The Fermi Chronicles - Part 14: Neutron Moderation

Not moderation in the political sense, as that word is equivalent to 'waffling,' or 'flip-flopping.' In the nuclear industry, moderation of neutrons necessary for fission to occur is extremely important in controlling the core reaction rate. If left to its own devices, nuclear fission would not only be self-sustaining, but would chain-react to consume all the fuel in a very short time period releasing such a large amount of energy that the core would melt down, or, depending on design, would halt the nuclear reaction as some reactors are dependent upon slower neutrons. Thus, some means of control must be available. As a backdrop, recall from earlier that there are two types of neutrons - fast and thermal. When fission occurs, all neutrons are fast (about 10% of the speed of light!), but slow down as they bump into other atoms (neutron scattering). Once a few bumps occur, the fast neutrons have lost pretty much all of their kinetic energy and have the same speed as the ambient atoms floating around. They have been thermalized, or slowed down to thermal neutrons. Also, recall that fast neutrons can cause fission in all fissionable material, while thermal neutrons can only cause fission in fissile material (U-235, P-239), of which there is very little in a typical nuclear reactor core. The material used to slow down or thermalize the neutrons is called a moderator.

A good moderator should slow down fast neutrons in very few collisions, and should not absorb them to any significant extent. If it took too many collisions to thermalize the fast neutrons, then there would be a good chance of what is termed 'neutron leakage.' The ability of a material to moderate, absorb, detect, etc. neutrons is quantified in terms of a cross-section, and has units of area. For instance, in the case of a moderator, ideally it should have a large scattering cross-section, meaning that their is a very good probability that a majority of fast neutron is thermalized. A small cross-section here would be undesirable since it would certainly lead to neutron leakage. Other aspects of the moderator, however, such as the absorption cross-section, should be absolutely minimal (thus maintaining a constant neutron flux). A moderator should ideally scatter the neutrons elastically, as I wrote about in a prior entry. The most ideal moderator would be one with equal mass to the neutron - the hydrogen atom (not practical).

As it turns out, H2O ('light' water) is an excellent moderator and is used in BWRs. Heavy water, D2O is used in some PWRs, including the CANDU reactor types. Russian reactors use graphite, which was the moderator of the infamous Chernobyl reactor. Helium has been used in GCRs (gas-cooled reactors don't seem to be ideal candidates in my book). Taking standard water as an example, it takes on average about 19 collisions to thermalize a neutron, and the H2O was given a great deal of energy in the process. The only downside is that water has a greater absorption cross-section than say heavy water, D2O, but D2O has a smaller scattering cross-section. So there is typically a trade-off with most moderators. This causes the water to increase in temperature. Water is also a good moderator from the standpoint of safety. In a PWR, if a loss of coolant accident were to occur, moderation would be lost and the reaction shut down simply from the fact that thermal neutrons would no longer be available. Loosing coolant, however, would negate the cooling of the hot fuel rods and lead to loss of geometry rendering the fuel rods useless.

Unlike the moderator, control rods have a high absorption cross-section as they are not meant to moderate, but rather to stop. I'll talk about control rods, typically made of mostly boron (I'll try to make the discussion more lively), with some halfnium, in a later installment.

Previously:
The Fermi Chronicles - Part 13: Nuclear Reactor Types
The Fermi Chronicles - Part 12: Generating Electricity
The Fermi Chronicles - Part 11: Worldwide Uranium Availability
The Fermi Chronicles - Part 10: Utilizing Nuclear Reactions To "Breed" More Fuel
The Fermi Chronicles - Part 9: Nuclear Fission
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post

The Fermi Chronicles - Part 12: Generating Electricty

Almost all power plants that utilize a heat source rely on a single form of energy that is converted to electricity. The feeder energy is that contained in high-pressure steam. How that steam is generated depends upon the individual power plant and heat source. But in the end, the steam is generated by burning coal, oil, wood chips and other biomass, geothermal boiling, solar concentrator boiling, or nuclear power (amongst others). That high pressure, high temperature steam turns a steam turbine that rotates a shaft that spins a generator creating an electromagnetic field from which electrical energy is converted. It is simply a number of energy conversions from thermal to electrical energy. Prior to the generator, the cycle is called the Rankine cycle. Here is the very basic layout:

So we have water in a loop that gets pressurized to a high pressure through a liquid pump, then goes to a boiler (steam generator) where it is converted to steam. That steam flows through a turbine that contains many fan blades that turn a shaft as the steam flows through it. The turbine essentially converts the pressure energy into work (the turning of a shaft). The steam leaves the turbine lower-temperature and low-pressure with possibly some water present in some designs. That low-grade steam then goes to a condenser that converts the steam back into a liquid so that the pump can take it right back into the boiler and the cycle repeats over and over again. The heat rejected from the condenser goes into the environment and is lost. Each component is not 100% efficient, nor is the entire cycle. In fact, the most efficient cycle operating between two temperature extremes is called the Carnot cycle. For most plant designs, the Carnot cycle predicts a maximum efficiency allowed by the physical laws of nature in general, and the 2nd law of thermodynamics specifically, predicts a maximum possible efficiency in the range of 60%. Of course, we cannot design anything near that boundary, and thus most plants run between 30-40% efficient. That's simply a limit of the laws of physics, boundaries that we have to work within and really can do nothing about. Only He who imposes the law can repeal the law, and since we haven't imposed laws of nature, we cannot repeal them either, notwithstanding the wishful thinking of those ignorant to such laws. (more on that later)



Back to the Rankine cycle, the turbine is the primary generator of work in the form of a rotating shaft that then spins in a generator converting the work to electricity. The turbine is a very large device, in many cased the size of a large conference room. Here's a schematic and pic of one such turbine:

They get bigger than that too. That big shaft you see in the above pic goes directly into the electrical generator, which is basically an induction motor still basically similar to what Michael Faraday invented back in 1831:

It's too bad that Faraday didn't have a big turbine to turn the shaft. It was all manual back in that day... More on generators at HowStuffWorks.

Previously:
The Fermi Chronicles - Part 11: Worldwide Uranium Availability
The Fermi Chronicles - Part 10: Utilizing Nuclear Reactions To "Breed" More Fuel
The Fermi Chronicles - Part 9: Nuclear Fission
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post