[zdragnar]

Construction costs for fission are largely bloated due to safety, activist and political concerns that span the realm of pragmatic and necessary to pure fantasy as well.

Aneutronic fusion would (hopefully) bypass a lot of that.

[htag]

* The safety is massively different than fission because there is no 'run away' reaction. If you turn off the power, fusion will stop. Fission will keep going once started, and can be very dangerous if the cooling system fails.

* I do believe the fusion paths Helios is using emit neutrons, which is a big safety concern. Not only is neutron radiation directly deadly to humans, it's also a challenge to maintain containment. Neutrons are not magnetic, so matter must be used as a shield. Most materials that absorb a neutron will itself become radioactive. I'm unsure about the number of neutrons Helios is emitting, or will emit as they scale up.

[DennisP]

Their combined D-D/D-He3 reaction will emit 6% of its energy as neutron radiation, compared to 80% for D-T. Mostly they'll be lower-energy neutrons, and it's only a 50MW reactor. They don't plan to make it bigger, just to make lots of them.

They're considering doing the D-D reaction in a separate reactor to produce the He3. The D-He3 reaction is purely aneutronic, and while some D-D will still happen, they can tune it so it doesn't happen much. That would mean very little neutron radiation at the power plants.

[htag]

> The D-He3 reaction is purely aneutronic, and while some D-D will still happen, they can tune it so it doesn't happen much. That would mean very little neutron radiation at the power plants.

Yea. I'm just very curious as to what these numbers look like in practice. Neutrons are essentially waste emissions with their plan, but physics and engineering constrains will determine how much they can tune out D-D reactions.

[schiffern]

> very curious as to what these numbers look like in practice

At their stated "optimum" temperature of 200 million kelvin[0] or ~17 keV, the selectivity of the two cross-sections (D-H3/D-D) is about 0.7[1].

Let x = the fraction of D (we'll generously assume no T contamination, so the remainder 1-x is He3), plug in the reaction energies[2], and rearrange.

Graph is here: https://www.wolframalpha.com/input?i=plot+y+%3D+1.225+x%5E2%...

The top line is total non-neutron power, and the bottom line is neutron power. The horizontal axis is the fraction of D in the mix, and the vertical axis is proportional to power.

It might help compare the relative tradeoffs if you zoom in on the rising slope of the power curve, and normalize both curves to 1: https://www.wolframalpha.com/input?i=plot%3A+y%3D1%2F3.9503+...

[0] https://www.reuters.com/technology/microsoft-buy-power-nucle...

[1] https://commons.wikimedia.org/wiki/File:RxDT-DD-DHe3.jpg

[2] https://en.wikipedia.org/wiki/Nuclear_fusion#Criteria_and_ca...

[DennisP]

In fact, even for D-T fusion the NRC recently decided to regulate reactors like accelerators instead of like fission reactors.

Also, we never mass-produced fission reactors. Helion wants to build a factory making twenty 50MW reactors per day, shippable by rail.

[rcpt]

When it becomes clear that fusion can displace coal those safety, activist and political concerns will immediately pop up again

[DennisP]

And any country that chooses to keep using a dirty, unhealthy, more expensive, 250-year-old energy supply over helium-3 fusion will rapidly fall behind.

[rcpt]

Ok but that's a different story.

Just look at what's going on with solar https://twitter.com/ArmandDoma/status/1656323112349179907

Yes in theory it should be easy to shut these people up with fusion but historically it hasn't been.