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 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.
[1] https://en.wikipedia.org/wiki/Isotopes_of_sodium#Sodium-24