3.3 Up-Grade of the Cold Moderator with a Study of Flow and Temperature


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Fig. 3-7 Layout of the spallation target system and structure of the cold moderator

When a high energy proton beam strikes the mercury target, high intensity neutrons are generated by spallation reactions. The moderators divide the neutrons into several energy levels so as to utilize them in neutron scattering experiments. In particular, a cold moderator using supercritical hydrogen is necessary to make the moderator vessel's shape flat like a lunch box while realizing high neutronic performance and suppressing the hydrogen temperature rise.


Fig. 3-8 Experimental and analytical flow patterns in the cold moderator

The flow patterns in the vessel were visualized with a PIV system under water flow conditions simulating the hydrogen flow. A recirculation flow and a stagnant flow region are clearly seen in the figure. The analytical results predicted by a design code agree very well with the experimental result.


Fig. 3-9 The analytical temperature distribution under 2 MW proton beam operation

The temperature distribution in the cold moderator vessel was analyzed by using the verified design code. A temperature rise of about 2K is seen in the stagnant flow region.



In the spallation target system, the moderator is an important component that directly affects neutronic performance (Fig. 3-7). In particular, we are developing a cold moderator using supercritical hydrogen which aims at the highest neutronic performance
(5 x 1012 n/cm2/sr/MW) in the world. In order to realize such a cold moderator, it is necessary to suppress the local temperature rise of hydrogen in the moderator vessel, which affects neutronic performance. A full-scale model made of acrylic resin, simulating the inner structure of the moderator vessel, was fabricated to measure flow patterns affecting the temperature rise. Flow patterns measured by a PIV (Particle Image Velocimetry) system showed recirculation flows and stagnant flow regions that were induced by the impinging jet flow. Also, analytical flow patterns predicted with a design code agreed very well with the experimental results (Fig. 3-8).
The thermal-hydraulic behaviors of hydrogen in the moderator vessel were estimated with the design code. It is possible to limit the local temperature rise to within 3K, which is indispensable for realizing high neutronic performance under 2 MW proton beam operation (Fig. 3-9). Currently, we are improving the structural design of the moderator vessel to secure a much higher neutronic performance.


Reference
T. Aso et al., Development of Cold Moderator Structure, Proc. of ICONE-7, Apr. 19-23, 1999, Tokyo, Japan (1999).

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