atomic power

[J Rindt]

...Maybe I did not look hard enough but..... I cannot find an explanation of what how a nuclear reactor works in your "typical" power plant.
My question is..."How do they initiate the fission.?"
I understand there are Uranium "pellets" in a tube, and there is  controlled fission, and they harness the heat to make steam, turbine, generator, etc etc.
But how do they get the fission started.?
Thank You

[PRR]

> how do they get the fission started.?

Atomic guitar amplifier? Play a lot of Atom Heart Mother?

Put a small piece of high-reactive Uranium on the table. Put another small piece of high-reactive Uranium next to it. Do this over and over until it glows and goes BOOM.

(One guy did this, knowing the danger; but slipped (oops). Coulda blown-up the lab. Instead he died in a few days and his corpse was hazardous.)

There is a Critical Mass (a good search term). Depends on the Uranium, the lump geometry, also certain substances are "moderators" which lever the action in different ways.

Like throwing a party. Three roommates isn't a party. 20 semi-friends is a party. 50 friends and strangers and neighbors and police is a PARTY.

Before computers, calculating the critical mass was very difficult and very secret. Today all the math is online, and I don't doubt there are java-calcs to do the hard work for you.

[J Rindt]

OK...Thanks.
I thought there was some type of implosion, compression, or physical disruption (like in a bomb I guess) required to get things stared.
But if the Uranium is "pure" enough it becomes a self fulfilling prophecy. I did not know that
Thank You

[PRR]

> I thought there was some type of implosion, compression

Stacking small bits of U gives you a big fizzle. It is not convenient or effective as a bomb.

For a bomb you compute the Critical Mass, then make two half-masses. Keep them many feet apart, typically each end of a bomb-tube. To go boom, bring them together. If they come together slow, you get a mild fireball that blows all the U all over the place. Poor boom, though fairly lethal. For a really good BOOM you have to THROW the two half-masses together faster than the fission builds-up. A TNT-like explosion behind one half-mass works. Variations include hollow masses that crumple to higher density when slammed, and a dozen part-masses around a sphere.

This needs a fast precise detonation of the TNT. Typically this means special high-current discharge tubes and capacitors. Outside the usual range of industrial capacitors. At least one hush-hush bomb project has been revealed when they contacted specialist cap-makers for a custom job and no real details of application.

No, if you want a fizzle you just keep stacking refined U until it glows.

The reaction is unstable. Just like a wood fire. If too small it goes out, if too large it grows to burn all the wood very fast. Except the fission reaction will typically "grow" in a milliSecond. When you get close to criticality it wants to run-away and burn-up real quick (and violently). In power reactor control, to increase power you pull-out the moderator rods then put them almost back to the same position. Like a heavy car where you have to goose the gas violently to get from 50 to 55 MPH, then back-off to hold 55.

If you want to do this at home (you almost can), read-up on David Hahn. As a teenager he collected mild radioactives and refined them in the garden shed.

Getting useful power is not cheap or easy, partly getting enough refined U, but mostly getting the control systems to stay stable in ALL conditions. The pioneers at the Hanford(?) reactor research facility have a few "ooops" hidden deep in the woods. Reasonably mature refined commercial designs sometimes "burp": Salem tends to shut-down, but remember 3-Mile and the Russian graphite-bomb. External factors matter: a tidal-wave on a coal burner will need repairs, but not like the Japanese atomic plants.

FUsion bomb is different. You compress H hotter than the sun and it condenses to He and a lot of heat. The only low-tech way to do this is a fission bomb. Fusion reactor could be possible with high-energy beam colliders, but this has been "just around the corner" most of my life.

[J Rindt]

So.....in "Fat Man And Little Boy" they needed Implosion (and so a sphere) for the Plutonium Bomb.
How did they activate the Uranium bomb.?
Will Uranium go off that fast all o its own.? That is to say, will Uranium go into Fission all by itself, in a manner that would be a weapon, or does it need some kind of accelerator.?
Thank You
...Obviously, while I was forming my question, PRR was somehow already in the process of publishing the answer.
Thanks Again
I Appreciate It

[J Rindt]

Speak of the Devil...
I just watched a video that showed/explained Explosion Welding.
What an amazing Idea/Process.
Who'd have thought.....?
Thanks for your info, it has been a pleasure...
best

[thermion]

some type of implosion, compression, or physical disruption

may i beat this dead horse some more? i love this stuff...
bombs need the neutron initiator to get enough shakes in a fast enough timeframe otherwise the pit fizzles.
for reactors in power production they load U-based fuel pellets in tubes and back out the neutron-absorbing control rods from the pile. one must make sure there is enough unspent fuel loaded to overcome fission product poisoning (an isotope of xenon, i believe, does a great job of absorbing neutrons and quenching the reaction). otherwise the pile dies...the system is closely monitored to push through this relatively short-lived isotope's half-life and you're off. i don't think reactors are typically taken totally off-line, just partially unloaded and refilled.
reactor grade U is not very pure...highly diluted weapons-grade Pu can be made into MOx pellets, also useful in civilian power apps. this may be where most of the draw-down Pu pits will end up.
yes, most nuclear reactors are huge, expensive, high-maintenance hot water heaters.
back to the military side: U gun barrel type bomb was never tested before deployment over japan, trinity was Pu.
U gun barrel weapon was modified howitzer hardware. gun barrel design will not work for Pu.
about trinity and Pu projects: all Pu is synthetic. once we made enough Pu to do more than just look at under a microscope, it was found that metalurgy of Pu to make a stable, machinable alloy was to be one of the biggest challenges. this sucker has like 6 solid phase transistions (some of which actually contract) in phase:temp plots with increased T. the room temp alpha phase is quite brittle and not suitable for machining bomb parts.
another wrinkle: the chemistry of elements as you cross the actinides does not really change (unlike smaller elements with fewer electron orbitals), confounding separation efforts. a stroke of luck at dupont chemical engineering was key.
creating highly symmetrical implosion was crucial. focusing "lenses" were needed to concenterate the triggering charges and jetting was a eminent threat to useful yields. i think fat man was only ~20% apparent yield. little boy was only ~2%!
6LiD surely enabled DELIVERABLE fusion weapons. otherwise the weapon is more of a fusion factory/ installation than a bomb. indeed the fusion trigger is a fission explosion in close proximity to the fusion fuel, but only one design has ever worked: the teller-ulam.
thanks, drs. oppenheimer and teller!
 

