The thermal/hydraulic analysis focused on the behavior of the high- power channel at steady state, low-power pulse, and high-power pulse.[60] The analysis yielded estimates of:

• Coolant flow rate; and

• Temperatures at the fuel centerline, the axial/radial temperature profile, and the minimum departure from nucleate boiling ratio (DNBR).[61]

The U. S. Nuclear Regulatory Commission’s (USNRC’s) RELAP5/ Mod3.3 code was used to perform the thermal/hydraulic analysis. A single channel analysis was performed with the highest-power channel, involving 20 axial nodes (15 in the fuel meat) and 27 radial nodes (21 in the fuel meat). To model the reactor pulsing mode, a two-channel model was used, with the two channels defined as (1) the hot channel and (2) the rest of the core. For the pulsing analysis, a RELAP point reactor kinetics model was used, with temperature coefficients obtained from the MCNP5 analysis that was described previously. Finally, a two-channel model was used to model

Figure 1, tori’ Мэр: HEU on Left, LEU on Right

FIGURE 3-1 Core map of the University of Wisconsin reactor before (left) and after (right) conversion from HEU to LEU fuel. Fuel elements are shown in red, and beryllium reflector elements are shown in grey. SOURCE: Austin (2010).

a loss-of-coolant accident using three phases to represent different water levels remaining in the pool and assuming axial conduction in the fuel.

The thermal/hydraulic analysis faced four major challenges. First, the analysis was very sensitive to gap thickness, so additional sensitivity analy­ses needed to be carried out. Second, a discrepancy was found between the two critical heat flux correlations used to analyze the natural circulation mode.[62] [63] Third, there was some uncertainty in the natural convection heat transfer models. Finally, it was challenging to determine appropriate air­cooled temperature safety limits for the new LEU 30/20 fuel type.

The overall outcome of the thermal/hydraulic analysis was encourag­ing. The average fuel assembly power increase associated with the use of fewer assemblies caused small changes to appear in the models of the steady-state operation of the reactor following conversion. However, the definition of the fuel temperature-limiting safety setting11 was updated, ensuring that the fuel temperatures remained below the set points. The temperatures were calculated to be within technical specifications for the reactor: The maximum fuel temperature under pulsing operation at 1 kW and at 1.3 MW was calculated to be within maximum allowable tempera­tures, and the loss-of-coolant fuel temperatures were less than 700 oC.