Analysis of dpa Rates in the HFIR Reactor Vessel using a Hybrid Monte Carlo/Deterministic Method*

Abstract

Abstract. The Oak Ridge High Flux Isotope Reactor (HFIR), which began full-power operation in 1966, provides one of the highest steady-state neutron flux levels of any research reactor in the world. An ongoing vessel integrity analysis program to assess radiation-induced embrittlement of the HFIR reactor vessel requires the calculation of neutron and gamma displacements per atom (dpa), particularly at locations near the beam tube nozzles, where radiation streaming effects are most pronounced. In this study we apply the Forward-Weighted Consistent Adjoint Driven Importance Sampling (FW-CADIS) technique in the ADVANTG code to develop variance reduction parameters for use in the MCNP radiation transport code. We initially evaluated dpa rates for dosimetry capsule locations, regions in the vicinity of the HB-2 beamline, and the vessel beltline region. We then extended the study to provide dpa rate maps using three-dimensional cylindrical mesh tallies that extend from approximately 12 in. below to approximately 12 in. above the height of the core. The mesh tally structures contain over 15,000 mesh cells, providing a detailed spatial map of neutron and photon dpa rates at all locations of interest. Relative errors in the mesh tally cells are typically less than 1%.

Preliminary Assessment of the Impact on Reactor Vessel dpa Rates Due to Installation of a Proposed Low Enriched Uranium (LEU) Core in the High Flux Isotope Reactor (HFIR)

“…As part of an effort sponsored by the National Nuclear Security Administration (NNSA), conversion to LEU cores is being considered for future HFIR operations. The HFIR LEU configurations analyzed are consistent with the LEU core models used by Ilas and Primm and the HEU balance-of-plant models used by Risner and Blakeman [2] in the latest analyses performed to support the HFIR materials surveillance program [3][4][5][6].…”

“…The dpa rates presented in Section 4 consist of both neutron-and photon-induced components. Risner and Blakeman [2] performed similar analyses for the current HEU core configuration. A comparison of the HEU and LEU dpa rates in the HFIR RV demonstrates that the calculated dpa rates with the proposed LEU core installed are lower than those calculated for the current HEU core and thus support a preliminary conclusion that conversion of HFIR to LEU core operations will have no negative impact on vessel damage.…”

“…The HEU analyses of Risner and Blakeman [2] and the LEU analyses documented herein are the first to apply the hybrid transport methods available through the Automated Variance Reduction Generator (ADVANTG) [7] code to the calculation of dpa rates in the HFIR RV. ADVANTG is a powerful suite of software that combines the strengths of the deterministic transport code, Denovo [9], and the stochastic transport code, MCNP5 [8], to minimize both the user's effort and computational burden involved in the calculation of local or global flux solutions (or any related response functions like dose, dpa rates, etc.)…”

“…Risner and Blakeman [2] applied the hybrid methods of an earlier version of ADVANTG (ADVANTG2) to the latest analysis of dpa rates in the HFIR RV. Their analyses present a wide range of calculations based on the cycle 400 model of HFIR with an HEU core [19].…”

“…Comparisons of the MCNP fluxes to measured values were also made. While the analyses of Risner and Blakeman [2] were underway, the proposed LEU core design of Ilas and Primm [1] was finalized. Blakeman developed shielding models that incorporated the LEU core of Ilas and Primm [1] and used the older ADVANTG2 code to calculate dpa rates in the reactor vessel of these models.…”