"liquid sodium, whose boiling point is more than 8 times higher than water’s"
Ew, multiplicative temperature comparisons in unspecified units.
Sodium's boiling point is 882'C.
"so it can absorb all the extra heat"
Confusion of temperature and heat? Sodium's specific heat capacity is ~1/3 of water, so sodium's higher boiling point doesn't by itself mean that it can absorb more heat, though ofc the combination is still in sodium's favour.
Sodium is such a nightmarish coolant. Yes, there are advantages in building and engineering a sodium cooled reactor, but from an operating point of view... oh boy.
Sodium catches fire when exposed to air.
Sodium reacts violently with water (used in some designs as secondary cooling cycle).
Sodium absorbing a neutron creates a strong gamma emitter with a low half time.
Sodium reactors have always had low availability times, caused by constant technical problems.
However unlike lead as coolant, no advances in material sciences are required to get an operational Gen 4 reactor. On the upside, you can pretty much use Sodium (fast) reactors right now... but maybe you shouldn't?
Sodium catches fire when exposed to air. That's why you don't expose it to air. Generally you fill the reactor with argon, which is an inert gas, and very heavy (dense) compared to air. Leaks can still happen, and the few sodium cooled plants around the world have all experienced them. But the fire the sodium catches when in contact with air is a very mild one. It's nothing like the violent reaction sodium has with water.
Which brings up your next point, that sodium reacts with water. For some reason you said "in some designs [water is used] as secondary cooling cycle". Which makes me think you are fully aware that this particular design does not actually use water in its secondary cooling cycle, but rather molten salt (point mentioned by Gates in the article). It's a bit disingenuous of you to bring up this point (arguably the greatest negative point about sodium) when it is actually irrelevant in the reactor discussed here.
Sodium absorbs a neutron and becomes a strong gamma emitter with a low half time. To be more precise, the half life of Na-24 is 15 days [1]. It decays in a stable isotope of Magnesium, which is not radioactive. Any leak will result in radioactivity that will naturally completely disappear after about one year.
As for the "strong gamma rays" (all gamma rays are strong, by the way), they are contained in the containment vessel.
Edit: the half life of Na24 is 15 hours, not 15 days. Even better.
No. But I have a hope that the US engineers can do at least as well as the Russian ones. Russia has been running BN-600 for more than 4 decades now, and BN-800 for 8 years. There were incidents but not huge. Here's what wikipedia has to say [1]
In the first 15 years of operation, there have been 12 incidents involving sodium/water interactions from tube breaks in the steam generators, a sodium-air oxidation/"fire" from a leak in an auxiliary system, and a sodium "fire" from a leak in a secondary coolant loop while shut down. All these incidents were classified at the lowest level on the International Nuclear Event Scale, and none of the events prevented restarting operation of the facility after repairs. As of 1997, there had been 27 sodium leaks, 14 of which resulted in sodium-air oxidations/"fires". The steam generators are separated in modules so they can be repaired without shutting down the reactor. As of 2020, the cumulative "energy Availability factor" calculated up to year 2019 and recorded by the IAEA was 75.6%.
Something as seemingly safe as water can be as dangerous as molten Sodium. The following is an explanation of the Chernobyl accident, which in part was due to using water as a coolant:
Efforts to increase the power to the level originally planned for the test were frustrated by a combination of xenon poisoning, reduced coolant void and graphite cooldown. Many of the control rods were withdrawn to compensate for these effects, resulting in a violation of the minimum operating reactivity margin (ORM, see Positive void coefficient section in the information page on RBMK Reactors) by 01:00 – although the operators may not have known this. At 01:03, the reactor was stabilised at about 200 MWt and it was decided that the test would be carried out at this power level. Calculations performed after the accident showed that the ORM at 01:22:30 was equal to eight manual control rods. The minimum permissible ORM stipulated in the operating procedures was 15 rods. The test commenced at 01:23:04; the turbine stop valves were closed and the four pumps powered by the slowing turbine started to run down. The slower flowrate, together with the entry to the core of slightly warmer feedwater, may have caused boiling (void formation) at the bottom of the core. This, along with xenon burnout, could have resulted in a runaway increase in power. An alternative view is that the power excursion was triggered by the insertion of the control rods after the scram button was pressed (at 01:23:40). At 01:23:43, the power excursion rate emergency protection system signals came on and power exceeded 530 MWt and continued to rise. Fuel elements ruptured, leading to increased steam generation, which in turn further increased power owing to the large positive void coefficient. Damage to even three or four fuel assemblies would have been enough to lead to the destruction of the reactor. The rupture of several fuel channels increased the pressure in the reactor to the extent that the 1000t reactor support plate became detached, consequently jamming the control rods, which were only halfway down by that time. As the channel pipes began to rupture, mass steam generation occurred as a result of depressurisation of the reactor cooling circuit. Two explosions were reported, the first being the initial steam explosion, followed two or three seconds later by a second explosion, possibly from the build-up of hydrogen due to zirconium-steam reactions.
