1. Analyses to Support Waste Disposition of SNS Inner Reflector Plug
SATIF-13
October 10-13, 2012, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
Analyses to Support Waste Disposition of SNS Inner Reflector Plug
I. Popova, F. X. Gallmeier, S. Trotter, M. Dayton

2. SNS layout
The Spallation Neutron Source in Oak Ridge, Tennessee, is an accelerator driven neutron scattering facility for materials research
SNS operates presently at up to 1.4 Megawatt (MW) proton beam power incident on a mercury target
Target building is designed to house 24 instruments
Presently 19 instruments are operating
10 years beam on target
Proton beam power on target
1.4 MW
Proton beam kinetic energy on target
1.0
GeV
Average beam current on target mA
Pulse repetition rate 60 Hz
Protons per pulse on target
1.5x1014
protons
Charge per pulse on target 24 ˜C
Energy per pulse on target kJ
Proton pulse length on target
695 ns

3. Introduction
According to the SNS operations plan Target System components are replaced:
To avoid excessive degradation of components materials – target vessels and proton beam windows;
When beam line optical components are ready to be installed Core Vessel Insert (CVI) plugs are extracted;
When reach planned end-of-life – Inner Reflector Plug (IRP).
Components must be:
Safely removed;
Placed in containers for storage on site in order to cool down;
Placed into shipping container packages to transport off-site.

4. Introduction
Inner Reflector Plug (IRP) is a central component of the SNS target monolith that is exposed to high-level radiation fields
Builds up significant activity
IRP needs to be replaced due to damage of structural materials caused by high particle fluxes, which reflects in the moderator poisoning
Life-time of IRP was assumed to be about 40,000MWh
IRP will be extracted from the target monolith and segmented into three components, each which will be disposed separately
Scheduled for replacement in March, 2017

5. Introduction
Upper Segment is pulled into
cask

6. Introduction Neutronics analyses for IRP deposition are performed to:
Predict isotope composition for spent structures to support waste characterization and transportation analyses;
Predict dose rates after cool down;
Ensure choice of proper shipping container/package;
Develop shielding for container/package;
Perform transport analyses with real irradiation history for chosen container/package
Analyses are performed for each segment

7. Methods and Codes
Reaction rates and neutron fluxes below 20 MeV are calculated using MCNPX
Calculated results are fed into an Activation Script that provides the interface between MCNPX and the transmutation code CINDER’90
The isotope inventory is obtained for the date of provisional transport of the package, if component has few materials ALLCODE is applied
Photon spectra are extracted by the GAMMA_SPECTRA script from the transmutation analyses output and formatted into a source description for MCNPX
Dose rates are calculated in the package vicinity using MCNPX
Report is generated

8. Methods and Codes
The latest 3D as-built target station and proton beam window model in MCNPX language are used
Effective dose rates are obtained by folding fluxes with flux-to- dose conversion coefficients, which are taken from standardized neutron and gamma flux-to-dose conversion coefficient libraries for the SNS
2D mesh-tallies were defined in the vertical and horizontal planes
Surface tallies were set at requested key locations
For waste classification purposes a report containing isotope inventory, dose rates at specified locations is generated

9. Waste characterization
Report for waste management contains:
History of irradiation;
Radionuclide inventory;
Time line of component activity after beam termination;
Gamma dose rate contour plots;
Predicted dose rates at locations required by the U.S. Department of Transportation (DOT);
Distances from the side and the bottom of the spent component package for which the dose rates fall below 100 mrem/hr and 5 mrem/hr

10. Geometry
Large component - the IRP is cylindrical and about about 39” (100- cm) in diameter
Split into three segments, each segment will be disposed separately

11. Geometry The SNS as-built target station model Target module
Cylindrical IRP, including beryllium and steel reflectors and moderators
Cylindrical outer plug, and the proton beam window assembly

12. Analyses
Analyses are performed for 40,443MWh distributed over 11 years of operation
Isotope inventories are presented for up to one year of cooling down
Dose rates are calculated for 100, 182 and 365 cool down for the top and for the middle segments
Dose rates are calculated for 100 days for the lower segment

13. IRP top segment
Material is stainless steel 316
MCNPX model

14. IRP top segment

15. IRP middle segment
Material is stainless steel 316
MCNPX model

16. IRP middle segment

17. IRP lower segment
Material is stainless steel 316

18. IRP lower segment Beryllium reflector Stainless steel reflector
Aluminum enclosure
Four moderators
Aluminum
Cadmium
Gadolinium
Stainless steel piping
Stainless steel shielding
Moderators
Moderators
Shielding
Aluminum enclosure
Beryllium reflector
Stainless steel reflector

19. IRP lower segment The most irradiated segment Complex geometry
Over 700 cells
Transformations
For residual analyses
Small cells with insignificant distribution were omitted
Due to limitation in the MCNPX source description, two decay gamma sources were prepared
Two transport simulations were performed
Results from the analyses were combined

20. IRP lower segment

21. Conclusions
An accurate estimate of the radionuclide inventory and the dose rates associated with radiation fields of the spent IRP were performed
Awaiting characterization and classification of each segment and to determine the appropriate storage and transport package.
Radiation fields at 30-cm from IRP lower segment after 100 days cool down are above 1000Rem/h.