That was a great article. May I ask where you got your nuclear expertise from?
That is, each fission event creates only one new fission event on average, giving an overall fission rate that is stable.
Ahh yes, but that’s a neat trick, isn’t it? How do you arrange for there to be exactly one new fission event on average? 1.000 is a very precise number and one wonders how engineers can achieve such stability.
My impression is that in general, the coefficient of reactivity (i.e. the tendency for atoms to fission) tends to fall as the temperature rises, due to basic physical phenomena such as thermal expansion and doppler broadening. As a result, if a reactor has approximately the correct amount of “reactivity” (i.e. U-235 or plutonium fuel plus moderator effects minus poisoning effects), the coefficient of reactivity will remain steady around 1.000, and so will the reactor’s temperature. This is because if it gets too hot the reactivity will naturally fall (thus reducing temperature, provided pumps are carrying heat away) and if it gets too cool the reactivity will naturally increase (thus increasing the temperature). This tendency toward stability is known as a “negative temperature coefficient of reactivity” and is a standard feature of all reactors.
However, controlling reactivity isn’t exactly easy in standard Light-Water Reactors (LWRs), e.g. because unstable Xenon produced by fission has a strong “poisoning” (reducing) effect on the nuclear reaction, causing a tendency for the reactivity to rise and fall over several hours, and so, control rods are required to counteract this effect and increase or decrease reactivity as necessary. Also, correct me if I’m wrong, but in a LWR I don’t think the nuclear fission reaction can be stopped without inserting control rods, because nothing short of a meltdown can cause the temperature inside the reactor chamber to get hot enough for the “negative temperature coefficient of reactivity” to stop the fission itself. Plus, adding fuel is a bit of a pain, so they put somewhat more fuel in the reactor than strictly necessary, in order to refuel less often, but this increases the potential maximum reactivity, which is a potential hazard.
a strongly negative temperature coefficient of reactivity,
a wide thermal margin, so that if they overheat, the rate of fission drops almost to zero without any manual or automatic intervention
no Xenon poisoning, because Xenon simply bubbles out of the salt as soon as it is produced, which makes reactivity easy to control; thus there is no need for control rods, though shutdown rods can be used to shut down, if desired...
an easy way to add fuel (usually? I can’t remember how Moltex manages this), and
a reliable system for passive shutdown, such as a drain tank.
Thus MSRs take maximum advantage of the laws of physics to remove hazards: more safety, less complexity.
(The most it can do is get so hot that it melts.)
Correction: the most nuclear fuel can do is get so hot that it causes a steam explosion. That’s what happened at Chernobyl.
I gather that there were several contributing factors to the Chernobyl disaster, but among these were two primary flaws that made the accident so serious. These flaws (1) never existed in any reactors outside the Soviet Union and (2) were both intentional. The first flaw was the lack of a containment building; the second was a reactor design with a substantially positive void coefficient.
Obviously, containment buildings cost money and you can save money by not having one. Mystery solved!
But what is a “positive void coefficient” and why did Chernobyl have one? This phrase is short for “positive void coefficient of reactivity”, which means that if an empty space (void) appears in the reactor chamber, the reactivity (rate of fission) increases, which increases the amount of heat being created. If this heat causes the void itself to get bigger, then the heat causes more heat, in a dangerous feedback loop. The interesting thing about this design flaw—I think—is that it was intentional. I put two and two together after watching lots of videos on MSRs and then reading Bernard Cohen’s book (Chapter 7); I inferred that the positive void coefficient is a cost-saving measure.
It’s related to a curious property of neutrons. You might expect that a neutron needs to hit a nucleus hard and fast in order to fuse with it, but it turns out that the opposite is true: nuclei can absorb slow-moving neutrons more easily than fast neutrons. But when atoms split, the released neutrons are moving very fast, so they are not easily absorbed by other uranium atoms, which makes it hard to keep a chain reaction going. Nuclear engineers use materials called “moderators” to solve this problem. A moderator is any material that slows down neutrons (but rarely absorbs them).
In Western light water reactors, the light water itself is used as a moderator (“light water” means “normal water”, i.e. not enriched, which would make it “heavy”). But light water has two different effects: it can slow down neutrons (increasing reactivity) but it can also absorbs neutrons (decreasing reactivity). Normally, the moderating effect is the dominant one, so if the water boils (causing steam bubbles, which are “voids”) the amount of moderator decreases, which decreases the fission rate. But water’s “poisoning” effect of absorbing neutrons is big enough that light water is incompatible with natural uranium.
Natural uranium has only 0.7% U-235 content, so it’s challenging to get a fission chain reaction out of it. For that reason, most western reactors use enriched uranium instead. But enriching uranium costs a lot of money, so, you know, why bother?
There are two ways to run a reactor with natural uranium: either you can use heavy water moderator (expensive), or graphite moderator (cheap). Well, you can guess which moderator the USSR chose for Chernobyl!
Since graphite was used as the moderator, the light water in the reactor was only there for the purpose of cooling, not moderating. When the reactor overheated in 1986, the coolant water boiled, which created steam bubbles (voids) in the reactor. So now, the reactor is losing a neutron poison that slows down the chain reaction, and simultaneously losing the ability to cool down the reactor. Meanwhile, it is not losing enough moderator to make a difference. Therefore, the nuclear reaction speeds up, which soon leads to the steam explosion that destroyed the reactor. Oh, and plus, their control rods were slow and may have had a design flaw that increased reactivity during insertion. Oops!
So there you have it. Chernobyl happened because the Soviets were cheapskates, and a similar event cannot happen in the modern world because no regulatory agency today would allow the combination of natural uranium with graphite moderator and light water coolant. Even in the 1970s this design was already off-limits in the West.
