低浓缩铀靶辐照后裂变核素产量的理论研究

用低浓缩铀靶代替高浓缩铀靶辐照进行⁹⁹Mo、¹³¹I等医用放射性核素生产是一个必然的趋势。本文利用输运计算程序DRAGON研究了靶件²³⁵U富集度、中子注量率、辐照时间对⁹⁹Mo、¹³¹I、⁹⁰Sr、⁹⁵Zr、²³⁹Pu等核素比活度变化的影响，以及不同²³⁵U富集度下裂变体系组成和总比活度的变化规律。计算结果表明，本文考察的10余种核素比活度的变化随辐照时间的不同而有所不同，其中⁹⁹Mo、¹³¹I、¹⁴⁷Nd和¹³³Xe等核素的比活度可快速达到饱和，⁸⁹Sr、¹⁰³Ru、⁹⁵Zr和¹⁴¹Ce等缓慢达到饱和，而⁹⁹Tc、⁸⁵Kr和⁹⁰Sr、²³⁹Pu在计算时间内达不到饱和，但所有核素的比活度随时间的变化趋势与靶件²³⁵U富集度无关；⁹⁹Mo、¹³¹I、⁹⁰Sr、⁹⁵Zr等核素的比活度均随靶件²³⁵U富集度提高而增加，而²³⁹Pu比活度则随着靶件富集度的减少而显著增加，提示改用低浓缩铀靶进行⁹⁹Mo、¹³¹I等医用放射性核素生产时应特别关注²³⁹Pu带来的影响；核素比活度随中子注量率的增加而线性增加，且斜率基本相同；靶件辐照时间的改变不会明显影响裂变体系的组成，在低浓缩铀（²³⁵U含量≤20%）区域，靶件²³⁵U富集度对裂变体系的组成影响很小。

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 147Nd 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%).