SCIENCE & ENGINEERING BEHIND THE ATOMIC BOMB

        In this article we will talk about the science and engineering behind the weapon and the series of events that happen before the bomb explodes.

                       

 SCIENCE  BEHIND  THE  ATOMIC  BOMB

        Have you ever noticed that what happens when we put blocks of brick over the other?you keep on putting the blocks one over the other more the blocks you put more unstable it becomes if the height of the stock is too high then a slight force will collapse the entire structure. 

Similarly more number of protons and neutrons in the atom more unstable it becomes. All the elements heavier than lead (82Pb) are usually unstable so they eventually decay and convert into lead to lower their internal energy & get their nucleus stable.

        Some of the materials and their isotopes like Uranium-235 and Plutonium-239 are called fissile materials because their atoms can be split if it is supplied with a small energy by colliding with free neutron. Already unstable nucleus of the Pu-239 atom completely splits into two smaller atoms Xenon and Zirconium this is called nuclear fission.

        This fission or splitting of the Pu-239 atom also releases three more neutrons which collide with another plutonium atoms and split them and the reaction continues at room temperature and eventually release tremendous amount of destructive energy.

        The Pu-239 sphere must be at critical mass i.e., the smallest amount of fissile material needed for sustained nuclear chain reaction one of the ways to make it critical from subcritical is to compress it and increase its density by two and a half times at normal densities.

        Due to the large spaces between atoms the neutron released from the fission reaction will lose its energy before reaching another atom and the chain reaction will not sustain but if the density of the Pu-239 is increased by more than two times and if we release the neutron this time due to smaller gaps the chain reaction will sustain.

        In "FAT MAN" the amount of Pu-239 used was 6.19 kg to make it critical the sphere had to be compressed to increase its density. This is not an easy task the pressure required to do so is about 300000 atmospheres it is equivalent to a pressure exerted by 500000 elephants not only that the pressure applied should be symmetrical to compress the sphere symmetrically.

        It can be done by arranging explosives around the sphere and detonating it simultaneously but there is a problem the explosion wave or the detonation wave is spherical if these spherical waves hit the Pu-239 sphere the compression will not be symmetrical.

        To make the compression symmetrical the chemical compositions of the explosive materials is modified instead of a single material the explosives are made with different layers of fast and slow explosives. 

        This changes the curvature of diverging detonation wave to converging wave. This is called the explosive lens. 

        The explosive lens when detonated it increases the density of the Pu-239 sphere by two and a half times by compressing it this state of the plutonium sphere is supercritical.At the same time a radioactive element called Polonium-210 releases alpha particle which in turn absorbed by the element called Beryllium which in turn emits neutron. If a free neutron emitted by beryllium is injected in the supercritical sphere of Pu-239, a chain reaction will occur and spontaneous energy will be released.

ENGINEERING  & WORKING  OF  ATOMIC  BOMB

        This is "FAT MAN" used by USA on 9th Aug1945 on Nagasaki, Japan. It's length is 3.3 meters and has a diameter of 1.5 meters it weighs about 4.6 tons let's open the coverings to see what's inside. Let's start with the core of the assembly.

        This is called the Internal Neutron Initiator or urchin it is responsible for emitting neutrons required for the chain reaction inside there is a Beryllium pallet and also a Beryllium shell the Beryllium shell has spikes pointed inwards between the Beryllium pallet and Beryllium shell there is Polonium this substance is radioactive and emits alpha particles the whole neutron generator assembly is two centimeters in diameter.

        Let's see what is surrounding the neutron generator there is a main fusion material of the atomic weapon a 6.19 kilogram Pu-239 sphere it is 9.2 centimeters in diameter.

         The Pu-239 sphere is enclosed inside another concentric sphere of U-238 this sphere of U-238 acts as a neutron reflector which reflects the emitted neutrons back in the Pu-239 sphere and increases the fission efficiency it also partly takes part in the fission reaction.

        On the U-238 sphere there is a thin layer of Boron plastic shell this is the shell of acrylic thermoplastic with enriched Boron it absorbs the slow moving neutrons and avoids pre-detonation.

        Everything that is responsible for the fission reaction is enclosed in a thick layer of Aluminium this is called an "Aluminium Pusher" which transfers the shock waves from the explosives to the U-238 uniformly.

        The pusher sphere of Aluminium is enclosed by 32 blocks of faster explosives. The faster explosive is a combination of 60%RDX & 40%TNT. 

        Outside of this there is another layer of 32 explosive blocks these blocks are made using combination of fast and slow explosives to shape the detonation wave so it is called an explosive lens.

        To detonate each of the explosive block a detonator was inserted inside it the whole assembly is called the "Physics Package".

        There is a thick steel case surrounding this physics package to contain the explosion of fast and slow explosives inside. Before the actual fission reaction simultaneous explosions of all the explosive blocks are crucial for the symmetric explosion.

        This is X unit, the electronic component with capacitors which is responsible to produce high current at high voltage and to distribute it to all the 32 detonators at the same time.

          Wires with thick insulation carry this current from the X unit to all the detonators.

         All the sensors timers and radar controllers are mounted on this plate and on the other side there are batteries which supply power to the X unit and all the sensors everything is placed inside the outermost shell of the bomb.

            The structure at the tail of the bomb is called "California Parachute" this is to stabilize the bomb in the air.

        On 9th Aug-1945, when "FAT MAN" was dropped a timer inside it started as the timer counted 15 seconds the bomb had fallen far away from the plane after 15 seconds the barometric sensors and the radar system were enabled this was done to prevent any interference of the aeroplane's radio signals with the radar system.

        After another 28 seconds the radar recorded distance of 500 meters and the firing circuit closed. The X unit on the side of the bomb sent high current at 5000 volts to the detonators outering of the 32 exclusive blocks detonated they further detonated the inside ring of 32 detonators creating a concave shock wave with a pressure over 300000 atmospheres. This intense pressure was then transferred to the U-238 and C spheres by the Aluminum Pusher this reduced the volume of Pu-239 sphere and increased the density by two and a half times. At this time the Pu-239 core became super critical this pressure also crushed the gold-plated Beryllium neutron generator. The spikes went through the Polonium layer into the pallet this mixed the Polonium and Beryllium together the alpha radiation emitted by radioactive Polonium was absorbed by the Beryllium after absorbing the alpha radiation Beryllium then emitted free neutrons these free neutrons were enough to start a chain reaction inside the supercritical Pu-239 the sustained fission reaction started inside the 6 kg Pu-239 sphere and within a millionth of a second  24.5 gigawatt hour equivalent energy released in the form of explosion and heat. This tremendous energy created a huge fireball of temperatures more than 4000 degrees celsius and powerful shock waves with velocities over one thousand kilometers per hour and a huge burst of gamma radiation.

        An estimated 35,000–40,000 people were killed outright by the bombing at Nagasaki. Another 20000-30000 died later from blast and burn injuries.

Richard P. Feynmann (Renowned Physicist who played a crucial role in the development of the very first Atomic Bomb) —— ONCE IN HAWAII I WAS TAKEN TO SEE A BUDDHIST TEMPLE. THERE A MAN SAID, "I AM GOING TO TELL YOU SOMETHING THAT YOU'LL NEVER FORGET.... TO EVERY MAN IS GIVEN THE KEY TO THE GATES OF HEAVEN. THE SAME KEY OPENS THE GATES OF HELL.... AND SO IT IS WITH SCIENCE."

Thankyou For Reading!!