Neutron has highest penetrating power and low ionizing power. 

Heavy water ($D _{2}O$) is used as moderator. Most of the fast neutrons produced escape from the rod and enter into the moderator. These neutrons make collisions with particles of moderator and slow down.

Surrounding a spherical critical mass with a neutron reflector reduces the mass needed for criticality.
A common material for a neutron reflector is beryllium metal.

Fragments produced in the fission of $U^{235}$ are radioactive.
The fragments have the neutron of proton ratio about 1.6.
So, Assertion is correct but reason is false.

Nuclear forces is charge independent
i.e. $F _{pp} = F _{pn} = F _{nn}$
If the K>1 the fission reaction will be in accelerating state.
So, correct choice is option C.

Cadmium and Boron rods are used as control rods in a nuclear reactor as , they are capable of absorbing many neutrons without themselves fissioning.
So, correct choice is option D.

Slow moving neutrons are called thermal neutrons.
So, correct choice is option B.

In a critical chain reaction $K=1$,
where $K$ is the reproduction factor defined as the ratio of number of neutrons in any particular generation to the number of neutrons in the preceding generation.
As $K$ is $1$, energy is released in steady state.

The man-made element which was made in the nuclear reactor is plutonium. Polonium, thorium and uranium are naturally find in the nature.

Cadmium, Boron and Steel rods are used as control rods in a nuclear reactor.
Commonly moderators are Heavy water, graphite and Beryllium and Berillium oride.
Common fuels are Uranium $(^{238}U)$, enriched Uranium $(^{235}U)$ and plutonium $(^{236}{Pu})$ and $^{232}{Th}$

The function of a moderator is to slow down the fast moving neutrons.
The function of control rod is to absorb (capture) the neutrons.

Nuclear fission is caused by both fast and slow neutrons.

The fuel in nuclear power plant is U-235 Uranium-235 Is an isotope of Uranium making up about 0.72% of natural uranium. It is the only fissile isotope with a primordial nuclide found in significant quantity in nature.

Control rods are used in the nuclear reactor to control the fission rate of uranium and plutonium. These are composed of chemical elements such as cadmium, silver and indium that are capable of absorbing many neutrons without  fissioning themselves.

The process by which the percentage of U-235 in U-238 is increased is called enrichment.

Both coolant and moderator.

BWR stands for Boiling Water Reactor.

Highly radioactive spent fuel at nuclear power plant are buried underground for sake of safety.

Controls the nuclear fission by slowing down the neutrons.

Beryllium

In a nuclear reactor core from a physical standpoint, the best reactor core shape is a sphere, but design considerations often dictate a cylindrical shape.

Controlled fission reaction takes place in nuclear reactor.

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving U-235 or a fissile nuclide. Graphite is used as moderator in nuclear reactor.

Heavy water $(D _2O)$ is used as moderator in nuclear power plants.

Boiling water is used as both coolant and moderator is a medium that reduces the speed of fast neutrons.

$D _2O$

The process of increasing the percentage of fissionable $U-235$ in naturally occurring Uranium is called enrichment.

D

In fusion a heavy nucleus is splitted into two nuclei of comparable masses. The mass of parent nucleus is greater than the total mass of daughter nuclei, some mass is disappeared, this is mass defect and this mass appeared as the binding energy.

Heavy water is water that contains heavy hydrogen or deuterium. Deuterium differs from the hydrogen usually found in water, protium, in that each atom of deuterium contains a proton and a neutron. Heavy water may be deuterium oxide,$D _2O$ or it may be deuterium protium oxide, $DHO$. Heavy water occurs naturally, although it is much less common than regular water. 

Water can be used as coolant and moderator

Control rods are used in nuclear reactors to control the fision rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning.

Neutron 

Lead rods 

To absorb neutrons 

Fermi 

Moderator 

A

A

Controlled nuclear fission

The nuclear reaction that takes place in a nuclear reactor is controlled fission reaction.

