#Nuclear_weapons
https://en.wikipedia.org/wiki/Nuclear_weapon_design

I don't mean to detract the thread too much but nuclear reactors are physically impossible to be a bomb

you cannot have a reactor and a bomb mechanism in the same place; nuclear reactors cannot just stick in pure U-235 for the reactor. The fuel is in a geometry that includes spacing, cladding, etc etc etc (or even more simply, the fuel itself is uranium oxide, TRISO pellets, etc; the fuel is not just a chunk of single uranium mass). you cannot get a bomb from the geometry.

going supercritical also does not make a bomb. a good example is this:
http://en.wikipedia.org/wiki/Demon_core

which was Pu-239 pit (weapons grade plutonium) that went supercritical
http://en.wikipedia.org/wiki/Pit_(nuclear_weapon)

this is of interest to me because I am a nuclear engineer student**

*not quite finished yet, should clarify that

well its less about the criticality stuff and more about what a reactor actually needs to do. Uranium metal won't have the properties good for a reactor or as a fuel form since it can oxidize readily and burn/melt, plus uranium is mined in the form of uranium oxides anyways (in the near future, maybe 15-20 years, japan and US should be good at uranyl extraction from seawater, which basically will mean infinite uranium supply). And briefly googling myself uranium metal isn't a good waste form.

Having all the fuel chunked together in 1 place isn't going to be a good way to heat up a shitton of water as it flows by, at least in my mind. All a reactor does is heat up water in a fancy way, so it's most important on how to heat up the water the best---having the UO2 pellets in easily assembled fuel arrays spaced apart just makes the heat distribution across the reactor not all concentrated in one place, which is good for the water and good to not melt your reactor core and have a good temperature profile. The fuel assemblies in a reactor is going to expand and have a lot of thermal stress on it because it's going to be in a very hot environment, and you don't want to have the fuel experience an actual meltdown instead of what a "Pacific Rim" meltdown is I suppose
http://en.wikipedia.org/wiki/Nuclear_meltdown

uranium metal melts at lower temperatures than the oxide forms; reactor's aren't really about criticality conerns, it's about heat removal from the system (which hopefully get turned into electric generation!). Of course, the heat will be determined by the power level the plant is working at, so you control the power level by controlling criticality (i.e. number of fissions going on)

as far as being able to control solid materials, criticality goes though the first nuclear "reactors" or criticality tests are just various chunks of material afaik like chicago pile 1 was a bunch of uranium sitting around or the other stuff I linked to [demon core plutonium], but those aren't actually "reactors" for electric generation.

i should clarify I am a student in nuclear engineering, really only know the "basics". Of course, people who have worked 40 years in engineering still feel like they only know the "basics"

then there is proliferation concerns always.