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Experience with Inner Reflector Plug exchange in SNS

by Irina I Popova, Franz X Gallmeier, Scott O Schwahn, Michael J Dayton, Christi L Elam
Publication Type
Conference Paper
Book Title
Shielding Aspects of Accelerators, Targets and Irradiation Facilities (SATIF-14)
Publication Date
Page Numbers
167 to 179
Publisher Location
United States of America
Conference Name
14th Specialists' workshop on Shielding aspects of Accelerators, Targets, and Irradiation Facilities
Conference Location
Gyeonju, South Korea
Conference Sponsor
OECD Nuclear Energy Agency (NEA), Korea Multi-purpose Accelerator Complex (KOMAC), Pohang Accelerator Laboratory (PAL)
Conference Date

The Inner Reflector Plug (IRP) is a central component of the Spallation Neutron Source target monolith, which houses the mercury target and four liter-sized neutron moderator units. It is exposed to high-level radiation fields during routine operation and builds up significant activity. The IRP needs to be replaced due to moderator neutron poison and decoupler burn-out, which is used for shaping neutron pulses. The first IRP exchange took place in March 2018. The old IRP was extracted from the target monolith, providing space for the new one. It was split into three segments, each of which was handled separately. The lowest section of the IRP is the largest segment in size and in activity and is temporarily stored on-site for cool down before conduction post irradiation examination. In support of planning the replacement activities, a wide range of activation and transport analyses were performed. This included calculating isotope inventories and the radiation fields for each segment as it is extracted in storage casks, and the radiation field from the empty IRP pit in the target monolith. While the replacement was taking place, measurements were performed and later on compared to the calculations. During these studies, it was discovered that a significant contributor to the radiation field from the lower IRP segment is from photo-neutrons. Photo-nuclear physics was added to the analyses and calculated results compared well with measured dose rates.