| Summary: | The MIT Research Reactor (MITR) is in the process of conducting a design study to
convert from High Enrichment Uranium (HEU) fuel to Low Enrichment Uranium (LEU)
fuel. The currently selected LEU fuel design contains 18 plates per element, compared to
the existing HEU design of 15 plates per element. A transitional conversion strategy,
which consists of replacing three HEU elements with fresh LEU fuel elements in each
fuel cycle, is proposed. The objective of this thesis is to analyze the thermo-hydraulic
safety margins and to determine the operating power limits of the MITR for each mixed
core configuration.
The analysis was performed using PLTEMP/ANL ver 3.5, a program developed
for thermo-hydraulic calculations of research reactors. Two correlations were used to
model the friction pressure drop and enhanced heat transfer of the finned fuel plates: the
Carnavos correlation for friction factor and heat transfer, and the Wong Correlation for
friction factor with a constant heat transfer enhancement factor of 1.9. With these
correlations, the minimum onset of nucleate boiling (ONB) margins of the hottest fuel
plates were evaluated in nine different core configurations, the HEU core, the LEU core
and seven mixed cores that consist of both HEU and LEU elements. The maximum radial
power peaking factors were assumed at 2.0 for HEU and 1.76 for LEU in all the analyzed
core configurations.
The calculated results indicate that the HEU fuel elements yielded lower ONB
margins than LEU fuel elements in all mixed core configurations. In addition to full
coolant channels, side channels next to the support plates that form side coolant channels were analyzed and found to be more limiting due to higher flow resistance. The
maximum operating powers during the HEU to LEU transition were determined by
maintaining the minimum ONB margin corresponding to the homogeneous HEU core at
6 MW. The recommended steady-state power is 5.8 MW for all transitional cores if the
maximum radial peaking is adjacent to a full coolant channel and 4.9 MW if the
maximum radial peaking is adjacent to a side coolant channel.
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