What is difference between moderator and coolant of nuclear reactor?

Answer 1

The control rods absorb the nuetrons which keeps the reaction rate relatively constant (rather than letting it grow exponentially). They create a situation where roughly one neutron per fission goes on to split another atom.

Moderators slow down the neutrons. Fast neutrons are more inclined to bounce/deflect off of the surface of a nucleus so slower neutrons actually lead to a greater number of succesful fissions i.e. moderators don't slow the reaction down, they just help it to take place.

Answer 2

Sometimes (as in water moderated reactors) they are physically the same material, but they have two different functions. Moderator is present in low enrichment uranium reactors to "moderate" or slow down neutrons so they cannot be captured by uranium-238 nuclei before they can fission uranium-235 nuclei, moderator is not needed in reactors using high enrichment (>20%) uranium or using plutonium fuels. Coolant is used to extract the heat from the reactor core.

Here are some reactor examples:

• CP-1, moderator graphite, coolant none as reactor never operated higher than 1/2 watt thermal power.
• X-1, moderator graphite, coolant forced air.
• Hanford-B, moderator graphite, coolant Columbia river water with immediate direct return.
• EBR-1, moderator none, coolant NaK (molten sodium/potassium metal alloy).
• Submarine and Ship propulsion reactors, moderator water, coolant water.
• Homogenous Uranium Slurry reactor, moderator graphite in core, coolant Homogenous Uranium Slurry mixture circulating in primary coolant loop between core and heat exchanger (in other words, the fuel was the coolant).
• CANDU natural uranium reactor, moderator heavy water, coolant water.
• RBMK (Chernobyl, etc.), moderator graphite, coolant water.
• HTRE (nuclear jet engines), moderator uncertain, coolant compressed air.
• Pebble Bed Reactor, moderator pyrolytic graphite coating on fuel pebbles, coolant forced air.
• NRE (nuclear rocket engines), moderator uncertain, coolant/propellant hydrogen.

(Where moderator is uncertain, these were never put into production so there was no final decision made, several were probably experimented with in development.)

Just about everything was tried as a coolant (including molten lead) and anything containing light elements was tried as a moderator (even paraffin and other hydrocarbons).

Answer 3

The moderator is used to slow down the fission neutrons to increase its efficiency in producing more fission in the nuclear fuel. The control rods are used to control the nuclear reactor produced power and also to shut down the reactor.