Fuel assemblies

The introscopic image of power reactor fuel assembly reveals the presence the fuel elements, the tomographic image reveals their filling by fuel and location. 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

Fig. 5. Fuel assembly of Model 412 PWR having a 17 x 17 array of rods (48). Courtesy of Westinghouse Electric Corp. (48).

In the design of the container, factors being considered are fuel assembly size, weight, enrichment, amount of bumup, and age. Production of canisters must fit the schedule of fuel acceptance (18). 

HLW still looks like a fuel assembly, a collection of long, skinny rods, each filled with fuel pellets, held m a rack that allows water to pass through and pick up thermal energy. The assembly is kept tindeiwater to cool and also to shield the workers from the longer-... 

Rod consolidation can permit space saving by dismantling the fuel assembly racks. The rods are still maintained underwater. Rod consolidation is not a routine practice due to issues of heat dissipation and criticality, the possibility of a chain reaction continuing. Dry cask storage is the more common approach for the oldest fuel assemblies and calls for removal of the fuel assemblies from the cooling pool. Designs of the casks vary m detail, but... 

Since the amount of fissile material in the fuel assemblies is only about 3 percent of the uranium present, it is obvious that there cannot be a large amount of radioactive material in the SNF after fission. The neutron flux produces some newly radioactive material in the form of uranium and plutonium isotopes. The amount of this other newly radioactive material is small compared to the volume of the fuel assembly. These facts prompt some to argue that SNF should be chemically processed and the various components separated into nonradioac-tive material, material that will be radioactive for a long time, and material that could be refabricated into new reactor fuel. Reprocessing the fuel to isolate the plutonium is seen as a reason not to proceed with this technology in the United States. 

The relative activity of americium isotopes for a typical pressurized-water reactor (PWR) fuel assembly are 1,700, 11, and 13 Ci for241 Am, 242Am, and 243Am (DOE 1999). The respective activity ratios for a typical boiling water reactor (BWR) are 680, 4.6, and 4.9 Ci. There are 78 PWR and 41 BWR reactors in the United States, several of which have ceased operation. Total projected inventories of these three radionuclides for all reactors are 2.3x10s, 1.4xl06, and 1.7xl06 Ci, respectively. The post irradiation americium content of typical PWR and BWR reactor fuel assemblies are 600 g (0.09%) and 220 g (0.07%), respectively. 

Rod bundle heat transfer analysis (Anklam, 1981a) A 64-rod bundle was used with an axially and radially uniform power profile. Bundle dimensions are typical of a 17 X 17 fuel assembly in a PWR. Experiments were carried out in a steady-state mode with the inlet flow equal to the steaming rate. Generally, about 20-30% of the heated bundle was uncovered. Data were taken during periods of time when the two-phase mixture level was stationary and with parameters in the following ranges ... 

Bowing effect on CHF of a P WR fuel assembly The bowing effect (Eb0VJ) on a PWR fuel assembly can be defined as... 

The following measurements were made on a PWR fuel assembly at 2,200 psia (15 MPa) (Hill et al 1975) ... 

Figure 5.62 Becker s analysis of CHF for damaged fuel assembly in BWR (g"cril predversus

C-E Report, 1975, C-E Critical Heat Flux Correlation for C-E Fuel Assemblies with Standard Spacer Grids, Part I, Uniform Axial Power Distribution, CENPD-162, Combustion Engineering Co., Winsor, CT. (5)... 

Granziera, R., and M. S. Kazimi, 1980, A Two-Dimensional Two-Fluid Model for Sodium Boiling in LMFBR Fuel Assemblies, Energy Laboratory Rep. No. MIT-EL-80-011, Massachusetts Institute of Technology, Cambridge, MA. (4)... 

Israel, S., J. F. Casterline, and B. Matzner, 1969, Critical Heat Flux Measurements in a 16-Rod Simulation of BWR Fuel Assembly, J. Heat Transfer 97 355, ASME, New York. (5)... 

Tong, L. S., A. A. Bishop, J. E. Casterline, and B. Matzncr, 1965, Transient DNB Test on CVTR Fuel Assembly, ASME Paper 65-WA/NE-3 Winter Annual Meeting, ASME, New York. (4)... 

Waters, E. D., 1963, Fluid Mixing Experiments with a Wire-Wrapper 7-Rod Bundle Fuel Assembly, Rep. HW-70178 Rev., Hanford Lab., General Electric Co., Hanford, WA. (App.)... 

The reactor core, the source of nuclear heat, consists of fuel assemblies and control rods contained within the reactor vessel and cooled by the recirculating water system. A 1,220-MWe BWR/6 core consists of 732 fuel assemblies and 177 control rods, forming a core array 16 feet (4.8 meters) in diameter and 14 feet (4.2 meters) high. The power level is maintained or adjusted by positioning control rods up and down within the core. The BW R core power level is further adjustable by changing the recirculation flow rate without changing control rod position, a feature that contributes to excellent load-following capability. 

Each fuel assembly that makes up the core rests on an orificed fuel support mounted on top of the control rod guide tubes. Each guide tube, with its fuel support piece, bears the weight of fuur assemblies and is supported by a control rod drive penetration nozzle in the bottom head of the reactor vessel. The core plate provides lateral guidance at the top of each control rod guide tube. The top guide provides lateral support for the top of each fuel assembly. 

Control rods occupy alternate spaces between fuel assemblies and may be withdrawn into the guide tubes below the core during plant operation. The rods are coupled to control rod drives mounted within housings welded to the bottom head of the reactor vessel. The bottom-entry drives do not interfere with refueling operations. A flanged joint is provided at the bottom of each housing for case of removal and maintenance of the rod dri ve assembly. 

Fig. 6. Fuel assembly of contemporary boiling water reactor, (General Electric)...

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