risk assessment

[frostyk72]

hey guys

i have to write a risk assessment for doing fusor experiment at tafe.
so what kind of risks are involved?

i know there are x-rays at 5kv+
with a fuel source at about 10kv - 15kv there will be fast neutrons.
possibility of vacuum chamber breaking and possibly causing damage if not made strong enough to handle low pressures.
risk of electrocution working with high voltage and amps

have i got everything to write a report from?
any safety tips for overcoming these problems.

have thought of a wall of bricks 2 bricks thick around chamber to stop neutrons, possibly some containers of water with some sort of cooling to prevent it heating up to much. still got to find out how thick i will need chamber to stop x-rays.

any help would be great.
thanks in advance.

steven w

[Steven Sesselmann]

Steven W.

In order of priority, your risks are..

Electrocution
X-rays
Gas explosion (Hydrogen)
Vacuum Implosion
Gamma rays
Bankrubtcy
Neutrons

Take care, be pedantic with all electrical wiring, always use the same colour cable for your earth and HV if possible, and double check everything before you turn the power on.

Use a small sheet of lead plumbers flashing, between you and any x-ray source (under 50 kv), X-rays under 20 kv are really only a problem where they can shine through glass. A geiger counter or ion chamber proportional counter is absolutely essential.

Don't forget that Deuterium if you get some, is really just another Isotope of Hydrogen, and reacts violently if ignited in the atmosphere.

Shield against implosion of glass vacuum chambers if you use one, polycarbonate would be the material of choice

Gamma rays can be an issue if you are successfully achieving high fusion rates.

Personal bankruptcy while chasing illusive neutrons can be a significant risk.

Neutrons are highly unlikely to be a risk, even the most successful fusors running at high volts and amps, only put out around 10^6 neutrons per second, which are unlikely to harm you unless you sit on top of the fusor for a long time.

Your educational institution would rather not know about neutrons, as the word will almost certainly result in a big NO.

Steven S.

[frostyk72]

thanks for reply steven

well i dont know how far will be able to push it.
i'm sure if we are under the threshold for neutrons we should be fine. to fiddle.
aim is to just to get a little glow happening for starters.
what is the voltage threshold for fusion in a fusor?

vacuum chamber will probably be steel. with a small view port for camera.

what sort of gas would you recommend for least explosive, and good to make a good glow for starters.
maybe good fusion later if i am allowed.

shielding against gamma rays?
what would you recommend?

bankruptcy, well havent got much money to start with so thats fine.

and yeah will be double checking everything and i'm sure teacher who helping me will do the same.
vacuum pump and diffusion pump should get here next week.
all i need now is vacuum chamber, dad said he will build me one , will be small but should work

thanks again

steven w

[Tyler Christensen]

The voltage level necessary to actually hit fusion is not a hard number, I've detected trace fusion as low as 15kV with extremely sensitive neutron detection equipment (2 foot 3He tube), however odds are you'll needs 25kV+ unless you start out with large 3He tubes.

For just plasma, you can use any gas really, most people in a demo fusor just run atmospheric air in which works fine. You can also use any inert gas which will not be flammable, such as argon. To actually perform fusion, the only fuel is deuterium which can be treated as hydrogen as far as flammability goes (aka very explosive in an oxygen mixture).

I don't think gamma shielding is a big deal especially if you're at a bit of a distance, but if you were concerned about it, thick lead shielding should suffice.

[Steven Sesselmann]

Steven

For a nice glow discharge, and to study plasma through your viewport, air vacuum will do just fine, no need to introduce any special gas.

Allthough for most of us Air is a pretty special gas

Steven S.

[frostyk72]

well if i can get plasma in air will just do that for starters.
that will lower risk alot

[bk8509a]

It will lower risk, and money.

Check out number 6 on that list: "Bankruptcy".

[Doug Coulter]

Yes, I was rolling on the floor over that number 6 one....this is a hobby for the rich or the very resourceful, or both, for sure.

You don't need real thick lead to stop power supply energy photons (X rays), here a couple mm does that just fine. If you get to copious fusion, you'll just want to stand back farther, as the amount of lead needed to stop the really hot gamma rays from the rare (1:10,000) DD->He reaction is in the inches range.

Here's a guide the radiologists use for shielding.

[bk8509a]

Steve,

I don't know how good you are at math, but here's ALL you need to calculate x ray safety from THE most reliable source when it comes to standards: NIST.

http://www.nist.gov/physlab/data/xraycoef/index.cfm

Everything you need is there. I have a paper on X ray shielding that I gave to my school in order to use the fusor, which I can give to you, but it's much more important that you do these calcs yourself, if you have the know-how.

You just need to understand
N0=NI e^-x

[Doug Coulter]

Well, that's interesting indeed and does describe how to attenuate X rays by some factor.

But it has nothing whatever to do with getting them to a safe level for humans -- that level (which is variable vs energy) isn't even mentioned.