That was a great article. May I ask where you got your nuclear expertise from?
Ahh yes, but that’s a neat trick, isn’t it? How do you arrange for there to be exactly one new fission event on average? 1.000 is a very precise number and one wonders how engineers can achieve such stability.
My impression is that in general, the coefficient of reactivity (i.e. the tendency for atoms to fission) tends to fall as the temperature rises, due to basic physical phenomena such as thermal expansion and doppler broadening. As a result, if a reactor has approximately the correct amount of “reactivity” (i.e. U-235 or plutonium fuel plus moderator effects minus poisoning effects), the coefficient of reactivity will remain steady around 1.000, and so will the reactor’s temperature. This is because if it gets too hot the reactivity will naturally fall (thus reducing temperature, provided pumps are carrying heat away) and if it gets too cool the reactivity will naturally increase (thus increasing the temperature). This tendency toward stability is known as a “negative temperature coefficient of reactivity” and is a standard feature of all reactors.
However, controlling reactivity isn’t exactly easy in standard Light-Water Reactors (LWRs), e.g. because unstable Xenon produced by fission has a strong “poisoning” (reducing) effect on the nuclear reaction, causing a tendency for the reactivity to rise and fall over several hours, and so, control rods are required to counteract this effect and increase or decrease reactivity as necessary. Also, correct me if I’m wrong, but in a LWR I don’t think the nuclear fission reaction can be stopped without inserting control rods, because nothing short of a meltdown can cause the temperature inside the reactor chamber to get hot enough for the “negative temperature coefficient of reactivity” to stop the fission itself. Plus, adding fuel is a bit of a pain, so they put somewhat more fuel in the reactor than strictly necessary, in order to refuel less often, but this increases the potential maximum reactivity, which is a potential hazard.
In contrast, the proposed Molten Salt Reactors (MSRs) have
a strongly negative temperature coefficient of reactivity,
a wide thermal margin, so that if they overheat, the rate of fission drops almost to zero without any manual or automatic intervention
no Xenon poisoning, because Xenon simply bubbles out of the salt as soon as it is produced, which makes reactivity easy to control; thus there is no need for control rods, though shutdown rods can be used to shut down, if desired...
an easy way to add fuel (usually? I can’t remember how Moltex manages this), and
a reliable system for passive shutdown, such as a drain tank.
Thus MSRs take maximum advantage of the laws of physics to remove hazards: more safety, less complexity.
Correction: the most nuclear fuel can do is get so hot that it causes a steam explosion. That’s what happened at Chernobyl.
I gather that there were several contributing factors to the Chernobyl disaster, but among these were two primary flaws that made the accident so serious. These flaws (1) never existed in any reactors outside the Soviet Union and (2) were both intentional. The first flaw was the lack of a containment building; the second was a reactor design with a substantially positive void coefficient.
Obviously, containment buildings cost money and you can save money by not having one. Mystery solved!
But what is a “positive void coefficient” and why did Chernobyl have one? This phrase is short for “positive void coefficient of reactivity”, which means that if an empty space (void) appears in the reactor chamber, the reactivity (rate of fission) increases, which increases the amount of heat being created. If this heat causes the void itself to get bigger, then the heat causes more heat, in a dangerous feedback loop. The interesting thing about this design flaw—I think—is that it was intentional. I put two and two together after watching lots of videos on MSRs and then reading Bernard Cohen’s book (Chapter 7); I inferred that the positive void coefficient is a cost-saving measure.
It’s related to a curious property of neutrons. You might expect that a neutron needs to hit a nucleus hard and fast in order to fuse with it, but it turns out that the opposite is true: nuclei can absorb slow-moving neutrons more easily than fast neutrons. But when atoms split, the released neutrons are moving very fast, so they are not easily absorbed by other uranium atoms, which makes it hard to keep a chain reaction going. Nuclear engineers use materials called “moderators” to solve this problem. A moderator is any material that slows down neutrons (but rarely absorbs them).
In Western light water reactors, the light water itself is used as a moderator (“light water” means “normal water”, i.e. not enriched, which would make it “heavy”). But light water has two different effects: it can slow down neutrons (increasing reactivity) but it can also absorbs neutrons (decreasing reactivity). Normally, the moderating effect is the dominant one, so if the water boils (causing steam bubbles, which are “voids”) the amount of moderator decreases, which decreases the fission rate. But water’s “poisoning” effect of absorbing neutrons is big enough that light water is incompatible with natural uranium.
Natural uranium has only 0.7% U-235 content, so it’s challenging to get a fission chain reaction out of it. For that reason, most western reactors use enriched uranium instead. But enriching uranium costs a lot of money, so, you know, why bother?
There are two ways to run a reactor with natural uranium: either you can use heavy water moderator (expensive), or graphite moderator (cheap). Well, you can guess which moderator the USSR chose for Chernobyl!
Since graphite was used as the moderator, the light water in the reactor was only there for the purpose of cooling, not moderating. When the reactor overheated in 1986, the coolant water boiled, which created steam bubbles (voids) in the reactor. So now, the reactor is losing a neutron poison that slows down the chain reaction, and simultaneously losing the ability to cool down the reactor. Meanwhile, it is not losing enough moderator to make a difference. Therefore, the nuclear reaction speeds up, which soon leads to the steam explosion that destroyed the reactor. Oh, and plus, their control rods were slow and may have had a design flaw that increased reactivity during insertion. Oops!
So there you have it. Chernobyl happened because the Soviets were cheapskates, and a similar event cannot happen in the modern world because no regulatory agency today would allow the combination of natural uranium with graphite moderator and light water coolant. Even in the 1970s this design was already off-limits in the West.