Experiments and Calculations of 14 MeV Neutron Streaming through Multi-Layered Slit Assembly

A streaming experiment using a D-T neutron source was carried out to verify the calculational technique for neutron transport in a shield assembly with multi-layered slits. Reaction rate distributions of a small spherical NE213 scintillation detector to fast neutrons were measured in the slits made of 304SS and in the mortar surrounding the slits. The energy spectrum of fast neutrons in the slit was also measured with the same detector. These measurement were compared with calculations using the continuous energy Monte Carlo transport code MCNP. The calculated reaction rates in the slits agreed with the measured ones within experimental and calculational errors. Besides, it is suggested that the attenuation of fast neutrons in the mortar is significantly different from that in the slits and the behavior is nearly traced by the calculation with the MCNP code. The measured and calculated spectra at a position close to the exit inside the lower slit agreed within the both errors.     

Improvement of a Simple Evaluation Method for Duct Streaming by Taking Account of the Deep Penetration Component

A simplified formula for radiation streaming through bent ducts was previously derived by optics analogy, considering only components passing through the duct and without enormous preparatory calculations. Now, by taking account of deep penetrating component, the streaming formula is improved in order to estimate not only γ-ray flux but also neutron flux with accuracy, factor 2–3, applicable to preliminary shielding designs. Dose rates at the exit estimated by the improved formula are compared with experimental values. As for γ-ray evaluated values become conservative by improvement, while evaluated values were underestimated before improvement. Also for neutron, the discrepancy between the calculated values by the present formula and observed ones becomes within factor 2 after improvement from factor 3 before improvement. From these results, it is concluded that the improved formula is applicable to preliminary shielding designs.     

Monte Carlo Analysis of Streaming Effect through Annular Gaps around Piping Legs in Sodium Pool to Secondary Sodium Activation of the Demonstration Fast Breeder Reactor

A top-entry loop-type reactor is one of the favorable options for Demonstration Fast Breeder Reactor (DFBR) which is now under development in Japan as a part of conceptual design study. Annular gaps around top-entry piping raise neutron streaming in upward direction in the sodium pool of reactor vessel. It enhances neutron flux level around decay heat exchangers (DHX's) in the pool and at the penetrations in primary biological shield for piping connecting to the vessels of intermediate heat exchangers (IHX's), and consequently enhance secondary sodium activations in these heat exchangers, which is one of the main issues for shielding design. In this study, three-dimensional Monte Carlo analysis method was applied to make precise evaluation of the neutron streaming effect with combination of some techniques for reduction of statistical error within reasonable CPU time. It is established that the contribution of neutron streaming to the secondary sodium activation in DHX's hardly reaches a level of 30% and that design analysis with two-dimensional discrete ordinates method gives conservative evaluation of the secondary sodium activations in DHX's and IHX's.