Question Set 64

1. How the analyses and calorific values of fuels can be reported ?

Ans: It may be reported as

(a) as received or fired (wet) basis

(b ) dry or moisture free basis

© combustible or ash and moisture free basis

2. What is the difference between nuclear fission and fission chain reaction.

Ans: The process of splitting of nucleus into two almost equal fragments accompanied by release of heat is nuclear fission. Self sustained, continuing, sequence of fission reactions in a con¬trolled manner is fission chain reaction.

3. Explain difference between fissile and fertile materials.

Ans: The materials which can give nuclear fission e.g. U 35, Pu 39, U 33 are fissile materi¬als. Fertile material itself is not fissionable, but it can be converted to a fissionable material by irradiation of neutrons in a nuclear reactor.

4. What do you understand by fuel cycle in nuclear plants ?

Ans: Fuel cycle a series of sequential steps involved in supplying fuel to a nuclear power reactor. The steps include : Mining, refining uranium, fabrication of fuel elements, their use in nuclear reactor, chemical processing to recover remaining fissionable material, re-enrichment of fuel from recovered material, refabrication of new fuel elements, waste storage etc.

5. What is heavy water and what is its use in nuclear plants ?

Ans: Water containing heavy isotopes of hydrogen (Deuterium) is known as heavy water. Heavy water is used as a moderator. Heavy water has low cross section for absorption of neutrons than ordinary water. Heavy water slows down the fast neutrons and thus moderates the chain reaction.

6. What is a converter reactor ?

Ans: A reactor plant which is designed to produce more fuel than it consumes. The breeding is obtained by converting fertile material to fissile material.

7. Explain nuclear reactor in brief.

Ans: A plant which initiates, sustains, controls and maintains nuclear fission chain reaction and provides shielding against radioactive radiation is nuclear reactor.

8. What is the difference between conversion and enrichment ?

Ans: The process of converting the non fissile U 38 to fissile U-35 is also called “Conversion”. The material like U 38 which can be converted to a fissile material by the neutron flux is called “fertile material”. The conversion is obtained within the nuclear reactor during the chain reaction.

Enrichment is the process by which the proportion of fissile uranium isotope (U-35) is increased above 0.7% (original % in natural uranium).

The concentration of U-35 in the uranium hexafluoride is increased from the 0.7% in natural uranium to to 4%. This is called enrichment and is accomplished in an enrichment plant.

9. Disposal of radioactive waste materials and spent fuel is a major and important technology. How the waste radioactive material is disposed off ?

Ans: Nonusable fission products are radioactive and take short/medium/long time for radioactive decay to reach safe level of radioactivity. Accordingly three methods of disposal are :

(a) Zero or low radioactivity material is dispersed or stored without elaborate shielding.

(b ) Medium radioactivity material is stored for short duration of about 5 years to allow decay of radioactivity.

© High radioactive material. They are stored in water for several months to permit radioactive decay to an accepetable low level.

10. Which nuclear reactor uses water as a coolant, moderator and reflector ?

Ans: Pressurised water reactor.

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