Question

Which of the following nucleus is fissionable by slow neutrons?
A) $_{92}{U^{238}}$
B) $_{93}N{p^{239}}$
C) $_{92}{U^{235}}$
D) $_2H{e^4}$

Answer 1

Hint: Try to figure out the reactions that take place in fission and analyze the characteristics of atoms that take part in the process. Most heavier atoms undergo fission but the nuclear force should also be kept in mind as higher nuclear force makes it difficult to break the atom. Lighter atoms usually fuse to form heavier atoms. So, we have to characterize the given options according to light and heavy atoms and find the process undergone by them.

Complete answer:
$_{92}{U^{238}}$atom is bigger in size. It contains $92$ electrons and protons and 146 neutrons. So, to break it so that it can undergo fission, we need to bombard it with neutrons having energy more than $1.2MeV$and such neutrons are called fast neutrons. But, the fast neutrons have fewer chances to interact with the uranium atom as they will escape without causing fission. Due to this, it cannot sustain a chain reaction in a thermal neutron reactor.
On the other hand, $_{92}{U^{235}}$ is a fissile isotope of uranium. When a lower energy neutron is bombarded on it, it splits the uranium atom. The neutrons released from the fission of the uranium atom further collide with other atoms and thus forming a chain reaction. The neutrons are bombarded on $_{92}{U^{235}}$possessing energy of $0.25MeV$ and are also called slow neutrons. The following reaction takes place in a fission reaction:
$_{92}{U^{235}} + n{ \to _{92}}{U^{236}}$
$_{92}{U^{236}}{ \to _{56}}B{a^{144}}{ + _{36}}K{r^{89}} + 3n + 177MeV$
Nuclear fusion takes place when two lighter elements fuse to form a heavier element. Isotopes of hydrogen: deuterium and tritium fuse together to form a helium atom.
Therefore, option (C) is correct.

Note: The fission reactions are used in nuclear power plants to produce electricity. The fission reactions release a large amount of energy which is used to heat the water to produce steam. The steam is used to rotate the turbine connected to the generator to produce electricity.
The sun gains its energy due to nuclear fusion. When four hydrogen atoms fuse to form a helium atom, some of the mass is converted into energy. The light energy produced keeps bouncing around inside the sun.