NUCLEAR WEAPON VS NUCLEAR REACTOR 2

As we all know nuclear bomb and nuclear reactor use the same fission method to produce large amount of energy in a short amount of period. So therefore what makes a nuclear reactor safe compared to the destructive force of the nuclear bomb? Well the answer lies in the ability of man to control this nuclear fission process various types of methods. The most widely used fission control technique is the use of moderator and the use of control rods.

 A moderator is a medium which is used to slow down the fast neutron to the thermal state (slow neutron state), by controlling the speed of the neutrons in the reactor core, eventually we can control the reaction rate or the reactivity. By controlling the reactivity we can ensure that the nuclear fission is taking place in a safe amount which is suitable for a nuclear reactor. Although there are many different types of moderator like the D2O (heavy water) and C2O (carbon dioxide) but the best moderator is pressured H2O or in simple word is the pressured water.

The efficiency of the reactor is called reactivity and the reactivity is measured using the k_eff. Moderator to fuel ratio is the amount of moderator in the reactor with respect to the fuel. In the modern reactors nowadays, they operate in the under moderated region. From the graph it shows that when the moderator to fuel ratio increases the k_eff  increases and if the moderator to fuel ratio decreases the k_eff decreases. This feature of the nuclear fission process is very useful when the reactivity of the nuclear fission start to reach the dangerous level so when we enter the control rod (such as graphite) the amount of moderator to fuel ratio decreases and according to the graph the k_eff also decreases and the reactor will reach a safe operating state.

This ability certainly ensures the reactor is safe and not hazardous to anyone. This use of moderators certainly gives us the ability to control the nuclear reactivity fission process; unlike the nuclear bomb whereby, the nuclear reactivity is uncontrollable and becomes destructive. This proves that the nuclear fission process is controllable for the use of power generation and it is also safe.   

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.