Initially unchecked radio boxes in a form on load
#checked is not a property used from radios form fields, nor radio form fields. It was not even used from Drupal 6.

I used the following code, and none of the radio buttons were selected.

/**
 * Form builder.
 */
function custom_module_form($form, &amp;$form_state) {
  $form['r_8'] = array(
    '#type' =&gt; 'radios',
    '#options' =&gt; array(
      '1' =&gt; t('Yes'),
      '0' =&gt; t('No')
    ),
    '#title' =&gt; t('Test'),
    '#default_value' =&gt; NULL,
  );

  return $form;
}  


  

In this case, if the user doesn't select one of the radio buttons, the form field would not have a a value associated. Your code could handle it as if the user selected a third value (such as "undecided") or what your code considers the default value (e.g. "yes"); if the latter is the case, users should see a note about that.

It makes sense to use the following code, at least in a case: When you want the users voluntary select a value for the field, and not use the default one selected by the module. For example, it would make sense if the user is expressing an agreement, and you want to be sure the user noticed that field, and clicked on one of the radio buttons. 

function custom_module_form($form, &amp;$form_state) {
  $form['r_8'] = array(
    '#type' =&gt; 'radios',
    '#required' =&gt; TRUE,
    '#options' =&gt; array(
      '1' =&gt; t('Yes'),
      '0' =&gt; t('No')
    ),
    '#title' =&gt; t('Test'),
    '#default_value' =&gt; NULL,
  );

  return $form;
}