c# - ラジオボタンのクリック時にモーダルポップアップを表示

Question

<tr>
    <td align="left">Have you asked the employee to leave without serving the notice period</td>
    <td align="left">
        <asp:RadioButton ID="rdoBtnleaveWoNoticeyes" runat="server" GroupName="Replacement" 
             Text="Yes" Font-Names="Arial" Font-Size="Small" AutoPostBack="true" 
             oncheckedchanged="rdoBtnleaveWoNoticeyes_CheckedChanged" /> &nbsp;
        <asp:RadioButton ID="rdoBtnleaveWoNoticeno" runat="server" GroupName="Replacement" 
             Text="No" Font-Names="Arial" Font-Size="Small"  AutoPostBack="true"
             oncheckedchanged="rdoBtnleaveWoNoticeno_CheckedChanged" />
    </td>
    <td align="left" colspan="2"><%--Remark--%>
    </td>
</tr>
<cc1:ModalPopupExtender ID="cc18_ModalPopupExtender" 
     runat="server" BackgroundCssClass="modalBackground" TargetControlID="rdoBtnleaveWoNoticeyes" PopupControlID="Panel18" CancelControlID="btnwonoticeperiod">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panel18" runat="server" CssClass="modalPopup" Style="display: none;top:50px">
    <table width="90%" border="0" style="border-collapse:collapse;font-family: Arial; font-size: small" cellpadding="2" cellspacing="2">
        <tr><td align="left">
            <asp:Label ID="lbl_leaveWoNotice" runat="server" ></asp:Label>
             </td>
        </tr>
        <tr><td align="center">
            <asp:Button ID="btnwonoticeperiod" runat="server"  Text="Save" OnClick="btnPLAdjSave_Click" CssClass="clsButtonpopup" />&nbsp;
            <asp:Button ID="Button2" runat="server" Text="Cancel" CssClass="clsButtonpopup" />
        </td></tr>
    </table>
</asp:Panel>

上でチェックしたラジオボタンでモーダルポップアップを表示したいのがデザインです。以下はコードです。出力が得られません。何をすべきか教えてください

protected void rdoBtnleaveWoNoticeyes_CheckedChanged(object sender, EventArgs e)
{
    if (rdoBtnleaveWoNoticeyes.Checked == true)
    {
        lbl_leaveWoNotice.Text = "As you have asked the employee to leave without serving notice, there will be no Notice recovery from employee and employee will receive his PL encashment treating this as “Ask to leave” case.";
        trNoticePayRecovery.Visible = true;
        cc18_ModalPopupExtender.TargetControlID = "rdoBtnleaveWoNoticeyes";
        cc18_ModalPopupExtender.Show();
    }
}

protected void rdoBtnleaveWoNoticeno_CheckedChanged(object sender, EventArgs e)
{
    lbl_leaveWoNotice.Text = "As process there will be Notice period shortfall recovery with PL bal adjustment from employee.";               
    trNoticePayRecovery.Visible = false;
    cc18_ModalPopupExtender.TargetControlID = "rdoBtnleaveWoNoticeno";
    cc18_ModalPopupExtender.Show();
}