What is a nuclear chain reaction? Explain the difficulties and their removal in its success.
Answers
Answer:
Nuclear Chain Reactions. A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).
Answer:
From the fission of the nucleus by a single neutron more than one neutron are released.
1 Averagely 2- neutrons are released per fission of uranium nucleus. It is a fraction since in some 2 fission events 2 neutrons are produced and in some 3.
The extra neutrons in turn can initiate fission processes, producing still more neutrons.
This leads to the possibility of a chain reaction as was first suggested by Enrico Fermi.
If the chain reaction is uncontrolled it leads to explosive energy output as in a nuclear bomb which would prove explosive. QUA UM PAPER
If the chain reaction is controlled properly then the energy we get at a fixed rate can be used for good purposes and this happens in a nuclear reactor.
The following difficulties occur as the chain reaction continues to receive energy at a fixed rate but it can be removed.
(i) In nuclear fission slow neutrons are much more likely to cause fission in 235U then fast neutron.
Also fast neutrons liberated in fission would escape instead of causing another fission reaction.
For this, the walls of the nuclear reactor are made of concave and with reflective materials for neutrons. The neutrons produced in the fission of 235U is fast neutron and its average energy is 2 MeV. If neutrons are not slowed down, then they exist without interacting with other uranium nuclei unless a very large amount of fissionable material is used for sustaining the chain reaction. But large amounts of such material cannot be used so neutrons emitted should slow down.
What one needs to do is to slow down the fast neutrons by elastic scattering with light nuclei. In fact, Chadwick's experiments showed that in an elastic collision with hydrogen the neutron almost comes to rest and the proton carries away the energy.
(This is the same situation as when a marble hits head-on an identical marble at rest.)
The thing that slower down neutron is called moderator and the reactor uses water, heavy water (D2O) and graphite as the moderator.
(ii) Because of the use of moderator it is possible that the ratio, K of number of fission produced by a given generation of neutrons to the number of fission of the preceding generation may be greater than one. This ratio is called the multiplication factor, it is the measure of the growth rate of the neutrons in the reactor.
For a successful chain reaction, the value of K should be one, when K = 1 , the reactor is said to be critical.
If K becomes greater than one (K > 1) , the reactor will become supercritical, the reaction rate and the reactor power increases exponentially and explosion destroys more fissile material.
If K becomes less than one (K < 1) , the reactor will become subcritical of the reaction rate and the reactor power decreases exponentially and the process eventually stops.
Cadmium or boron rods which absorb neutrons to keep the value of multiplication factor 1 in the reactor. They are kept automatically for longer or shorter lengths.
If the value of the K > 1 , the rods by themselves are more inside the reactor when the value of the K < 1 the rods by themselves come out of the chamber. Such rods are called controlling rods.
(iii) In a chain reaction enormous heat energy is produced and the temperature is likely to become (106 K). Hence, the moderator in this is used for cooling down the fission material and reactor. For this coolants are used. Gases, water, molten sodium etc. are passed as coolants through the tube in reactor.
(iv) The more abundant isotope 233 U is naturally occurring uranium is non-fissionable. When it capture a neutron it produces the highly radioactive plutonium through these reactions,
238 92 239- U + ¦n → 232U → 233Np+ē+v 92
233 Np→ 239Pu +ē+v 94
where è is ẞ-particle (electron) and v = antineutrino and plutonium undergoes fission with slow neutrons.
Explanation:
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