Neutronic studies on decoupled hydrogen moderator for a short-pulse spallation source

Harada, Masahide; Watanabe, Noboru; Teshigawara, Makoto; Kai, Tetsuya; Ikeda, Yujiro

Nuclear Instruments and Methods in Physics Research A, 539(1-2), p.345 - 362, 2005/02

https://doi.org/10.1016/j.nima.2004.10.013

Neutronic studies of decoupled hydrogen moderators were performed by calculations taking into account para hydrogen content, decoupling energy, moderator dimensions/shapes and reflector material. Low-energy parts of calculated spectral intensities with different para hydrogen contents were analyzed by a modified Maxwell function to characterize neutron spectra. The result shows that a 100% para hydrogen moderator gives the highest pulse peak intensity together with the narrowest pulse width and the shortest decay times. Pulse broadening with a reflector was explained by time distributions of source neutrons entering into the moderator through a decoupler. Material dependence of time distribution was studied. A decoupling energy higher than 1 eV does not bring about a large improvement in pulse widths and decay times, even at a large penalty in the peak intensity. The optimal moderator thickness was also discussed for a rectangular parallelepiped shape and a canteen shape moderators.

Premoderator optimization of decoupled hydrogen moderator

Harada, Masahide; Teshigawara, Makoto; Kai, Tetsuya; Sakata, Hideaki*; Watanabe, Noboru; Ikeda, Yujiro

JAERI-Research 2001-016, 32 Pages, 2001/03

JAERI-Research-2001-016.pdf:1.74MB

An optimization study on the premoderator, the reflector material choice and a length of the liner is carried out for the design of high performance decoupled hydrogen moderator. NMTC/JAM and MCNP-4C are used for the neutronics calculation. The result indicates that, assuming premoderator dimensions and decoupling energy is controlled, the decoupled hydrogen moderator with a premoderator can provide better pulse characteristics than that without the premoderator for a Be reflector. On the selection of the reflector material, it is clearly shown that Pb and Hg reflectors give merits in using the premoderator for higher intensity and reduction of energy deposition in moderator. It is also shown that a H2O premoderator provides a short tail while a D2O premoderator provides the high peak intensity. Minimum liner length is evaluated to be 20 cm from the viewpoint of neutronics.