NUCLEAR WEAPONS-HOW DO THEY WORK?

 

With the current battle between Israel and Iran and the US involvement in the war day before yesterday, the news about Iran’s nuclear facilities and enriched Uranium have come to the fore.

What is the process through which Nuclear Weapons work?

For knowing this we have to go to the atomic structure of the atoms. Now there are two varieties of nuclear devices; Fission and Fusion. Both of them depend on the principle of release of energy from the atoms. Fission involves the energy released when an atom is split into different smaller atoms while fusion involves the energy released when two atoms fuse together to produce another bigger atom. In both cases there is release of energy. The latter process takes place in our Sun and it loses 42 lac tons of matter each second on account of fusion which is turned into energy and the Sun shines with that radiation.  

But why does an atom split in the first place? An atom consists of particles called protons (positive charge) and neutrons (neutral) in the nucleus with the electrons (negative charge) going round  them in different orbits.

The Periodic table starts with the Hydrogen atom.  A Hydrogen atom has 1 proton and no neutrons. It has one electron orbiting the nucleus.

The next element Helium has 2 protons and 2 neutrons in the nucleus. Now how can that be?,  because 2 protons mean 2 positive charges staying close together in the nucleus while like charges repel each other? Moreover, there are 2 neutrons in the nucleus as well. Why are all these 4 particles, 2 protons and 2 neutrons stay together in the nucleus? What is the force holding them? On top of that, the nucleus of an atom is highly dense, and has a density that is a few lakh crore times that of water. All the 4 particles 2 of whom have the same charge packed like that in the nucleus? Scientists call the force holding them together cohesive force or nuclear force.   

On the periodic table the elements from Hydrogen and Helium onwards which has 2 protons and 2 neutrons to Silver which has 47 protons and 61 neutrons, the cohesive force is greater than the repealing force between the protons in the nucleus. However, after Silver in the higher elements the repulsive forces slowly strengthen while the cohesive force starts decreasing. Therefore there is a tendency for the protons and neutrons in the nucleus to try and free themselves from the nucleus. But this tendency strengthens only very gradually

For example in Gold which has 79 protons and 197 neutrons and Mercury 80 protons and 200 neutrons, this process is so slow that in a few centuries only one atom splits. The same thing continues till the element Lead which has 82 protons and 207 neutrons. But the elements that come after that are more unstable and the repulsive forces strengthen over the cohesive forces. As a result those higher elements keep on losing particles from their nucleus and finally end up as lead where stability again comes into the nucleus. In this process energy is released with each breakup. This process of successively losing charges from its nucleus till the element lead is realised, radiating energy is known as radioactivity. 

It was in 1886 that Henri Becquerel discovered radioactivity from a mineral of Uranium known as Pitchblende.  Uranium was the first discovered radioactive element. Becquerel discovered that such elements continuously give out radiation from their nucleus as they disintegrate. Even after removing Uranium completely from Pitchblende Becquerel found that it is still giving out radiation and requested Madam Curie to find out the reason. Madam Curie along with her husband Pierre Curie researched on it for 2 years and isolated 2 more radioactive elements named Polonium and Radium from it.  Radium had a radioactivity that is 20 lac times that of Uranium. Thereafter, Thorium was discovered and also other radioactive elements. However, the most stable and naturally available radioactive element is Uranium.  It has 92 protons in its nucleus. The most abundant isotope (different number of neutrons) of Uranium is U-238 which has 146 neutrons and U-235 which has 143 neutrons. The latter isotope is very rare in nature. Then there is the U-234 which has 142 neutrons. This is even rarer in nature.  

In U-238 the nucleus is forever in flux and it deteriorates on account of Radioactivity. As already stated earlier all the unstable radioactive elements deteriorate to finally become lead. The time taken for half the material taken to turn into lead is called the half-life of that element. U-238 has a half-life of 450 crore years, but this is a mere 1600 years for Radium.

Till now all we discussed earlier was natural radioactivity. We earlier talked about the cohesive forces that hold the nucleus of protons and neutrons together. When radioactive elements deteriorate to the element lead, the cohesive force that holds the nucleus together gets released in the form of energy, with some of its matter getting completely being destroyed.  

When scientists first estimated the force that is so released, the human race was surprised because some mass of the original element for deterioration was completely lost in the action. And that entire mass got converted into energy. Using the Einstein’s formula of e=mC² the energy so released was enormous because the C in this equation is equal to 299,792,458 and this has to be squared for calculating of energy released.

The mass of an atom is defined in Atomic Mass Units (AMU). The unit is also called the Dalton. It is one twelfth the mass of a C-12 Carbon atom. The atomic mass of a Carbon atom is 1.993*10^-26Kg.

