The flux pinning potential and critical current of a grain and a twin
boundary are evaluated for a S-S'-S junction model and a configuration
of parallel magnetic field and junction plane. In the grain boundary
model, the S' domain is assumed to be disordered, to mimic oxygen diso
rder. Large critical currents almost-equal-to O(10(6) amp/cm2) are est
imated for both models. The grain boundary model exhibits a ''nonweak'
' link behavior and moderate critical current dependence on the grain
misalignment angle. We briefly dicuss an approach to enhance critical
currents based on our grain boundary model.