[en] The National Research Universal (NRU) reactor is heavy water cooled and moderated, with on-power fueling capability. TRIAD, a 3D two-group diffusion code, is currently used for support of day-to-day NRU operations. Recently, an MCNP full reactor model of NRU has been developed for benchmarking TRIAD. While reactivity changes and flux and power distributions from both methods are in reasonably good agreement, MCNP appears to eliminate a k-eff bias in TRIAD. Beyond TRIAD's capability, MCNP enables the assessment of radiation in the NRU outer structure. Challenges include improving TRIAD accuracy and MCNP performance, as well as performing NRU core-following using MCNP. (author)

[en] Highlights: • 77 Heavy-water moderated critical ZED-2 experiments analyzed with MCNP and KENO. • SCALE used to find nuclear data adjustments minimizing MCNP a priori biases. • MCNP patch used to repeat analysis using nuclear data adjustments from SCALE. • Present a detailed code-to-code comparison of of a priori and a postiori k_eff biases. • SCALE results consistently underestimate impact of proposed corrections by 1–2 mk. - Abstract: Many nuclear data uncertainty propagation methods are implemented on the basis of first-order perturbation theory. These methods are complex and integral verification using direct perturbation of the nuclear cross section data is difficult due to the structure of nuclear data files. We present a new implementation of the direct perturbation method, which eliminates the need to modify these files. The method was implemented in DPERT, a patched version of MCNP5. Using the DPERT patch, we present an integral verification of TSURFER based on 77 heavy water moderated ZED-2 critical experiments. TSURFER is a module of the SCALE code suite and applies first-order perturbation theory to propagate nuclear data uncertainties. The experiments were modeled using the standard MCNP5 code to establish the a priori k_eff calculation biases. TSURFER was used to minimize these biases by adjusting the underlying nuclear data. The proposed cross section alterations were then applied to the experiment models, and the DPERT patch was used to verify TSURFER’s evaluation of the a posteriori k_eff biases. The study confirmed the TSURFER bias reduction prediction, but suggests TSURFER may underestimate the impact of the nuclear data corrections by 1.35 ± 0.05 mk on average

[en] The National Research Universal (NRU) reactor is heavy water cooled and moderated, with on-power fueling capability. TRIAD, a 3D two-group diffusion code, is currently used for support of day-to-day NRU operations. Recently, an MCNP full reactor model of NRU has been developed for benchmarking TRIAD. While reactivity changes and flux and power distributions from both methods are in reasonably good agreement, MCNP appears to eliminate a k-eff bias in TRIAD. Beyond TRIAD's capability, MCNP enables the assessment of radiation in the NRU outer structure. Challenges include improving TRIAD accuracy and MCNP performance, as well as performing NRU core-following using MCNP. (author)