Nuclear Fission

NUCLEAR FISSION

In 1939, German scientists Otto Hahn and F. Strassman discovered that when uranium nucleus is bombarded with a neutron, it breaks up into two smaller nuclei of comparable masses with the release of energy. The process of breaking up of the nucleus of a heavier atom into two smaller nuclei with the release of a large amount of energy is called nuclear fission. The fission is accompanied by the release of neutrons. The energy that is released in the nuclear fission is of many orders of magnitude greater than the energy released in chemical reactions.

Uranium undergoes fission reaction in 90 different ways. The most common fission reactions of ²³⁵₉₂U nuclei are shown here.

Here Q is energy released during the decay of each uranium nuclei. When the slow neutron is absorbed by the uranium nuclei, the mass number increases by one and goes to an excited state ²³⁶₉₂U ^* . But this excited state does not last longer than 10^-12s and decay into two daughter nuclei along with 2 or 3 neutrons. From each reaction, on an average, 2.5 neutrons are emitted. It is shown in Figure 8.27

Energy released in fission:

We can calculate the energy (Q) released in each uranium fission reaction. We choose the most favorable fission which is given in the equation (8.45).

²³⁵₉₂U + ¹₀n → ²³⁶₉₂U ^* → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3₀¹n + Q

Mass of ²³⁵₉₂U = 235.045733 u

Mass of ¹₀n = 1.008665 u

Total mass of reactant = 236.054398 u

Mass of ¹⁴¹₅₆Ba = 140.9177 u

Mass of ⁹²₃₆Kr = 91.8854 u

Mass of 3 neutrons = 3.025995 u

The total mass of products = 235.829095 u

Mass defect ∆m = 236.054398 u – 235.829095 u

 = 0.225303 u

So the energy released in each fission =

0.225303 × 931MeV ≈ 200 MeV

This energy first appears as kinetic energy of daughter nuclei and neutrons. But later, this kinetic energy is transferred to the surrounding matter as heat.

Chain reaction:

When one ²³⁵₉₂U nucleus undergoes fission, the energy released might be small. But from each fission reaction, three neutrons are released. These three neutrons cause further fission in another three ²³⁵₉₂U nuclei which in turn produce nine neutrons. These nine neutrons initiate fission in another 27 ²³⁵₉₂U nuclei and so on. This is called a chain reaction and the number of neutrons goes on increasing almost in geometric progression. It is shown in Figure 8.28.

There are two kinds of chain reactions: (i) uncontrolled chain reaction (ii) controlled chain reaction. In an uncontrolled chain reaction, the number of neutrons multiply indefinitely and the entire amount of energy released in a fraction of second.

The atom bomb is an example of nuclear fission in which uncontrolled chain reaction occurs. Atom bombs produce massive destruction for mankind. During World War II, in the year 1946 August 6 and 9, USA dropped two atom bombs in two places of Japan, Hiroshima and Nagasaki. As a result, lakhs of people were killed and the two cities were completely destroyed. Even now the people who are living in those places have side effects caused by the explosion of atom bombs.

It is possible to calculate the typical energy released in a chain reaction. In the  first step, one neutron initiates the fission of one nucleus by producing three neutrons and energy of about 200 MeV. In the second step, three nuclei undergo fission, in third step nine nuclei undergo fission, in fourth step 27 nucleus undergo fission and so on. In the 100th step, the number of nuclei which undergoes fission is around 2.5 ×10⁴⁰ . The total energy released after 100th step is 2.5 ×10⁴⁰ × 200MeV = 8 ×10²⁹ J . It is really an enormous amount of energy which is equivalent to electrical energy required in Tamilnadu for several years.

If the chain reaction is controllable, then we can harvest an enormous amount of energy for our needs. It is achieved in a controlled chain reaction. In the controlled chain reaction, the average number of neutron released in each stage is kept as one such that it is possible to store the released energy. In nuclear reactors, the controlled chain reaction is achieved and the produced energy is used for power generation or for research purpose.

EXAMPLE 8.15

Calculate the amount of energy released when 1 kg of ⁹³⁵₉₂U undergoes fission reaction.

Solution

235 g of  ⁹³⁵₉₂U  has 6.02 ×10²³ atoms. In one gram of  ⁹³⁵₉₂U , the number of atoms is equal to 6.02 ×10²³ /235 = 2.56 ×10²¹ .

