Fuel swelling

The buffer layer is a low-density pyrocarbon layer. The density is less than 1.05 g em 3 and the thickness is 95 20 pm. This layer is deposited with C2H2 as the reactant gas. Due to the relatively low density, the buffer layer can provide the void volume for the gaseous fission products and fuel swelling as well as the accommodation for the kernel dimensional change. 

Three tensile bars and two volume change specimens of each elastomer were exposed In a test tube to each of the pure fuels and the mixtures. Exposure for 72 hours at room temperature was determined to be sufficient for most elastomers to approach equilibrium swell. Tensile properties were determined using the ASTM D412 procedure for original samples and fuel exposed samples Immediately after removal from the fuel. Swell measurements were conducted per the ASTM D471 procedure. To obtain the amount of extractables the fuel swelled samples were dried In a vacuum oven at 100°C for 24 hours and then weighed. In most samples constant weight was achieved within this time period. 

The length of time that fuel can be used in a reactor before it must be discharged depends on the characteristics of the reactor, the initial composition of the fuel, the neutron flux to which it is exposed, and the way in which fuel is managed in the reactor, as described in more detail in Chap. 3. Factors that eventually require fuel to be discharged include deterioration of cladding as a result of fuel swelling, thermal stresses or corrosion, and loss of nuclear reactivity... 

Unfortunately, the high emitter temperatures required for unignited operation eliminate most heat sources because of materials limitations. For example, most nuclear heat sources caimot be used on a long-term basis at these temperatures because of either fuel swelling or fuel-emitter incompatibility. 

The highest Pu consumption rates can be achieved only if uranium is eliminated from the core. Nitride appears to be a possible non-uranium fuel material, and the performance of a core fuelled with pure PuN has been studied. AEA-T have studied the vaporisation behaviour of nitride fuels, surveyed the extant data on the physical and chemical properties of PuN and (U,Pu)N, and set up a calculational model of a nitride fuel pin. Preliminary results indicate that acceptable bumups can be achieved provided potential problems of fuel swelling can be solved. 

Figure 18.21 Percent volumetric swelling (VW) of UN as function of bumup and temperature. Based on Ross, S.B., El-Genk, M.S., Matthews, R.B., 1990. Uranium nitride fuel swelling correlation. Journal of Nuclear Materials 170, 169—177.

Fuel pin gas gaps to accommodate lifetime fuel swelling... 

Fuel swelling contributes to the pin sizing (fission gas gap) and the reactivity defect... 

When a fuel pin is first placed in the reactor the fuel is subjected to a large radial temperature gradient. If the fuel is coherent, large thermal stresses are produced. The solution of this problem enables the pellet crack distributions to be described and can also be extended to include the effects of differential fuel swelling, and, by allowing Poisson s ratio to approach 0.5, include the creep behavior. 

Goldberg, L. L. Lynn, and C. D. Sphar, FIGRO—Fortran IV digital computer program for the analysis of fuel swelling and calculation of temperature in bulk oxide cylindrical fuel elements, WAPD-TM-514, Bettis Atomic Power Labs., 1966. 

The maximum clad temperature is determined by mechanical criteria. A combination of fission gas pressure, fuel swelling pressure, and thermal stress results in the maximum material stress at the end of the fuel lifetime. This may lead to pin failure by the clad exceeding the yield strength in the 650° to 750° C temperature range either as a result of strain cycling or creep deformation leading to rupture. 

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