I believe this should help -
A brief of the link is as below, The link for Configure NPS Event Logging should be what you are looking for in particular.
NPS logging is also called RADIUS accounting, and should be configured to your requirements whether NPS is used as a RADIUS server, proxy, NAP policy server, or any combination of the three configurations.
To configure NPS logging, you must configure the events logged and viewed with Event Viewer and determine other information you want to log. In addition, you must decide whether you want to log user authentication and accounting information to text log files stored on the local computer or to a SQL Server database on either the local computer or a remote computer.
The following Help sections provide the information you need to know to deploy RADIUS accounting.
RADIUS Accounting - http://technet.microsoft.com/en-us/library/cc725566(v=ws.10).aspx
NPS Events and Event Viewer - http://technet.microsoft.com/en-us/library/cc753898(v=ws.10).aspx
Configure NPS Event Logging -