From symmetric meson theory, the formation of helium nucleus from two deuterons, i.e., fusion, requires necessarily a direct force due to exchange of two neutral pions, which do not actually compose the deuteron nucleus. The neutral pions are provided by two photons, which are produced by emission of excited collective electrons derived from the palladium atoms. The introduction of the pions makes it possible to reduce remarkably an internuclear distance, enhancing the fusion rate for helium formation. The dynamic interaction is interpreted as the result of condensation of deuterons into octahedral interstitial sites by electrolysis and contraction of the deuteron octahedra around the Pd10- atom with the help of the electron-phonon charge-density wave coupling.