Introduction
Some of the electricity supplied to homes and businesses is generated by steam driven turbine generators at Boiling Water Reactor (BWR) power stations. The steam to drive the turbine generators is produced in a nuclear reactor core.
Boiling Water Reactor (BWR) Parts
The reactor core consists of uranium oxide pellets that are collected in tubes called fuel rods which are arranged into an array called a fuel bundle. When the fuel bundle is surrounded by a rigid sheath with other components, it is called a fuel assembly. Four fuel assemblies are arranged into a square pattern called a control cell in a typical BWR core design. The control cell is oriented vertically inside the core shroud. A neutron absorbing control rod at the intersection of the four fuel assemblies in the control cell is used to control the nuclear reaction of the four fuel assemblies. The control rod moves vertically inside the control cell. Multiple control cells are arranged inside a metal shroud creating the reactor core that produces the steam for the turbine generators.
Boiling Water Reactor Fuel Assembly
Parts/Components
Fuel Pellet
The fuel pellet consists of various ratios of Uranium-238, Uranium-235, Gadolinium-155 and Gadolinium-157 in an oxide form. The fuel pellets are typically about .5 inches in length and 0.45 inches in diameter. The fuel pellets have chamfered edges to limit the pellet-to-fuel rod interaction during pellet expansion. Uranium-238 and Uraniuim-235 provide the fuel for the nuclear fission reaction. The Uranium-235 undergoes fission directly while the Uranium-238 undergoes a nuclear transformation to Plutonium-239 which undergoes fission directly. Gadolinium-155 and Gadolinium-157 act as a temporary neutron absorber which allows for the addition of the Uranium needed to operate the reactor for 12-24 months. Without the addition of Gadolinium, additional control rods would need to be added to ensure safe operation and shutdown capability of the reactor.
Fuel Pellets
Fuel Rods
Standard Fuel Rod
The standard fuel rod consists of a hollow tube approximately .5 inches in diameter and 160 inches in length. The fuel rod is made of Zircaloy-2 for its material properties and superior heat transfer properties when compared to other zirconium alloys. Each fuel rod has upper and lower Zircaloy-2 plugs which are welded to the ends of the fuel rod. The upper and lower end plugs for eight (8) fuel rods are threaded. Each fuel rod is filled with helium at 44 psig (300 KPa) to improve the heat transfer from the fuel pellet to the cladding. A standard fuel rod is designed to withstand an internal pressure of 1800 psia (1.24e4 KPa) and operate with external pressures between 1000 (68.e3 KPa) and 1100 psig (75.8e3 KPa).
Rod Positions
Partial Length Fuel Rod
Partial Length Fuel Rods are fuel rods that only extend approximately 40% of the length of a Standard Fuel Rod from the bottom of the core. The Partial Fuel Rod allows Plutonium-239 to be produced in the fuel rods adjacent to the steam filled region above the top of the Partial Fuel Rod. Plutoniun-239 contributes to the heat production of the reactor core and is created by neutron absorption in Uranium-238.
Water Rod
Water rods are placed in the center of the bundle to increase power production in the center of the fuel bundle. This arrangement provides a more even fuel burn across the fuel bundle and greater margin to fuel thermal limits. The outside diameter of the water rod is larger than that of the Standard Fuel Rod. Depending on the fuel design, a fuel bundle could contain zero to three water rods.
Fuel Bundle Parts
a. Upper Tie Plate and Bail Handle
The upper tie plate fixes the top end of the fuel rods into the appropriate position, supports the channel and provides a handle by which the bundle can be lifted and moved. Fuel rods with threaded plugs are bolted to the Upper Tie Plate to lock the fuel bundle in place.
Fuel Assembly Labelled Parts
b. Lower Tie Plate and Nose Piece
The lower tie plate fixes the position of the bottom end of the fuel rods and supports the bundle weight. Its bottom end centers the bundle in the core fuel support (i.e., ensures the bundle properly sits in the core) and it provides the entrance for the coolant flow into the bundle. The nose piece of the lower tie plate fits precisely into the fuel support piece and directs coolant flow up through the fuel assembly. Fuel rods with threaded plugs are bolted to the Lower Tie Plate to lock the fuel bundle in place
c. Fuel Spacers
The function of the spacer is to hold the fuel rods in the proper location. The spacers provide lateral support needed to suppress fuel rod vibration and fretting wear associated with vibration.
d. Finger Springs
Finger springs are incorporated at the lower end of the fuel bundle to provide positive contact between the lower tie plate and the fuel channel. This arrangement minimizes any change in channel flow over the fuel assembly lifetime.
Fuel Channel
A fuel channel has several purposes:
- Flow distribution.
- Bearing surface for the control rod blade rollers.
- Improved fuel bundle rigidity.
- Fuel rod protection during fuel handling.
- Acts as a thermal mass heat sink during Loss of Coolant Accident (LOCA) conditions.
The fuel channel encloses the fuel bundle. The fuel channel provides a barrier to separate two parallel flow paths. Approximately 90% of the coolant flows within the fuel channel to remove heat from the fuel rods. Approximately 10% of the coolant flow is directed to the region between fuel assemblies. The bypass flow is required to cool the control rods and nuclear instrumentation.
BWR Fuel Channel
Control Rods
Control rods are designed for sufficiently rapid control rod insertion to avoid fuel damage from any abnormal operating transient. The position of the control rod from fully inserted to fully withdrawn contributes to the amount of power produced in the adjacent fuel assemblies. When the control rod is fully inserted, the adjacent fuel assemblies are not able to sustain a fission chain reaction. A typical control rod consists of a cruciform array of stainless-steel tubes filled with the powdered form of boron carbide (B4C) poison.
