Excellent Paper on Neutron Shielding

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Andrew Robinson
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Excellent Paper on Neutron Shielding

Post by Andrew Robinson »

While not critical in our application (neutron shielding), my work and professors in the past have drilled the importance of shielding into my head to the point of pure habit. As a result, I now shield some of the silliest things haha. I started thinking about neutron shielding and had a neat (albeit not original) idea. We use standard concrete blocks for our above ground reactor which is great for aBy, however the attenuating properties of concrete are not as friendly for neutrons. So the thought was to make boron doped concrete blocks similar to those used in industry. Thankfully I found a very nice paper detailing the exact same tests with proportions correlated with results. Should be interesting to test my own mix proportions. I plan on building some break apart molds that will form the block in an interlocking fashion similar to the modular concrete shielding used today in the field. I'm curious to see how effective something like quickcrete is structurally after mixed with additives. Should make for some interesting tests.

Here is the paper for anyone interested in some general reading, or for conducting their own tests.
Turgay Korkut.pdf
Gamma and neutron shielding
characteristics of concretes containing
different colemanite proportions
(295.79 KiB) Downloaded 451 times
Also just a quick Solidworks concept
Boron Concrete Block 1
Boron Concrete Block 1
Boron Concrete Block 2
Boron Concrete Block 2
Boron Concrete Block 3
Boron Concrete Block 3
Boron Concrete Block 4
Boron Concrete Block 4
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Re: Excellent Paper on Neutron Shielding

Post by Richard Hull »

I think there is a FAQ on shielding that I wrote a while back. The shielding would stop virtually all radiation from X-rays to neutrons from a fusor. This assumed a fusor that presented a genuinely serious x-ray and neutron hazard such as one operating in excess of 60-70kv. (virtually non-extant in the amateur world).

I presented it as a "shadow cone shield"

viewtopic.php?f=31&t=5783&p=34227#p34227

The shielding offered up here is more or less standard fare for intense neutron environments where there are real flux levels of some concern.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Excellent Paper on Neutron Shielding

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Thanks Richard and I totally agree. This concept was intended for use in much more serious environments. I completely agree with the shadow cone design as well. That is what I am most used to designing and I already have plans in place for some similar and more basic shielding techniques to be integrated into the reactors design and carriage system. This is thrown out simply to spark some discuss and enthusiasm on the subject.
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Re: Excellent Paper on Neutron Shielding

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Richard Hull wrote:serious x-ray and neutron hazard such as one operating in excess of 60-70kv.
Hehehe, wait till you see our progress on the mutant, overkill, Tesla inspired supply. It's quite scary :)
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Re: Excellent Paper on Neutron Shielding

Post by Steven Sesselmann »

Andrew,

It is indeed a good idea to make shielding part of any high voltage experiment, and if you consider the shielding right from the beginning, it is much easier and cheaper to incorporate it. Start by considering where you locate your apparatus, ie. What is behind the wall behind the devise? , is it someones bedroom, or is it uninhabited bushland?

By choosing an appropriate corner to build your device you may only need to shield two sides, in order to make it safe. Then consider what kind of radiation you expect to generate, and choose appropriate materials for the shielding, to sufficiently attenuate the radiation.

As Richard has pointed out time and time again, the neutrons are unlikely to be a risk, unless you are the "Lucky Donkey" as he calls it, who achieves runaway fusion, and it is well documented how that won't happen with a fusor.

X-rays are by far the biggest problem, most of which come from electrons or cathode rays slamming into something hard, usually a stainless steel flange at ground potential, and these x-rays simply have the same energy as the potential voltage difference between the cathode and the anode, sure you can get some secondary X-Rays from fluorescence, but I have done experiments with scintillation detectors and a gamma x-ray spectrometer to show that the x-ray peak generated is right on your voltage setting.

Just remember that the electrons come from the cathode, but it may not be so obvious where the x-rays come from, the electrons will find the shortest way to ground even if this means going around a corner, and it's where they stop, you get an x-ray, I found this out the hard way, so if you are using partial shielding, make sure it's in the right place.

I agree Borated concrete blocks should provide an economic shield if you have the space to lay them. A simple way to test the x-ray attenuation of your concrete blocks is to use a collimated Am241 source from a smoke detector, and a geiger counter, and simply do a count with and without the concrete block between the detector and the source. Am241 emits a gamma ray at 59.6 keV which is in the same region where x-rays from fusors become an issue.

Steven
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Re: Excellent Paper on Neutron Shielding

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Great advice from Steven!

