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Multiple choice chemistry nuclear physics nuclear reactor the nuclear power station isotopes and nuclear chemistry

In a nuclear reactor

  1. the chain reaction is kept under control by rods of cadmium, which reduces the rate

  2. the thick concrete shield is used to slow down the speed of fast neutrons

  3. heavy water (or graphite) moderate the activity of the reactor
  4. out of $U^{238}$ and $U^{235}$ natural uranium has less that 1% of $U^{235}$
Reveal answer Fill a bubble to check yourself
A,C,D Correct answer
Explanation

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

Multiple choice chemistry nuclear physics nuclear reactor the nuclear power station isotopes and nuclear chemistry

The phenomenon of nuclear fission can be carried out both in a controlled and in an uncontrolled way. Out of the following, the correct statements vis-avis these phenomena are

  1. the fission energy released per reaction is much more than conventional nuclear reactions and one of the products of the reaction is that very particle which initiates the reaction

  2. it is the "surface to volume" ratio of the sample of nuclear fuel used which determines whether or not the reaction would sustain itself as a "chain reaction"

  3. the "control rods" in a nuclear reactor must be made of a material that absorbs neutrons effectively

  4. the energy released per fission as well as energy released per unit mass of the fuel in nuclear fission are both greater than the corresponding quantities for nuclear fusion
Reveal answer Fill a bubble to check yourself
A,B,C Correct answer
Explanation

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

$1)$ If nuclear reactions are to be continued, it is necessary that one of the products works for initiating the reaction further.
For example, in the fission $^{235}U$, $3$ neutrons are released which cause fission of further three $^{235}U$ atoms.
$2)$ There are certain value of surface to volume ratio of the sample of nuclear fuel used which should be satisfied so as the fission can occur.
$3)$ Basically, the fission is continuously carried out due to production of neutrons.
In order to control the reaction, the neutrons should be absorbed.