The kid's mother is dating a scientist played by John Lithgow, Dr. Mathewson. Mathewson works for the government--for the DoD--and has developed a process for purifying plutonium. I'm pretty sure they mean isotopically pure, rather than chemically pure, since chemistry has long established how to purify particular elements or compounds to perfect purity...and had even in 1986.
(Never mind the fact that the plutonium used in atomic bombs is essentially pure PU-239; that's just a detail. But remember this, because there's a plot point coming up....)
Mathewson's process is revolutionary. He is demonstrating it to some Generic Military Men and one of them says, "So the plutonium is 99% pure?"
"99.997," Lithgow says proudly. (This is why I always say "99.997%" when most people would just say "99.9%" to mean "essentially 100%".)
The plutonium is stored in clear plastic bottles in a green gel. It looks like glitter. (remember this, too.)
Mathewson starts dating the main character's mother. Paul Stephens is a genius, and his father has either died, left, or divorced; in any event, Paul resents the replacement of his father, especially with a man he sees as supercilious and annoying. To try to get on his good side, Mathewson takes Paul on a tour of the ultra-secret government lab which is purifiying plutonium for (gasp!) nuclear bombs, unbeknownst to the innocent townsfolk around the lab. (The lab has a cover story which most townspeople have accepted without comment.)
So Paul--being a smart young lad--is outraged at the secret nuclear weapons facility being operated secretly in his home town, and sets out to expose it...how?
Uh...by stealing plutonium from the facility and building a nuclear bomb.
Where most people would, I don't know, call some reporters, you know, attend a town hall meeting or two, do some picketing, maybe try to get some lawyers to do pro-bono work for them and file a lawsuit, or--hell!--contact Greenpeace and other activist groups, this super-genius decides that the best way to "out" the secret weapons laboratory is to go steal plutonium from it and build an atomic bomb. Besides, this way he is certain to win the regional science fair!
If you look at the IMDB entry I linked above, you can read the comments section for the biggest and most obvious plot holes; I am going to comment about the science.
Working in his garage, this kid manages to build a suitcase nuke--a real one, a man-portable device which he never seems to tire of carrying.
He separates the "99.997% pure" plutonium from the green gel by washing it in a vegetable strainer. (I hope he didn't just put that thing back in the dishwasher when he was done with it...) He casts a plutonium sphere by heating a hollow steel ball and dumping the plutonium flakes into it. He's working in a glove box made out of acrylic. That's fine; plutonium is largely an alpha-emitter. But I didn't see if he had anything to ensure that he didn't inhale any vaporized plutonium; and plutonium is pyrophoric--it'll burst into flame on contact with oxygen, particularly if heated. Those little plutonium flakes would have burned as they dried and he wouldn't have had a chance to cast them into any shape. In fact, he would have contaminated the house with plutonium oxide and died of both radiation and plutonium poisoning long before he could complete his little science project. But we'll ignore that little detail for a moment.
He solves the problem of producing an implosion by patterning his explosive blocks after a soccer ball. He does test this configuration. But when making an implosion bomb, you need several kinds of plastic explosive configured precisely to shape the blast wave just so. I'm not sure a kid working in his garage without an explosion-proof machine shop could do it. But we'll ignore that little detail for a moment.
He needs neutron reflectors. Dr. Mathewson is impressed when the kid tells him he used stainless steel salad bowls. Unfortunately, the typical stainless steel salad bowl won't reflect more than 0.003% of the neutrons passing through it. 1-0.99997=0.003--in other words, I made up that number. But I would wager that stainless steel won't reflect much more than 10% of the neutrons passing through it, anyway. The scientists of the original Manhattan Project had to use large blocks of U-238--depleted uranium--as tamper and neutron reflector in the implosion bomb, "Fat Man", and it weighed tons as a result. But we'll ignore that little detail for a moment.
He needs to ensure all the explosive blocks go off at once. He whips up a timer circuit and uses flash units from cameras. You see, he is a super-genius. The best physicists of the 20th century had to invent the kryten switch in order to time their implosion device. implosion is a fussy thing to get right, and if the explosive blocks do not go off simultaneously, implosion won't happen. The kryten switch was precise enough to ensure that the timing was nanosecond-sharp. But those guys--! All they needed was a few goddamned camera strobe circuits! What a shame they didn't have them back in the 1940s! But we'll ignore that little detail for a moment.
And related to that, the dozens of wires leading to the explosive blocks must all be the exact same length. And when I say that, I mean exact. Beyond the precision of a kid working in his garage. But we'll ignore that little detail for a moment.
How does he build the detonator? With six camera strobe circuits. Six! Not "dozens". His implosion device works fine with six initiation points. Well, what do you expect? Paul is a super genius and is much smarter than the war-mongering scientists who originally developed the implosion bomb. They had to have dozens of initiation points on their implosion device. But we'll ignore that little detail for a moment.
This super genius gets stuck in a bind and actually puts the plutonium core into his device, making it a live bomb. He's conveniently designed it for field-activation; all he has to do is place the core into the bomb and put a section of explosive in place, and press a few buttons. "Fat Man" took a day to assemble (and weighed tons when complete), but because Paul is a super genius he can engineer a man-portable device which can be field-assembled in about twenty seconds. This makes him much smarter than the scientists and engineers of the real Manhattan Project, who had four years and an unlimited budget to build their device. Paul did it in three months in his garage, with his allowance money. But we'll ignore that little detail for a moment.
