Water as Neutron Shield

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Jason C Wells
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Water as Neutron Shield

Post by Jason C Wells » Fri Jul 22, 2016 6:06 pm

I saw Mr. Hull's post on the super fusor location. That caused me to think of neutrons. So I went searching the forums to see if anyone mentioned using a water tank as a neutron shield. I didn't find mention of water for neutron shielding. Your rich Uncle Sam puts a lot of credence in using water to shield neutrons for his toys. If such a thing as solid hydrogen existed, it would be optimal for moderating neutrons. Liquid water is about as close as we can get, and it's cheap.

So, why no water moderating neutron shields in fusor land? Is the production rate not large enough to warrant neutron shielding?

I read some comments about using boron bearing compounds here so there is apparently some interest in neutron shielding.

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Jason

JohnCuthbert
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Re: Water as Neutron Shield

Post by JohnCuthbert » Sat Jul 23, 2016 1:55 pm

Water does not play nicely with high voltages.
It's a pity- because water is cheap and effective.

Dan Knapp
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Re: Water as Neutron Shield

Post by Dan Knapp » Sat Jul 23, 2016 6:39 pm

Water is one of the best materials for neutron shielding and one of the most used materials. Borated water (water saturated with boric acid and borax) is even better. The liquid nature can be a disadvantage.
"Solid" water and borated water can be made with water extended polyester, a resinthat can be mixed with up to 60% water and hardens to a plaster-like consistency. It has lower total hydrogen concentration than water but has the advantage of being a solid.

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Jason C Wells
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Re: Water as Neutron Shield

Post by Jason C Wells » Sun Jul 24, 2016 3:02 am

Seems like keeping the water separate from the electrons should be doable. You don't have to immerse the gear. You could make a donut shaped tank. All the metal parts stay nice and dry inside the donut hole.

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Richard Hull
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Re: Water as Neutron Shield

Post by Richard Hull » Sun Jul 24, 2016 9:43 am

Water shields without dissolved borax are just moderators. (prevents hot beaming)

Water is a nasty thing to contain and weighs a lot per unit volume. A leak in anything less than an ideal tank or vessel will flood your home or lab work area while you sleep or are at work. In short, water is a terrible amateur fusor shield, in practice. (impractical).

I have a FAQ all about practical neutron shielding. For a controlled amateur lab space, (no humans within 30 feet except for the operator), a shadow cone shield is all you will ever need for serious x-ray and serious neutron protection. A box of wood of a proper size, (as small as 12 inches by 12 inches and 3 inches thick), with molten parafin mixed with borax cast into it and a lead plate of 1/4" thickness on the face of the box, placed within inches of the fusor can supply a large area shadow cone of safety for an operator within 6-8 feet from the hard driven, radiation producing fusor body.

I do use a pure water moderator for my 3He neutron detector tube made from a 26 inch length of 6" PVC schedule 40 pipe with a pressure cap base. (Seen in many images of fusor IV.) As it is in my unheated lab, I do add about a pint of auto anti-freeze to the water. Before a major run, I just pour in fresh water to replace any that has evaporated over time between infrequent runs. Anti-freeze, effectively, never leaves or evaporates.

Check the FAQs for my shadow cone exposition.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Water as Neutron Shield

