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College Park, Maryland      June 6 - 10 , 2004

TP21: ZrH as a moderator for pulsed spallation neutron sources

B. J. Micklich, J. M. Carpenter (Argonne National Laboratory)

Several types of neutron scattering instruments, including spectrometers such as HRMECS at IPNS and perhaps TOF-USANS, could profitably use a source of neutrons prolific in the few hundred meV range. A moderator of ZrH2, which has energy levels at n\hω where ħω = 0.137 eV, could provide such a neutron energy spectrum when operated at elevated temperature. ZrH2 has seen application as a moderator for nuclear reactors, either alone or in the form of ZrH-U fuel elements of a TRIGA reactor, because of its unusual temperature-dependent thermalization properties and its high hydrogen density. ZrH2 (0.07066 atoms/bn-cm) has a higher hydrogen atom density than water (0.0662 atoms/bn-cm). Metal alloy phase diagrams for the Zr+H system indicate that the material is stable (defined here as a hydrogen pressure less than or equal to about 1 atm) for ZrH1.9 at 800 K or ZrH1.6 at 1200 K. This pressure could be supported by a vacuum-insulated stainless steel container.

We calculated neutron spectral intensities and neutron emission time distributions for a ZrH2 moderator in the H moderator position at IPNS using the radiation transport code MCNPX and ZrH2 neutron scattering kernels available for temperatures between 300 K and 1200 K. The moderator was decoupled and poisoned with 9.71020 at/cm2 gadolinum at the midplane. No attempt was made to optimize the moderator thickness or the location of the poison plate. Our results show that the spectral intensity over the energy range 0.1 eV to 1 eV increases as the temperature of the moderator increases. Increases up to a factor of three over the installed solid methane moderator are seen in this energy range. These additional neutrons show up in neutron time distributions that have both higher peak intensities and a broader distribution in time.

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