Declassified 12 Sep 1973. Development of UN in four major areas is presented: 1) powder synthesis and powder metallurgical fabrication, 2) physical and mechanical property testing, 3) fuel-cladding compatibility and barrier development, and 4) irradiation evaluation. Synthesis and sintering methods for producing large quantities of high-densily and high-purity UN are described. Results of experimental studies of the following properties are summarized: melting point, thermal conductivity, thermal expansion, vapor pressure, and thermodynamics, heat capacity, hot hardness, compressive creep, nitrogen self- diffusion, and electrical resistivity. Compatibility tests are described which demonstrated the need for a diffusion barrier. Lithium soak tests for up to 11,000 h at 2200 deg F demonstrated the stability and practicality of vapor- deposited tungsten-lined Nb--1 Zr alloy over the projected life and temperature of SNAP50. Similar static tests of purposely defected simulated fuel pins indicate a relatively high degree of stability of UN towards lithium. Instrumented capsule irradiation tests of simulated Nb--1 Zr alloy - clad fuel pins are described under 2 Mw(t) and 8 Mw(t) SNAP-50 conditions. Under 8 Mw(t) conditions, 20% fission gas release and 2% diametral cladding growth were observed after 2750 h at 2200 deg F (2.0 at.% U burnup). In-pile operation under 2 Mw(t) conditions was achieved for 5940 h at 2000 deg F (1.0 at.% U burnup) and 3360 h at 2200 deg F (1.5 at.% U burnup) while experiencing less than 0.2% fission gas release and less than 0.4% diametral growth. (66 figures) (auth).