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

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

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.

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

Introduction Upper Segment is pulled into cask

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

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

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

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

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

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

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

IRP top segment Material is stainless steel 316 MCNPX model

IRP top segment

IRP middle segment Material is stainless steel 316 MCNPX model

IRP middle segment

IRP lower segment Material is stainless steel 316

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

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

IRP lower segment

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.