As already stated the most abundant element of Uranium, the U-238 is not very fissile and takes a long time to deteriorate. It can be bombarded with fast moving neutrons to induce speedier radioactivity. However, its isotope U-235 is more fissile and can get split with bombardment of neutrons with lesser speed. The initial atomic devices made in the world including the one exploded on Hiroshima used this U-235 isotope. But this isotope is rare (at a mere 0.72% of all Uranium) and is not available to make many bombs. So scientists have artificially developed an element called Plutonium which is as fissile as U-235. The weapon used on Nagasaki was a Plutonium bomb. If the abundant U-238 is bombarded by fast neutrons then Neptunium is produced, it then deteriorates and produces Plutonium.

What happens when either the U-235 or Plutonium atom is bombarded with neutrons? They would split into two and the process releases 2-3 neutrons, which in turn bombard the other U-235 or Plutonium atoms in the lump and release more neutrons which in turn would bombard the other atoms. So the reaction proceeds without any further intervention and is known as the CHAIN REACTION. Each stage of this reaction releases big energy and as it continues, and as all the U-235 or Plutonium deteriorates massive amount of energy is produced in the form of radiation.

Now here comes another concept called the CRITICAL MASS. The chain reaction cannot continue uninterrupted unless certain amount of U-235 or Plutonium is available. This is known as the critical mass. In the atomic bombs the destructive power of the bomb is limited to the under twice the critical mass. At critical mass the chain reaction automatically takes place without any human intervention. Two quantities of U-235 or Plutonium, both below the critical mass (to prevent immediate Chain reaction) is taken so that neither part would be able to sustain a chain reaction. In an atom bomb an explosive apart from fissile material, is placed to explode which forces the two masses together and that initiates the chain reaction.  

The bomb that was exploded on Hiroshima contained less than 4.4 tons of U-235, but it had an explosive power of 20,000 tons of TNT. The minimum mass required to sustain a chain reaction of U-235 is 52 Kgs. Of course the size varies based on the purity of U-235 used.

Now, what is URANIUM ENRICHMENT? As already stated earlier while U-238 is less fissile, its isotope U-235 is more so and can sustain a chain reaction. Therefore a nuclear device can only be made with U-235 and not with U-238. But as the quantity of U-235 available in a quantity of Uranium is only 0.76%, the U-235 can be obtained by using a centrifuge to separate the two isotopes as the weights of the isotopes U-238 & U-235 are different. The percentage quantity of fissile Uranium 235 present in a Quantity of Uranium is known as the enrichment of Uranium. For nuclear power an enrichment of 3-5% is sufficient while for producing atomic bombs the enrichment should be 90% or more. Iran has enriched the Uranium to 60% or more as per its own admission and claims that it is for peaceful purposes. Actually for that they only require a maximum of 5% enrichment, so no reason whatsoever Iran should enrich Uranium to 60% unless they have an atom bomb in mind.  

Plutonium does not require any enrichment, but first it has to be artificially produced by bombarding U-238 with fast moving neutrons. In Plutonium the critical mass for a chain reaction is between just 10 to 15 Kg. The Nagasaki bomb contained only 6.2 Kg of Plutonium which is the reason why only about 1 Kg of Plutonium fissioned during the detonation.    

In the Hydrogen bomb the explosion is achieved by integrating two atoms into one and it is the opposite of fission. In this, 2 Hydrogen isotope atoms (Deuterium & Tritium) fuse into a Helium atom releasing energy. The bomb would be of more compact size and lesser in weight compared to the atom bombs. Unlike in the atom bombs there is nothing called a critical mass for the Hydrogen bomb and any amount of material can be used, with the capacity of the carrier of the weapon being the only constraint?  Tsar Bomba exploded by the Soviet Union in 1961 is a Hydrogen bomb ( also known as a Thermonuclear device)  and it packed the explosive power of 50 Megatons or 5 crore tons TNT. It is 2500 times more powerful than the Hiroshima bomb. The Tsar Bomba weighed a stunning 27 tons. For detonating a Hydrogen bomb, an atom bomb acts as a trigger by creating immense amount of heat in order for the fusion to occur.    

These nuclear bombs release massive amounts of energy and cause tremendous amount of destruction. Any war involving them would put entire humanity at risk and is not confined to just one or two countries. Therefore, the lesser countries have them, the better. Okay, while nothing can be done to the countries who already have them (9 in number), there is every reason for the world to nip in the bud a nuclear weapons program of any country, if any new country attempts to acquire them. The more the number of such countries, the more the risk to the world.