Nuclear
Interesting facts about nuclear science
Chernobyl's Accidents
Of Chernobyls four reactor blocks that were finished and in use, only block 3 never had a bigger accident.
In 1982, a fuel element in block 1 was destroyed and a lot of radioactivity escaped, the incident was ranked 5 on the INES scale.
The big disater in block 4 we all know happened in 1986 and it was the first and till Fukushima happened the only incident that was ranked a 7, the highest number on the INES scale.
Another explosion happened in 1991 in block 2. The reactor itself wasn't harmed this time luckily, therefore (almost) no radiation escaped.
All this shows very well how bad Chernobyl was designed and built.
However, they changed very little about the remaining reactors, even after number 4 exploded, but they still kept using the power plant, that's almost a criminal act imo.
Even after 1991, they still didn't learn anything. Reactor 1 was shut down in 1996 and reactor 3 in 2000. Btw, they even kept building the reactor blocks 5 and 6 after 1986, till they finally realized after two (!!) years that they had to stop bc of the high radiation.
Gamma Rays & cameras
If you'd be in a highly radioactive place (I mean like inside chernobyls sarcophagus or in a reactor pool near the fuel rods) and you take a video, you'll be able to see gamma rays hit the camera. The sensor of the camera can't deal with their high energy (about 1 million times higher than the energy of visible light) and just shows them as tiny white flashes.
Cherenkov radiation
Nothing is faster than light, right? Wrong.
Light is a lot slower in water (for example) than in a vakuum and the charged particles that "fly around" in a nuclear reactor are often faster than light.
Such a particle polarizes the electronic shells of the atoms in its way and a glow occurs. This process is normal and can happen pretty much everywhere, but we can only see the glow when the particle is faster than the light around it. That blue glow, called Cherenkov radiation, can be pretty strong and it's one of the most beautiful things I've ever seen tbh. It doesn't occur in all kinds of reactors tho.
Void coefficient
A very important factor for the safety of a nuclear reactor is whether its void coefficient is positive or negative. This will be pretty hard to explain for me, so lets only talk about boiling water reactors and the graphite moderated pressure tube boiling water reactor type RBMK.
To understand this, you have to know two things:
Both of these reactors are water cooled. However, a normal boiling water reactor also uses the water as moderator, while RBMK reactors pretty much only use graphite as moderator. Moderators have a bunch of functions, but for this post you only need to know that the charged particles in a reactor have to be slowed down a bit by the moderator, otherwise the desired chain reaction wouldn't work out properly.
Steam is a lot worse at cooling the reactor and it doesn't stop far as many particles as water would.
Okay so, but what does "void coefficient" mean now?
RBMK reactors have a positive void coefficient, bc the reactivity increases when there are steam holes in the water (you remember, this reactor type doesn't use the water as moderator, and since the graphite rods are still there, the particles still get slowed down and can react. This adds up to the worse cooling situation with all the steam in the water, which means the core gets hotter and hotter. We all have seen in Chernobyl what this can lead to. Soviet/Russian RBMK reactors like Chernobyl are the only reactor type I could find with such a high void coefficient. Only CANDU reactors have a positive coefficient too, but theirs is a lot lower and easier to control.
Okay, and what's the alternative?
A negative void coefficient means that the reactivity sinks the more steam is in the water bc the particles aren't slowed down enough anymore. Some call reactors like this "inherent stable" bc the less water it has, the more the reaction is slowed down. And yes, that's indeed a very good thing, but it doesn't change anything about the fact that steam can't properly cool the reactor AND it might become critical when there's too little steam (it's normal and desired for a boiling water reactor to produce steam tho). So I wouldn't call it "inherent stable", but nuclear reactors should only exist with a negative coefficient.
Radium as superfood
Y'all have heard of "superfood" like chia seeds and other magical ingredients like vitamin c and activated charcoal, but get ready for the superfood of the uhhh maybe like 30s to 60s? Radium!
The "mild radioactivity" of a little of that element was believed make you younger, stronger, more beautiful and so on and so on. They put radium in everything. In chocolate, in skin cream, in beer, in boot polish, in condomes, in cigarettes, in water...