Post by Dan Knapp » Sun Jul 24, 2016 1:06 pm

Water alone is used as a neutron shield, but it can require a considerable thickness depending upon the neutron energy and the desired attenuation. A disadvantage of a pure water shield is that ultimately the neutrons undergo a capture reaction with hydrogen, which yields a hard to shield energetic gamma. Concrete is often used as a "solid" form of water, but typically concrete neutron shields are very thick (as much as four feet thick). People at the Jefferson Laboratory developed a concrete that uses boron carbide in place of sand and aggregate (U.S.Patent 8,450,707). When I visited Phoenix Nuclear, they were using stacked plastic water containers for shielding D-D neutrons. The (approximately) rectangular containers enable stacking with relatively few gaps. A good, relatively inexpensive solution for the amateur would be to stack plastic containers filled with saturated borax solution. Saturated boric acid (more expensive, but can be found on the web in bulk at reasonable cost) solution gives a higher boron concentration, and both are used in "borated water" shielding.
Interestingly, even wood (cellulose [CHO] and water) has been used for neutron shielding. Wood was used in the early years of operation at the Bevatron at the Lawrence Radiation Laboratory and found to be as effective per unit length as concrete for shielding intermediate energy neutrons.
A question is always "how thick does it need to be?" There is no simple answer, it depends upon do the neutron energy and how much attenuation you want. One can pick a desired attenuation value and consult published plots of attenuation vs. length. Hand calculations can be used for a gross estimate, but good numbers require complex computer calculation.
To further complicate things, one must not only consider line of sight exposure, but also bounced neutrons from "sky shine" and "groundshine" reflections.
The bottom line is that a shadow cone shield as Richard suggested is probably the best bet for the amateur. The neutron shielding could be borated paraffin (or plastic) or the cheaper stacked plastic containers of borated water.

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Richard Hull
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Re: Water as Neutron Shield

Post by Richard Hull » Sun Jul 24, 2016 9:49 pm

An (amateur) fusor at 60-70kev will not fuse enough to make a neutron flux of any significance, even without a shield if operated at a range of 12 feet. This same fusor as operated is a very nasty x-ray emitter and a lead shield, alone, would be all that might be demanded. Amateurs never run their fusors for any considerable period to receive any kind of serious acute neutron dose and never run their fusors frequently enough to get a worrisome chronic neutron dose. A fusor producing 1 million neutrons, (2million fusions), per second, (isotropic), is no radiation threat to an operator 8 feet distant from the reactor vessel. This is from x-rays or neutrons.

Nervous nellies and all, (100%), of government, academic, and museum organizations with the ALARA mantra and worry regarding litigation will have shielding no matter what!! ALARA = safe and tight assed. Smart folks, who know radiation and are free of ALARA brained oversight, know when they need shielding based on real honest science and common sense.

Unless you operate over 40 kv applied, x-rays are not even a concern. It is hoped that all amateur fusioneers operate their fusors from a minimum of 6-8 feet from their fusor shell to their bodies. My gas metering and D2 valves need rather frequent adjustment until stable operation is achieved and all are within 2 feet of the reactor shell. I turn the HV off before making adjustments.

Shielding is for super fusors that are run frequently at large power levels for protracted periods of time.....Something know one here has built yet or operated in such a manner.

I feel very sorry for anyone in one working a fusor in the ALARA oversighted environments. VCU, (Virginia Commonwealth University), operated its fusor at ultra low levels (30kv applied) for a year and a half. Their CHP, (certified health physicist), exploded and all activity has stoped as they are now forced to go, hat in hand, and raise fresh monies to spend 1.7 times what they spent on the entire fusor project with all the NIM based rad gear and vacuum system to build a custom shield and pass muster from the ALARA authority so they can resume operation. Stupid and sad, but it is their obligation to do so in an academic environment.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Water as Neutron Shield

Post by Dennis P Brown » Wed Jul 27, 2016 12:39 pm

Water(?), really! Why in the world would this even be mentioned when large, thick paraffin sheets are available and cheap? Paraffin is far more effective against all neutron energies and is very light and a great insulator. Also, easy to heat and bend or even melt in order to form complex shapes to fit around equipment. Sorry, but besides being heavy, water isn't easy to contain (plastic jugs are easy to break when filled/stacked and tend to create a leaking hazard that would a nightmare around an operating fusor ... overall, just an overly complex approach for a home fusor (and just adds another possible electrical danger.) As for borated paraffin ... really, these are not fission reactors we are talking about here (lol.) Gamma from a stopped neutron via a fusor just isn't a flux that would pose the slightest danger.

As for x-ray shielding - I've talked about this before. No need for lead but slate panels are available from Home Depot, are extremely cheap, easy to cut (and drill), are great insulators (can be in contact with hot wires and no danger), very dense and best of all, non-toxic. While one needs the exact same total weight, of course the slate shield is thicker - will need 4 mm of slate for evey 1.0 mm of Pb to get identical shielding protection from x-rays.

