Hydrogen vs. Water Shielding?

[Aeronaut]

The size, weight, and amount of materials involved in building the water shielding jacket keeps bugging me. Since the water is used to provide hydrogen atoms to slow the occaisional low energy neutrons, the obvious question is “why not use hydrogen as the first shielding layer?”

The obvious objection is how a leak would be flammable and/or explosive. But the hydrogen car fuel tanks don’t seem to worry about those hazards. How thin could a hydrogen-filled inner shielding layer be? Iow, could this make the FF small enough to fit inside a semi-truck’s tractor? If so, this would also be very important to making FF fit in a standard shipping container.

[Henning]

You mean like molecular hydrogen (H2)? Liquid hydrogen? Solid hydrogen? Molecular hydrogen which is stored in a tank, like those in a hydrogen powered car? Those tanks contain a resolvent (or whatever) that bounds to hydrogen, and the mass of the resolvant exceeds that of hydrogen a few times.

Otherwise you have to cool the hydrogen to a liquid, or have a very high pressure.

The easiest resolvent (or whatever, well not quite) for that is oxygen, which makes it H2O, water. But then it’s not molecular hydrogen anymore. But it stays liquid up to 100°C.

For methane and -40°C you get more hydrogen atoms per mol or per kg than with water, and it’s solid.

So cool the shielding to 100°C with water or -40°C with methane.

Or longer strands of hydrocarbonates. But that makes it more explosive again. Maybe some plastics? More hydrogen atoms per mol, but you can’t replace them when they get shot out of their molecular bindings. Water just changes to OH when it looses an hydrogen atom.

Thinking of it, a liquid is preferable to a solid, so you can replace the modified molecules (H2O -> OH + H). And a gaseous substance doesn’t comes near to a density of a liquid.

[HermannH]

Needing a ton (or several) of water is a problem if you want to use the device to power a truck or a plane. Otherwise, nothing could be simpler, cheaper, safer and more elegant than a blanket of water. Also, water molecules pack tightly, you would have to apply a lot of pressure to hydrogen gas to get the same density of protons that water gives you in its natural state.

The container for the water can be shipped in pieces and assembled on site. At the end you hook up your garden hose and fill it up. Well, you may have to use distilled water.

Keep in mind, even in the best case scenario there will be extensive site preparation and assembly required: You don’t just plug a 5 MW power supply into an outlet! Getting rid of around 5 MW of waste heat is also not trivial.

[Aeronaut]

Re: Henning: You mean like molecular hydrogen (H2)? Liquid hydrogen? Solid hydrogen? Molecular hydrogen which is stored in a tank, like those in a hydrogen powered car? Those tanks contain a resolvent (or whatever) that bounds to hydrogen, and the mass of the resolvant exceeds that of hydrogen a few times.

Yeah, I was originally wondering about pressurized hydrogen gas to make it denser. Didn’t realize there were so many forms of hydrogen.

Re: HermannH: The container for the water can be shipped in pieces and assembled on site. At the end you hook up your garden hose and fill it up. Well, you may have to use distilled water.

Seems that the water tank would have to be one piece so that the water is a 360 degree shield instead of having a bunch of seams. Hadn’t thought about distilled water, but it sounds safer than guessing what’s in the local water… that just might be able to react…

How about no shielding in some applications, such as where the core could be lowered into a silo roughly 1 meter in diameter? Them pesky neutrons would then have a boronated concrete wall as a partial shield, followed by several meters of dirt.

[HermannH]

The seams would be in the walls of the water containment vessel, which could be any shape. All you have to ensure is that the total travel distance through water is 1 meter or more in any direction.

Of course you could bury the thing underground and not worry about shielding it with water. That is until you have to service it. Depending on the surrounding material you may encounter some neutron activation. i.e. the neutrons have made the material radioactive.

That residual radiation should be rather small, after all it is mostly aneutronic fusion. In the end, however, I would expect that water (with a blanket of boron) provides the cleanest solution.

[Aeronaut]

Yep, water is the best all-around solution, since the power output wiring would need an access tunnel below the silo- with little shielding from some angles. But water’s winning argument is the ability to pick the module up with an overhead crane and move it around just like any other machine in a growing factory. Thank you both for the reality checks.

PS- I’ve been assuming that the water jacket has to enclose the drift tube as well as the vacuum chamber. Would it still be shielded to below background levels if the water jacket only surrounded the vacuum chamber?

[HermannH]

I suppose by drift tube you mean the ion beam converter. I have no idea how large an opening you need for the ions.
At any rate, both neutrons and x-rays will escape through that opening. So you need to provide some shielding. I have no idea what material you would use and what the geometry should be.
If I understand correctly, the entire room where the reactor sits is off-limits for personnel during operation and possibly for an hour or so afterward. I assume the concrete walls of the room will take care of any x-rays and neutrons that do escape the shielding.

All of this is important: In order to gain widespread acceptance and avoid overly restrictive regulations you need to demonstrate that the amount of radiation that can escape the ‘reactor room’ is comparable to, say, the radiation that gets out of a radiology lab in a hospital.

[Aeronaut]

HermannH wrote: I suppose by drift tube you mean the ion beam converter. I have no idea how large an opening you need for the ions.
At any rate, both neutrons and x-rays will escape through that opening. So you need to provide some shielding. I have no idea what material you would use and what the geometry should be.
If I understand correctly, the entire room where the reactor sits is off-limits for personnel during operation and possibly for an hour or so afterward. I assume the concrete walls of the room will take care of any x-rays and neutrons that do escape the shielding.

All of this is important: In order to gain widespread acceptance and avoid overly restrictive regulations you need to demonstrate that the amount of radiation that can escape the ‘reactor room’ is comparable to, say, the radiation that gets out of a radiology lab in a hospital.

Yes, the drift tube encloses the ion converter coiled tubing. Since we have a tightly focused ion beam, we may be able to stop most of the stray X-rays with a one or two inch opening. Just musing here- I’m an electronics technician by training.

I fully agree about the safety factor- measurable radiation has to remain below cosmic background radiation in operation, and it has to do that as a passive, designed-in feature. My understanding is that the boron and lead outer shielding layers provide that level of attenuation. I was just hoping I could cut the length of the water jacket about half to around 4 feet, then use that volume to mount much of the support gear like vacuum pumps, fuel system, much of the cooling system, power conditioning, etc. on a square steel tubing frame.

Maybe this system is going to end up between 2 and 4 modules to fit under 14 foot ceilings and avoid the oversized load shipping hassles. Then there’s 14 foot bridges to route around…

[Author]

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