Another great example of the reckless dealing with radioactivity are x-rays. After the German physicist W. C. Röntgen discovered them and the first x-ray machines were built, people were fascinated by this new technology that enabled them to look under their skin without having to open it. The main use was in the medical field ofc, but the doctors didn't protect themselves at all.
But the more spetacular (mis)uses of x-rays were for example the huge machines they set up at fairs, so the guests could see their own skeleton. Just imagine that. Or that some shoemakes x-rayed the shoes someone was wearing to see if they fit. That's so stupid, but they wanted to use this new technology.
Germany has a memorial that reminds of the first doctors, nurses, technicians and researchers that died from radioactivity btw and it actually stands right next to my radiography and nuclear medicine school.
chernobyl's problematic construction
I wanna explain the problems with Chernobyls construction and system which led to the terrible accident in block 4. It will also explain a little bit why it's unlikely for a such a disaster to happen again.
Chernobyl, and the RBMK reactor type in general, was really badly designed, but the accident started with the deputy chief engineer Djatlov insisting on doing the test they had to execute in that night on low reactor power. The saftey rules said that the power had to be between 700 and 1000 megawatt and Djatlov knew this just as well as everyone else in the control room, but he was in charge and ignored that (actually pretty important) rule.
The problem is that the reactor became really unstable at low power and hard to control bc of its design. Also it lacked a real emergency shut down system, it took between 12 and 18 seconds until the control rods got from completely pulled out into their fully retracted state. Every reactor we have today has a way better system to interrupt the nuclear chain reaction in an emergency.
Another disadvantage of Chernobyls control rods is that their tips were made of graphite which increases the reactivity for a few seconds in the moment the control rods are inserted in the reactor. Usually that's not a real problem, but Chernobyls reactor was already in a critical condition and the graphite was basically the last drop that caused the glass to overflow. The core got hotter, the water vaporized abruptly and bc of the positive void coefficient (1⟶) of the reactor, the reactivity increased even more and it exploded.
RBMK reactors are the only reactor type with such a highly positive void coefficient, but the RBMK reactors that are still in use today are improved. The void coefficient got lowered from +4.5% beta to +0.7% beta, which is a lot better. Also an emergency shutdown system was installed and todays RBMK reactors have more control rods but less graphite (it's not possible to take all graphite out tho bc the risk of a xenon poisoning of the reactor would be too high. I'll explain this in another post).
However, one problem with RBMK reactors that they still have nowadays is the lack of a containment that encases the reactor and prevents radioactivity from escaping after an accident in the core of the reactor. They have a confinement, but that's not as good as a real containment. But most other reactor types to have a containment, dw.
And the last two problems with Chernobyl I wanna mention here were that they made the roof out of combustible material bc the provided flame proof material wasn't available when reactor block 4 was built and they wanted to finish the reactor as soon as possible to get premiums from a political party. Also the Soviet Union always tended to trust the human operator of the reactor more than the machines, even tho they usually make less mistakes.
(1⟶) See ⟶ VOID COEFFICIENT
xenon poisoning
To understand the Chernobyl accident, you have understand what a xenon poisoning is.
With the nuclear fission in a power plant, the element idione-135 and a few other elements develop. Iodine-135 (I'll just call it Iodine from now on) decays with a half life of 6.6 hours to xenon-135, a noble gas that absorbs neutrons very well. When that happens, the unstable xenon-135 becomes the stable xenon-136.
When the nuclear reactor is running normally, there's a balance between the creation and the degradation of xenon-135.
But let's say a reactor gets shut down completely. Because the idione is still there, new xenon-135 forms, but less and less gets degraded. The reactor is full of xenon and that state is called a xenon poisoning.
The xenon absorbs the neutrons that are needed for the desired chain reaction, which leds to the reactor being unable to be start up again for the next 1 - 2 days.
A xenon poisoning has played an important role in the disaster of Chernobyl, let's take a look at it.
To completely understand this, I would recommend you to read "VOID COEFFICIENT" (1⟶) and "CHERNOBYL'S PROBLEMATIC CONSTRUCTION" (2⟶) first (if you haven't already).
At the beginning of the test, Djatlov forced the plants staff to reduce the power of the reactor way under the required power level (2⟶) and a xenon poisoning happened. Because of reasons no one really knows, the power level sunk even more, until the reactor was almost completely shut down, which made the poisoning even worse.
