亮点

Highlights

1. 文章提出了燃料球内的裂变产物精细扩散计算方法，以提高堆芯裂变产物释放源项计算精度。采用一维稳态导热模型考虑非均匀扩散系数分布的影响。耦合盘存量计算程序NUIT考虑现实运行历史下的核素产生率变化。
2. 文章基于蒙特卡罗随机抽样方法提出了一种精细到球的全堆裂变产物释放计算模型，有效解决了传统方法均匀化、集总化引起的简化误差。并提供了释放率空间分布的计算思路。

1. The paper proposes a refined calculation method for fission product diffusion within fuel pebbles to improve the accuracy of the core's fission product release source term. It uses a one-dimensional steady-state heat conduction model to account for the impact of non-uniform diffusion coefficient distribution. It is coupled with the inventory calculation program NUIT to consider changes in nuclide production rates under realistic operational histories.
2. The paper introduces a full-core fission product release calculation model, refined down to the individual pebble level, based on the Monte Carlo random sampling method. This effectively addresses the simplification errors caused by homogenization and lumping in traditional methods and provides a framework for calculating the spatial distribution of the release rate.

研究背景

Research Background

高温气冷堆的裂变产物释放行为对安全评估至关重要。我国现有方法（如FRESCO-II）采用保守参数，均匀化处理，导致释放率估算偏高，影响经济性和计算精度。国际上已发展更精确的模拟方法（如STACY、MELCOR）。本文提出改进模型，结合精细扩散计算和蒙特卡罗全堆释放模拟，以提高释放率预测的准确性。

The fission product release behavior in High-Temperature Gas-Cooled Reactors (HTGRs) is crucial for safety assessment. Existing methods in China (such as FRESCO-II) use conservative parameters and homogenization, leading to overestimated release rates, which affects both economic efficiency and calculation accuracy. More precise simulation methods (like STACY, MELCOR) have been developed internationally. This paper proposes an improved model that combines refined diffusion calculations with a Monte Carlo full-core release simulation to enhance the accuracy of release rate predictions.

研究方法

Research Methods

1. 精细化球内扩散计算方法：利用盘存量计算程序NUIT，平衡堆芯中子注量率分布，计算球内裂变产物产生率历史，功率历史。利用稳态导热方程、平衡堆芯温度分布，计算基体温度分布，将燃料区分为多个子区，计算每个子区中颗粒温度分布。
2. 随机球全堆释放计算方法：设定堆芯为平衡堆芯状态，划分二维堆芯网格。新鲜随机球不断投入堆芯，在网格内进行燃耗计算、温度场计算、扩散计算，并留下状态点。随机球在网格间跳跃，于堆低检测燃耗决定是否卸出。对网格状态点加权获得平均值，全部网格属性代表着平衡堆芯属性分布。

1. Refined Intra-Pebble Diffusion Calculation Method: Utilizes the NUIT inventory calculation program to balance the core's neutron fluence rate distribution and calculate the history of fission product generation rates and power within the pebble. It uses the steady-state heat conduction equation and the equilibrium core temperature distribution to calculate the matrix temperature distribution. The fuel zone is divided into multiple sub-regions to calculate the particle temperature distribution in each.
2. Random Pebble Full-Core Release Calculation Method: The core is set to an equilibrium state and divided into a two-dimensional grid. Fresh random pebbles are continuously fed into the core, where they undergo burnup, temperature field, and diffusion calculations within the grid, leaving behind state points. These random pebbles "jump" between grid cells and are checked for burnup at the bottom of the core to determine if they should be discharged. A weighted average of the state points in each grid cell is calculated, and the properties of all grid cells collectively represent the attribute distribution of the equilibrium core.

研究结果

Research Results

1. 精细化球内扩散计算方法：长寿命金属裂变产物浓度呈阶梯形增长，温度分区模型中基体裂变产物需更远的平均扩散距离。
2. 随机球全堆释放计算方法：100个随机球产生了30902个状态点，堆芯释放率相对偏差小于5%。
3. 长寿命核素堆芯释放率分布主要受堆芯温度分布影响，短寿命核素堆芯释放率分布受堆芯中子注量率分布及温度分布的共同影响。

1. Refined Intra-Pebble Diffusion Calculation Method: The concentration of long-lived metallic fission products shows a step-like increase. In the temperature zoning model, fission products in the matrix require a longer average diffusion distance.
2. Random Pebble Full-Core Release Calculation Method: 100 random pebbles generated 30,902 state points, with the relative deviation of the core release rate being less than 5%.
3. The core release rate distribution of long-lived nuclides is primarily influenced by the core temperature distribution, while that of short-lived nuclides is affected by both the core neutron fluence rate distribution and the temperature distribution.

结论

Conclusion

1. 精细化球内扩散计算方法：精细扩散计算方法能考虑燃料球内非均匀温度分布对核素扩散快慢的影响，能考虑变化核素产生率对浓度积累的影响，并进行更现实的扩散释放计算。
2. 随机球全堆释放计算方法：随机球堆芯释放计算方法能反映燃料球真实的运行历史，产生多种可能的释放率状态，并基于此获得更现实的堆芯释放率分布。核素产生率变化对长寿命核素浓度积累影响较大，温度分布对扩散速度的影响较大，精细扩散计算方法较传统方法燃料球释放率更低。

1. Refined Intra-Pebble Diffusion Calculation Method: The refined diffusion calculation method can account for the effect of non-uniform temperature distribution within the fuel pebble on nuclide diffusion rates. It can also consider the impact of varying nuclide production rates on concentration accumulation, enabling more realistic diffusion and release calculations.
2. Random Pebble Full-Core Release Calculation Method: The random pebble core release calculation method can reflect the actual operational history of fuel pebbles, generating a variety of possible release rate states. Based on this, a more realistic core release rate distribution can be obtained. Changes in nuclide production rates have a significant impact on the accumulation of long-lived nuclides, while temperature distribution has a major effect on diffusion speed. The refined diffusion calculation method results in a lower fuel pebble release rate compared to traditional methods.