Non-fissionable uranium-238 is 140 times more abundant than the fissionable U-235 and can be efficiently converted into Pu-239 by the neutrons from a fission chain reaction.

All commercial reactor is based on nuclear fission. An atomic reactor means a great explosion of power that results come from a nuclear fission reaction of heavy nuclei (uranium, plutonium). 

Uranium-23, do not usually undergo fission when struck by slow neutrons, but do split when struck with neutrons of high enough energy. The fast neutrons produced in a hydrogen bomb by fusion of deuterium and tritium have even higher energy than the fast neutrons produced in a nuclear reactor. This makes it possible to increase the yield of any given fusion weapon by the simple expedient of adding layers of cheap natural (or even depleted) uranium. Fast fission of uranium-238 provides a large part of the explosive yield, and fallout, in many designs of hydrogen bomb.

Let the temperature be $T$.

Thermal neutrons are slow down having kinetic energies similar to those of the surroundings molecules.

Proton will be experiencing repulsive forces because of presence of positive charges in the nucleus.
Hence, nucleus is more likely to be absorbed.

Boron rods are used as control rods in nuclear reactors to control the fission rate of uranium and plutonium. These are capable of absorbing many neutrons without fissioning themselves. Composition of control rods is designed for the neutron spectrum of the reactor.

Slow the neutrons thereby controlling the fission reaction.

The temperature inside the core of nuclear reactor is so high that the gas of heavy hydrogen used becomes the fourth state of matter which we call as plasma.

Cadmium rods are used as control rods in nuclear reactors to control the fission rate of uranium and plutonium. These are capable of absorbing many neutrons without fissioning themselves. Composition of control rods is designed for the neutron spectrum of the reactor. Rods are composed of boron,silver,indium and cadmium which can absorb excess neutrons.

Nuclear power plants rum almost all the time and refuel only once every two years or so. Outages typically occur in the spring or autumn when electricity demand is lowest and there is little winter heating or summer air conditioning.

Both (1) and (2)

Pb is the most stable element, it never disintegrates, hence it has no radioactivity and provides very less energy in fission

Base load power plants operate at maximum output. They shut down or reduce power only to perform maintainance or repair. Base load power plants include coal, fuel oil, almost all nuclear, geothermal, hydroelectric, biomass and combined cycle matural gas plants.

Controlled fission reaction is carried out in reactor to produce electricity.

The nuclear reactor of the power plant must be shielded with lead and steel walls so as to stop radiations from escaping out to the environment during its normal operation.

The nuclear reactor at Kaiga is Kaiga Atomic Power Station.

Controlled nuclear fission 

A nuclear reactor must consist:
 II. Fissionable fuel elements such as U-235 and Pu-239.
III. Moderator (heavy water) which slows down the speed of neutrons.

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. It is achieved by removing the excess neutrons from the reactor core. The only way to remove the excess neutrons through some elements which posses strong neutron absorption capacity. One such element is boron. By regulating the movement or insertion of boron rods into the reactor core while fission process is in progress, the excess neutrons produced during the chain reaction are absorbed by boron which rods regulate the availability of neutron. 

Pure water 

Nuclear fuels are non-renewable sources of energy and large amount of radioactive materials, which are toxic in nature, are released in the environment which are extremely hazardous to the health of living being for thousands of years.

Commonly used moderators include regular(light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

A nuclear reactor produces and controls the release of energy from splitting the atoms of Uranium. Nuclear plants do not burn anything to create steam. Instead they split uranium atoms in a process of fission

Slow moving/low energy neutrons are used to initiate the fission reaction

Moderator is used in nuclear reaction as  it can slow down the fast nuclear chain reaction by slowing down the neutron from fission to thermal energies.

Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons.

Heavy water or Graphite are used as coolants and thick concrete shields are used to prevent radiation.

Statements (a), (b) and (c) are correct because of following reasons: