Guide thimbles

The factors recognized so far are such as excessive axial forces on FAs induced by holddown springs, upper internals and guide thimble tube growth, initial (FA) deformation, bumup, lateral FA stiffiiess as well as absorber swelling and its cladding wear, corrosion deposits on the rodlets and guide tubes, etc. 

Russian WWER-1000 fuel vendor is considering a new FA design avoiding stainless steel used for guide thimbles and guides so far, higher lateral stiffness and heavier RCCAs. 

Only minor deformation of fuel assemblies has been observed at German PWRs as a result of radiation induced creep. This caused damages at some fuel assemblies (FA) spacer grids during refueling, but did not affect movement or fast insertion of control rods. New improved FAs have been constructed to minimize bending. For this purpose, very stiff blind guide thimbles have been implemented within the FAs. 

A single rod cluster control assembly occupying all 24 guide thimble tubes within a fuel... 

Guide thimble tubes that do not contain any other core components are plugged, to minimise the cooling water flow that bypasses the core. 

The core of CAREM-300, a 300 MWe module, has 199 fuel assemblies of hexagonal section and about 2.85 m active length. Each fuel assembly contains 108 fuel rods, 18 guide thimbles, and 1 instrumentation thimble. Fuel assembly components are similar to those of a conventional PWR. The overall thermal power is 900 MWth, the average linear power is 147 W/cm [5]. 

Guide thimble material SUSS 16 stainless steel... 

FIG. Ill-10. Fuel assembly diagram. Fuel rods, guide thimbles and instrumentation thimble distribution. 

Reactivity control is achieved through the use of control rods with in vessel drives no soluble boron system is foreseen. To reduce the reactivity at the beginning of the cycle, the loaded portion of fuel contains burnable poison. As in standard PWRs, the rod clusters are moved in guide thimbles but since the steam generator acts as a vessel head, there is no possibility of... 

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