Although you have set the default value of the field to "N" in MySQL, when using the radio button on the form, if it is not selected, the record is null. How to make the record inserted, in which the radio button is not checked, come as "N"?
The table configuration looks like this:
Andtherecord,onceinsertedwiththeuseofradiobuttonlookslikethis: 
I'musingthefollowingencoding:
<labelfor="b_ant">Base Anterior</label><input name="b_ant" type="radio" value="S" />
<label for="b_grade">Base na Grade</label><input name="b_grade" type="radio" value="S" />