Premoderator extension effect for a decoupled hydrogen moderator

To improve neutronic performance of a decoupled super-critical hydrogen moderator, which is one of main moderators in a high intensity pulsed spallation neutron source presently under development, neutronic characterization of the moderator surrounded by lead reflectors was studied by using NMTC/JAERI97 and MCNP4A codes. In a case of a flat premoderator installed between a target and the moderator, the gain of a neutron intensity reached to its maximum at l.5 cm of the premoderator thickness. Then, the nuclear heat in the moderator decreased with increasing premoderator thickness and the pulse time FWHM was not changed much. These results indicate that the introduction of premoderator is very effective for improvement of a thermal moderator. The premoderator which surrounded the moderator and extended to the neutron beam hole was turned out to be effective. Also, it was shown that the premoderator in lead reflectors was effective on neutronic performance in comparison with those in beryllium reflectors. Moreover, a poison was not favorable for the MW-class spallation neutron source because of a great deal of nuclear heat generation. And it was found that best decoupling energy was 1 eV for moderator characterization. Then, the optimized moderator size and viewed surface size were shown to be 12 x 12 x 5 cm3 and 10 x 10 cm2, respectively. Based on these results, we proposed a standard decoupled hydrogen moderator concept. (author)