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The Micro-Pocket Fission Detector (MPFD) is a small-form-factor real-time fission chamber. MPFD performance has been simulated in the Advanced Test Reactor Critical Facility located at Idaho National Laboratory. The neutron and gamma-ray flux profiles and magnitudes were simulated using MCNP in the near-core B-8 irradiation position. These simulations were performed at 69 discrete axial locations inside the B-8 position 55 for three separate orientations of the nearby hafnium outer shim control cylinders and at a power level of 700 W(thermal). The resulting neutron and gamma-ray flux values were used to determine the MPFD response for various fissile masses and detector gas pressures. The optimal gas-operating pressure was determined to be between 30 and 60 psig. The required fissile layer mass was determined to be between 0.5 to 1.0 µg of 235U. Additionally, the gamma ray to fission fragment interaction rate was determined to be 1.43 × 103 with average energy deposition for gamma rays and fission fragments in 30 psig argon gas to be 1 keV and 3.5 MeV, respectively. [ABSTRACT FROM AUTHOR]