MadSci Network: Physics

Re: If you were to split atoms in a building, would the building explode?

Date: Sat May 31 00:06:04 2008
Posted By: Michael Kay, President and Consultant AMBRY, Inc., and
Area of science: Physics
ID: 1211228870.Ph


As with general questions, the usual answer is "It depends." In this 
case, it depends on which atoms you are splitting, how many you are 
splitting, and even on the definition of "splitting" you use. For 
background, and for a great read, I recommend "From X-Rays to Quarks" by 
E. Segre. This is a fascinating history of Nuclear Science from the 
discovery of radioactivity on to today.  

The equation that governs the amount of energy released when a nuclear 
reaction takes place is the famous Einsteinian 

E = mc^2 where E is energy, m is mass, and c is the speed of light 

Remember that E can be positive if energy is given off, or negative if 
more energy must be input than is given off.

In the history of nuclear chemistry (also called radiochemistry) Ernest 
Rutherford is credited with discovering the splitting of atoms. The first 
part was recognizing that radioactive decay is a form of splitting atoms. 
In particular, alpha decay is where a nucleus gives of an alpha particle, 
a helium nucleus, and becomes another element with Atomic Number A - 2 
and Mass M - 4 from the original nucleus. Since this reaction goes 
spontaneously, the E given off is positive, and is reflected in the 
kinetic energy of the alpha particle.

While you might be able to get heat out of radioactive decay, and indeed, 
alpha and beta radioactive sources have been used as heat sources in 
space and underwater applications. But in general this spontaneous 
splitting is not going to give enough energy to "make the building 

Rutherford also discovered the trasmutation of atoms by nuclear reactions 
where an atom is bombarded by a moving particle. He found that nitrogen 
in the air reacts with alpha particles formed by cosmic ray interations 
and from decay of radionuclides such as radon. The reaction 

He4 + N14 -> O17 + H1 + Energy (the kinetic energy of the hydrogen 
nucleus or proton).

This reaction is also not going to give enough energy to "make the 
building explode."

We can use the invention of E.O. Lawrence, the cyclotron, to accelerate 
charged particles to high energy and cause nuclear reactions. Since a lot 
of energy must be put into the charged particle (proton, helium nucleus, 
or ionized nucleus such as iron). In almost all cases, the energy given 
off by the splitting is not enough to raise the temperature enough to 
form a shock wave and make the building explode. 

But there is one class of nuclear reactions that does give off enough 
energy that if we make a device where a large number of atoms are split 
in a short time, we can release enough energy to not only "make the 
building explode", but a good part of the city around it. That is the 
atomic bomb, which gets its energy from the splitting of very heavy 

The most stable nuclide in the entire table of nuclides is Fe56. If there 
were no kinetic barriers, the entire universe would start reacting to 
form Fe56. You can look up "binding energy of a nucleus" on the web or in 
a general radiochemistry book such as "Radiochemistry and Nuclear Methods 
of Analysis" by Ehmann and Vance, or Nuclear and Radiochemistry by 
Friedlander, Kennedy, Miller, and Macias. 

There are a few reactions that can be "easily" initiated to split into 
smaller pieces. One that is used in nuclear reactors and bombs is 
U235 + neutron  --> 2 fission products, about 2 neutrons, and much energy.

The fission products are not about half of the U238, but have one group 
centered around Barium and the other around Krypton. These nuclei have 
high kinetic energy, are intensely radioactive giving off high kinetic 
energy particles and gamma rays. Now there are two ways to "make the 
building explode". The first way with a nuclear reactor is to have the 
controlled reaction become uncontrolled. This is what happened at 
Chernoble. The heat of the fission reaction is removed by a coolant--a 
gas, or liquid--that is heated. This heated medium is used to generate 
electricity. Helium, Water, and Liquid Sodium metal are all used as 
coolants. These coolants are circulating under pressure. If the reaction 
becomes uncontrolled, the coolant is not able to remove all the heat fast 
enough, and, for example with water, the coolant is flashed to steam and 
the rapidly rising pressure causes a steam explosion that "makes the 
building explode." This is not like an atomic bomb because the atoms that 
are splitting are not forced to stay together. The fuel is melted or 
blown apart by the steam explosion. Except for the radioactivity of the 
fuel and fission products, this explosion is just like a boiler 
explosion. At a real reactor, there may be a large release of 
radioactivity that can affect large areas (Chernoble radioactivity 
contaminated a large area around the reactor, and the wind took 
radioactive materials into other countries, and eventually measurable 
amounts went around the world).

Now to really "make the building explode", we need to keep the fission 
reaction going as long as possible to build up the maximum energy 
possible before the fissioning material blows itself apart with the force 
of an atomic bomb. This is accomplished by using conventional explosives 
(like dynamite) to form a powerful spherical compression on the uranium 
or plutonium and force it to stay together in a mass building up energy 
(in a very short time) until it blows apart as an atomic bomb. Now we 
have really "made the building explode" and taken out everything for a 
good distance around the building (several kilometers for a large bomb).

You can Google "Nuclear Physics" and the first hit is the Wikipedia for 
Nuclear Physics, and it is very good. I haven't found any glaring errors 
in the times I have used it. There will be many more hits, and some of 
the teaching resources are also very good. The professional society sites 
are some of the best sources, but they are a bit harder to find than the 
Wiki site.

My recommendation to you is to read about splitting atoms rather than 
trying it. You can Google "Radioactive Boy Scout" to see what can happen 
when things get out of control from a Health Physics perspective and 
create a major radioactive contamination.



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