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Shielded Building Construction
Posted: Mon Jun 30, 2008 4:09 am
Someone on the IEC Yahoo board brought up the fact that he did not know how to design a building that was properly shielded from radiation. Well here's one that I am familiar with, The FEL building at Jlab:
the trick is to put the experiment in the cellar and have dummy floor filled with sand on top. A berm helps. Entrances are serpentine with magnetic locks to keep people from entering the machine when operating and for installation there is a garage door that is covered in layers of big concrete blocks for operations. Control rooms and auxilary equipment go on the second floor with serpentine feedthroughs. The radiation from the FEL when operating(20-30 MEV) is higher than anything we would ever see from a BFR.
Posted: Mon Jun 30, 2008 5:41 am
I think MSimon calculated that a few feet of concrete would be adequate shielding for even a D-D fueled Polywell.
Posted: Mon Jun 30, 2008 10:03 am
I was thinking of a berm in front of the garage door. Maybe not a good idea, unless access to the door can be strictly controlled.
For a rough idea: 6" of concrete stops 90% of the radiation (that may not be true - it is roughly true). To go from 1E12 n/sq cm to 1 n/sq cm then requires 12 X 6" of concrete. 6 ft. For a D-D machine the n production is 100X higher. Add another ft of concrete.
We know how to do this.
The tricky part is getting the shielding light enough for a ship. Again consider - even for a D-D machine you only add about 20% to the amount of shielding required vs a fission nuke. By the same token using p-B11 only reduces shielding requirements by about 30% (vs a fission nuke) in terms of material.
Now we may be able to do better than that by tuning the shield to the radiation produced. A BFR is going to produce a narrower spectrum than a fission nuke. Especially when it comes to gammas.
For an aircraft: A long fuselage and superconducting cables to get the power to the motors. Low activation potential cargo near the reactor.
In any case there are folks working on 100 knot ships. That is about 24 hours across the Atlantic. About 3 days across the Pacific. Steer around storms.
Think about what 100 kt ships mean in terms of war fighting. For one thing CVs would not have to turn into the wind to launch aircraft. If the ship has enough maneuvering room to get up to speed, launches can be gentler. Lengthening airframe life. Landings can be safer - reduced delta V between the ship and the aircraft.
Deck activity may have to be automated (or at least "remote" controlled) to allow deck activity with a 100 to 150 kt wind. Set up may be strictly a hangar deck operation.
Interesting set of problems.
Re: Shielded Building Construction
Posted: Tue Jul 01, 2008 6:44 am
I had to look this up... (I know, I know, I'm not enough of a geek... yet...) So in case anyone is curious:
Posted: Tue Jul 01, 2008 8:53 am
For some reason even our local hospital's linac has insane radiation shielded rooms with concrete walls and massive 2ft thick door around a 20ft corridor. Its like going in to norad and and they don't even emit neutrons.
Posted: Tue Jul 01, 2008 1:06 pm
There's a good reason for that shielding. I'm betting the linac has 20 to 50 MeV protons which have a pretty good penetration in most things. It's all the secondary radiation of gammas you need to make sure don't come out when the beam misses due to a magnet failure.
That's the problem with safety - it's not just dealing with what happens when things are working. It's dealing with all the crap that might happen if something fails. It's a lot easier to argue that no matter what, the walls are thick enough than to go thru every possible failure mode.
Posted: Tue Jul 01, 2008 1:16 pm
Its an electron linac with top power of I think 20 something MeV.
Posted: Tue Jul 01, 2008 1:22 pm
Then it's shielding from the secondary gamma's and x-rays. Does sound like over kill though. But it was easy to get approval!
Posted: Thu Aug 07, 2008 2:55 am
In production the first criteria for a containment structure would be one strong enough to discourage men with funny accents and suicidal tendencies from flying planes into it to see what would happen. Radiation containment is a smaller problem by a couple of orders of magnitude. Someone else in this thread pointed out a couple of feet of packed earth would do for the kind of radiation likely to be emitted and I agree with his estimate. The boron fusion reaction isn't all that "hot". Putting the reactor in a chamber 15' underground and capping it with a 2' reinforced concrete cover would probably take care of the likely and expected/unlikely but most unfortunate mishaps imaginable.
(Oh, I wouldn't expect a mushroom cloud if someone flew a plane into it. But it would make a hell of a mess of a $100Million dollar facility and drop 1000-1500MW of power off the grid suddently and for an inderminate length of time. That in itself could cause a cascade of power failures across the country.)
Posted: Thu Aug 07, 2008 3:00 am
All they would have to do is drop aluminum chaff into the distribution station.