X-rays form a continuum with the bulk of the energy at the low end. Thus fully 90%++ of the x-rays generated within the device never leave the fusor as they cannot penetrate the shell. Those that do, and they are plentiful, are nasty provided you are over about 35kv applied. The fusor is an x-ray filter and only the most energetic of the x-rays shine through. Unless you are way up in fusor applied voltage, ( 80kv+) the simplest of lead shields should be effective. 1/8th inch, (3-4mm) of lead sheeting will stop almost all the X-rays that will bother you or attenuate them to the level of insignificance. At 80kv, placement of your device gets critical, as I have noted in my FAQs and Steven just re-iterated. Neutrons at this level will force a wax-boron shield. Remote operation (15+ feet) is advised with video observation.

Few, if any here, will ever need this sort of overkill.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Excellent Paper on Neutron Shielding

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Maybe I have been misunderstood. I am not recommending this for fusor shielding, nor will I be using it myself as it is blatantly overkill lol. Its just a neat concept that will be fun to experiment with and to spark discussion here on the fourms. I'm particularly looking forward to the chemistry and structural aspect of the block designs at different concentrations and thicknesses. Yes the results are right there in the paper, but then again how many people are truly doing original research. I do however appreciate all the feedback and suggestions for realistic shielding and have annotated them for our actual reactor which is obviously still under construction. I really value your guys expertise and appreciate the time taken to read and respond to my posts. Thanks again all!
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Re: Excellent Paper on Neutron Shielding

Post by Steven Sesselmann »

Andrew,

Sorry, that we went off topic, but as always, some safety advise comes naturally. Your block design is neat, and worth a try.

As someone who was a dental technician in a past life, I am interested in what material you have in mind for the mould, obviously it has to be something which will let go of the cement easily and which can be reused.

If I was to attack this problem, I would probably go old school make a model of the brick from wood first, but the younger guys out there would probably spend $3000 on a 3D printer before daring to chop into a block of wood, which when you think of it can be pretty scary if you only know how to operate a keyboard.

Then I would most likely make a two part mould from fibreglass mats and resin over the wood mould, and leave a flange around to clamp the moulds together.

From that one should be able to cast at least one or two borated concrete bricks per day.

The high water content of concrete makes a good moderator for neutrons, and the density of the brick will attenuate X-rays up to a certain energy as well.

Steven
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Re: Excellent Paper on Neutron Shielding

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S_Sesselmann wrote:Sorry, that we went off topic, but as always, some safety advise comes naturally.
Oh no need to apologize. The more safety that can be put on the site, the better. Everyone should be reminded of safety as often as possible. In fact, generally the people who have been around the block before are the ones that kick safety to curb. I always welcome safety discussions and soak up everything, possible. Thank you.
S_Sesselmann wrote:Your block design is neat, and worth a try.
Thank you. Should be interesting to try. As I said above, its not a new idea by any means. I am actually modeling the design off of a proven design (So I cannot take credit) used in industry for shielding. Where the interesting part for me comes in is seeing how the additives effect the concrete. I really know nothing about concrete. I'm curious if special mixes are required or special concretes considering the acidic nature of the additive.
S_Sesselmann wrote:As someone who was a dental technician in a past life, I am interested in what material you have in mind for the mould, obviously it has to be something which will let go of the cement easily and which can be reused.
Very good point, and all part of the learning process. I have no idea. That's part of the fun. I have absolutely no prior experience with mixing or setting concrete.
S_Sesselmann wrote:If I was to attack this problem, I would probably go old school make a model of the brick from wood first, but the younger guys out there would probably spend $3000 on a 3D printer before daring to chop into a block of wood, which when you think of it can be pretty scary if you only know how to operate a keyboard.
Hmm interesting idea. I do actually have access to a 3D printer. I had not even considered making a block first in order to build the negative. I was planning on just jumping right in and building the actual mold from scrap. My first thoughts were a steel hinged mold with angle iron to build the edges, but I was not sure if it would be possible to coat the steel in something to prevent the concrete bonding.
S_Sesselmann wrote:Then I would most likely make a two part mould from fibreglass mats and resin over the wood mould, and leave a flange around to clamp the moulds together.
So you think fiberglass will be the best material to inhibit the concrete from setting to the mold? Any other materials? I need to get some civil eng friends :)

Thanks again for the reply! Sorry for my late reply. Work got a little tied up.
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Re: Excellent Paper on Neutron Shielding

Post by Steven Sesselmann »

Andrew,

Cement will probably stick to fibreglass if you let it dry completely, but usually when you are casting into moulds like that, you remove the mould after 6-12 hours, after the cement is hard, but while it is still moist.

