No, you put the ‘‘waffle’’ right under the core and spread the melted fuel out before it forms the heated glob that melt through everything
What do you mean by “is far more radioactive than…”
Bq/kg of pure am-241 compared to bq/kg of fuel rod material?
Or Adsorbed dose in Grays / hr at distance X from a smoke detector compared to gy / hr at distance X from the same mass of generic nuclear fuel rod waste as there is of mass of Am-214 in a smoke detector.
Or equivalent dose of the above?
Or same whole body dose for ingesting the Am-241 accounting for associated body transit time and accumulations around sensitive organs (Iodine - thyroid gland?) to lead to a life time dose (equivalent?) compared to the same for ingesting the equivalent mass of generic fuel rod waste?
I am a nuclear power supporter, however I feel that comparing a smoke detector to fuel waste based on a poorly defined measurement to demonstrate nuclear power is safe, is not helping. The analogy is so easily proved to be useless. I know I roll my eyeballs (in a polite way) whenever I see someone state nuclear reactor = controlled nuclear explosion, sure both involve fission, but to use the latter as a reason not to do the former is disingenuous as the physics don’t support it. The same should be said for saying a smoke detector is more radioactive than nuclear fuel rods (used or unused).
Sorry rant over.
Besides - if you get people to be concerned about putting smoke detectors in their house because 'it’s more radioactive than the uranium used in nuclear fuels or some kinds of nuclear waste " and that leads to a drop in smoke detector uptake you will have MILLIONS AND MILLIONS OF DEATHS ON YOUR HANDS.
Well, it was a worst-case scenario for what kind of thing could go wrong. It could still have gone more wrong, though, had operators not eventually figured out what was really happening and taken appropriate steps.
Interestingly enough, some reactors now have a heat-resistant, upward-pointing cone under the core, specifically so that potential melted fuel falling through the bottom of the vessel would spread out instead of pooling and concentrating (the hope is that it would eventually no longer be critical). So some nuclear engineers agree with you.
Sure. The specific activity of U-238 is 12,445 Bq/g, and the specific activity of U-235 is 80,011 Bq/g, whereas the specific activity of Am-241 is 1.295 x 10[sup]11[/sup] Bq/g. Thus, the americium in a smoke detector is nearly seven orders of magnitude more radioactive than a typical unused fuel element composed of 3-5% enriched uranium.
This is the basic definition of radioactivity, which is inversely proportional to the half-life of the nuclide in question.
Now you’re getting into measures of radioactive exposure, which is not the same thing as measures of radioactivity.
In humans, radioactive exposure is measured by the amount of energy that ionizing radiation deposits in a unit mass of tissue, referred to as the “absorbed dose,” and measured in grays (Gy). The biological effects depend not only on the absorbed dose, however, but on the type of radiation. The “equivalent dose” takes this into account, which is measured in sieverts (Sv).
That being said, Am-241 is indeed a highly radioactive substance. As it decays by alpha decay, it is particularly dangerous if it is ingested. The precautions you would have to take if you handled significant quantities of Am-241 are similar to the precautions that would have to be taken by comparable quantities of the byproducts of nuclear fission. The reason why it is safe to use in household smoke detectors is solely because of the minute quantity involved (a fraction of a microgram), not because it is inherently more safe than either nuclear fuel or nuclear waste products.
I’m sorry you feel that way, but all that you are demonstrating is your own ignorance of the facts.
In addition, I don’t see how you propose to easily prove the analogy to be useless. The usefulness of an analogy is a matter of opinion. You certainly haven’t disproved the veracity of the analogy.
The Am-241 in a smoke detector is far more radioactive than the uranium in unused nuclear fuel rods, whether you like it or not. It would likely be comparable to many fission products, depending on the length of time elapsed after the reactor shutdown.
Besides, I didn’t say that a “smoke detector” is more radioactive than fuel rods. I stated “[t]he Americium-241 contained in smoke detectors, with its half-life of 432.2 years, is far more radioactive than the uranium used in nuclear fuel, and also more radioactive than many types of nuclear waste,” which is a true statement. I would add that I am referring to a per-mass basis.
The point of this website is to “fight ignorance.” The fact of the matter is that the americium used in smoke detectors is indeed “more radioactive than the uranium used in nuclear fuels or some kinds of nuclear waste.” Period. What would be helpful is further explaining why it is a non-issue, not hiding the facts of the matter.
After all, people have enough misconceptions about nuclear power (and radioactivity) without adding more.