Paul's bomb does not contain an initiator core. The scientists of the Manhattan Project were pretty sure they needed one; their cores were made from hemispheres with a little hollow in the center. In the center, a little sphere of a couple of different elements would sit. When the implosion happened, the sudden increase in density (and the concomitant increase in the density of the alpha radiation from the plutonium) would cause this initiator to release a huge gout of neutrons, thus--surprise!--initiating the plutonium reaction which leads to a nuclear fireball. But since Paul is a super genius his bomb doesn't need an intiator!
But we'll ignore that little detail for a moment, because I'm going to talk about Hollywood's perception of atomic bombs.
When the bomb is set to go off, and the situation is resolved such that Paul won't be shot by the government, he wants to disarm the bomb, but can't. (I guess all his engineering expertise went into the other stuff; he didn't expect to have to turn the thing off?) They have to clip the six detonator wires simultaneously. Paul asks why don't they just take it to a nearby quarry and set it off?
Mathewson informs him that the ultra-pure plutonium will be far too powerful for that. And this little exchange (courtesy of the IMDB site) is too good:
Official: Sir, what about evacuation?
Lt. Colonel Conroy: Evacuation? Of who?
Official: The people.
Lt. Colonel Conroy: Oh, you mean New York, Pennsylvania, Vermont, Canada... those people?
In Hollywood, "nuclear bomb" means "end of the world". That mindset is what drove the writers of this movie; the movie is rife with leftist liberal assumptions. It was meant to make a political point (hot on the heels of War Games, which had come out two years earlier).
Paul's bomb is a single-stage device, made in a garage to dubious technical standards, by a high-school kid. It contains real explosive and real plutonium. In practical terms--real-world terms, using real physics, not Hollywood physics--such a device would produce an explosion on the order of about 20 kilotons, more or less. This assumes that it actually would work at all, owing to all the "little details" I mentioned above, but let's assume that it would. (We are, after all, ignoring those little details for the moment.) If the reaction failed, it would be a "dirty bomb" which would spread plutonium powder over a few square miles--inconvenient but relatively harmless, requiring decontamination of a town. If the reaction "fizzled"--in which fission occurred, but failed to achieve the full potential of the plutonium core--it would vaporize a couple blocks, start some huge fires, and contaminate the county with fallout. Bad, but not too bad.
But assuming that it really worked, and worked well, figure it would make 20 kilotons. 20 kilotons is bad. It would destroy a city. It would spread radioactive fallout for miles. It would cause other problems. Yes.
But evacuate "New York, Pennsylvania, Vermont, Canada"? (All of Canada?) You'd have to evacuate wherever the bomb was at, yes. One city. Then you'd have to warn all people living downwind of that location to stay indoors for the next two weeks, with the air conditioners turned off, and all the doors and windows closed and duct-taped. That's about it. The fallout wouldn't be all that damn bad. And if the bomb was set off in a quarry, there wouldn't even be all that much fallout to worry about.
The ultra-pure plutonium will mean a much more powerful explosion? Uh, no, since the plutonium used in nuclear bombs is already pure. But even if it did mean that, how much more powerful would it be? 30 kilotons? 40 kilotons? It would certainly not be on the order of megatons. You can't get megatons unless you build a hydrogen bomb, and there is no way in fucking hell that kid could build a man-portable hydrogen bomb in his garage, super genius or not.
Paul is a high school kid who is outraged that there's a secret nuclear weapons lab in his hometown. He's a super-genius, so of course he is outraged that there's a secret nuclear weapons lab in his home town! You can't be a super-genius and not be outraged about nuclear weapons! If you aren't outraged about nuclear weapons, you're not a super-genius!
Paul does something spectacularly stupid for dubious reasons and ends up nearly vaporizing the home town he was trying to "save". He is not arrested for all the felonies he committed in stealing plutonium from a government laboratory and constructing a nuclear bomb; of course not! His heart was in the right place! His intentions were good! He struck a blow against the evil government and the even more evil Department of Defense during the Reagan years! Paul isn't a nuclear terrorist; he's a peace activist!
The back of the box had a blurb: "A gripping and intelligent thriller." The movie was not intelligent...unless you define "intelligent" as "agreeing with liberal misconceptions about defense strategy, nuclear physics, and the relative moral defensibility of actions aimed at furthering the liberal agenda which are felonious in nature."
But I don't really mind all that. I'm used to Hollywood pushing a leftist view of the world. It's all they know how to do. What I do mind is them ignoring all the real facts of science and physics in order to do it. All they had to do was hire a couple of physicists and let them fix the movie.
Mathewson's process? Make it a uranium process. It's a cheaper and easier way of separating U-235 from U-238; instead of the huge gaseous diffusion and mass spectrometer plants at Oak Ridge, this lab is refining weapons-grade U-235.
Paul's bomb? Make it a gun bomb. A gun bomb has none of the problems associated with the implosion device. You fire one slug of U-235 into another slug of U-235. It forms a super-critical mass and you get a big explosion. There are still problems with building such a device, but they are much easier to solve than the problems of an implosion device. You have none of the timing or initiation problems to solve, for one thing, and the precision of the device can be many orders of magnitude lower. Paul could realistically build a gun bomb in his garage, given the U-235.
But I still think he would die of radiation poisoning. Forget going to the science fair, Paul, unless you're entering yourself as the "glow-in-the-dark boy"....