The Method to Predict First Critical Core Loading for Nuclear Reactors

Nuclear power plants have an important role in carbon free electricity production in the world. One of the important steps of commissioning a nuclear power plant is the first core loading. This is also called approaching the criticality. Since the number of fuel elements for the criticality is not known, precautions must be taken to prevent safety incidents. Although the procedure is performed on-line such that the neutron counts are measured at each loading of fuel elements to calculate sub-critical multiplication and the number of fuel element to reach criticality were predicted, computer simulations can also be used. In this study, inverse subcritical multiplication method was applied to Istanbul Technical University TRIGA Mark II research reactor first criticality in 1979 by using Monte Carlo simulation code MCNP6.2. Full 3-D model of the reactor was generated for calculations. Both results, experimental and simulation, showed that reactor became critical with 62 fuel elements. The core excess reactivity of 23.1 cents was predicted as 21.7 with the code. The simulation results are in good agreement with experimental results. The methodology and simulations can be used for power reactor analysis as well.Keywords: Criticality approach, Monte Carlo method, sub-critical multiplication, reactor startup

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