People need to think outside the box (heads up to Mr. Wells for the joke) and stop following old, outdated ideas.

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Re: Water as Neutron Shield

Post by Dan Knapp » Fri Jul 29, 2016 12:48 am

The paraffin I've seen for sale is not particularly cheap. But worse, its a fire hazard. No environment with safety regulation is likely to approve use of any significant amount of paraffin shielding. When paraffin burns, it melts, and you then have liquid fuel. It does have slightly higher number density of hydrogen than water. If that is if significance to you, and you feel you need hydrocarbon shielding, polyethylene is a safer (although more expensive) alternative. Borated polyethylene is a commercial product that is extensively used for neutron shielding. Borated paraffin is not.
On what data do you base your advocacy of slate shielding for gamma radiation? It is only 4-8% iron oxide; the other significant constituents are relatively low mass silica and alumina. Based upon its elemental composition, one would not expect it to be a very good shielding material for gamma radiation.

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Dennis P Brown
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Re: Water as Neutron Shield

Post by Dennis P Brown » Fri Jul 29, 2016 9:51 am

First, water around high voltage is safe but paraffin isn't because in the event of fire, it can burn? Sorry, that doesn't make sense since electrocution is, without any doubt, the number one hazard I see with a fusor or a sim-fusor. Also, exactly what safety regulations are there for the average home fusor? Are you serious or forget this is a fusor forum for home based fusors? Yes, a few exist at schools (and dealing with fifty states and their regs is far, far afield here) but they do have standards and they can cover a huge range depending on the state or even country (lol.) These issues just are not relevant for this forum to deal with and frankly, not appropriate. Besides, if paraffin is so very dangerous how in the world can deuterium gas ever be used? The extremely wide explosive range of deuterium gas in air is an extreme hazard that dwarf's the idea that paraffin is a a hazard because it can burn (talk about safety risk; lol.)

Relative to stopping neutrons paraffin is excellent and readily available to the average home buyer - since a neutron flux is at issue, seems cheap compared to not having shielding. Also, borated polyethylene is a higher end commercial product that appears to be sold in rather large sizes (steep price) focused for commercial users, just from what I have seen. Not priced for a typical home fusor setup/application (a rather small area of coverage needed); as such, small paraffin sheets are available from McMaster-Car at a very reasonable price (under $60 for a sized sheet that can cover the main threat area of a small fusor.)

Are you suggesting we, here, cover neutron shielding issues for use in professional labs? If so, that is completely different; I am only interested in a home fusor application that make sense relative to this forum and I will always base my advice on this critical fact.

Next, you miss read my post on the gamma ray issue - I never said slate is useful for stopping gamma rays, only x-rays and by default, I mean in the energy range of a typical fusor. What fusor has gamma ray issues? This is a forum for fusors and people tend to forget this when discussing the full scope of the technical field - commercial labs can buy large quantities of very radioactive substances, build/operate machines that produce hideous levels of gamma's, but the average fusor person can't - very different situation and radiation problem. However, since you are very concerned about safety, slate is significantly superior to lead in that regards, no? Slate is non-toxic (that matters a lot), extremely cheap - especially so compared to metals; also, easy to install and avoids electrocution hazards. Also, unlike concrete, no molds are needed or mixing. While this is getting old, my points are relevant for the average home user NOT commercial users where regs and dangers are very different.

In no case does water make any sense what-so-ever for a home based fusor; besides the fact that the neutron flux of most fusors do not warrant shielding, all such fusors have extreme electrocution hazards associated with them, no? Putting large amounts of water next to that danger, for me at least, seems far worse then a possible fire with paraffin - I guess home cooking oil near a gas stove is even a greater risk compared to that ... wonder if that can be approved by the safety people (lol.)

No matter, if the flux from a person's home fusor is enough to worry about, then an easy to buy from company that sells to the average person in small quantities makes far more sense to me. But, to each their own.

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