Since the discovery of the nucleus and its properties, it has been always of special interest to the physicists. It's so wonderful and astonishing to think of how many phenomenons can happen in the core of the atom. Since has come to a place where we not only can observe these phenomenons but also can speed up or slow down, in short word influence the process. In my last post, I talked about radioactivity and the characteristics of the nucleus. I mentioned there that only at the beginning of the periodic table the nucleus is more stable as the mass number increases the nucleus become more unstable. In other words, the electromagnetic force starts taking over control from the strong nuclear force. The result is the nuclei with higher atomic mass will break apart to make two or more nucleus. This process is called fission reaction.

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Fission mechanism

There can be two types of fission. One can be spontaneous which means that a nucleus can decay spontaneously. But most of the elements which can decay radioactively doesn't exist anymore as their half-life is less than the age of our beloved earth. So, we can make them in the laboratory. But we can also make a nucleus unstable by adding energy to it.

As we know that nucleus is a mixture of proton and neutron which is bind together by nuclear force and Coulomb force try to tear them apart as positive charges repel each other. In the liquid drop model, the nucleus is compared with a spherical liquid drop. It can be in spherical shape. But in most of the cases, they are not. Their shape is distorted from the spherical shape. So, it is like a spring in the simple harmonic motion always tries to come back to the equilibrium position. But the problem happens when we distort it so much that it can never come back to its equilibrium position rather tear apart. Exactly the same happens for the nuclei. If the nuclei are so big, I mean the mass number is so big that it can't hold together by being stable rather break down to two or more small stable nuclei. This is called nuclear fission.

We can find out the condition for fission to happen. I won't go to the complications of mathematics. By using the Weisbecker formula we find that . Here z is the atomic number, A is proton number. For supermassive nucleus, z is more than or equal to 150.

Where does the energy go

The nucleus has its binding energy which can be found by Weisbecker formula. For example, Californium (Cf) is a nucleus where spontaneous fission happens. The mass number of the Cf is 254 and the total binding energy per nucleon is around & megaelectronvolt (MeV). Now let us imagine it breaks into two equal size nucleus. Now they the binding energy per nucleon is around 8 MeV. So, there is a huge amount of increase in binding energy. That is why the same amount of energy has to be released. So, around 254 MeV of energy is released from a single nucleus. This is around a billion times more energy released that in chemical reactions.

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The outcome of the fission reaction from the same nucleus is not always the same. It basically depends on the statistical probability. The question is if that much amount of energy is released, where does the energy go?

When the parent nucleus become two new fragments, because of the Coulomb repulsion go far from each other. So, this energy is used as the kinetic energy of the fission fragments. But these fragments cannot go so far before they collide with other atoms and molecules of the system which can be seen as the temperature increase of the system. We can use this temperature to generate electrical energy. So, 80 percent of fission energy goes to the kinetic energy of the fission fragments and other 20 percent goes in decays like gamma, beta etc and neutron can also be released.

Above I talked about Cf which can be produced in laboratory and fission happens spontaneously. But we can hit a nucleus like Uranium 235 by a neutron and make unstable Uranium 236 isotope which will create two fission fragments and neutrons as well. These neutrons can hit another 235 uranium and does the same as before and create a chain reaction. There is a critical number of neutrons which should be available after one reaction to hit another nucleus. If the available neutron is more than 1 the situation will be uncontrollable. If it is exactly 1 then we can control the reaction rate and use the energy produced to generate electrical energy.

Producing electric power from fission reaction

An enormous amount of energy is released in a fission reaction which we can use to heat up to boil water which can be converted later to electric energy but we can not use the usual uranium. Here are some reason why. First, in normal uranium, 99.3 percent is uranium 238 and 0.7 % is uranium 235. Uranium 238 does not participate in a fission reaction, rather it absorbs the neutrons produced in the fission from 235 which doesn't allow to happen further chain reaction. To get the chain reaction we need at least 3 to 5 percent of uranium 235. It is really hard to separate between these two isotopes as both of them are identical looking and there is a small mass difference.

Another is reason is the neutrons from the uranium are very highly energetic. High energy neutron cannot interact with further uranium and lead the chain reaction again. So, we need to slow them down. The way to slow it down is to make it interact with some light nucleus so that it loses its energy and later interact with the uranium again. One way of doing it is by using hydrogen. So, we can use normal water to slow the neutron down but normal hydrogen can absorb neutrons and create deuterium. But instead, we can use heavy water which doesn't have any reputation for absorbing the neutron. Heavy water can be used with normal uranium as more neutrons are available.

The last reason is control. If the number of available neutron per reaction is more than one it will be uncontrollable and if less no further fission will happen. So, control is gained by inserting a control rod made of cadmium into the core. Cadmium has a high capacity for absorbing neutrons. Some fluctuation can happen when inserting or removing the rod but some of the neutron produced in fission reaction late neutrons. So, we keep it slightly under 1 for the prompt neutrons and prompt and delayed neutrons give exactly 1 neutron per reaction.

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I talked above that fission energy produces heat and this heat can be extracted as electric power. SO, how can we do that? Excessive heat can melt the core and total system and create a serious accident. For this reason, we need a cooling system. Here as cooling system water is used under great pressure not to allow some serious accident to happen. This water then heats up another water system which is not connected directly with the core. Therefore no radioactive material in there. The water from the second system creates steams to turn the turbine. This is how electric power is generated. Instead of water liquid sodium can be used as well.

Using nuclear power to produce electricity or source of energy is a highly sensitive issue. Technologically it should be perfect and also we have to take care of nuclear waste as well. Accidents like in Chernobyl and in Fukushima is an indication of how sensitive it is. But still, the amount of energy produced in nuclear fission is a very valid reason to take the risk and make it more perfect system to produce energy and not causing an accident at the same time. So, that's all for today. See you in my next post. Steem on.

References

1. Povh, Nuclei and particles

2. Jean-Louis Basdevant, James Rich, Michel Spiro, Fundamentals in Nuclear Physics: From Nuclear Structure to Cosmology

3. Burcham, Nuclear physics, an introduction

4. Kenneth Krane, Modern physics