Water as Moderator

Neutrons from fission have very high speeds and must be slowed greatly by water "moderation" to maintain the chain reaction. The uranium-235 is enriched to 2.5 - 3.5% to allow ordinary water to be the moderator.

Loss or the water coolant kills the chain reactionsince the fuel configuration is not "critical" without water moderation. Even with the moderator, the fuel is not "critical" without the inclusion of the "delayed" neutrons which may be emitted several minutes after the fission.

Water as Moderator

Enough spontaneous fission events occur io initiate a chain reaction if the proper moderation and fuel density is provided.

Loss or the water coolant kills the chain reactionsince the fuel configuration is not "critical" without water moderation. Even with the moderator, the fuel is not "critical" without the inclusion of the "delayed" neutrons which may be emitted several minutes after the fission. These neutrons come from the radioactive fragments from previous fissions.

Moderation of Fast Neutrons

The neutrons emitted in nuclear fission reactions have high energies, typically in the range of 1 MeV. But the cross section for neutron capture leading to fission is greatest for neutrons of energy around 1 eV, a million times less. Neutrons with energies less than one electron volt are commonly referred to as "thermal neutrons" since they have energies similar to what particles have as a result of ordinary room-temperature thermal energy. It is necessary to slow down the neutrons for efficient operation of a nuclear reactor, a process called moderation. While neutrons are efficiently slowed by inelastic scattering from U-238 , the non-fissionable isotope of uranium, when their energies are higher than 1 MeV, the remainder of the process of slowing them down must be done by elastic scattering from other nuclei. When a neutron collides elastically with another nucleus at rest in the medium, it transfers some of its energy to it. The maximum transfer of energy occurs when the target nucleus is comparable in mass to the projectile. Water and carbon (graphite) are commonly used moderators. Water is a good moderator, but the hydrogens in the water molecule have a fairly high cross section for neutron capture, removing neutrons from the fission process. Heavy water, used as moderator in Canadian reactors, avoids this loss.

Conceptually, the effectiveness of water as a moderator can be compared to what happens on a pool table when the cue ball strikes another ball on the table head-on. The head-on elastic collision with an equally massive target ball at rest stops the cue ball and sends the target ball forward with the cue ball's original speed. The hydrogens in the water play the role of the target ball and are effective in dramatically slowing the fast neutrons, even when the collision is not head-on.

Another conceptual image which may help with understanding the need for moderation is the nature of a short putt on the green of a golf course. The original experiments in the laboratory of Otto Hahn in Germany tried unsuccessfully to get uranium to absorb neutrons by bombarding them with fast neutrons - ²³⁵U just has a very small probability of absorbing fast neutrons. But it has a high probability of absorbing slow ones. If your golf ball is a few centimeters from the hole, you don't get out your driver and hit it as hard as possible - it just will not go into the cup that way. But a gentle tap with your putter has a high probability of success. Moderation to slow the neutrons by collisions with nuclei of similar mass dramatically increases the probability of neutron capture leading to fission.

Critical Mass

For a chain reaction of nuclear fission, such as that of uranium-235, is to sustain itself, then at least one neutron from each fission must strike another U-235 nucleus and cause a fission. If this condition is just met, then the reaction is said to be "critical" and will continue. The mass of fissile material required to achieve this critical condition is said to be a critical mass. The critical mass depends upon the concentration of U-235 nuclei in the fuel material as well as its geometry. As applied for the generation of electric energy in nuclear reactors, it also depends upon the moderation used to slow down the neutrons. In those reactors, the critical condition also depends upon neutrons from the fission fragments, called delayed neutrons. For weapons applications, the concentration U-235 must be much higher to create a condition called "prompt criticality". This means that it is critical with only the neutrons directly produced in the fission process. For U-235 enriched to "bomb-grade" uranium, the critical mass may be as small as about 15 kg in a bomb configuration.

Delayed Neutrons From Fission

One of the safety factors built into the nuclear reactors which are used for electricity generation is that they are only critical with the inclusion of the delayed neutrons which are emitted by some of the fission fragments. Some of these fragments emit neutrons as a part of their radioactive decay, and these neutrons can contribute to fission of any U-235 nucleus they strike. A nuclear power reactor controls the fission chain reaction by moderating the neutrons and with the use of control rods which may be inserted in the reactor core to absorb neutrons and slow down the reaction. These control rods may be adjusted so that the reaction remains critical only with the inclusion of the delayed neutrons. About 0.65% of the neutrons are delayed by an average of 14 seconds, giving significant increase in the generation time and the time for reaction to an emergency in such a power reactor.

Of course in a weapons application, these delayed neutrons are not significant, so weapons-grade uranium is enriched to over 90% U-235.