Physics study of liquid hydrogen moderator of CMRR reactor cold neutron source

The neutronic performance of a liquid hydrogen (LH) moderator is affected by its constituent ratio of para-hydrogen (pH) to ortho-hydrogen (oH) due to the difference of their scattering cross sections, as well as by the moderator geometric structure. Initially, the transmission and reflection properties of pure pH and oH slabs were analyzed. These indicated that the reflection method would generate a higher cold neutron flux (CNF) than the transmission method for both pH and oH, and that oH has a better reflection performance than pH. Secondly, the slab LH moderator located at the neutron channel entrance in the reactor was investigated under different conditions viz. pH percentage, slab thickness and neutron channel transparency. Results show that the slab thickness of the LH moderator in the range of 3-5 cm is appropriate for mixed pH and oH, that the optimum value of the pH proportion decreases with neutron channel transparency (optimum pH proportion for absorbent, translucent and transparency neutron channel walls are 100%, 75% and 50% respectively), and that the CNF with a transparent wall is 2-3 times higher than that with an absorbent wall. Bending the LH slab to various degrees showed that the CNF with an absorbent wall channel is determined primarily by the curvature of a concave moderator-viewed surface. However, the CNF for the transparent wall case is mainly determined by the viewed surface area due to a contribution of neutrons reflected from the heavy water. In the case of a 2 cm thick aluminum-alloy channel wall, we obtain an optimized geometry of a half-cylinder shape, balancing the area and curvature of the viewed surface. For 75% pH and 100% pH components, compared with the reference LH moderator configuration, the CNF of our optimized moderator increased by 41% and 20%, respectively.