Shielding Analysis Of A Small Compact Space Nuclear Reactor

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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DeltaV
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Shielding Analysis Of A Small Compact Space Nuclear Reactor

Post by DeltaV »

Much less power than required for a flying Polywell, but maybe there's something useful here:
Shielding Analysis Of A Small Compact Space Nuclear Reactor ~6.3MB
(100 KWe)
10.0 Conclusion

Two shields were identified in this analysis that are of primary concern for the launching of a space nuclear reactor. A W/LiH* configuration was found to be the lightest weight shield. Its mass was calculated to be 528.39 kg. The SP-100 shield was calculated to be about 681 kg. The difference in mass can be accounted for by noting that the LiH in the SP-100 is in a stainless steel honeycomb, and there is aluminum in the shield for thermal conduction. There is also much more W in the SP-100 shield. Also, the shield has an insulation material which adds more weight. The shield with the smallest volume was found to be a W/B4C** shield with the W at a position of 10 cm from the core. This shield's mass was calculated to be 655.35 kg. The volume for the W/B4C shield was found to be 211,176 cc. The volume of the W/LiH optimum weight shield was calculated to L)e 437,407.84 cc. Since the W/LiH shield has twice the volume than that of the B4C shield, the 126.96 kg difference in mass may be more acceptable when considering a volume constraint for the launch vehicle or the material characteristics of the different shielding materials.
* Tungsten/Lithium-Hydride
** Tungsten/Boron-Carbide

DeltaV
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Post by DeltaV »

Related material copied from the General thread:

"Gamma-Ray Interactions with Matter"
http://library.sciencemadness.org/lanl1 ... 326397.pdf
Table 2-2 shows that at energies above 500 keV the tungsten alloy has a significantly
higher linear attenuation coefficient than lead because of its higher density. Thus,
the same shielding effect can be achieved with a thinner shield. At energies below
500 keV, the difference between the attenuation properties of the two materials is
less significant; the higher density of the tungsten alloy is offset by the lower atomic
number. The tungsten alloy is used where space is severely limited or where machinability
and mechanical strength are important. However, the tungsten material is over
thirty times more expensive than lead therefore, it is used sparingly and is almost
never used for massive shields. The alloy is often used to hold intense gamma-ray
transmission sources or to collimate gamma-ray detectors.

Table 2-2. Attenuation properties of lead and tungsten

...........................Attenuation Coefficient (cm-1).....Thickness (cm)*
____________________________________________________________
Energy (KeV)........Lead...Tungsten**.......................Lead....Tungsten**
1000....................0.77....1.08.................................2.98.....2.14
500......................1.70....2.14.................................1.35.....1.08
200......................10.6....11.5.................................0.22.....0.20
100 .....................60.4....64.8.................................0.038...0.036

* Thickness of absorber with 10% transmission.
** Alloy: 90% tungsten, 6% nickel, 4% copper.
Polywell gammas are expected to go up to 16 MeV, I believe.

taniwha
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Post by taniwha »

DeltaV: thanks for that shielding pdf. It looks like it will be an interesting read. I should be able to do a better (more accurate) shielding design for my ships :)

DeltaV
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Post by DeltaV »

Glad to be of help. Radiation calcs are not really my gig, but, assuming Polywell works, shielding mass seems to be the biggest hindrance to putting it in a flying machine. I'm still hoping for some sort of nanotech breakthrough that results in drastically lighter shields. Layers of different materials seems to be a step in the right direction.

Nik
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Fresnel approach

Post by Nik »

Must wonder if the 'basic' shield could be complemented by a lightweight fresnel arrangement with glancing incidence reflectors.

There's no real substitute to hangin' the Power Section as far up-Sun (*) as practicable...

(*) Get double duty if CME calls by...

