This thesis addresses the calibration and testing of the Oak Ridge Institute for Science and Education's (ORISE) Holdup Measurement System 4 (HMS4) for use in quantifying U-235 holdup contained within the Process Gas Piping (PGP) of the K-25 Building. In addition to the calibration and testing performed the feasibility of measuring U-238 as a surrogate for U-235 quantification was conducted. A Performance Testing and Validation Plan (PTVP) was developed for confirming the calibration of the system and collecting test data to qualify the HMS4 system for the intended application (ORISE 2009). Tests performed were in accordance with the PTVP and the results were documented in the Performance Testing and Validation Report (PTVR). In turn, the results have been examined to verify that the HMS4 system functions properly and can be used to reliably measure the residual U-235 in pipes. Measurement parameters such as the Lower Level of Detection (LLD), the Minimum Detectable Activity (MDA), and the Total Measurement Uncertainty (TMU) have been determined and reported. To support decommissioning activities, a rapid reliable radiation detection system is needed to assess the amount of residual U-235 within the PGP of the K-25 Building in Oak Ridge, Tennessee. The HMS4 system has been selected to make the required measurements. Verification measurements are considered necessary to assess the reliability and adequacy of the PGP characterization results to ensure criticality incredibility. The purpose of this document is to evaluate the characteristics of the HMS4 system regarding its applicability for use on the east side of K-25. Additionally, to define the level of confidence that should be associated with each HMS4 measurement.