Nice to know how to attenuate by factor 100, for example. But if your source is say, 10,000 times what's good for a human -- does that do you enough good? Not mentioned at the above source, which is why I quoted from a source of *human health* radiology that also figures in what's required to keep you healthy. According to radiation doctors anyway -- they may or may not know something about X ray effects on human health. NIST simply does not address that at all.

Not as informative on pure attenuation, but it's what it takes in an average hospital X ray room (which runs a lot more power than we do for a lot more hours per day, and generally at higher voltages), for people exposed day in and day out, and so is pretty conservative. In practice, the table I quoted above is what we use, and is what it takes to make my survey meter read as though the fusor wasn't on even right at the shield. A lower threshold scintillator detector still can see if the fusor is on, meaning some low (sub 5kv) x rays still get out. As the NIST source indicates (and so do all the physics books) X ray absorption is not a one-shot and a certain thickness of substance X won't stop them all, but tends to down-scatter X rays in energy and finally stop some, but not all completely, no matter how thick, in theory.

I half expected Richard to jump in here and say something along the lines of "don't worry, be happy, below about 30-40kv the fusor shell gets them all" which would also be more or less correct. Since he didn't, I am.


Sorry Brian ;~) The NIST source is definitive, but not fully useful unless you take human dose tolerances into account. In practice -- we don't run like a hospital in any of power, voltage, or duration, so we in general are safer and their safety procedures are fine for us. The attenuation ratio is of interest, but incomplete where human exposure is involved....

[myID]

Hi Doug-

I agree with the "shell gets them all" BUT:
Xray "field" around my fusor measured with a sensitive counter (TBM-3S) is "not nice" even at voltages above 20kV - not only in "shine direction" of the viewport.
I guess it does not penetrate the shell (around 10mm stainless) but shines through every gasket...
Distance, time, so no worries,... but I would recomend at least a "shadow cone" lead shield still.... Lead sheet is cheap and super easy to work with so go for it!

Greets
Roman

P.s.: The TBM-3S is scary- EVERYTHING is radioactive I was sure the piece of Trinitite I have is not radioactive- since I have the TBM I know it IS (at least sort of...)

[Doug Coulter]

Yeah, I have to agree. BillF who has a super sensitive scint counter that seems to start at about long UV range photons, still can see some even through the lead when I run. But I feel like if a pretty sensitive geiger that is at half scale on cosmics (full scale pegged at some other fusor's labs, no names) doesn't show anything, it's probably OK. As I mentioned, I did put lead, about 2mm over the whole affair (heck of a job too, a lot of cutting, soldering, etc), and don't see stuff out of the gaskets (all CF on that system). But do have leaks around things like ceramic feedthroughs and such. Some of that I can cover, but the rest I simply have aimed to outdoors with no way to get into the field myself. Eg, all the X ray leaks are on the back with the high voltage, and that's just not a place to go when running -- the HV is much more instantly lethal anyway.

While "outdoors" may mean something kinda nasty if you're in a city (the apartment next door perhaps), where I am you'd have to be trespassing well inside my borders to get a count at all -- So, no worries here on that kind of thing. 50 acres also makes a nice fume hood for casting Cd alloys....just stay upwind.

[Richard Hull]

A scintillation counter is just no good in x-ray dosimetry. The final arbiter about x-ray intensity and exposure is a thin aluminum windowed ion chamber which allows most x-rays over 10kv to enter the chamber.

Most of us are proof against the X-ray issue due to the inverse square law alone, provided our view ports are not facing us or any potential audience. It looks at how much actually air is being torn apart by any supposed X-ray blast.

Remember, at 40kv applied, a very small amount of the X-radiation has an energy of 40kv at the instant the x-ray photon is produced. A 40.00 kv fusor is not a real source of 40.00 kev x-rays.

While a well designed x-ray tube in continuous operation at 40kv - 10ma is a frightfull thing, the fusor is not an x-ray tube and, by design, in common use, is a terribly rotten emitter of that pesky radiation.

Finally why isn't this valuable discussion in the Radiation forum???

Richard Hull

[Doug Coulter]

Yes, without sophisticated analysis, a scintillator is a pretty bad way to check for human-harming X rays. I note that BTI does make a scintillator-based dosimeter, but it needs a computer in it to match what the PHA from the scintillator shows to actual dosage. Just a counter -- no good -- thresholds are always too low, and a count is a count whether it's a "hot one" or something safely ignored. If it doesn't light off one of my thin-window geiger sensors (eg ion chamber with gas gain), I'm not worrying.

And unless there are for some reason a bunch of heavy metals in the fusor where the electrons hit, yes, it's a lousy DC to X ray converter as the K,L lines are fairly low energy for the main components of stainless steel. Thank heavens! So don't make the tank out of tungsten! I've used both Ti and W for grids and don't notice much difference there, for what it's worth. Seems like the high energy electrons mostly hit the tank walls, not the grid.

I suspect that people who still need to ask this question aren't yet hip enough to put it in the correct forum. (or use the search function as this one's been done to death here already).

Didn't you do a good FAQ on this one awhile back, Richard?