Which moderator is used in PHWR?
heavy water
A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator.
What is a moderator in NPP?
The moderator of a nuclear reactor is a substance that slows neutrons down. In traditional nuclear reactors, the moderator is the same thing as the coolant: it’s water! When fast neutrons strike the hydrogen atoms in H2O, they slow down a lot (like a billiard ball striking another).
Which material is the most commonly used moderator?
Water (sometimes called “light water” in this context) is the most commonly used moderator (roughly 75% of the world’s reactors). Solid graphite (20% of reactors) and heavy water (5% of reactors) are the main alternatives.
Why is d2o used as moderator?
Water containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.
Why PHWR is over moderated?
Over-moderation means a higher than optimum amount of moderator between fuel plates or fuel rods. An increase in moderator temperature and voids increases the keff of the system and inserts positive reactivity. An over-moderated core would create a positive temperature and void feedback.
What is the use of moderator?
The function of the moderator is to slow down the fast neutrons from energies of few MeV to thermal neutrons of ~0.025 eV. For efficient slowing down of these neutrons, the atoms of moderator material need to have a size close to neutrons and thus the clear choice is H2O since hydrogen has the smallest atomic size.
Why do we not use breeder reactors?
Breeder reactors are costly to build and operate. Although it could be expected that once in production this cost ratio would decline, today few, if any, experts argue that breeder reactor capital costs could be less than 25 percent higher than that of similarly sized water cooled reactors.
Why is heavy water used as a moderator?
Moderator is required in a Nuclear reactor to slow down the neutrons produced during the fission reaction so that the chain reaction can be sustained. Heavy Water is an excellent moderator due to its high moderating ratio and low absorption cross section for neutrons.
Why is heavy water a good moderator?
Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.
What is an over moderated reactor?
Why void coefficient is positive in Phwr?
The physical characteristics of PHWR, i.e. the low temperature heavy water moderator, the high temperature heavy water coolant, and the natural uranium fuel, lead to a positive coolant void coefficient.
Which of the following material is used for moderator?
Detailed Solution Option 4 is correct, i.e. Graphite.