Re: how does escape of radioactivity from the nuclear reactor is prevented

The escape of radioactive material from a nuclear reactor is prevented by 
what is commonly called the "defense in depth" approach.  In this approach 
there is no single barrier betweeen the radioactive fission products 
produced as a result of nuclear fission and humans or the environment.

The first barrier is the design of the fuel itself.  In today's commercial 
nuclear power plants the fuel is a uranium oxide ceramic.  Most of the 
fission products are retained within the ceramic and go no further.  The 
second barrier is the cladding that surrounds the fuel pellets, which is a 
corrosion resistent barrier such as zircalloy or stainless steel.  The 
third barrier is the pressure boundary of the reactor coolant system.  The 
forth is the containment building.  Finally, reactors are typically located 
within exclusion areas where members of the public have no access.  

All of these features are engineered barriers that would have to fail 
before radioactivity could reach people and the first four prevent relaese 
of radioactivity to the environment.  

There are other engineered features, such as emergency cooling systems, 
that prevent the barriers from failing.

For more information I recommend one of the following web sites:
 http://www.virtualnucleartouri
st.com/
 http://www.uilondon.org
/factsheets/index.htm
 http://www.uic.com.au/nip.htm