You can also use a silicone release spray if necessary, even a bit of old sump oil would do the trick

Steven
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Re: Excellent Paper on Neutron Shielding

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Mam's a civil engineer...If you want I can get her to read the post and offer any advice?

Tom
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Re: Excellent Paper on Neutron Shielding

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Sure, all comments are welcome. What we could really use is a concrete/cement expert lol.
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Re: Excellent Paper on Neutron Shielding

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Andrew Robinson wrote:Sure, all comments are welcome. What we could really use is a concrete/cement expert lol.
Seriously guys, mixing concrete is not rocket science, sand, cement, gravel, then add water, hardly need for an expert or a civil engineer, unless you are building bridge, and need to know the specific strength of the material. You can even buy ready mix concrete in bags, just add water.

For neutron capture add a few scoops of borax..

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Re: Excellent Paper on Neutron Shielding

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The cause for question is due to all of the reports (particularly within the nuclear industry) of boric acid leaching and degrading concrete structures. Upon mixing concrete with water, depending on the cement chosen, there could be materials in the mix that may react negatively to the addition of boric acid. Depending on the concentration added it may adversely affect pH which could effect the structural integrity of the cement. You're right, it's not rocket science, but at the same time, it still merits discussion from someone who has had a little more experience in the area. As I have said above, I intend to at some point just experiment with mixes to see the effects first hand, but until that time comes, discussion is cheap and can't hurt.

In my opinion, anything worth doing is worth doing right. No harm in analyzing all aspects and details.
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Re: Excellent Paper on Neutron Shielding

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In my shadow cone device FAQ I opted for the thin lead in front to stop x-rays, parafin or polyehtylene in the next layer to thermalize and simple store bought boxes of Borax to back that up and absorb the thermal neuts. The concrete issue is just for really big operations and way overkill for a fusor, even a big amateur fusor.

We should have discussed this at HEAS in more detail.

Richard
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Re: Excellent Paper on Neutron Shielding

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Just on the subject of borax shielding, some time ago I was testing some of my neutron detectors at the University of Sydney third year physics lab, where they have a 10^6 AmBe neutron source, and we discovered that a 25 kg (50 lb) bag of industrial borax between the source and the detector almost completely stopped the neutrons. Not much more than background behind the bag.

Granular Borax in bulk is fairly cheap, about a dollar a pound.

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Re: Excellent Paper on Neutron Shielding

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Thanks Richard, but I think we may be going in circles haha
Richard Hull wrote:I think there is a FAQ on shielding that I wrote a while back. The shielding would stop virtually all radiation from X-rays to neutrons from a fusor. This assumed a fusor that presented a genuinely serious x-ray and neutron hazard such as one operating in excess of 60-70kv. (virtually non-extant in the amateur world).

I presented it as a "shadow cone shield"

viewtopic.php?f=31&t=5783&p=34227#p34227

The shielding offered up here is more or less standard fare for intense neutron environments where there are real flux levels of some concern.
Andrew Robinson wrote:Thanks Richard and I totally agree. This concept was intended for use in much more serious environments. I completely agree with the shadow cone design as well. That is what I am most used to designing and I already have plans in place for some similar and more basic shielding techniques to be integrated into the reactors design and carriage system. This is thrown out simply to spark some discuss and enthusiasm on the subject.
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Re: Excellent Paper on Neutron Shielding

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S_Sesselmann wrote:Just on the subject of borax shielding, some time ago I was testing some of my neutron detectors at the University of Sydney third year physics lab, where they have a 10^6 AmBe neutron source, and we discovered that a 25 kg (50 lb) bag of industrial borax between the source and the detector almost completely stopped the neutrons. Not much more than background behind the bag.

Granular Borax in bulk is fairly cheap, about a dollar a pound.

Steven
Thanks Steven, excellent addition. This is the kind of discussion I was hoping for. You wouldn't happen to have any recorded experimental data would you, ie was this an official test, or more of a "just for kicks" kind of quick observation?

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Re: Excellent Paper on Neutron Shielding

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Since this is open source........What kind of harsher environment is this concrete shield intended for? What kind of neutron source that you could have would require this? The circle stops here. You are working on a massive source. I finally understand. What is it?

Richard Hull
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Re: Excellent Paper on Neutron Shielding

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I am in talks with some good friends out in NM at Sandia and UNM that said they would be willing to take some measurements for me. I plan on going out there to visit some friends early next year. Since I'll already be there, gives me the opportunity to play with some fire :) Not sure what they have in mind yet as a test source. ACRR would be cool, but don't know if any of my friends have access to that lab. They have something in mind to surprise me with I'm sure.
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