Figure 530 shows some examples of BWR fuel assem­blies. BWRs have 110-140 mm square full-core height assemblies which, unlike their PWR counter­parts, are contained within thick-walled channel boxes of zirconium alloy. They contain arrays of 6 x 6 to 10 x 10 fuel elements, usually with eight elements acting as tie rods that screw into upper and lower tie plates. Some of the element posi­tions are occupied by unfueled water-filled tubes (called water rods) or water channels and are used to control local flux peaking. Element separation is maintained by grid spacers that are attached to the water rods and evenly distributed along the entire length. The square duct is attached to a top-end fixture, relative to which the remainder of the sub­assembly may slide. The bottom-end fitting has a mechanized orifice to control flow in the subassem­bly and this is located in the core grid plate. The upper end fixture has a handle for loading and unloading against which the hold-down bars rest to prevent levitation.

There are no absorber elements in BWR assem­blies and reactor control is achieved by having cruciform-shaped absorber blades throughout the core which move vertically in the clearance between

GNF

Areva Nuclear fuel GNF2

ATRIUM 10XM industries NFI 9 x 9B Figure 5 Example boiling water reactor fuel assemblies. Reproduced from Tarlton, S., Ed. Nucl. Eng. Int. 2008, 53, 26-36.

sets of four subassemblies. Power peaking is mini­mized on the local scale by having fuel elements with different enrichments and burnable poisons (gener­ally Gd2O3) dispersed within each assembly. Various fuel design improvements have been adopted, such as a debris-filtering structure for better reliability, opti­mized distribution of water channels, fissile material with partial length fuel rods and burnable poison use to improve fuel cycle economy and to extend reactor cycle length.

2.15.3.2.2 VVER fuel assembly

Figure 630 shows an example of a VVER fuel assem­bly. The VVER uses hexagonal fuel assemblies of 3200-4690mm length and 145-235mm width. The assembly is used such that it is contained in a hexag­onal shroud, but shroudless assemblies are available for the VVER-1000.30

2.15.3.2.3 CANDU reactor fuel

Figure 730 shows an example of a CANDU fuel bundle. Twelve fuel bundles fit within each fuel chan­nel that is horizontally aligned in the reactor core.

2.15.3.2.4 AGR fuel

AGR fuel assemblies typically have 36 rods contained within a graphite sleeve. Twenty fuel assemblies are placed in a skip inside a flask.

2.15.3.2.5 LWR MOX fuel assembly

Plutonium recycling has so far been limited to partial loading in LWR cores. A primary design target of the MOX fuel assembly is compatibility with the UO2 standard fuel assembly. In the neutronic design for partial loading of LWR cores, significant thermal neutron flux gradients at the interfaces between the MOX and UO2 fuel assemblies have to be considered. The increase in thermal neutron flux in the direction of an adjacent UO2 assembly is addressed by a grada­tion in the plutonium content of the MOX fuel rods at the edges and corners of the fuel assembly. There are three typical rod types for PWR MOX fuel assemblies. Optimized BWR fuel assemblies are more heterogeneous: wider water gaps and larger water structures within a BWR fuel assembly result in MOX fuel assembly designs with an increase in the number ofdifferent rod types. Examples ofMOX fuel assembly designs are shown in Figure 8.2 There are plans for recycling weapons grade plutonium in PWRs in the United States.33

Plenum

spring

Enriched UO2 fuel pellets

Fuel rod

Natural

uranium

axial

blanket

Zirc-4

cladding

Zirconium

diboride

integral

fuel

bundle

absorber

Figure 6 Example Westinghouse VVER-1000 fuel assembly. Reproduced from Tarlton, S., Ed. Nucl. Eng. Int. 2008, 53, 26-36.

The 100% MOX cores permit an increase in the amount of plutonium under irradiation at a reduced level of heterogeneity of the core. An advanced boil­ing water reactor (ABWR) to be constructed in Ohma, Japan, will be the first plant with an in-built 100% MOX core capability.

2.15.3.2.6 FBR fuel assembly

Figure 92 shows an example of an FBR fuel assembly. FBR fuel assemblies have a hexagonal fuel rod arrangement with small gaps provided by a wire spacer, helically wound around each of the fuel pins or by hexagonal grid spacers. The fuel bundle is

Spacer pad (0.8t, 0.6t), □ Bearing pad (1.32t)

^w2 ^

(2 Bearing pad (2 Sheath

(3) End plate

(4) UO2 pellet (2 Spacer pad (2 End plug

encased in a wrapper tube, in order to form a sodium flow channel for efficient cooling and to prevent fuel failure propagation during an accident.

Austenitic or ferritic steels or nickel alloys are selected as materials for structural components because of their good compatibility with sodium and their ability to cope with high temperatures and high levels of fast neutron exposure. These features of FBR fuel assembly design result from the unique design require­ments of the FBRs, including the hard neutron energy spectrum, compact core size, high power density, high burn-up, high temperature, and plutonium breeding. The fuel structure and actual fuel design vary with the reactor scale, design targets, and the design methodol­ogy. Table 3 summarizes the fuel assembly design specifications of the SUPERPHENIX, BN-600, and MONJU.34

2.15.3 Uranium Oxide Production

Uranium oxide has become the primary fuel for the nuclear power industry today. As of April 2010, there
are some 438 commercial nuclear power reactors operating in 30 countries, with a total capacity of 374 000MWe.* Most of these reactors are of the LWRs, AGRs, or the CANDU reactor types, and they are fuelled with sintered pellets of UO2 contain­ing natural or slightly enriched uranium.

