Nuclear physics anyone?

[diddi]

Can anyone shed some light on a bit of a grey area for me...

The terrible events in Japan have prompted me to ponder: if there is a 'meltdown' in a reactor core, which presumably means that all the fuel accumulates at the bottom of the vessle before burning its way through, why does the fuel not exceed its critical mass and a undergo nuclear fission explosion?

 

[Norie]

No expert but I think it's something to do with how 'enriched' the uranium is.

 

[shg]

It doesn't have the density to reach critical mass. It's physically impossible for a reactor core to explode, ever, ever, ever.

 

[Sandeep Warrier]

I think that the meltdown is precisely the reason why there is no nuclear explosion. The amount of Uranium/Plutonium used in the nuclear fuel cells is quite small (also called enrichment).

Because of low U/Pu enrichment, the fuel cells do not undergo an uncontrolled chain reaction that leads to a nuclear explosion.

 

[Atroxell]

Also not a physicist, but...

After the accident, I did a LOT of surfing in an attempt to find out what was going on and what might happen.

It appears that, although any nuclear accident is not something to be taken lightly, this type of reactor can never truly have a "meltdown". A meltdown is where the fuel actually melts and cooks its way through the containment vessel. Even the Chernobyl "meltdown" only made it to the sub-floors of the building before stopping the burn. The reason why that happened was because there was not containment at Chernobyl. The melt left a melted pile of nuclear material called "the elephant's foot". Google it, there's pics... The radiation from that pile was so sever that it killed several robots before they were able to get one close enough to take a picture. Somewhere around 15 or 20K rads if I remember correctly.

I don't understand the precise details, but from what I read the light water in these reactors is actually part of the medium that controls the reaction. As water levels drop in the core, the sleeves (made of zirconium oxide) holding the fuel can heat up and melt, exposing the fuel, which could possibly drop to the floor of the vessel. But the material itself, a ceramic form of uranium, which from what I have read, is incapable of the heat levels of the Chernobyl reaction. Also, with the water level drop, they supposedly cool down on their own after a few weeks...

But when you factor in an earthquake, tsunami and fire, things probably get a bit more complicated. In all likelihood, at least some of the fuel has dropped to the bottom of the containment vessel. They really won't know for some time, as there are so many variables that have to be considered and eliminated before they can even go in to take a look.

Further, if these reactors should somehow find a way to "meltdown", their design is such that the fuel would burn through containment into a specially designed cavity under the reactor vessel that disperses the fuel to the point where the reactivity would diminish to the point of making it a "cleanup job".

Luckily for everyone, they don't appear to be heading that way...

But again, I am not a physicist and there's always the chance that I have misunderstood or misinterpreted something, so there's my two cents...If there are any physicists out there, love to hear from a pro.

 

[diddi]

The amount of Uranium/Plutonium used in the nuclear fuel cells is quite small

thats what i thought too, BUT last night on a reputable current affairs program a UK expert indicated that reactor 4 contains 98 Tonnes of uranium. the hiroshima bomb contained 60kg of uranium which in physical size is about a 15cm (6") cube.

also it was stated that one of the reactors was running on plutonium, not uranium, which from my understanding is even more active than uranium.

the expert observed that should the reactor 'meltdown' then there would be an explosion, but that it would be due to overheating and the result would produce the equivanent of what is termed a 'dirty bomb' which is basically an ejection of radioactive waste which causes contamination rather than a fission chain reaction. and that brings me back to my OP

is it that the critical mass of the various isotopes is widely variable?

Luckily, as has been noted, it seems that the worst has passed.

 

[shg]

The enrichment for uranium as fuel is much less than enrichment for weapons. The plutonium is a byproduct of the fission reaction, which is why it appears in spent fuel. The ability to extract it in reprocessing is why non-nuclear countries are 'encouraged' not to do their own reprocessing.

At high temperatures (think uncovered fuel rods sprayed with water), the zirconium reduces the water, producing zirconium dioxide and hydrogen gas. The hydrogen looks for any excuse to explode.

 

[diddi]

OK, so heres my latest take on the subject.

it seems that reactor grade uranium is about 5% U-235 which is the highly reactive bit and the rest is U-238 which actually slows the chain reaction effect exhibited by the U-235.

U-238 does not support chain reaction decay because it absorbs neutrons, but does not re-emit them (ie the product of the reaction is a single atom) whereas U-235 fissions into Kr and Ba and releases more neutrons than it absorbs which is therefore self propogating.

 

[chuckchuckit]

In my opinion (not a nuclear guru) the real problem all along has been, if there is a serious breach all personel would have to be totally evacuated from the plant. There would be no one to cool down "the other reactors" and many if not all six reactors could each overheat and breach their containment also.

Was watching for this early on, glad they have "perhaps" cooled enough now so there may not be such an event. Think the after effects are still going to be worse than most think. Too bad they don't error on the side of caution, instead of continually moving out the evacuation perimiters and trying to down play it, or avoid initial panic.

Chernobal had to use robot bulldozers to finish the job burying the core area due to no containment. And heard that even today the radiation levels around those still quarantined robot bulldozers is extremely high.

 

[chuckchuckit]

Zirconia Oxide trivia...

Many moons ago (back in the 1970's) I worked for a small company that made at that time, the best oxygen analyzer in the world. It was based upon a Zirconia Oxide ceramic tube that was heated to 750 degrees centegrade. Had platinum electrodes pasted to inside and outside of the tube. The outside of the tube had stable room air, and inside the tube we would flow an inert (non-explosive) gas that had the unknown amount of oxygen. This would create an exact voltage differental instantly which represented the differental oxygen concentration in the gas inside the tube compared to the outside of the tube air. Extremely fast, and extremely accurate. Provided you kept the temperature stable on the hot end of the Zirconia tube. And kept the temperature of the analog calculating circuitry stable too, as we were pushing the electronics of analog circuits to their max back then to get the high precision measurments we were getting.

The Zirconia Oxide ceramic tube was a little bit bigger than a person's finger. Extremely strong for a ceramic as we would have to cut them with diamond saws using water to keep the saw cool.

But once heated to 750 degrees centegrade, if we wanted to work on the Zirconia tube for any reason, we had to let it slowly cool for a couple days in its containment housing, or it would immediatly crack if exposed to room air anywhere near the heated end.

I have a hard time imagining pouring water on those reactor Zirconia tubes filled with hot uranium etc does not crack them. Especially once they are exposed to air and thus the zirconia surface temperature is raised with huge temperature gradients through them. Our Zirconia tubes had huge temperature gradients also from the hot 750 degree celsius tip, to the other end closer to room temperature. But this was not a problem as long as the "temperature changes were not made quickly". Otherwise they would crack open every time.

I would think most all those reactor Zirconia tubes are cracked that were exposed and then had much colder water directly sprayed on them in those cooling ponds. And maybe also inside the reactor itself if they were exposed and then cooled too quickly. Their leaking radiation remaining inside containment hopefully. It has got to be quite a mess in all those reactors by now as I am sure all of them had exposed hot zirconia at one time or an other, that they desperatly tried to cool down with water cracking or even shattering their Zirconia enclosures.

Been a while since I have thought about Zirconia, but this brought back a lot of old memories.