[dongobongo]

Meltdown accident: basically, reactor is turned off, however heat continues to be generated because of a thing called decay heat which is when isotopes generated by the fission reactions decay to more stable isotopes and release energy. It's about 7% of a fission reactor's power and continues for a few hours until it's negligible. 7% of a gigawatt reactor is like having a couple of jet engines going full blast inside the core. This heat has to be removed, and meltdowns happen when people fail to do so - basically pumps break, coolant leaks, or coolant is blocked from cooling down the core. Recent micro reactors get around this because they don't need active coolant or people to cool down the reactor - they just cool off by conduction or simple heat rejection systems. I read recently that fusion reactor will also generate decay heat from all the activated components and this is comparable to a fission reactor. The difference is there's a lot less radioactive crap in a fusion reactor - but the fusion reactor will still meltdown and they are expensive...

[mlindner]

> I read recently that fusion reactor will also generate decay heat from all the activated components and this is comparable to a fission reactor.

I'd like to know where you read that as the entire idea is to build a reactor out of things that don't activate or are very hard to activate. i.e. things that thermalize or reflect neutrons.

Fission reactors produce tons of neutrons too (they kind of have to to work more so than fusion even) and that doesn't leave the containment vessel anywhere near as radioactive as the nuclear waste itself.

[lven]

I think they are talking about me.

Here's two papers about decay heat in ITER: https://www.sciencedirect.com/science/article/abs/pii/092037..., and https://www.sciencedirect.com/science/article/pii/S092037961...

I used their data to find power density and compared it the micro modular fission (MMR) fission reactors.

“MMR has a lower decay heat power density than fusion systems like SPARC or ARC, DEMO, or ITER and orders of magnitude lower than other advanced fission reactors as show in the figure below. UNSC's MMR has the lowest decay heat power density at 0.075 W/cm3, less than DEMO's 0.083 W/cm3 in the blanket and divertor. A lower decay heat is more manageable by passive cooling systems, allowing the reactor to dissipate heat more easily and without damaging the reactor. The other aspect to consider is the maximum temperatures that can be safely maintained in the reactor. Gas-cooled reactors like the MMR have all-ceramic cores that can withstand much higher temperatures than a fusion's reactors metals, molten salts, and magnets. MMR's low power density is a paradigm shift in nuclear safety, more foundational than fusion, for it can be accomplished cost effectively today.”

[mlindner]

Isn't ITER specifically not designed for that issue given it's not intended for long term operations? They even have the coil magnets directly in the neutron flux.

I don't think using ITER as an example here is relevant.

[lawrenceyan]

If there's truly no meltdown risk with micro reactors, I'm all for it.

In terms of fusion, I'd much rather make the tradeoff of increased cost in order to remove issues of vulnerability completely. I want to be able to not even have to think about / plan for dealing with a meltdown scenario.

[lven]

This is it: https://usnc.com/mmr/

I work on it.

[lawrenceyan]

This is pretty cool!