2.15.4.1 Uranium Oxide Powder Production

Prior to UO2 pellet fabrication, the enriched uranium feed, UF6, is converted to UO2 powder. Although a number ofconversion processes have been developed, only three are used on an industrial scale today. Two of these are wet processes: ADU and ammonium uranyl carbonate (AUC) and the third is a dry process.

The selected conversion process and its process parameters strongly influence the characteristics of UO2 powder and the resulting UO2 pellets.

2.15.4.1.1 ADU process

The ADU process has been widely used for many years. It uses ADU as an intermediate product in

■ ■ X X X X x|x X X X X X X X ■ ■ ■ X X X X X ■ X • • • X X • • X X X X X • • • l« • X X X X X X X X • • Co) I* • X X X X X X X X • • • • • X X X X X • • X X • • • X ■ X X X X X ■ ■ ■ X X X X >ф< X X X X X X X ■ ■

и Fuel rod, 3.7 wt% Pu m Fuel rod, 5.2 wt% Pu □ Fuel rod, 8.2 wt% Pu m Guide tube @1 Instrumentation tube

Ц 2.5wt% 235U Q2.8wt% Pu I I 3.8 wt% Pu

I I 5.1 wt% Pu 5.1 wt% Pu (part length)

Figure 8 Example light water reactor mixed oxide of uranium and plutonium fuel assemblies. The upper is pressurized water reactor design of the 17 x 17-24 type with a fuel assembly averaged plutonium concentration of 7.2% Pu. The lower is boiling water reactor design of the 10 x 10 -9Q type with a fuel assembly averaged plutonium concentration of 5.4 wt% Pu. Reproduced from IAEA. Status and Advances in MOX Fuel Technology, Technical Reports Series No. 415; IAEA: Vienna, 2003.

a two-step process. First, UF6 is vaporized and injected into an ammonia solution. UF6 hydrolyzes and precipitates as ammonium diuranate (NH4)2U2O7. The ADU precipitate is collected on filters and dried to get the ADU powder.

UF6 + 2H2O! UO2F2 + 4HF

2UO2F2 + 6NH4OH! (NH4)2U2O7 + 4NH4F + 3H2O

Secondly, the ADU powder is calcined and then reduced to UO2 with hydrogen.

(NH4)2U2O7 + 2H2 ! 2UO2 + 2NH3 + 3H2O

The properties of the resulting UO2 are strongly depen­dent on the processing parameters of precipitation,
calcinations, and reduction and equally on material contents, and reacting temperatures. For example, the amount of NH3 is critical in the precipitation step: too much will yield gelatinous ADU which is difficult to filter; if there is too little then the result­ing UO2 powder will be difficult to press and sinter into pellets.

2.15.4.1.2 AUC process37

In Europe, the AUC process is widely used for fabri­cating UO2 fuels. The precipitation of AUC is done in a precipitator, filled with demineralized water. The vaporized UF6, CO2, and NH3 are added as gases through a nozzle system. Reaction occurs according to the following equation:

UF6 + 5H2O + 10NH3 + 3CO2 ! (NH4)4{UO2(CO3)3g + 6NH4F

Fuel assembly Fuel pin

Top end plug Figure 9 Example fast breeder reactor mixed oxide of uranium and plutonium fuel assembly design of MONJU. Reproduced from IAEA. Status and Advances in MOX Fuel Technology, Technical Reports Series No. 415; IAEA: Vienna, 2003.

The AUC precipitates in the form of yellow single crystals. The grain size depends on the precipitation conditions. Instead of UF6, uranyl nitrate solution can also be used as a feed material.

The AUC precipitate is filtrated and washed with a solution of ammonium carbonate and methyl alco­hol. Then, the AUC powder is pneumatically trans­ferred to a fluidized-bed furnace, decomposed, and reduced to UO2 with hydrogen according to the following equation.

(NH4)4{UO2(CO3)3} + H2

! UO2 + 4NH3 + 3CO2 + 3H2O

The transformation of AUC to UO2 gives rise to desirable UO2 powder properties: it is free-flowing and has a high sintering activity.

The resulting UO2 powder is made chemically stable by a slight oxidation to about UO210.

2.15.4.1.3 Dry process38

The dry process was developed in the late 1960s and is widely used today. UF6 is vaporized from steam or hot-water-heated vaporizing baths, and vaporized UF6 is introduced into the feed end of a rotating kiln. Here, it meets and reacts with superheated steam to give a plume of uranyl fluoride (UO2F2). UO2F2

passes down the kiln where it meets with a counter­current flow of steam and hydrogen and is converted to UO2 powder. The reaction sequence follows the equations below.

UF6 + 2H2O! UO2F2 + 4HF
4UO2F2 + 2H2O + 2H2 ! U3O8 + UO2 + 8HF

U3O8 + 2H2 ! 3UO2 + 2H2O

The UO2 powder resulting from dry processes is of low bulk density and fine particle size. Therefore, granulation before pressing and the employment of a pore former process are usual during the pellet fabri­cation process.

A dry process has preferable advantages: the pro­cess is simple and the equipment is compact; the criticality limitation is less required; and liquid waste treatment is not necessary.