Blanket assemblies

Figure 10.28 shows the principal steps in reprocessing LMFBR fuel. Feed quantities are for a plant fed with 5 MT/day of irradiated heavy metal (uranium plus plutonium). Feed is combined core and blanket assemblies from LMFBRs operated under conditions nearly the same as those on which Fig. 3.34 and Tables 8.8 and 10.20 were based. The head-end steps 1 through 6 follow one alternative of several sketched in Report ORNL-4422 [05]. 

Figure 10.28 Principal head-end steps in preparing irradiated LMFBR core and blanket assemblies for Purex process. F.P. = fission products S.S. = stainless steel.

After spent fuel has decayed for about 100 days in the EVST, it may be loaded into the SNF shipping cask. Control, radial shield, and some low-power blanket assemblies can be shipped off-site before the 100-day cooling period, but fuel and high-power blanket assemblies are held until they decay to within the spent fuel shipping cask heat rejection capability. 

The core design aims at passive shutdown capability based on the features of metallic fuel and the small-size core. A homogeneous core is used to achieve the compact radial core size which has a marked influence on the vessel size. The core consists of driver assemblies, blanket assemblies, shielding assemblies, control rods and in-vessel storage (for spent fuel). A quarter of the core (drivers and blankets) is changed every two years. 

S. S. GLICKSTEIN et al., "Thermal Disadvantage Factors and Fast Advantage Factors in Uranium-233-Thorium Seed-Blanket Assemblies, Trans. Am. Nucl. Soc., 9, 180 (1966). 

CRBRP (USA) all fuel and inner blanket assemblies replaced every two cycles (2 years) ... 

The fest flux test facility (FFTF) was a 400 MW(th) sodium cooled last reactor specifically designed for development and testing of fast breeder reactor fuels, materials, and components. The reactor was a loop-type plant with three parallel heat transport system loops. The plant has neither steam generators nor blanket assemblies for fissile breeding, consistent with its role as a test reactor. The FFTF was equipped with a great deal of instmmentation. Each core assembly was provided with instruments for measurement of sodium flow rates and sodium outlet temperature. Three instrument trees, one of which serves each of the three core sectors, provide outlet instrumentation for all fiiel assemblies, control and safety assemblies, and selected reflector assemblies. In addition, 8 of the 73 core positions were equipped for full in-core instrumentation. Two of these eight positions were available for closed-loop facilities. 

The core is annular and grouped concentrically its central part is composed of steel assemblies and fertile fuel blanket assemblies (Fig. XVI-5). At the end of life, the core configuration is changed, with its external diameter being reduced only steel assemblies remain in the centre, and the thickness of the external fertile blanket is increased (Fig. XVI-6). At shuffling, the reactivity margin is made up for the next interval of 1 year. 

The radially heterogeneous core layout (i.e. with fertile material blanket assemblies interspersed in the core itself) is used to flatten radial power profile and to enhance internal breeding so as to reduce bum-up reactivity loss. 

Core and fuel The hexagonal core consists of fuel assemblies, control rods and blanket assemblies, and a neutron shield which surrounds the blanket. 

No. of fuel assemblies No. of blanket assemblies Conversion ratio Cycle length (months)... 

The reactor has a provision for on-line refuelling. On average, 12 driver fuel assemblies about 11 blanket assemblies will be replaced assuming 310 effective full power days (EFPD) of operation per year, with an average residence time of the driver fuel assemblies and the internal blanket assemblies of the core being approximately 4.5 years, while it will be 9 years for the outer blanket assemblies. The average discharge burn-up of the driver fuel (without reconstitution) is 87.6 MW-d/kg U, and the maximum one is 120.7 MW-d/kg U. 

Reactor. The general characteristics of the reactor are illustrated by Fig. 9-12, which shows the annular arrangement of the Zircaloy-2 blanket assemblies around the core region. The thorium-oxide pellets within these assemblies are cooled by the reactor fuel solution, which is pumped up through the packed beds from the supply header. To reduce the pressure drop across the pebble bed, the solution is introduced through a tapered perforated pipe the same length as the assemblies, flows into the bed and by means of baffles is directed back to the center outlet pipe, which is concentric with the inlet. 

The core consists of driver fuel assemblies, internal blanket assemblies, radial blanket assemblies, control rods, ultimate shutdown system (USS) assembly, gas expansion modules (OEMs), reflector assemblies, B4C shield assemblies, shield assemblies, and in-vessel storages (IVSs). There are no upper or lower axial blankets surrounding the core. A fission gas plenum is located above the fuel slug and sodium bond. The bottom of each fuel pin is a solid rod end plug for axial shielding. The reflector assemblies contain solid Inconel-600 rods. The control assemblies use a sliding bundle and a dashpot assembly within the same outer assembly structure as the other assembly types. 

The neutron spectrum of the Super FR is compared with those of LWRs and the sodium cooled fast reactor in Fig. 1.56. The blanket assemblies of the Super FR are equipped with zirconium hydride layer for the negative coolant void reactivity. The spectrum near the layer is similar to that of LWRs. Both fast and thermal neutron spectra are available in the Super FR. Availability of both will be suitable for the transmutation of long-lived fission products as well as minor actinides [95,96]. The improved core design for the high power density was reported [87]. 

Fig. 1.54 Two-pass flow scheme with downward flow in blanket assemblies and part of the seed assemblies...

Placing a thin zirconium hydride layer between the seed and blanket fuel assemblies was found effective in changing the reactivity with steam density in the study of a steam cooled fast reactor [103, 104]. The typical geometry and calculation result are shown in Figs. 1.58 and 1.59, respectively. The effectiveness was explained in the subsequent studies [105,106]. The mechanism is described in Sect. 7.3. The fast neutrons are generated in the seed assemblies. They are moderated by the thin zirconium hydride layer between the seed and blanket. The layer is installed in the blanket assemblies in the present Super FR design. The moderated neutrons are effectively absorbed in the blanket fuel by the capture of U-238. The... 

The local peaking factor is considered in the three-dimensional core depletion calculation of the Super FR, while it is separately considered by the assembly bumup analyses coupled with the subchaimel analyses in the Super LWR (see Chap. 2). The reason is that the local power peaking is mainly caused by the zirconium hydride (ZrHi 7) layers located in the blanket assemblies, introduced in Sect. 7.3. The local power peaking must be calculated along with the radial power distribution considering the arrangement of both the seed and blanket assemblies in the whole core, while it instead depends on the control rods and burnable poisons inside a fuel assembly in the Super LWR. 

Concept of Blanket Assembly with Zirconium Hydride Layer... 

Figiue 7.28 [1] compares radial power distributions with ordinary blanket fuel rods or thick walled duct tubes in the outer region of blanket assemblies. Simplified R-Z calculations for radial heterogeneous cores are used to clarify the effect of the duct tubes. The relative power peak near the seed and blanket interface is significantly reduced by replacing the blanket fuel rods with the duct tubes. 

The flow rate at each fuel assembly inlet is controlled by an orifice and is not changeable during the operation, while the power share of the blanket assemblies gets larger as bimiup proceeds. Flow rate that is large enough to cool the blanket... 

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