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VNS Tokamak OpenMC-Serpent Validation for Medical Isotope Studies

Christopher Ehrich, Christian Bachmann, Pavel Pereslavtsev, Christian Reiter

TL;DR

The study validates cross-code reliability for a Volumetric Neutron Source (VNS) tokamak model by comparing Serpent and OpenMC in neutron-photon coupled transport, focusing on vacuum vessel and blanket regions and extending to a demonstrative $^{99}$Mo medical isotope production case. Using high-statistics simulations and ENDF/B-VII.1 data, the work finds overall good agreement in neutron flux and $(n,T)$ reaction rates, with notable discrepancies in $ (n,2n) $ and photon flux that depend on tally estimators and tracking methods. Delta tracking improves photon agreement but increases runtimes, while default tracking favors Serpent for coupled calculations. A preliminary $^{99}$Mo production scenario illustrates VNS's potential as a medical isotope supplier, supporting the use of VNS as a versatile platform for isotope production and reactor component qualification. Overall, the paper demonstrates robust cross-code validation for a complex fusion-relevant geometry and highlights estimator-driven differences that guide future modeling choices.

Abstract

The Volumetric Neutron Source (VNS) tokamak is a proposed fusion reactor for testing and qualification of reactor components for future use in a fusion power facility, and has potential use for radioisotope production. The VNS geometry is modeled in the Serpent and OpenMC neutronics codes. Analog neutron-photon coupled simulations are carried out to compare the model's vacuum vessel and blanket components across codes. In the vacuum vessel, neutron and photon flux maps are calculated, while in the blanket region, neutron and photon spectra, (n,T), and (n,2n) reaction rates are calculated and compared between models. The detector response comparisons found the following: neutron flux and (n,T) reactions achieved excellent agreement, the (n,2n) detector response had good agreement, and photon flux had regional discrepancies depending on Serpent tracking used. Hybrid tracking lead to a relative difference of about 20% in the outboard side blanket, where as employment of delta tracking resulted in less than 1% relative difference. On an HPC cluster, Serpent was found to have shorter computation time than OpenMC in neutron photon coupled simulations using both hybrid tracking and delta tracking, but longer in neutron only simulations. An exemplary radioisotope production case is presented for the demonstration of additional VNS capabilities.

VNS Tokamak OpenMC-Serpent Validation for Medical Isotope Studies

TL;DR

The study validates cross-code reliability for a Volumetric Neutron Source (VNS) tokamak model by comparing Serpent and OpenMC in neutron-photon coupled transport, focusing on vacuum vessel and blanket regions and extending to a demonstrative Mo medical isotope production case. Using high-statistics simulations and ENDF/B-VII.1 data, the work finds overall good agreement in neutron flux and reaction rates, with notable discrepancies in and photon flux that depend on tally estimators and tracking methods. Delta tracking improves photon agreement but increases runtimes, while default tracking favors Serpent for coupled calculations. A preliminary Mo production scenario illustrates VNS's potential as a medical isotope supplier, supporting the use of VNS as a versatile platform for isotope production and reactor component qualification. Overall, the paper demonstrates robust cross-code validation for a complex fusion-relevant geometry and highlights estimator-driven differences that guide future modeling choices.

Abstract

The Volumetric Neutron Source (VNS) tokamak is a proposed fusion reactor for testing and qualification of reactor components for future use in a fusion power facility, and has potential use for radioisotope production. The VNS geometry is modeled in the Serpent and OpenMC neutronics codes. Analog neutron-photon coupled simulations are carried out to compare the model's vacuum vessel and blanket components across codes. In the vacuum vessel, neutron and photon flux maps are calculated, while in the blanket region, neutron and photon spectra, (n,T), and (n,2n) reaction rates are calculated and compared between models. The detector response comparisons found the following: neutron flux and (n,T) reactions achieved excellent agreement, the (n,2n) detector response had good agreement, and photon flux had regional discrepancies depending on Serpent tracking used. Hybrid tracking lead to a relative difference of about 20% in the outboard side blanket, where as employment of delta tracking resulted in less than 1% relative difference. On an HPC cluster, Serpent was found to have shorter computation time than OpenMC in neutron photon coupled simulations using both hybrid tracking and delta tracking, but longer in neutron only simulations. An exemplary radioisotope production case is presented for the demonstration of additional VNS capabilities.

Paper Structure

This paper contains 12 sections, 1 equation, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Volumetric Neutron Source (VNS)
  3. Codes
  4. Serpent
  5. OpenMC
  6. Model
  7. Simulation and Results
  8. Reaction rate comparison
  9. Spectral flux comparison
  10. Simulation time comparison
  11. Medical isotope generation
  12. CONCLUSIONS

Figures (5)

  • Figure 1: VNS Overview: Neutral Beam Injector (NBI), Inboard Blanket (IBB), Outboard Blanket (OBB), Test Blanket Module (TBM), Diagnostic Modules (DIAG) Plasma,, Vacuum Vessel (VV), Central Solenoid (CS), Toroidal and Poloidal Field Coils (TFC's and PFC's), Blanket Cooling Pipes, Divertor, and Cryostat
  • Figure 2: OpenMC and Serpent ("O" and "S" subscripts) neutron and photon flux in cut of vacuum vessel
  • Figure 3: OB blanket OpenMC to Serpent relative difference of neutron flux, $(n,T)$, and $(n,2n)$ reacitons
  • Figure 4: Flux spectra comparison in blanket regions
  • Figure 5: Simulation times on HPC