Sodium has a melting point slightly below 100 degrees Celsius. Magnesium has a melting point of 650 Celsius. I don't know the exact temperature that sodium will have in the Natrium reactor, but in the French Phenix reactor it was about 560 Celsius. But that's the highest one. The temperature at the bottom of the sodium pool is most likely much lower. In any case, much, much lower than the "freezing" point of magnesium, which will particulate. So, I imagine it can be filtered by a sieve made of steel, not much different than the sieve in my kitchen to filter my tea.
Won't the magnesium just dissolve in the sodium?
I'd expect distilling the sodium or chemically processing it being a requirement for removing accumulated magnesium from the sodium coolant.
this process (neutron capture and decay) contributes to only a small amount of magnesium in the reactor coolant, as the probability of neutron capture by sodium-23 is relatively low compared to other reactions involving the reactor's fuel, coolant, and/or structural materials.
>But the fire the sodium catches when in contact with air is a very mild one. It's nothing like the violent reaction sodium has with water.
Yep, and then someone will go and build the reactor beside the ocean and a tsunami will swap it, or by a river for its cooling waters, and oopsie a flood.
Or Arizona, Texas and New Mexico with an even more dry climate than Wyoming. Although, water is becoming a much more serious issue in these regions given the influx of migration from other states over time. I still think that Nuclear + Water for Hydrogen as a general fuel storage for transportation is a better option than battery packs... Though would require more investment in water infrastructure and transport as well as desalinization.
> Sodium absorbing a neutron creates a strong gamma emitter with a low half time.
Isn't that a feature? Is it 24Na that decays to 24Mg in 14 hours? In case of an accident, you can run away for a week and it will magically disappear. You don't need a long term storage of the waste.
Actually this might be sodium's biggest advantage.
Everyone knows that Uranium (or Plutonium) can sustain a chain reaction: when hit with a neutron, they split in 2 or 3 lighter atoms, and 2 or 3 new (and fast) neutrons. That can be used to produce a bomb, because the fission events grow exponentially. But it's not all that useful for a reactor, where you want the number of fission events per second to stay constant in time. Which means, on average, each out of the 2 or 3 neutrons produced in a fission event, exactly one will trigger another fission event, and the remaining 1 or 2 neutrons have to find some way to disappear. Roughly speaking they can be absorbed by: 1. by some heavy nucleus like uranium 2. some control rods 3. some neutron poison introduced in the reactor on purpose, such as boron, gadolinium or hafnium, 4. the moderator, like water, or sodium in this case, 5. the walls of the containment vessel.
If you think of it, it's such a waste. Many of these options result in radioactive elements. Some result in material embrittlement.
Given that, it may very well be that sodium could be the best option out there.
There’s an argument to be made that we should do a much better job of directing those neutrons to make tritium, since fission isn’t even something to discuss until we know how to make abundant tritium.
Yes. But fusion is not quite here yet. It's a difficult business proposition to store large quantities of tritium in the hope that you'll be able to sell to an operator of a fusion reactor three decades down the road (after about 80% of it has decayed).
On the other hand, when fusion is ready for prime time, one could use liquid lithium in a reactor. Liquid lithium has quite a number of advantages over sodium: in that temperature range it has more specific heat capacity than any metal, higher even than water. It has excellent conductivity.
And if it captures a neutron, it splits in helium and tritium plus energy. It could increase the energy production of a fission reactor by more than 5%. And you get that tritium for free, and ultra-rare helium-3 if you fancy some aneutronic fusion at some point.
A half-life of a few seconds and it will probably decay before it gets near a human.
A half-life of thousands years and it will give out it's energy so slowly you will probably be more worried about it's toxicity (e.g. plutonium).
