Fuel irradiated, density

Reactions with fast neutrons, such as (n, 2n), (n, p) and (n, a) reactions, are only of minor importance for production of radionuclides in nuclear reactors. However, special measures may be taken for irradiation of samples with high-energy neutrons. For instance, the samples may be irradiated in special fuel elements of ring-like cross section as shown in Fig. 12.1, or they may be irradiated in a receptacle made of enriched uranium. In both cases, the fast neutrons originating from the fission of enter the samples directly and their flux density is higher by about one order of magnitude than that at other places in the reactor. 

The accident at Chernobyl Nuclear Power Plant and considerable release of radionuclides in particulate fraction renewed interest in hot particles (HPs)—tiny objects of pm dimensions, having density of activity comparable with the one of irradiated nuclear fuel. They pose radiological risk, especially when inhaled with the air after resuspension from the soil. Studies (Osuch et al., 1989 Piasecki et al., 1990), performed on a quite large set of HPs (over 200 species) collected in Autumn 1986, indicated the existence of two, roughly equally populated groups of HPs ... 

The problem of UO2 pellet densification under irradiation causes contraction and leads to collapse of the cladding in axial gaps in sections of the fuel columns. The solution is to control the manufacturing process to ensure the production of pellets with higher density and stabilized pore structures (pore size and grain size). Prepressurizing the fuel rod with helium also avoids clad flattening,... 

The fuels for fast breeder reactors include alloys such as U-Pu-Zr and the ceramic materials UO2-PUO2, UC-PuC, and UN-PuN, but the mixed oxides, UO2-PUO2, are the choice for prototype fast breeder fuel elements because of their high melting temperature, compatibility with cladding and coolants, and relatively good irradiation stability and fission product retention. The disadvantages are the relatively low metal density, the... 

To point to the importance of using improved methods of fuel and poison management, we shall discuss qualitatively the multiple drawbacks of the simplest method, which is batch irradiation of fuel initially uniform in composition, with spatially uniform distribution of boron control poison and with complete replacement of fuel at the end of its operating life. An example of this would be a PWR charged with fuel of uniform enrichment containing 4 percent and 96 percent and controlled by adjusting the concentration of boric acid dissolved in the water coolant to keep the reactor just critical at the desired power level. When this reactor starts operation, the compositions of fuel and poison are uniform throughout the core, and the flux and power density distribution are very nonuniform. 

During the MK-II operation, extensive data were accumulated from start-up and core characteristics tests. These core management and core characteristics data were compiled into a database [2]. The core management data includes core specifications and configurations, atomic number densities before and after irradiation, neutron and gamma flux, neutron fluence, fuel bumup, and temperature and power distributions. The core characteristics data include excess reactivities, control rod worths, and reactivity coefficients, e.g., temperature, power and bumup. These core characteristics data were recorded on CD-ROM for user convenience. 

Aluminium and its alloys are used extensively in research reactors as core components such as fuel element claddings, channel tubes and pipes, and this is mainly because of aluminium s low thermal neutron absorption cross-section and low density, and the absence of any structural transformation up to its melting point. In addition, upon irradiation it does not produce any long lived radionuclides except for Al, which has a half-life of only 2.24 minutes. Radiation damage in aluminium and its alloys is insignificant because of their low recrystallization temperatures. 

The choice of mononitride fuel for fast reactors was dictated by its certain, if small, advantages over carbides from the viewpoint of density, swelling, and confinement of fission gas. An important factor in the final decision made in favor of mononitrides was the higher pyrophorosity of monocarbide, which causes problems in fuel fabrication and handling of irradiated fuel with failed claddings (Rogozkin et al. 2003). 

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