When pressing the Alt key, the access key of a given element must be displayed. Inside the Header of a TabItem it is not. The issue is not reproducible with the standard MS TabItem. It seems that it is related to the Foreground binding of the control. There is a workaround as shown below.

<telerik:RadTabControl>
    <telerik:RadTabItem >
        <telerik:RadTabItem.Header>
            
                <AccessText Text="_Tab 1"
                            Foreground="{Binding RelativeSource={RelativeSource AncestorType=telerik:RadTabItem},
                            Path=Foreground}"/>
        </telerik:RadTabItem.Header>
    </telerik:RadTabItem>
</telerik:RadTabControl>