it will take some hand coding, out side of the loop that creates the radio button add this code:
<?php $selectIt = "0"; ?>
then change the radio button code to:
<input name="RCUK_2_Options_Add" type="radio" value="<?php echo $row_Rig1sw_option['OptionName']; ?>" <?php if($selectIt == "0") { ?>checked="checked"<?php } ?> />
<?php $selectIt++; ?>