The staff pulled out the control rods to start up the reactor again, but bc of the poisoning, nothing happened. Now they closed the steam feeds of the turbines, which means that the steam was "trapped" inside the reactor and bc of the positive void coefficient (1⟶), the reactivity suddenly rose, the xenon-135 got degraded again, which increased the reactor power even more, a vicious cycle. The staff couldn't control the reaction anymore and the core exploded.
(1⟶) See ⟶ VOID COEFFICIENT
(2⟶) See ⟶ CHERNOBYL'S PROBLEMATIC CONSTRUCTION
pilots of chernobyl
When Chernobyl exploded, literally no one was prepared for a war against an invisible enemy. The Soviet Union refused international help and helped itself with their best resource: Humans.
I wanna tell y'all what these people did and how they safed all of us and all the generations after us. I'll begin with the pilots and the firefighters and other liquidators who flew with them.
The liquidators fought the fire and the extremely radioactive molten corium with water, sand, boracic acid and lead. Pretty primitive methods and not the best solution, but they had to do something and they did. They flew over the reactor with minimal saftey distance, right through the radioactive smoke. The dosimetrists measured 1800 roentgen 300 meters over the reactor, which is in general considered to kill a human in seven days. But the liquidators opened the windows anyway and threw out sand bags into the open reactor with their bare hands. Many pilots got fits of dizziness while flying, one passed out.
Beside the fight against the fire itself, it was also important to fixate the radioactive dust on the ground to prevent it from being blown off and being spread over Europe by the wind. They used water and a coagulating mass the people just called "бурда", sludge. People tend to forget what an important role the people who did this played. Without them, a lot more dust would have contaminated way more areas.
Radiation doses
I thought it would be useful to give you some examples of how much radiation is low dose and what's (too) high. There are many units for radiation, but I'll only talk about Sievert bc it's the most important unit when it comes to humans and their exposure to ionizing radiation.
First of all:
1 microsievert µSv = 0.001 millisievert mSv
1 millisievert = 0.001 Sievert Sv
10 µSv: near Chernobyl block 4, 2018
~50 µSv: long distance flight Frankfurt - NY
87 µSv: Fukushima plant area, 2014
0.1 mSv: at the wall of chernobyl 4, 2018
0.1 mSv: pelvic xray
0.2 mSv: inside Chernobyl, near reactor hall, 2018
0.2 mSv: thorax xray
0.5 mSv: thoracic spine xray
1.0 mSv: mammography
2.0 mSv: skull CT
2.4 mSv: natural radiation per year
10 mSv: thorax CT
20 mSv: limit for a radiation exposed workers (like me) per year in Germany
from 0.2 Sv: possible long term effects, not measurable
0.2 - 0.5 Sv: no symptoms, slight reduction of red blood cells
0.5 - 1.0 Sv: headache and temporary higher risk of infection
1 - 2 Sv: mild radiation sickness, nausea, loss of appetite, fatigue, temporary male infertility 10% fatalities after 30 days
2 - 3 Sv: severe radiation sickness, new symptoms are: hair loss, discomfort loss of white blood cells, maybe permanent female infertility, 35% fatalities after 30 days
3 - 4 Sv: severe radiation sickness, new symptoms are: diarrhea, random bleedings in mouth, under the skin and in kidneys, 50% fatalities after 30 days
4 - 6 Sv: worst radiation sickness, first symptoms appear after 30 to 120 min, 60% fatalities after 30 days
6 - 10 Sv: worst radiation sickness, first symptoms appear after 15 - 30 min, bone marrow is almost completely destroyed, gastrointestinal skin is heavily damaged, 100% fatalities after 14 days
10 Sv: inside Chernobyl, near reactor hall, at desaster
10 - 20 Sv: worst radiation sickness, first symptoms appear after 5 - 30 min, the gastrointestinal cells begin to die a few days later, which leads to intestinal bleeding, loss of water and disturbance of the electrolyte balance, 100% fatalities after 7 days
18 Sv: Chernobyl, shortly after desaster, 300m over destroyed reactor
20 - 50 Sv: worst radiation sickness, 100% fatalities after 3 days
over 50 Sv: immediate disorientation, coma after seconds or minutes, death after a few hours
about 100 Sv: Chernobyl, the radiation on the roof top that was cluttered with remains of the reactor itself sometimes reached this
Some important comments about this list:
It's hard to find out how much radiation there really was at the Chernobyl desaster, and not all of the numbers you'll find can be trusted. I only used the numbers of persons who were actually there at that time. Also just because I wrote that 100% of all people who were exposed to this dose died after so and so many days doesn't mean that no one ever survived such a high radioactivity. I mean, people were 40 seconds on the rooftop, many more than one time, and some are still alive or just died a few years ago.
