Abstract Nondestructive gamma-ray spectrometry of nuclear fuel is routinely performed in axial gamma scanning devices and more recently with gamma emission tomography. Following the irradiation of a fresh nuclear fuel with high intensity neutron flux in a nuclear reactor core, a great number of gamma-emitting radionuclides are created. These can be utilized for gamma spectrometric techniques. However, due to the high density and atomic number of the nuclear fuel, self-attenuation of gamma-rays is a challenge, which requires attenuation correction in order to perform accurate analysis of the source activity in the fuel. In this study, the degradation of the gamma-ray mass attenuation with burnup was investigated and, in addition, a predictive model was created by investigating the attenuation change at various gamma energies caused by the burnup of the nuclear fuel. This model is intended for use by spectrometry practitioners inspecting nuclear fuel. To this aim, the energy-dependent gamma-ray mass-attenuation coefficients were investigated as a function of burnup for UOX, and three MOX fuels having different initial Pu contents. The Serpent 2 reactor physics code was used to simulate the burnup history of the fuel pins. The nuclide inventory of the Serpent 2 output is combined with the NIST XCOM database to calculate the mass attenuation coefficients. The mass attenuation coefficient of the fuel was found to decrease with the fuel burnup, in the range of a few percent, depending on the burnup and gamma energy. Also, a theoretical burnup dependent swelling model was imposed on fuel density to see how linear attenuation coefficient of fuel material is changed. Furthermore, greater effect may be expected on the transmitted intensity, where a simulation study of a PWR assembly revealed that the contribution from the inner rods in a scanned fuel assembly increased by tens of percent compared to the one with non-irradiated fresh fuels, when shielded by the outer rods of the assembly. A sensitivity analysis was performed in order to test the effect of a number of geometrical and operational reactor parameters that were considered to potentially effect the mass attenuation coefficient. Finally, a simple-to-use predictive model was constructed providing the mass-attenuation coefficient [cm2/g] of fuel as a function of burnup [MWd/kgHM] and initial Pu content [wt%]. The resulting predictive model was optimized by using the nonlinear regression method.

中文翻译：

---

UOX 和 MOX 燃料的伽马射线质量衰减随核燃耗的降解

摘要 核燃料的非破坏性伽马射线能谱分析通常在轴向伽马扫描设备中进行，最近使用伽马发射断层扫描进行。在核反应堆堆芯中用高强度中子通量对新鲜核燃料进行辐照后，会产生大量发射伽马的放射性核素。这些可用于伽马光谱技术。然而，由于核燃料的高密度和原子序数，伽马射线的自衰减是一个挑战，需要衰减校正才能对燃料中的源活动进行准确分析。在这项研究中，研究了伽马射线质量衰减与燃耗的退化，此外，通过研究由核燃料燃烧引起的各种伽马能量的衰减变化，创建了一个预测模型。该模型旨在供检查核燃料的光谱从业人员使用。为此，研究了与能量相关的伽马射线质量衰减系数与 UOX 和具有不同初始 Pu 含量的三种 MOX 燃料的燃耗的函数关系。Serpent 2 反应堆物理代码用于模拟燃料棒的燃耗历史。Serpent 2 输出的核素清单与 NIST XCOM 数据库结合计算质量衰减系数。发现燃料的质量衰减系数随着燃料的燃耗而降低，在几个百分点的范围内，这取决于燃耗和伽马能量。还，对燃料密度施加了理论燃耗相关膨胀模型，以查看燃料材料的线性衰减系数是如何变化的。此外，可能会对透射强度产生更大的影响，其中压水堆组件的模拟研究表明，与未辐照的新鲜燃料相比，扫描燃料组件中内杆的贡献增加了几十个百分点，当由组件的外杆屏蔽。进行了敏感性分析以测试被认为可能影响质量衰减系数的许多几何和运行反应堆参数的影响。最后，构建了一个简单易用的预测模型，提供燃料的质量衰减系数 [cm2/g] 作为燃耗 [MWd/kgHM] 和初始 Pu 含量 [wt%] 的函数。