[J Rindt]

Thanks for the physics lesson. I appreciate it.
Was Teller the guy with the H-Bomb.? And then in a historic case of back-stabbing.....ratted on his old comrade.?
Oppenheimer is totally unappreciated.
Those were the days.....vacuum tubes Every Where.
best

[PRR]

Getting back to the Question:

You don't have to "do anything" to start fission.

U-238 fissions naturally, but slowly. It is possible to make a reactor with this stuff using graphite or heavy water to moderate the fast neutrons.

U-235 fissions better, but is only <1% of the Uranium found today (most of it fissioned eons ago). U-235 is rather essential for bombs; useful for reactors after the gigantic U-235 extraction plants were built.

However chain-reaction fission has happened *without* (and long before) human actions.

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

[J Rindt]

That is very interesting.
Had no idea stuff like that took place.
Kind of scary...Uranium.
Arguably the Worlds dirtiest and most expensive way to generate a power grid.....rather than the Cheapest and Cleanest.
Thanks for the Wiki link.
If you can "think it"... it seems that somewhere in the Universe, Nature can do it.
best

[Geezer]

With all this "bomb" talk, I guarantee Big Sis/Homeland Security is now monitoring our humble little forum

 

[jjasilli]

I was fascinated by this as a youngster.  Now there's lots of info on the web.  Fuel rods of fissionable material are inserted into the reactor core to create a "critical mass".  This means that a chain reaction of atomic fission takes place spontaneously.  When one atom splits, the parts fly apart like bullets and hit other atoms, causing them to split, and so on.  To prevent a run away reaction and explosion, the chain reaction is held in check by control rods.  The control rods are inserted into the reactor's core to absorb some of the fission particles so they can't cause more chain reactions.  That way, the chain reaction is kept at a constant rate instead of accelerating out of control.  The whole thing is cooled by heavy water. Water is H₂O.  Hydrogen comes in 2 different isotopes; when it combines with Oxygen to form water the Hydrogen's proton may be alone, or with one neutron. The latter form of hydrogen is called deuterium. Heavy water is about 11% more dense than regular water, because it has more deuterium than usual.  The extra mass better cools the core and absorbs some stray radioactive particles.   

Usually the heat generated by the reactor is used to generate steam which runs a steam engine.  The steam engine can run an electric generator or turn propellers on a submarine. 

[PRR]

> The whole thing is cooled by heavy water.

Heavy Water is useful when you can only get natural U238 and need the extra moderation of fast neutrons.

When you can get U235-enriched stuff, ordinary water is used for cooling.

IIRC, the submarines run Sodium(??) to cool the reactor, then transfer heat to water for steam turbine. (Better check that before you DIY a nuke sub.... my info is old and poorly remembered.)

[J Rindt]

I am glad I asked.....
About the Heavy Water, and The Nazis...is it more likely then (pardon my ignorance of WWII science) that maybe The Nazis were more likely working on a Nuclear Power Reactor and maybe not working on a bomb.?
Thanks

[PRR]

One theory is that German professors said they were working on atomic power to get draft-exemptions for their grad students.

Realistically, nobody was ready to make a useful reactor or bomb in the 1940s. Enriching Uranium is MUCH too expensive to happen after a Depression in the middle of the War.

That the US did it anyway is still amazing.

It's far less likely the Germans could have done it, even if they'd stopped war in 1942 and avoided Allied assault.

So it was one of many back-burner projects every good government supports for long-term hope.

And the last thing those professors would want is instant success, because then they'd have to actually *work* to put the idea into practice. And by the time Brit Yank and Russian tanks were rolling into Germay it was much too late.

There was a Japanese project too. Much of the records were destroyed. They didn't get real far; again lack of resources.

[thermion]

problem is a chemistry problem

Totally, as in the actinides have no easily exploitable chemical differences between them as you increase mass. The nazi weizsacker did not appreciate this fact in his 1941 patent describing u separations in his "uranmaschine".
[/quote]IX[/quote]
This is or is related to the PUREX method developed/ employed in the 1960s.

dirtiest and most expensive

And highest energy density, by orders of magnitude. Nations don't go nuclear because of its ease, fun, or affordability.
As for subs (the real peacekeepers) yah liquid metals can be used as heat transfer medium. Be real careful welding those cooling lines though, this was the ultimately problem in soviet k19 (widowmaker). Hot chaff and spall during construction fell onto these lines, weakening them. Under use they failed. I read the poor seamen who made the heroic repair for their comrades received around 6000rad full body dose; 800rad is about the LD99 in humans.