The modified source multiplication method was used to determine the reactivity from the count rate data as fuel assemblies were removed from the engineering mock-up core for the Fast Flux Test Facility (FFTF). The count rate was monitored with a fission detector in the center of the core and in each of the three shield lobes [simulating the low-level flux monitor (LLFM)] as the ZPR-9 assembly was unloaded to simulate, in a reverse manner, the proposed initial loading to critical for the FFTF. Some conclusions from this interpretation are: 1. The inverse count rate from a fission counter in the center of the core is an excellent way to monitor the initial loading of the reactor. 2. The inverse count rates from each of the LLFMs are not adequate for monitoring the initial loading, since they were not a smooth function of the number of fuel assemblies loaded even after correction for changes in detection efficiency. 3. The reactivity versus fuel loading (obtained from the interpretation of the inverse count rate data from the LLFM detectors using an inverse kinetics rod-drop calibration at 0.8 dollar sub-critical) was not a smooth function of the fuel loading because of difficulties in calculating the required changes in detection efficiency for detectors in the shield. However, a similar interpretation for the in-core detector showed a smooth dependence of reactivity on fuel loading. 4. The reference asymmetric loading pattern for startup does not present any interpretational difficulties with a detector in the core, and, thus, the symmetric loading pattern has no real advantages with an in-core detector and requires more time. 5. The initial startup of the FFTF should be monitored with an in-core detector. These conclusions are consistent with those obtained with the prototype fast reactor in the United Kingdom (with its in-core detector) and with the Phenix reactor in France (with its detector outside the core).
