*3.1. Model Comparison of Each System Code*

In total, 219 steady-state bundle test cases were employed for the assessment. Figure 5 shows the comparison of results from the one- and multi-dimensional models of each system code by plotting the calculated void fraction with respect to the measured value. The results indicated that each system code showed no significant difference in the void prediction from the one-and multi-dimensional models. In fact, the multi-dimensional models generally showed a slightly higher void fraction than the one-dimensional models, but the difference was negligible. Even though the multi-dimensional model of MARS-KS simulated the additional turbulent mixing, no significant difference from the one-dimensional model was captured. Therefore, further assessment was made to evaluate the sensitivity of the *mixing length*. The results with the turbulent *mixing length* model were compared with the additional calculations without the model. Figure 6, where the comparison is depicted, revealed that no significant difference was observed in the void predictions from the two calculations. Furthermore, this was clearly supported by the statistical results by one-sample t-test, as listed in Table 2. For the statistical comparison, absolute error against the experimental data was calculated. In total, 657 error samples for each model were applied to the statistical test in order to assess the prediction quantitatively with respect to the allowed measurement error range, 2σ. In turn, the results were compared with each other. A hypothesis test was performed and divided into two categories based on a void fraction of 30%. This division was made due to the difference in the prediction tendency with respect to the condition, beyond which the general prediction tendency of the code showed underprediction, whereas an overprediction tendency was captured at low void regions less than 30%. The results indicated that there was no significant difference in the prediction capability, which was supported by the almost identical statistical values between the models. This means that the turbulent

mixing model in MARS-KS does not play an important role in predicting the void fraction in bundles. This is because the turbulent mixing model of MARS-KS applied the same equal mass method [14] used for single-phase flow. Thus, the code cannot model direct mass and energy exchange between channels under two-phase conditions. Therefore, the model is not appropriate to give effective influence to the void prediction.

**Figure 5.** Model comparisons: (**a**) Calculation results of MARS-KS against experimental data; (**b**) calculation results of TRACE against experimental data; (**c**) linear fit of the 3D model of MARS-KS against the 1D model of itself; (**d**) linear fit of the 3D model of TRACE against the 1D model of itself.


**Figure 6.** Model comparison of MARS-KS for sensitivity check to *mixing length*. **Table 2.** Results of one-sample *t*-test for assessing model predictions of MARS-KS.
