If you're using a simple array with radio buttons, the following should work.
Set up your survey to use JavaScript and place the following script in the source of the array question. Replace "QQ" with the array question ID.
The script will loop through all rows and if no checked radios are found, the last one in that row will be checked.
<script type="text/javascript" charset="utf-8">
$(document).ready(function() {
var qID = QQ;
$('#question'+qID+' table.question tbody tr').each(function(i) {
if($('input.radio:checked', this).length < 1) {
$('input.radio:last', this).attr('checked', true);
}
});
});
</script>