Spreading the corium out is good from a thermal management standpoint, in that more surface area (whether in contact with air or in contact with the concrete containment building floor) helps to dissipate heat. As far as criticality is concerned, it’s a non-issue. Neutron moderators are needed to provide thermal neutrons that can sustain the criticality; once the fuel melts down and ends up at the bottom of the pressure vessel (or on the floor of the containment building), the moderator is either missing (stuck up in the pressure vessel, or boiled away), or in an unfavorable geometry (e.g. water on top of the corium rather than distributed through it), and the fuel mass is no longer critical; the main nuclear reaction dies out. Yes, the control rods are also stuck up in the pressure vessel, but without neutron moderators, the level of enrichment of reactor fuel is not enough to sustain a critical mass all by itself. You’ll still be left to deal with decay product heating for a while (regardless of geometry), but that’s what the containment building is supposed to do.
The smoke detector thing is sort of my fault. Someone said coal ash was more radioactive than nuclear waste, but I pointed out that this is based on a SciAm article where they said as much. When questioned, they said their testing was for coal ash in the air (which does have a very small amount of radioactivity) vs shielded nuclear waste. I said that’s like saying the smoke detector is more dangerous than nuclear waste because it’s closer to me and unshielded. I think robby was just pointing out that I was using a bad example because, technically, the smoke detector probably is more radioactive.
Am I the only one that thought this was about waffles and bacon? mmmm
Uh, well, every once in a while this criticality that “cannot happen” happens, usually due to density of the radioactive material or due to the shape of nearby materials reflecting neutrons.
I agree that the fuel mass should go subcritical most of the time, but maintaining that it does do so because it can’t do otherwise flies in the face of actual experience.
That depends on whether you are talking about substained criticality or temporary criticality. Substained is hard to substain 
just due to the nature of the beast. And even then, there is substained and there is SUBSTAINED. As in, there are still some neutrons flying around and some fissioning still going on adding some small additional amount of heat/energy being added to the heat/energy being produced purely from the decay of the fission products produced by recently properly running reactor versus a buttload of heat being produced from sustained fission.
The China Syndrome molten blob of sustained criticality working its way to the Earth’s core has about as much scientific truthiness to it as the old bumblebees can’t fly in theory meme.
Most of the criticality accidents on that list involved nuclear research with a variety of radioactive materials, not necessarily nuclear reactor fuel. Of the few that incidents that mention reactor fuel materials, they also involved moderator materials, typically water. As billfish678 points out, any configuration of nuclear reactor fuel with moderator that happens to go critical won’t be sustained in that configuration for long. If water is the moderator, it will boil off long before the fuel gets hot enough to melt concrete. If graphite is the moderator, it’s going to stay up in the pressure vessel; in the unlikely event that it falls down to the bottom of the reactor vessel (or onto the containment building floor), what do you think the odds are that things will end up in a configuration that results in criticality? Try to imagine tossing a box of toothpicks on the floor, and having them all form a nice, orderly stack.
In the astronomically unlikely event that a critical mass of fuel+moderator is formed on the floor of the containment building, the fuel will melt, and then it will flow out of its critical-mass configuration, go subcritical and stop producing the massive amounts of heat that would be required to burn through the floor of the containment building. No China Syndrome.
China Syndrome refers to molten core material melting its way through containment, and then the reactor building.
Yes, China Syndrome. Well, if you’re talking about corium melting its way through the floor. Also, if it does make its way through the floor and hits the water table just beneath, it’s very bad. We’d be lucky if it kept going down, really far down.
I apologize for my lack of clarity (“No China Syndrome”), as it seems to have inspired a lack of clarity on your part as well. You have defined China Syndrome (even if your terms are somewhat unclear), but it’s not plain whether you are claiming this is a possible outcome of a meltdown. If you are, then we disagree.
Corium can (and has, in some meltdowns) breached the pressure vessel. This is why the containment building exists - to contain any corium that escapes the pressure vessel. The corium won’t burn through the containment building because once it’s gone subcritical, it just isn’t making enough heat to do so; the containment building floor is thick enough to tolerate the decay-product heat. If the containment building floor is cracked (e.g. due to a quake), some radioactive material may escape into the soil, but it will not keep burning its way down to the water table to cause a steam explosion as postulated in The China Syndrome.
If your molten blob melts its way past your concrete floor, you have China Syndrome. It’s not meant to literally mean all the way to China anymore than China is actually on the opposite side of the planet. They could just as well call it “Hell Syndrome.”
Then its just an engineering/safety factor problem then isn’t it and NOT a syndrome. Thats just a scary made up name.
The “syndrome” aspect of it was/it is an unstoppable mass of hot shit that STAYED hot due to continued high level heat production from sustained criticality.
Engineers know how much heat a molten blob (aka the former reactor) will produce and about how thick a layer of concrete would be needed to stop it.