The effects of radiation on a human being are unpredictable, almost random. That's why it's so hard to say where the lethal limit is. Also the liquidators of Chernobyl protected themself. Their protection wasn't the best, but in the end their bodies, especially their organs, were exposed to less radiation than the measuring instruments. Alpha radiation, despite being the most dangerous, barely comes through the layer of dead cells on our skin.
X-rays
Now I wanna talk about X-rays bc they're probably a bit different from what you think to know about them.
Basics first: X-rays are a form of ionizing radiation, just like normal radiactivity. They were discovered by the German physicist W.C. Röntgen. The first picture that was ever taken with xrays showed the hand of his wife. He xrayed her hand often in the following months and years and it's not unlikely that she and he experienced radiation damage, but without knowing the reason.
Röntgen remained modest and gave his discovery the name "X-Rays" and refused to have them named after him. However, in many countries xrays are called Röntgen Rays nowadays.
Okay, but where do xrays come from and are they different from "normal" radioactivity?
The two main differences between xrays and radioactivity are 1) xrays have to be generated while radioactivity is always around us and 2) the way xrays are generated is different from radioactivity.
The heart of every xray machine is the xray tube. Electricity at the tube's negative pole makes a wire made of tungsten/wolfram glow and electrons get flung out of the tungsten atoms. They have so much energy, they fucking yeet electrons of the positive pole out of their orbit and other electrons take their place. At this exact moment, the energy basically gets converted into 99% heat and 1% xrays.
The radiation that is created by this is called "characteristic xrays" and it only makes up a small amount of the xrays we use.
You can probably imagine, that not every electron always hits another electron. After all, these things are very small. But luckily, xrays are also created when an electron of the tungsten wire just gets slowed down by coming near a core of an atom. This radiation is called "braking radiation".
Radioactive gold
Did you know that every element can be radioactive? And I mean every element. There's radioactive gold, iron, helium, mercury, calcium...
Radioactivity occurs when the core of an atom is instable. Every element till (including) bismuth has a stable form that isn't radioactive and this stable form is also the "normal" form. This means, normal gold for example isn't radioactive bc the core (nucleus) is stable.
You might remember that a nucleus consists of protons and neutrons. Let's take gold as example. Gold ALWAYS has 79 protons. If you'd take away one proton, it would be platinum. If you'd add one, it would be mercury.
Buuuut you can change the number of neutrons. Normal, stable gold has 79 protons and 118 neutrons, so you call it Gold-197 (79 + 118 = 197). That's how the nucleus "feels comfortable", there's no need to change anything and the core is stable. But when you take away one neutron, you have Gold-196 and the nucleus isn't stable anymore. Congratulations, you have radioactive gold.
Important: Not every unstable nucleus reacts in the same way and some elements have more than one stable form, mercury for example is always stable between Mercury-196 and Mercury-202, also Mercury-204 is stable as well.
But that would be too much detail now.
Beta becomes X-ray
When you wanna protect yourself from beta (β) radiation, you might just exchange the type of radiation instead of getting into real safety - if you're doing it wrong. Let me explain.
If I'd ask you how you would shield radiation, I'm pretty sure the first idea most of you have is lead. Lead is a rlly dense material and it's used pretty often as radioprotection right? Right. BUT.
Let's say we have an element that gives of beta radiation. If you don't know, beta radiation is basically just the element shooting off an electron.
But do you know how x-rays are produced? They occur when fast electrons fly into a material and interact with the atoms of it. You should read "X-RAYS" (1⟶) about this, I explained it there.
The denser the "target" material is, the higher is the chance of the electron coming near a nucleus (atom core) and producing an xray.
So you might be safe from the electrons/β-radiation behind a lead wall, but you're very likely subjected to x-rays. You basically exchanged one ionizing radiation for another. RIP.
(1⟶) See ⟶ X-RAYS
notes