Speaker
Description
The inner reflector plug (IRP) at SNS, hosting moderators and reflectors, is an essential complex in delivering desired neutron pulses to all the instruments. The lengthy and complicated manufacturing process usually means a well advanced but very short window for physics and engineering design. Currently SNS just starts operating its third IRP (IRP-III), the fourth IRP (IRP-IV) is under manufacturing and a conceptual study on the future IRP (IRP-V) has been completed. It was found the decoupled and coupled moderators have different requirements on the energy spectra of the feed-in neutrons hence favor different reflector configurations. A combination of beryllium and stainless steel in different sizes and ratios serves best for the decoupled and coupled moderators. The beryllium volume was thus reduced by ~60%, or a saving of ~$ 0.75M at little or no loss to moderator performance. A recent study showed that the extinction effects due to crystallite in various grades of beryllium produced insignificant impact on the decoupled moderators while a slight drop on the performance of the coupled moderator. However, the moderator performance due to the volume reduction of the beryllium reflector was not impacted by the extinction effects for the same grade of beryllium. A full study of the poison and decoupler burn up history was also performed to find their right thickness for improving the lifetime of the IRP from 38 GWhr to 50 GWhr, the radiation damage limit on the Al-6061 structure of the IRP. These efforts proved the gadolinium poison plates have to be thickened from 0.8 mm to 1.1 mm for the decoupled hydrogen moderator, and from 1.3 mm to 1.7 mm for the decoupled water moderator. Furthermore, the cadmium decoupler thickness has to be increased from 1.4 mm to the estimated 1.9 mm to achieve the lifetime goal. Inevitably, it impacts initial moderator performance by up to ~7% in the decoupled moderators. However, an ~30% increase of lifetime for IRP would save ~$ 3M over an operation of 5 years.