Theoretical Study on Yield of Fission Nuclide after Irradiation of Low-enriched Uranium Target

The production of medical radionuclides such as ⁹⁹Mo and ¹³¹I with low enriched uranium as irradiation target instead of high-enriched uranium is an irreversible trend. In this paper, the lattice transport code DRAGON was used to analyze the yields of more than 10 nuclides (including ⁹⁹Mo, ¹³¹I, ⁹⁰Sr, ⁹⁵Zr and ²³⁹Pu, etc.) with different ²³⁵U enrichments, neutron fluence rates and irradiation time. And the composition and total specific activity of fission system were also studied. The results show that the specific activity of nuclides as a function of irradiation time is found to be different. Of which, the specific activity of ⁹⁹Mo, ¹³¹I, ¹³³Xe and 147Nd can saturate rapidly, the specific activity of ⁸⁹Sr, ⁹⁵Zr, ¹⁰³Ru and ¹⁴¹Ce saturates slowly, whereas the specific activity of ⁸⁵Kr, ⁹⁰Sr, ⁹⁹Tc and ²³⁹Pu can not saturate until 120 d. The specific activity of all nuclides as a function of irradiation time is unrelated to ²³⁵U enrichment, but the specific activity of the nuclides such as ⁹⁹Mo, ¹³¹I, ⁹⁰Sr and ⁹⁵Zr is linear function of ²³⁵U enrichment. Specially, the specific activity of ²³⁹Pu increases with the decrease of ²³⁵U enrichment, and it implies that the effect of ²³⁹Pu should be paid more attention if the low-enriched uranium is used to product medical radionuclide such as ⁹⁹Mo and ¹³¹I. Additionally, it could be noted that the specific activity of nuclides increases linearly with neutron fluence rate, and the slopes of different nuclides which are same group are similar. However, the irradiation time and enrichment of ²³⁵U have no remarkable effect on the composition of fission system, especially in the region of low-enriched uranium (content of ²³⁵U≤20%).