DeltaV
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Post by DeltaV »

Anybody know how Borated High Density Polyethylene or Ultra High Molecular Weight Polyethylene compare to lead or tungsten for neutrons/gamma?

http://www.radiationshieldingsolutions.com/index.html
http://www.professionalplastics.com/Bor ... yetheylene
http://www.professionalplastics.com/BORATEDPOLYETHYLENE
King Plasti-Shield® has been used as medical and industrial neutron shielding for more than 25 years. It is a light-weight, cost-effective and easily fabricated solution for a wide variety of neutron-shielding needs. Compared to paraffin-based products, King Plasti-Shield sheets are durable and perform well in a wider range of temperatures.
King Plasti-Shield® is offered in three grades:
Industrial Grade (PURPLE COLOR, 5%* Boron by Weight) W is effective in medical and other applications requiring attenuation of thermal neutrons.
Grade III (RED COLOR, 2%* Boron by Weight) W
Grade II (YELLOW COLOR, 1%* Boron by Weight) W - Virgin Polyethylene (natural color)
Grade II (yellow, 1% boron) and Grade III (red, 2% boron) are used as neutron shielding in the construction and repair of US Navy ships. These products are manufactured to MIL-P-23536, Rev. A. Certification and testing is available to meet MIL-Spec. requirements

Haven't found attenuation specs. Classified?

DeltaV
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Post by DeltaV »


MSimon
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Post by MSimon »

Classified?
Probably. You can work back from B10 loading density. Hydrogens/cc. To get a rough idea.

And since this is more or less out there: the secret to low weight shielding is layering. And getting the order of the layers and their thickness right. For instance B10 gives off X-Rays when it absorbs neutrons. OTOH poly is low melting point. So you have to allow for that.

The most secret thing about Naval Nukes is how the shielding is done. The Ruskies tended to kill their crewmen. The USN did not.
Engineering is the art of making what you want from what you can get at a profit.

DeltaV
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Post by DeltaV »

Thanks, MSimon. Watch your back.

I was reading in a NASA report on space shielding options that they wanted more research into carbon nanostructures loaded with hydrogen (compounds?). One idea was that carbon composites might do double-duty; load-bearing structures and radiation shields.

Maybe boron buckyballs or nanotubes could also be used. It would have to yield a really big improvement, however, to justify the added hassle/expense of nanostructures vs. macroscopic layers.

BenTC
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Post by BenTC »

DeltaV wrote:More info on various shield materials:
http://www-eng.lbl.gov/~dw/projects/DW4 ... r%20PE.pdf
What do they mean be "self-extinguishing"? Something to do with fire?
eg JC227
In theory there is no difference between theory and practice, but in practice there is.

MSimon
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Post by MSimon »

BenTC wrote:
DeltaV wrote:More info on various shield materials:
http://www-eng.lbl.gov/~dw/projects/DW4 ... r%20PE.pdf
What do they mean be "self-extinguishing"? Something to do with fire?
eg JC227
Yes. Anything sufficiently borated will tend to be self-extinguishing.
Engineering is the art of making what you want from what you can get at a profit.

DeltaV
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Post by DeltaV »

Interesting stuff about layered shield design for mobile lunar reactors.

http://www.csnr.usra.edu/archives/2008% ... utpost.ppt (~6.5 MB)

DeltaV
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Post by DeltaV »

http://www.google.com/patents/about?id= ... dq=7384576
Patent for a lead-free shield material containing at least 26% gadolinium.

http://en.wikipedia.org/wiki/Gadolinium
Gadolinium has the highest neutron cross-section among any stable nuclides, 61,000 barns for 155Gd and 259,000 barns for 157Gd.
http://en.wikipedia.org/wiki/Neutron_cross-section
~64 times the cross-section of Boron for thermal neutrons, fast neutrons are not mentioned.

kunkmiester
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Post by kunkmiester »

http://www.chemicool.com/elements/gadolinium.html

$100 a gram bulk. How much you need?
Evil is evil, no matter how small

DeltaV
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Post by DeltaV »

Enough to build my VTOL SSTO space hopper.
So it's a chicken and egg situation. I need the gadolinium to build the space hopper, but I need the space hopper to mine the gadolinium asteroid to get the price down.

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