Nuclear Weapon vs Nuclear Reactor

Figure 1: Fat man ( picture taken from :http://en.wikipedia.org/wiki/Nuclear_weapon)

Nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or the combination of fusion and fission. Usually a nucleus tends to change its form to stable state if it is in an unstable state.Fission is a process of an excited (high energy level) nucleus splitting into two fragments and thus releasing energy to be stable. This process also releases neutron and this free neutron if bombarded on another nucleus, would make the nucleus excited and unstable and the process repeats. This is where the chain reactions starts. 

                                                                                                                      

Figure 2: Fission Process (picture taken from : http://www.atomicarchive.com/Fission/Fission1.shtml)


Where fission is a process of splitting a nucleus, fusion is a process of two or more atomic nuclei joining together to form a single heavier nucleus and releasing an amount of energy and neutron as a by product. The released neutron might hit a nucleus and fission could occur and chain reaction starts.


Figure 3: Fusion Process 

Picture taken from http://mrbarlow.wordpress.com/tag/chemistry/

There are two basic type of  nuclear weapons and the first type is the Atom bomb which produces its explosive energy from fission. Usually for fission weapons, a mass of fissile material which is enriched uranium is assembled into a supercritical mass (k> 1). For this type of bombs, Uranium is enriched to a level of above 90%. Comparing this to a normal nuclear reactor, the enrichment level is just around 4-5%. This shows how much more safer the nuclear reactor is compared to the nuclear weapon. Adding to it, a nuclear reactor is usually kept at critical level where k=1. K is the neutron multiplication factor. What does K show is , if 

K= 1,  Then the mass is in critical state where the fissile material is self sustaining whereby there is no increase or decrease in power, temperature and neutron population. 

K<1, The mass is in subcritical state and the mass is said not to have the ability to sustain fission reaction, where the neutron population decays exponentially.

K>1, The mass is in supercritical state where there is an increasing rate of fission. 

So, in a bomb, the critical level is made to be in supercritical state so that the explosion would take place and continuously explode and create a massive effect on the area. In a nuclear reactor the criticality level is kept at critical state and this is done by the usage of control rods which would absorb excess neutron. From this, we could see that nuclear reactors are much more safer and is designed and maintained at a safe level.