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

The production of medical radionuclides such as ^99Mo and ^131 I with lowenriched uranium as irradiation target instead of high-enriched uranium is an irreversibletrend.In this paper,the lattice transport code DRAGON was used to analyze the yields of more than 10nuclides(including ^99Mo,^131 I,^90Sr,^95 Zr and ^239 Pu,etc.)with different^235 U 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,^131 I,^133 Xe and 147 Nd can saturate rapidly,the specific activity of ^89 Sr,^95Zr,^103 Ru and ^141 Ce saturates slowly,whereas the specific activity of ^85 Kr,^90 Sr,^99Tc and ^239 Pu can not saturate until 120 d.The specific activity of all nuclides as a function of irradiation time is unrelated to ^235 U enrichment,but the specific activity of the nuclides such as ^99Mo,^131 I,^90Sr and ^95 Zr is linear function of ^235 U enrichment.Specially,the specific activity of ^239 Pu increases with the decrease of ^235 U enrichment,and it implies that the effect of ^239 Pu should be paid more attention if the low-enriched uranium is used to product medical radionuclide such as ^99Mo and ^131 I.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 ^235 U have no remarkable effect on the composition of fission system,especially in the region of low-enriched uranium(content of ^235 U≤20%).