I made a couple of minor mistakes. (Well, maybe not "minor".) I mis-remembered a couple of details, which I'll correct here; but I have some other comments to make.
Paul did not use a kitchen strainer to wash the green guck off the plutonium flakes; he actually separated the plutonium flakes from the green guck inside his "hot box", using lab equipment. Score one for the writers.
The "hot box" was properly sealed from the "lab environment" (a rented garage)...but there was an exhaust hose plainly visible leading from the "hot box" to the outside. Minus one for the writers.
Before seeing that hose, I was thinking, "Hey, I was wrong! I bet Paul has filled that box with nitrogen."
To get the completed core out of the box, there is a passthrough filled with mineral oil or glycerin or something--completely airtight--and Paul uses a magnet on a chain to pull the core through.
He makes his core by heating a hollow steel sphere and then adding the plutonium flakes a little bit at a time. Presumably once the core has cooled, he has a plutonium sphere with a thin sheet of steel surrounding it.
All this is pretty reasonable, actually. But the big dryer exhaust hose leading to the outside puts the kibosh on it; and there was no obvious supply of inert gas, either.
As mentioned in prior posts on the subject, plutonium is pyrophoric. Plutonium flakes would--once exposed to air--catch fire. Most (if not all) of Paul's plutonium would have gone out that exhaust hose and contaminated whatever was right outside.
Making some "back of envelope" calculations I figure Paul has in the neighborhood of three kg of plutonium, plus-or-minus about 500 grams. Figure 2.5 kg at the minimum, anyway. Most atomic weapon designs call for at least 3 kg of plutonium; it can be done with less, but probably not by a kid working in a rented garage.
It's hard to assemble a supercritical mass; it took the Manhattan Project (the real one, I mean) to figure out how to do it the first time. Most of the information is out there in one form or another; it's physics, not magic, so a sufficiently intelligent and motivated person could realistically figure out how to build himself an atomic bomb.
Actually putting the theory into practice, however, is where it all falls down.
Paul uses C4, bought from a pal in the Army, as his explosive to build his core. The problem is, C4 isn't all that good an explosive, not for building an implosion bomb; and the explosive lensing effect requires two or three different types of high explosive, carefully worked to shape the shock wave. Merely cutting one type of explosive into the right shapes is insufficient. And anyway, he builds in a screw-plug removable tamper, to facilitate field assembly; even if he had done everything else correctly, that alone would ruin the implosion effect.
The writing plays fast and loose with the realities of radioactivity, too. Plutonium is primarily an alpha-emitter, though it also produces some gamma rays. Paul's acrylic "hot box" is actually fairly reasonable protection from alpha radiation, though I'm not so sure what he did--if anything--to protect himself from the gamma rays. Gamma rays won't leave you "glowing in the dark"--it's not an activating radiation--but they can kill you just as dead as any other radiation.
But after the bomb is armed, primed, and ready to go off, "neutron radiation" screws up the timer circuit and the bomb begins to count down.
What neutron radiation? Granted, there will be some neutron emission from plutonium (even John Lithgow's "99.997 percent pure" plutonium) but enough to damage a timing circuit built from salvaged mid-1980s components in a garage? The timer is somehow magically set to "999:99:99" and begins to count down from there, but because of the neutron radiation the timing circuit has degraded, such that the timer progressively speeds up and turns 999 hours into about three minutes. The problem is, a few stray neutrons (what you can reasonably expect from plutonium) would not cause that kind of trouble.
(What really annoys me about all this is that Paul is a "super genius" only long enough for the bomb to get put together. Then when the writers want a tense "disarm the nuke" scene, suddenly Paul is just a high-school kid. He's smart enough to build a functioning nuclear weapon but he's not smart enough to "harden" it?)
If the core's neutron flux is sufficient to wreak havoc on electronics, then it's also sufficient to wreak havoc on the people around it--Paul's going to die, and anyone who spent any time near that core is going to lose his hair and maybe his teeth, and get cancer at an early age.
John Lithgow says that the plutonium is "hot stuff", so hot they can't test it; it's a wonder Paul didn't "cook" himself just carrying it around, and that Paul's bomb is good for fifty kilotons.
Again, I say, what? If it's Pu-241, it has a half-life of 14 years (Pu-239, which is the primary isotope of plutonium used in atomic bombs, has a half-life of 24,000 years) and it's a beta emitter--and Paul is dead dead fucking dead and should have lost his hair a week after he brought the stuff home, and been in the hospital shortly thereafter. You need an inch of wood, at least, to isolate you from beta radiation; the stuff is in green goop in a plastic bottle. Not only would it kill him, but it would glow in the dark, a pretty blue glow--the pretty blue glow of Cerenkov radiation. "Pretty", yes, but deadly.
Fifty kilotons is not an unreasonable yield for a boosted fission weapon, even using 3 kg of plutonium. But just having "pure" plutonium (whatever isotope) won't get you fifty kt of yield.
That assumes Paul's bomb would go off properly; I don't think it would. I think it wouldn't do much more than produce a rather messy radiological release. But I explained all that in my prior entries.
Overall, although I mis-remembered a few points from the movie, it still stands that Manhattan Project suffers from all sorts of stupid Hollywood cliches and prejudices. Not exactly a "gripping and intelligent thriller", certainly.