A moderator is used in nuclear reactor in order to?

Answer 1

The moderator is there to slow down the neutrons produced by fission. These are produced with high energy, that is they move fast, but Uranium235 has a capture cross section much greater for slow neutrons, so they need to be slowed down to make the chain reaction more efficient. Graphite and heavy water are good moderators, and don't absorb too many neutrons, so they can be used even with natural (non-enriched) uranium. Normal water is not so good but it is ok if the uranium is enriched to about 4 percent U235.

Answer 2

The moderator in a nuclear reactor does not so much control the reactions as allow them to happen. In most reactors the reactions that are useful in producing heat output are in the U235 which is slightly enriched to about 3-4 percent. U235 experiences fission through what are called slow neutrons. When fission occurs, fast ie high energy neutrons are ejected from the fissioned nuclei. To allow the chain reaction to proceed, enough of these fast neutrons must be slowed down so that there are just enough slow neutrons to produce more fissions and allow a steady power level to be achieved. It is the function of the moderator to slow the fast neutrons, in fact we talk of thermalising them, and such reactors are sometimes called thermal reactors. Graphite is a very good moderator as if it is produced in very pure form it absorbs only a few neutrons. This enabled early reactors such as Hanford and Windscale in the UK to operate with natural uranium (0.7 percent U235), and this moderator was subsequently used in power reactors in the UK with natural U fuel (the so-called Magnox design).

In the US, because a compact reactor design was required for subs, the PWR was evolved, which requires enriched fuel because it uses normal water as moderator and this absorbs neutrons more than does graphite. This concept was then developed into larger sizes for power stations. In this design the water is both moderator and heat transfer medium.

In Canada a reactor was developed which used heavy water (deuterium) as the moderator and light water as the coolant. This has also been successful. (The Candu design).

In all these designs the minute-by minute control of the reactor fission rate (which is the same as the heat output rate) is achieved by control rods of a material which strongly absorbs neutrons. This enables a quick shutdown (scram or trip) should it be required, and ensures that the reactor will always remain shutdown until whatever the fault was has been assessed and rectified.

I said at the beginning most reactors-the exceptions are so-called fast reactors which utilise the fissioning of other fuels with fast neutrons. This is certainly possible and has been demonstrated in prototypes, but there are many problems with materials which make them doubtful for commercial use.

Answer 3

The moderator in a nuclear reactor is used to slow down neutrons that have been created in fission reactions. Slowing down those neutrons increases the probability that they can enter into neutron capture reactions to continue the chain.

Answer 4

In order to slow the neutrons down after they are released by fission. U-235 has a much larger capture cross section for slow neutrons than for fast ones. The moderator slows them by collisions and they slow down to the typical velocity of the moderator molecules, when they are said to be 'thermalised'. The moderator itself captures some neutrons, and heavy water is better than light water because it captures fewer. Graphite is also good but must be made to a very high purity, or impurities may capture enough to make it useless.

Answer 5

The moderator is there to slow down the neutrons produced by fission. These are produced with high energy, that is they move fast, but Uranium235 has a capture cross section much greater for slow neutrons, so they need to be slowed down to make the chain reaction more efficient. Graphite and heavy water are good moderators, and don't absorb too many neutrons, so they can be used even with natural (non-enriched) uranium. Normal water is not so good but it is ok if the uranium is enriched to about 4 percent U235.

Answer 6

The moderator in a nuclear reactor is the medium that slows down neutrons in the core. Fission releases neutrons, and these neutrons come away with a very high kinetic energy. They can be captured and cause more fissions, but when they are moving so fast, they have a low probability of this. By slowing the neutrons down, by thermalizing them, the probability that they will cause more fissions is greatly imcreases.

The term thermonuclear reactor is, in part, taken from the "thermalizing" idea. It's a thermonuclear reactor, which comes from the thermalizing or slowing down of the neutrons to improve the continuation of the chain.

Answer 7

A moderator is used to slow down the neutrons produced in nuclear fission in the core of the reactor. By slowing down those fission-generated neutrons (thermalizing them), we make it much more likely that they will be captured by a U-235 nucleus, thus continuing the chain reaction.

Answer 8

The purpose of the moderator is to slow down, or reduce the energy of, neutrons. This allows them to more easily interact with other nuclei; otherwise they could just "blow through" a nucleus and not destabilize it. The moderator also, when KEffective is not close to 1, can reduce reactivity to a subcritical level, shutting down the reaction.

Answer 9

It slows neutrons from fast to thermal velocities, preventing capture by uranium-238 in fuel that is not highly enriched.