PWR Fuel Rods and Control Rod
How Fuel Assemblies Work
Water entering the fuel assembly through the nosepiece in the lower tie plate collides with high energy fission neutrons and absorbs the heat produced from fission. As the water rises in the fuel bundle successive collisions occur and as more heat from fission is absorbed, the temperature of the water rises to the point of boiling. At the top of the fuel bundle, the water leaves the upper tie plate and channel as steam and is directed out of the reactor core. Water is constantly pumped to the fuel assembly to make up for water leaving the fuel bundle as steam. Almost every fission neutron experiences multiple collisions with the coolant causing the neutron’s energy level to drop to the point that the neutron can be absorbed by Uranium-235 or Plutonium-239. Some high energy neutrons leak out of the reactor core and some neutrons are absorbed in core materials other than fuel. A percentage of the neutrons that are absorbed in the fuel experience nuclear fission and approximately 2.5 high energy neutrons are born. The high energy neutrons from fission collide with the water and the process repeats itself.
Fuel Assembly Design Variations
GE-2 - 7x7 fuel bundle.
GE-3 - Improved 7x7 fuel bundle with 49 fuel rods, one of which is segmented.
GE-4 - 8x8 fuel bundle with 63 fuel rods and 1 water rod.
GE-5 - Retrofit 8x8 fuel bundle pre-pressurized with helium and having barrier fuel bundles containing 62 fuel rods and two water rods.
GE-6 & 7 - pre-pressurized at 3 standard atmospheres (300 KPa) with helium with barrier fuel
GE-8 - 8x8 array with 58 to 62 fuel rods and 2-6 water rods. Pre-pressurized at 5 standard atmospheres (510 KPa) with helium.
GE-10 - 8x8 array of 60 full length fuel rods and one large central water rod.
GE-11 & 13 - 9x9 array of 66 full length fuel rods, 8 part-length rods, and two large central water rods.
GE-12 & 14 - 10x10 array of 78 full length fuel rods, 14 part-length rods, and two large central water rods.
GNF-2 - 10X10 fuel bundle with a two water rods and partial fuel rods of two lengths; approximately 40% and 70% of the standard fuel rod.
GNF-3 - 10X10 fuel bundle with a single large central water rod and partial fuel rods of two lengths; approximately 40% and 70% of the standard fuel rod.
Triton11 - 11X11 fuel bundle with a three water rods and partial fuel rods of two lengths; approximately 33% and 67% of the standard fuel rod.
Fuel rods - In some designs, the inside of the fuel rod is coated with pure zirconium to absorb the force of expanding fuel pellets. Some fuel rods are made of Zircalloy-4 or advanced zirconium alloys.
Control rods - Some designs use hafnium or silver-indium-cadmium alloys
Boiling Water Reactor (BWR) Advantages
- The reactor vessel and associated components operate at a substantially lower pressure of about 1,000–1,100 psi (6.9e3 – 7.5e3 Kpa) compared to about 2,250 psi (15e3 KPa) in a Pressurized Water Reactor (PWR).
- The pressure vessel is subject to significantly less irradiation compared to a PWR and so does not become as brittle with age.
- Operates at a lower nuclear fuel temperature, largely due to heat transfer by the latent heat of vaporization, as opposed to sensible heat in PWRs.
- Lower risk (probability) of a rupture causing loss of coolant compared to a PWR, and lower risk of core damage should such a rupture occur. This is due to fewer pipes, fewer large-diameter pipes, fewer welds and no steam generator tubes.
- BWRs typically have N-2 redundancy on their major safety-related systems, which normally consist of four "trains" of components. This means that up to two of the four components of a safety system can fail and the system will still perform if called upon.
Boiling Water Reactor (BWR) Disadvantages
- BWRs require more complex calculations for managing consumption of nuclear fuel during operation due to "two-phase (water and steam) fluid flow" in the upper part of the core. This also requires more instrumentation in the reactor core.
- Larger reactor pressure vessel than for a PWR of similar power, with correspondingly higher cost, for older models that still use a main steam generator and associated piping.
- Contamination of the turbine by short-lived activation products. This means that shielding and access control around the steam turbine are required during normal operations due to the radiation levels arising from the steam entering directly from the reactor core. This is a moderately minor concern, as most of the radiation flux is due to Nitrogen-16 (activation of oxygen in the water), which has a half-life of 7.1 seconds, allowing the turbine chamber to be entered within minutes of shutdown. Extensive experience demonstrates that shutdown maintenance on the turbine, condensate, and feedwater components of a BWR can be performed essentially as a fossil-fuel plant.
- There have been concerns raised about the pressure containment ability of the as-built, unmodified Mark I containment – that such may be insufficient to contain pressures generated by a limiting fault combined with complete Emergency Core Cooling System (ECCS) failure that results in extremely severe core damage. In this double failure scenario, assumed to be extremely unlikely prior to the Fukushima I nuclear accidents, an unmodified Mark I containment can allow some degree of radioactive release to occur. This is supposed to be mitigated by the modification of the Mark I containment; namely, the addition of an outgas stack system that, if containment pressure exceeds critical setpoints, is supposed to allow the orderly discharge of pressurizing gases after the gases pass through activated carbon filters designed to trap radionuclides.