MadSci Network: Physics

Re: would a nuetron bomb destroy buildings due to thermal stress?

Date: Mon Jan 7 14:45:16 2002
Posted By: Scott Kniffin, Nuclear Engineer, Orbital Sciences Corporation
Area of science: Physics
ID: 1008800516.Ph


Good followup.  The answer is yes and no.  Near ground-zero, there will be
significant thermal stress and some fires are definitely expected.  However
the tactical significance of the bomb is seen at much greater range where
the radiation passes though targets and "disrupts" the "biologicals" in the
targets.  This is jargon for kills people and leaves the building standing.
 So why is that, you ask?  Here's a great example from one of my old text
books, Basic Nuclear Engineering by Foster and Wright (p146).  

The old (1928/1937) unit of radiation exposure was the Roentgen (R) defined
as the quantity of X- or gamma- radiation interacting such that 0.001293g
of air (1cm^3) will contain 1 esu of electrical charge (either sign) or, if
you prefer, 2.58x10^-4 coulombs/kg.  Thus 1 R after the proper conversion
factors equals 87.7 ergs/g of air.  What does that mean in terms of
something physical?  It takes 85.5 ergs to move an wooden #2 pencil 1/16 of
an inch.  In terms of thermal energy, this is trivial.  

Now with people, what form the radiation is makes a significant difference
in how it affects you. The unit for exposure was the rem (now old due to
the use of the Seivert, but since I have a reference handy, you get rem) or
the Roentgen equivalent man.  Rem represent the biological effect on a
person.  Various types of radiation have different quality or Q factors. 
From Introduction to Nuclear Engineering by Lamarsh (p. 404) Q factors for
X, gamma, and most beta particles is 1, alpha particles are 10, neutrons
range from 2 to 11 (if the energy isn't specified for the neutrons, you
assume a Q factor of 10).  Ok, so what?  Glad you asked.  The acute dose
LD50-30 for humans is about 450 rem.  This means that 50% of the people
exposed to that dose will die in 30 days (assuming no medical treatment). 
When you start talking about the ammount needed to kill people, with the Q
factors thrown in, the total thermal energy needed is still trivial.  If
all the energy were from gammas, the energy equivalent of 450 rem is that
of moving a pencil just over 28 inches.  Lets assume it's half neutrons,
then 450 rem is only moving the same pencil just over 15 inches.  All
neutrons?  Not quite 3 inches.  Yikes.  

Obviously, for some distance around ground zero, the dose will be amazingly
high (LD100-0) and will taper off over distance and there will be shielding
effects for things like big buildings, and hills.  But there is a second
little factor involved: secondary radiation.  As the neutrons and gammas
pass through matter (including air), they produce secondary radiation that
adds to the sum of the radiaiton in the area.  Gammas tend to make betas or
more gammas; neutrons however, will produce just about everything (alpha,
beta, gammas, protons, recoil nuclei, and combinations of the above).  This
ends in the target receiving several hits for the price of one produced by
the bomb itself.  That is the point of a neutron bomb unlike its cousin the
fission/fusion bomb who's purpose is to get hotter than the sun even if
only for a little while.  Not the most happy thinking there is, but that's
what it's all about.  

As for structural stress, yes there would be effects but in the time frame
of a war, these would be trivial as well.  You don't care if the building
collapses next month, you need to get the data/secret stuff/whatever out of
the building this week.  

I hope that adequately answers your question and thanks for asking! 

Scott Kniffin
Sr. Engineer
Radiation Effects and Analysis Group, Code 561.4

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