The reactivity effect of neutron interaction between two identical units containing low enriched (10% 235 U enrichment) uranyl nitrate solution without neutron isolator was measured in the STACY. The unit has 350 mm of thickness and 690 mm of width and distance between those two units was adjustable from 0 to 1,450 mm. Condition of the solution was about 290 gU/l in uranium concentration, about 0.8 N in free nitric acidity, 24-27• C in temperature and about 1.4 g/cm 3 in solution density. The reactivity effect was estimated from variation of critical solution level from 495 to 763 mm depending on the core distance. The reactivity effect was also evaluated by the solid angle method and a computational method using the continuous energy Monte Carlo code MCNP-4C and the nuclear data library JENDL 3.2. Those estimations were compared and the distance to isolate neutron interaction was overestimated by the solid angle method. The computational method considering neutron reflection from surrounding structures reproduced well the isolation distance given by the experiment.

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