Hydrolyzes UDP and GDP but not any other nucleoside di-, mono- or triphosphates. May promote reglycosylation reactions involved in glycoproteins folding and quality control in the endoplasmic reticulum. a ribonucleoside 5'-diphosphate + H2O = a ribonucleoside 5'-phosphate + H(+) + phosphate In the presence of 10 mM calcium. Optimum pH is 8.0 for UDP and 7.5 for GDP. By stress such as high temperature or ethanol. Belongs to the GDA1/CD39 NTPase family.