The Fermi Chronicles - Part 13: Nuclear Reactor Types

Now that you have down the basics of the Rankine cycle, and the necessity for generating steam, we can talk about the various types of nuclear reactors. There are 2. The burner reactor and the breeder reactor. There are 3 sub-classifications of reactors based on the primary coolant: the LWR, HWR, and LMR. Acronyms are common in the industry, but here is a general definition of each:

• LWR - Light Water Reactor. Uses plain 'ol H2O (distilled of course)
• HWR - Heavy Water Reactor. Uses the hydrogen isotope deuterium instead of plain 'ol hydrogen - D2O (known as heavy water because the molecular weight is 20 rather than 18 in standard H2O)
• LMR - Liquid Metal Reactor. Uses liquid metal (most commonly sodium) as the primary coolant. If there are any leftover T-1000s after John Conner saves the world, we can use 'em here...

The most common reactor type by far throughout the world is the LWR, and there are several types of those. They tend to be burner reactors, meaning that they simply use the uranium and then refuel. HWRs are not as common (except in Canada where the almost exclusively use CANDU reactors that are HWRs). Even less common is the LMR, but there are still some in operation as these tend to be designed as 'breeders,' creating fissile plutonium-239 during the usage of uranium-235. Fermi 1 was a LMR that was specifically termed a LMFBR (liquid metal fast breeder reactor). Fast breeder reactors don't necessarily have to be liquid metal cooled. There is also the LWFBR (using light water as the coolant) and the GCFBR (gas cooled. An experiment that didn't work out so well).

Because of the prevalence of LWRs, I will focus on these. There are two main types of LWRs, the BWR and the PWR:

• BWR - Boiling Water Reactor
• PWR - Pressurized Water Reactor

The PWR used to be the favored reactor in the U.S. The main reason for that is that it is the reactor used on all navy vessels. I suspect this is because there is no boiling in the nuclear core, just high-pressure water at 2,000 psi circulating through, and therefore the vessel orientation doesn't matter with core physics. If a BWR were present, how the vapor bubbles aggregate and move through the core will change when the vessel orientation changes since the bubbles will go straight up directly opposite gravity. that's just my theory though. In any case, when servicemen with nuclear experience leave the navy, many go into the civilian nuclear power generation industry, thus the prevalence of PWRs in the early going. However, almost all problems in the U.S. have occurred on PWRs. For example, 3-mile island is a PWR. BWRs for whatever technical reason are seen as the safer design today. Currently, there are 104 nuclear power plants based in the U.S. that are licensed by the NRC to generate power. From the NRC website:Out of those 104 plants, 69 are PWRs, while 35 are BWRs. All of the 104 plants run on 65 sites throughout the U.S. That means many sites have more than one plant, which makes licensing through the NRC easier as the geological survey and environmental impact study are already done. This trend will continue for cost reasons. Fermi 3 is proposed to be built on the same site as Fermi 2. Here's where nuclear reactors are located throughout the world:Now onto the good tech stuff. First on tap, the PWR. PWRs were designed specifically for the nuclear naval fleet but have been adopted throughout the world for civilian electrical power generation. There are more than 230 PWRs operating today thought the world. The unique feature of the PWR is that no boiling occurs in the nuclear core as the cooling water is kept above 2,300 psia. That is really high pressure! That very hot (600F) water then goes through a heat exchanger where it boils water in a different loop. Here's a good schematic of the PWR from the NRC website:One of the big advantages of the PWR is that all contamination remains in the containment building, and doesn't work its way around the turbine, condenser, pumps, etc. Thus, outside of containment, it is easier to maintain the equipment during operation. The disadvantage is the high pressure that the primary coolant water must be kept at, increasing the chance that a seal or pipe will cut loose some day. Thus the need for high maintenance and monitoring standards. The double coolant loop is more expensive than the BWR design, and the primary coolant loop is seeded with boric acid to moderate the nuclear reaction. Boric acid is highly corrosive. Also, the uranium fuel is enriched to 5-7%, making the fuel a lot more expensive than that used in BWRs that is enriched to only about 3%. Remember that weapons-grade uranium is about 90% enriched.

BWRs, on the other hand, have only one coolant loop through which power is generated. This costs less money. BWRs were designed by GE in the 1950s for civilian uses. Also, since boiling occurs in the nuclear reactor core, the pressure is lower than the PWR. For BWR, the reactor pressure is around 1,000 psia as opposed to the 2,300 necessary in PWRs. BWRs are currently the preferred design for new reactors. Fermi 2 is a BWR and has been running 20 years without major core incident. Here's a good schematic of the BWR from the NRC website:
Note the simpler design compared to the PWR above. That's one advantage. Another is that the bubble formation drives more cooling fluid up through the core. In fact, this tendency to bring about natural circulation is being utilized in the Fermi 3 design, which uses only natural circulation with no pumps at all! The biggest disadvantage of the BWR design is that, since the primary coolant is contaminated, it gets around to the turbine room, condenser and pumps, making maintenance an issue while the reactor is running.

UPDATE: Since Canada is our neighbor, I should probably mention that they have an affinity for HWRs in the form of a design known as CANDU. There are advantages and disadvantages to this design, but they are basically PWRs that use heavy water, D2O, rather than light water, H2O. The downside (besides all the downsides, and upsides, of PWRs) is that D2O is not dug up out of the ground. It must be manufactured and it is extremely expensive to do so. That being said, D2O allows fast neutron to penetrate deeper than in H2O (D2O is said to have a smaller neutron cross-section than D2O), allowing the nuclear fuel to be unenriched uranium with a U-235 content of the naturally-occurring 0.72%. That makes the fuel cheaper, and allows Canada to use its abundant stores of uranium without having to import the enriched variety since Canada lacks an enrichment facility. So perhaps not a bad idea from a national security standpoint.

Previously:
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