[en] The development of new low enrichment nuclear fuels for research and radioisotope production reactors imposes the knowledge of properties and behaviors about a series of alloys which the reducing of U²³⁵ (fissionable) concentration is compensated with a greater density of this element inside the fuel. One of these series is composed by U alloys with different contents of alloying, that allow to retain the body centered cubic structure solid solution recognized as phase α in metastable condition at low temperatures. For the present work 10 U based alloys were manufactured with different concentrations containing up to 43,7 % zirconium weight and up to 7,3 % niobium weight. An arch oven was utilized with argon atmosphere. The identification of the present phases in massive samples from the melting was carried out through X-rays diffraction analysis. The results obtained in this work are compared with others results published since the year 1957. In the samples melted the intermetallic UZr₂ diminishes in quantity with the reduction of the composition of Zr in the alloys. In all of them were identified, besides, Zr₆Fe₃O, ZrO_0,35, α and U₃O₈ present in quantities reduced. The quantity of the two last phases diminishes at the same time with the content in Zr. The parameter of network of the cubic phase γU in these alloys can be represented for the equation: α=(3,5796 -0,1616.x_Nb+0,1155.x_Zr)/(1.0306+0,003.x_Nb-0,0068.x_Zr. The parameter of network of the γ phase was measured. Comparing it measured with the value calculated, for eight alloys, the proposed equation showed a very good adjustment (HC)