The most dangerous ones are generally those that have half-lives of days or weeks. That is long enough to get into a human body and give out a lot of it's energy. The is particularly the case for elements that are readily absorbed by the human body (such as strontium which replace calcium IIRC).
On that basis radioctivity from sodium probably isn't too much of a threat. I would be more worried about it's reactivity.
(not an expert on this, but did consultancy for the nuclear industry some time ago)
> The most dangerous ones are generally those that have half-lives of days or weeks. That is long enough to get into a human body and give out a lot of it's energy. The is particularly the case for elements that are readily absorbed by the human body (such as strontium which replace calcium IIRC).
You're mistaken, the most dangerous waste is that with half lives measured in decades, like cesium-137 or strontium-90 which both have half lives of about 30 years. That 30 year half life means that it can take centuries for the waste to decay away to safe levels. More than hot enough to kill, and with the longevity to do so for several generations. Strontium-85 and strontium-89 half half lives measured in tens of days, but after a few years you don't have to worry about those anymore. It's the isotopes like strontium-90 that are the major concern.
for that specific waste, the coolant in a water cooled reactor isn't a huge problem either. The things that are "complicated" are the fuel rods that some people refuse to be convinced we can bury in storage until reprocessing becomes necessary.
The advantage is 24Na's short (14.9 hour) half-life, not the lack of solubility in water. Half of the original radiation will be gone in 15 hours, and close to 90% in two days.
I understand half life. My point moisture currents can spread water soluble things very far very quickly.
Think of it this way, what happens if there's a catastrophic sodium leak? The winds carry it far and the Na gets into everything because it will be diluted into the wind's moisture. Won't you breath the radioactive Na from the air's moisture?
I imagine drinking only bottled water for a week, and perhaps increase the intake of salt for the same time. (Be careful if you have hypertension, it may be more dangerous the additional salt than the radiation.) Beer and salted peanuts looks like a wonderful anti-radiation plan.
If you get enough radioactive sodium salts to get covered in a dust layer, you are probably in trouble anyway. It may help that sodium is soluble an it can be washed easily.
If the small hidden sodium salts leak to water streams, my guess is that the concentration will be smaller than the natural sodium, and eating some additional non-radioactive sodium may help to remove it from inside the body even faster. Something like the potassium iodine pills.
Are you saying they use raw sodium as the coolant? I thought they used a sodium-based salt liquid? In which case, most of the sodium in the mixture is too tied up to react with air or water.
“High-temperature properties such as the volumetric storage density, viscosity and transparency are similar to water at room temperature. The major advantages of molten salts are low costs, non-toxicity, non-flammability, high thermal stabilities and low vapor pressures. The low vapor pressure results in storage designs without pressurized tanks (Fig. 1). Molten salts are suitable both as heat storage medium and heat transfer fluid (HTF). In general, there is experience with molten salts in a number of industrial applications related to heat treatment, electrochemical treatment and heat transfer for decades.”
I don’t know anything about this but it does seem that things are not as clear cut as your comment made it seem.
“ One of the most commonly used molten salts in nuclear reactors is a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2), commonly referred to as FLiBe. FLiBe is used as both a coolant and a neutron moderator in some types of nuclear reactors, such as molten salt reactors (MSRs) and some advanced small modular reactors (SMRs).
FLiBe has several advantages as a coolant in nuclear reactors, including its good heat transfer properties and its ability to operate at high temperatures without evaporating. Additionally, FLiBe is not highly corrosive to many materials commonly used in reactor components, which can help reduce maintenance and replacement costs.
However, FLiBe does have some potential disadvantages, such as its relatively high viscosity, which can make it more difficult to pump and circulate, and its high melting point, which can increase startup times for reactor systems. Additionally, FLiBe can be corrosive to some materials, such as aluminum and some types of steels, so care must be taken in selecting materials that are compatible with FLiBe.”
The reactor that Gates talks about (Natrium) uses sodium as moderator and coolant. In other words, the uranium fuel is submerged in a pool (literally, a pool) of liquid sodium. There are some pipes that circulate the liquid (and very hot) sodium to a separate place, where it heats up a secondary circuit of molten salt. That molten salt then goes on to heat some water and make it steam, which then drives some turbines and generate electricity. Or that molten salt can be left molten for a number of hours, as some form of energy storage solution.
So, the molten salt does not get in contact with the nuclear fuel in any way in this design.
