用于核素在线瞬发伽马数据测量的热中子源设计(3)
图8 热柱孔道及其屏蔽门中心准直器优化设计结构图Fig.8 Structure chart of optimization design for thermal column and centre collimator of shielding door
图9 采用不同准直器方案时热柱孔道出口处中子通量密度和中子、伽马总剂量率的空间分布Fig.9 Space distribution for neutron flux density and total dose rate of neutron and gamma at the exit of thermal column by using different collimator schemes
表2 采用优化方案5时热柱孔道束流出口处中子、伽马参数的蒙特卡罗计算结果Table 2 Calculation results of neutron and gamma parametersat the exit of thermal column by using optimization collimator scheme 5计算参数中心束孔计算值束孔外侧最大计算值热中子通量密度,cm-2·s-14.11×1058.22×102总伽马通量密度,cm-2·s-13.27×1042.42×102中子伽马通量密度比值12.56中子剂量率,mSv·×10-2诱发伽马剂量率,mSv·h-11.88×10-13.54×10-3直接伽马剂量率,mSv·h-16.87×10-11.25×10-3总剂量率,mSv·×10-2
图10示出了热柱孔道出口平面半径分别为2、6、8.25、15、25 cm的不同网格内中子通量密度的能谱分布曲线。由图10可知,热柱孔道出口中心束孔位置处(r≤2 cm)的中子能谱呈高斯分布,且中心束孔0.4 eV以上的快中子以及束孔外侧的本底中子均较小,即热柱孔道出口处的中子源为准直性能较好的平行热中子源。
图10 热柱孔道出口平面不同半径区域内的中子能谱分布曲线Fig.10 Neutron energy spectrum distribution at different rings of exit plane of thermal column
4 结论
本文热柱孔道屏蔽门中子束流引出屏蔽装置采用图8中的优化设计方案5,可在热柱孔道出口处得到热中子通量密度大于4.0×105cm-2·s-1、中子伽马通量密度比值大于10、束孔外围中子和伽马总本底剂量率较低的平行热中子束。同时,热柱孔道出口处直径4 cm中心束孔的中子、伽马剂量率较大,须设计中子、伽马捕集器,有效捕获准直束流。束流外围的环境本底剂量率稍大于0.025 mSv·h-1,需在准直束流外围增加必要的屏蔽措施,降低束流外围的环境本底剂量率,确保反应堆稳态2 MW满功率运行时,堆大厅环境剂量率满足辐射安全要求。
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