PWR internals vibrational mode shapes calculation and tests

In the case of Pressurized Water Reactors (P.W.R.) the core is contained in a cylindrical core barrel, surrounded by a thermal shield, with its top rim at the vessel flange. The coolant flow along those internals is highly turbulent and induces some structure vibrations.

The barrel and thermal shield motion change more or less the neutron transmission through the water annulus : consequently, the resonant frequencies of structural vibrations may induce resonances on the power spectral density of ion chamber current. The relative amplitude of the neutron noise and mechanical resonances depends on the mode shapes. A good knowledge of these modal shapes is necessary to make a good interpretation of neutron noise result.

Obtained from a french computer system called AQUAMODE-TRISTANA, using the modal coupling of substructures, taking into account the liquid effects by finite elements and using experimentally deduced turbulent pressure forcing functions, the resonant frequencies, modes shapes and amplitudes of P.W.R. internals (900 MW, three loops) are presented in the paper.

These results are in good agreement with experimental results obtained on the SAFRAN Loop which consists of a reduced scale model of three loops P.W.R. primary circuit and reactor internals and on the FESSENHEIM reactor during cold flow tests.