There are other designs where this happens, and especially, there are designs where the uranium (or thorium) is itself part of the molten salt. Even Gates's company, Terrapower, has such a design in the works. But the Natrium reactor is not that.
"Sodium reacts violently with water (used in some designs as secondary cooling cycle)."
Wow. This statement is ludicrous. It's like me saying this wind-turbine is dangerous because of solar-panel hazardous waste...Seemed pretty obvious (guy in the article goes on for a while talking about why water was a bad choice) that the design for this reactor is using some variation of molten-salt as a coolant (not pressurized water).
This seems overly harsh - is there not water involved at the plant to drive the steam turbines?
That the design of the plant does not directly heat water from sodium is great, but it still is useful to know the plant becomes sensitive to water issues like flooding.
His understanding appears worse than that. Heat capacity is important, but as a second order. Absolute temperatures is the real deal since it sets the Carnot efficiency.
Another way to look at it, if I were doing a back of the envelop calculation (the most important calculations), I wouldn't look up the Cp of Na. Id only look up its boiling T
High Cp, low Cp... that can be remedied to some extent by running the pump faster or slower.
The text also kind of blanks the part where 2/3rd of world's nuclear reactors are PWRs, with typical water boiling temperature around 275°c.
Overall, the piece is a very flattering take on the technology, where all drawbacks are forgotten, and the comparison with competitors is not really honest.
A higher boiling point means that the coolant can absorb more heat without turning into a gas, which is crucial for efficient heat transfer in the reactor.
The statement about the boiling point being "8 times higher" refers to the boiling point comparison, not the specific heat capacity (which is +/- 3.4 times higher).
I don’t think the specific heat matters so much. The key is the boiling point is so high, the thermal flux will cool the system before pressurizing it. IIRC, thermal flux is proportional to the temperature differential.
Ah, TIL that Celsius is "non-multiplicative". From ChatGPT:
"When using Celsius as the unit for temperature, it is not meaningful to say that one temperature is a certain number of times higher than another. The Celsius scale is based on the freezing point (0°C) and boiling point (100°C) of water, which are not absolute values. Thus, the Celsius scale has negative values, and simply multiplying temperatures does not provide an accurate representation of relative differences.
If you want to compare temperatures in a more meaningful way, you should use the Kelvin scale. The Kelvin scale is an absolute temperature scale, with its zero point (0 K) representing absolute zero. In this scale, it is appropriate to say that one temperature is a certain number of times higher than another because the scale starts at an absolute zero point. To convert Celsius temperatures to Kelvin, add 273.15 to the Celsius value. Once the temperatures are in Kelvin, you can then make meaningful comparisons using multiplication or division."
Yeah, kelvin strictly speaking, but for reactors on earth (not in 3 kelvin outer space) with a baseline earth-ambient temperature, perhaps multiplicative C is valid, very approximately? I don't know, I guess I'm asking.
Temperature, especially in this context, relates to energy. The relative energy isn't 8x.
> boiling point is more than 8 times higher than water’s, so it can absorb all the extra heat
This is a bit like saying that a skyscraper is 8 times as tall as an apartment building, so the gravity is much weaker up there. Obviously not as extreme a mistake, but that's why people are jumping on it.
The Celsius scale is an "interval scale" while the Kelvin scale is a "ratio scale". One cannot take ratios of Celsius values as ratios are not even defined for that.
https://en.wikipedia.org/wiki/Level_of_measurement
Howso? If the conversion to Kelvin is additive (as opposed to something more complicated), and multiplying in Kelvin is fine, then how is multiplying effectively-unsigned in Celsius not fine?
For a given baseline, perhaps yes?. Kelvin has its baseline inherent, for C you can pick what you like if you're clear about it. Perhaps. (Edit: and using only positive or negative temps as pointed out above)
When I say multiply by n, what I'm really doing is applying a delta of (n - 1) times my distance to the origin.
The origin with celsius isn't at zero, so you're prevented from doing what you were pretending was multiplication (at least in the most natural way of doing it).
Obviously you can negate your offset, multiply and reapply your delta.
Ew, multiplicative temperature comparisons in unspecified units.
Sodium's boiling point is 882'C.
"so it can absorb all the extra heat"
Confusion of temperature and heat? Sodium's specific heat capacity is ~1/3 of water, so sodium's higher boiling point doesn't by itself mean that it can absorb more heat, though ofc the combination is still in sodium's favour.