So the number of atoms in 1 kg of ⁹³⁵₉₂U = 2.56 ×10²¹ ×1000 = 2.56 ×10²⁴_.

Each ⁹³⁵₉₂U nucleus releases 200 MeV of energy during the fission. The total energy released by 1kg of ⁹³⁵₉₂U is

Q = 2.56 ×10²⁴ × 200MeV = 5.12 ×10²⁶ MeV

By converting in terms of joules,

Q = 5.12 ×10²⁶ ×1.6 ×10^-13 J = 8.192 ×10¹³ J .

In terms of Kilowatt hour,

 Q = 8.192 ×10¹³ / 3.6 ×10⁶ = 2.27 ×10⁷ kWh

This is enormously large energy which is enough to keep 100 W light bulb operating for 30,000 years. To produce this much energy through chemical reaction, around 20,000 tons of TNT(tri nitro toluene) has to be exploded.

Nuclear reactor:

Nuclear reactor is a system in which the nuclear fission takes place in a self-sustained controlled manner and the energy produced is used either for research purpose or for power generation. The first nuclear reactor was built in the year 1942 at Chicago, USA by physicist Enrico Fermi. The main parts of a nuclear reactor are fuel, moderator and control rods. In addition to this, there is a cooling system which is connected with power generation set up.

Fuel: The fuel is fissionable material, usually uranium or plutonium. Naturally occurring uranium contains only 0.7% of ²³⁵₉₂U and 99.3% are only ²³⁸₉₂U. So the ²³⁸₉₂U must be enriched such that it contains at least 2 to 4% of ²³⁵₉₂U . In addition to this, a neutron source is required to initiate the chain reaction for the first time. A mixture of beryllium with plutonium or polonium is used as the neutron source. During fission of ²³⁵₉₂U , only fast neutrons are emitted but the probability of initiating fission by it in another nucleus is very low. Therefore, slow neutrons are preferred for sustained nuclear reactions.

Moderators: The moderator is a material used to convert fast neutrons into slow neutrons. Usually the moderators are chosen in such a way that it must be very light nucleus having mass comparable to that of neutrons. Hence, these light nuclei undergo collision with fast neutrons and the speed of the neutron is reduced (Note that a billiard ball striking a stationary billiard ball of equal mass would itself be stopped but the same billiard ball bounces off almost with same speed when it strikes a heavier mass. This is the reason for using lighter nuclei as moderators). Most of the reactors use water, heavy water (D₂O) and graphite as moderators. The blocks of uranium stacked together with blocks of graphite (the moderator) to form a large pile is shown in the Figure 8.29 (a) & (b).

Control rods: The control rods are used to adjust the reaction rate. During each fission, on an average 2.5 neutrons are emitted and in order to have the controlled chain reactions, only one neutron is allowed to cause another fission and the remaining neutrons are absorbed by the control rods.

Usually cadmium or boron acts as control rod material and these rods are inserted into the uranium blocks as shown in the Figure 8.29 (a) and (b). Depending on the insertion depth of control rod into the uranium, the average number of neutrons produced per fission is set to be equal to one or greater than one. If the average number of neutrons produced per fission is equal to one, then reactor is said to be in critical state. In fact, all the nuclear reactors are maintained in critical state by suitable adjustment of control rods. If it is greater than one, then reactor is said to be in super-critical and it may explode sooner or may cause massive destruction.

Shielding: For a protection against harmful radiations, the nuclear reactor is surrounded by a concrete wall of thickness of about 2 to 2.5 m.

Cooling system: The cooling system removes the heat generated in the reactor core. Ordinary water, heavy water and liquid sodium are used as coolant since they have very high specific heat capacity and have large boiling point under high pressure. This coolant passes through the fuel block and carries away the heat to the steam generator through heat exchanger as shown in Figure 8.29(a) and (b). The steam runs the turbines which produces electricity in power reactors.

India has 22 nuclear reactors in operation. Nuclear reactors are constructed in two places in Tamilnadu, Kalpakkam and Kudankulam. Even though nuclear reactors are aimed to cater to our energy need, in practice nuclear reactors now are able to provide only 2% of energy requirement of India.