Boiling water reactor fuel assembly

VAN DER GRAAF, R. et al, "Scaling laws and design aspects of a natural-circulation-cooled simulated boiling water reactor fuel assembly", Nucl. Technol., 105 (1994) 190-200. 

By contrast, uranium fuels for lightwater reactors fall between these extremes. A typical pressurized water reactor (PWR) fuel element begins life at an enrichment of about 3.2% and is discharged at a bum-up of about 30 x 10 MW-d/t, at which time it contains about 0.8 wt % and about 1.0 wt % total plutonium. Boiling water reactor (BWR) fuel is lower in both initial enrichment and bum-up. The uranium in LWR fuel is present as oxide pellets, clad in zirconium alloy tubes about 4.6 m long. The tubes are assembled in arrays that are held in place by spacers and end-fittings. 

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. 

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

The primary consequence of burnup is a drop in /c-effective as the fuel bums out and fission products are built up. This drop is compensated by the build-up of new fissile isotopes (notably Pu-239 from U-238 neutron absorption in uranium-fueled reactors). Generally, boiling water reactors and pressurized water reactors replace the fuel in stages, with fresh fuel assemblies replacing the most burned-out assemblies at scheduled shutdowns with nonreplaced assemblies often moved (shuffled) to new positions to optimize the reactor operating characteristics. 

In light water reactors, Zircaloy is commonly used as the fuel rod cladding material, a zirconium alloy with various metallic constituents. Pressurized water reactors use Zircaloy-4 (Zry-4), while in boiling water reactors Zircaloy-2 (Zry-2) is the preferred cladding material the compositions of both alloys are shown in Table 1.2. In German PWRs the mass of Zircaloy amounts to about 290kg/Mg HM (heavy metal), in BWRs to about 320 kg/Mg HM (including the fuel assembly channels). 

Advanced Nuclear Fuels Corporation Report, Critical power methodology for boiling water reactors - Methodology for analysis of assembly channel bowing effects, ANF-524(P), Revision 2, Supplement 1, submitted by letter dated November 30,1989. 

Fig. 9.6, Sectional view of the Grand Gulf boiling water reactor (courtesy of General Electric Company and Nuclear Engineering International). A, Vent and head spray B, steam dryer C, steam outlet D, core spray outlet E, steam separators F, feedwater inlet G, feedwater sparger H, L.P. coolant injection inlet J, core spray pipe K, core spray sparger L, top guide M, jet pump N, core shroud O, fuel assemblies P, control blade Q, core plate R, jet pump water inlet S, recirculation water outlet T, vessel support skirt U, control rod drives V, in-core flux monitor.

The control element assemblies consist of an assembly of 4. 8, or 12 fingers approximately 0.8-inch (2-centimeter) outside diameter and arranged as shown in Fig. 14. The use of cruciform control rods, as in boiling water and early pressurized water reactors, necessitates large water gaps between the fuel assemblies to ensure that the control rods will scram (prompt shutdown) satisfactorily. These gaps cause peaking of the power in fuel rods adjacent to the water channel compared to fuel rods some distance from the channel. 

Channel tubes (88 mm o.d. and 4 mm thick) are of welded design and contain fuel assemblies which are cooled by boiling light water. The upper and lower parts of the channel are made of stainless steel and the central part, located in the active zone, is made from a zirconium/2% niobium alloy. The central part is joined to the upper and lower parts by vacuum diffusion-welded stainless steel/zirconium transition joints. The channel tube is attached to the upper duct by a welded joint, and to the lower one by a compensator unit, which is necessary to compensate for the difference in thermal expansion of the channels and ducts without destroying the leak-tightness of the reactor cavity. This type of joint makes it possible to replace a channel during reactor shutdown. 

An experimental program was carried out in the Czech Republic, where Boiling crisis and Critical Heat Flux (CHF) were measured on the facilities that simulated the fuel assemblies of the former Soviet s Pressurised Water Reactors (PWR) WWER-440 and WWER-1000. The large part of experiments related to the CHF was performed at Skoda Plzen Ltd, Nuclear Machinery Plant. The NRI started a complex of research activities in this field at the end of seventies. 

Boiling water acts as coolant and moderator, while separated steam is used to drive the turbine. Maintenance of the water chemistry is performed by the systems that are conventional for reactors of BWR type. The coolant circuit equipment is made of stainless steel, or has a welding deposition made of stainless steel. The fuel assemblies are also made of stainless steel. The outer coatings of micro fuel elements are made of silicon carbide (SiC). 

Limits on the power distribution of the core and margins to these limits must be established to preclude fission product release from the fuel due to fuel and cladding failure. In pressurized water reactors (PWRs) the ultimate limit is the limit on the departure from nucleate boiling ratio (DNBR), which quantifies how close the core is to experiencing fuel melting. Inherent to the DNBR determination are core power distribution parameters such as assembly average powers and hot channel factors (HCFs). Since these parameters help make up the DNBR, limits placed on the DNBR can be translated into limits on these power parameters. 

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