Research on Obtaining Fast Neutron Spectrum through Radiation Testing of Fuel Rods for Fast Reactor in Thermal Reactor
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摘要: 基于国内高通量工程试验堆（HFETR），对快中子增殖反应堆（简称快堆）燃料棒的中子能谱改造方法及物理特性进行研究。主要研究分析了不同中子屏材料，多种燃料短棒辐照装置，在燃料辐照过程中对中子能谱的改造效果，以及中子屏特性的变化以及对反应堆的影响。通过MCNP程序进行单组件计算，在4种候选中子屏吸收体材料中筛选出碳化硼和氧化铕2种材料，镉和铪因能谱改造效果不佳被排除。比较了不同厚度的吸收体后，发现吸收体在0.3 mm或以上的厚度可实现有效的能谱改造。将装置置于堆内中心孔道计算，装置A内的燃料棒线功率密度较高，引入负反应性较小;装置B内的燃料棒线功率密度较低，引入负反应性较大。考虑燃耗对吸收体的影响，在只考虑燃料和吸收体材料燃耗的条件下，得出装置A的吸收体寿命均小于100 d；装置B的碳化硼有效寿命约300~450 d，氧化铕有效寿命约500~700 d。本文初步提出了可行的能谱改造方案，该方案可以满足在热谱研究堆上开展快堆燃料辐照试验的条件。Abstract: Based on China’s High Flux Engineering Test Reactor (HFETR), research has been conducted on the neutron spectrum modification method and physical characteristics of fast reactor fuel rods. This study analyzed the effects of various neutron screen materials and multiple fuel pin irradiation devices on spectrum modification during fuel irradiation, as well as the changes in neutron screen characteristics and their influence on the reactor’s performance. Single-assembly calculations were performed using the MCNP code, and two materials (boron carbide and europium oxide) were selected from four candidate neutron screen absorber materials, while cadmium and hafnium were excluded due to their suboptimal spectrum modification results. After comparing absorbers of varying thickness, we found that a thickness of 0.3 mm or above achieved effective spectrum modification. When placed in the reactor’s central channel, device A exhibited a higher fuel rod linear power density and a relatively small negative reactivity, while device B displayed a lower fuel rod linear power density and a greater negative reactivity. Considering the impact of burnup on the absorbers under conditions accounting only for fuel and absorber material burnup, we determined that the lifetime of absorber in device A was less than 100 days, while the effective lifetime of boron carbide in device B ranged from 300 to 500 days, and europium oxide had an effective lifetime of 500 to 700 days. This paper preliminarily proposes a feasible spectrum modification scheme, demonstrating that the modification method can meet the requirements for conducting fast reactor fuel irradiation tests in a thermal-spectrum research reactor.
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Key words:
- Spectrum modification /
- Fast reactor fuel /
- Neutron screen /
- MCNP code
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图 1 燃料辐照装置试验段横截面示意图
Figure 1. Cross Section of the Test Section of Fuel Irradiation Devices
图 2 不同装置采用不同吸收体材料的能谱图
α—对应能量区间的中子注量率占总中子注量率的份额。
Figure 2. Neutron Spectrum of Different Devices Using Different Absorbers
图 3 辐照装置MCNP计算模型图
a、b、c分别表示3处位置。
Figure 3. MCNP Calculation Model for Irradiation Device
图 4 装置A、B采用不同吸收体材料的能谱图
Figure 4. Neutron Spectrum of Devices A and B Using Different Absorbers
表 1 不同装置3个能量区间的中子注量率占比
Table 1. Proportion of Neutron Flux in Three Energy Regions for Different Devices
装置类型 中子能量区间 中子注量率占比/% 镉 铪 碳化硼 氧化铕 无吸收体（水） A E ≤1 eV 0.20 3.83 0.08 0.11 8.63 1 eV < E< 0.1 MeV 47.66 39.71 36.52 37.37 44.94 E≥ 0.1 MeV 52.14 56.46 63.40 62.52 46.43 B E ≤1 eV 0.10 3.18 0.03 0.06 7.29 1 eV < E< 0.1 MeV 47.71 36.83 34.29 35.06 41.83 E ≥ 0.1 MeV 52.19 59.99 65.68 64.88 50.88 C E ≤1 eV 2.82 3.56 1.93 1.99 6.34 1 eV < E< 0.1 MeV 45.71 41.12 40.34 40.26 42.07 E ≥ 0.1 MeV 51.47 55.32 57.73 57.76 51.59 
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表 2 不同吸收体厚度3个能量区间的中子注量率占比
Table 2. Proportion of Neutron Flux in Three Energy Regions for Different Absorber Thicknesses
材料 中子能量区间 中子注量率占比/% 0 mm 0.1 mm 0.3 mm 0.5 mm 0.8 mm 1 mm 2 mm 3 mm 4 mm 碳化硼 E ≤1 eV 8.63 2.85 0.81 0.31 0.12 0.08 0.02 0 0 1 eV < E< 0.1 MeV 44.94 45.90 43.51 41.03 38.10 36.52 31.05 27.54 25.40 E ≥ 0.1 MeV 46.43 51.25 55.69 58.67 61.79 63.40 68.94 72.46 74.60 氧化铕 E ≤1 eV 8.63 2.91 0.95 0.41 0.17 0.11 0.03 0.01 0 1 eV < E< 0.1 MeV 44.94 45.30 43.19 41.18 38.75 37.37 32.40 28.76 26.34 E ≥ 0.1 MeV 46.43 51.79 55.86 58.40 61.08 62.52 67.58 71.23 73.66
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表 3 不同吸收体、不同厚度装置A的keff
Table 3. keff of Device A with Different Absorber Materials and Thicknesses
材料 有效增殖因子keff 0 mm 0.1 mm 0.3 mm 0.5 mm 0.8 mm 1 mm 2 mm 3 mm 4 mm 碳化硼 1.39435 1.14889 1.04303 0.9924 0.94158 0.91544 0.84575 0.79439 0.74089 氧化铕 1.39435 1.13025 1.03185 0.98424 0.93567 0.90944 0.8412 0.79095 0.73837
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表 4 装置A中各燃料线功率密度
Table 4. Linear Power Density of Each Fuel in Device A
材料 燃料短棒位置 线功率密度/（W·m−1） MOX燃料棒 UZr燃料棒 碳化硼 上 18912 30679 中 25917 42991 下 25945 42202 氧化铕 上 17693 30852 中 24106 42245 下 24467 41898
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表 5 装置B中各燃料线功率密度
Table 5. Linear Power Density of Each Fuel in Device B
材料 燃料短棒位置 线功率密度/（W·m−1） MOX燃料棒 UZr燃料棒 碳化硼 第1层：a 5274 9028 第2层：a 9511 16038 第3层：a 11466 19157 第3层：b 8460 14395 第3层：c 9644 15735 第4层：a 8721 14754 氧化铕 第1层：a 4861 8653 第2层：a 8746 14765 第3层：a 10654 17800 第3层：b 7742 13067 第3层：c 8714 14592 第4层：a 8024 13994 燃料短棒位置中第几层表示从上到下第几层；a、b、c表示图3b中的位置。
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表 6 装置A、B吸收体的核素燃耗
Table 6. Isotope Burnup of Absorber Materials in Devices A and B
装置类型 核素 百分比燃耗/% 100 d 200 d 300 d 1000 d A 硼-10 65.73 98.71 100.00 100.00 铕-151 79.47 99.26 100.00 100.00 B 硼-10 19.10 37.38 54.42 99.87 铕-151 29.22 51.85 69.24 99.93
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