大型核设施三维辐射场快速耦合计算方法研究

The requirement of fast three-dimensional radiation field calculation is proposed during the decommissioning of large and complex nuclear facilities. But the traditional method could not meet this requirement. The Monte Carlo technique is difficult to give reliable results in a reasonable time for the large scale system and the deep penetration radiation transport. The analytical means like the method of Discrete Ordinates is limited in application to complex geometry and source distribution, and not suitable for fast simulation. The Point Kernel approach is fast but has integral divergence close to the source. Therefore, it is necessary to develop a coupled calculating scheme to meet the requirement of fast and whole space simulation. A coupled Monte Carlo-Point Kernel computational method is adopted in the project. The transport near the source region is simulated using the Monte Carlo method which is by nature good at treating complex geometries. The radiation field in the bulk shielding region is treated by the point kernel approach to avoid the extremely expensive computation for deep penetration problems. The project will study the key algorithms of coupled simulation scheme, and develop a fast three-dimensional radiation field simulation program. Finally benchmark examples will be selected to verify the effectiveness of the coupled program.

大型核设施退役过程中提出了对全空间辐射场快速计算的需求，而传统的辐射场计算方法不能满足这个需求。目前常用的蒙特卡罗方法对大系统深穿透问题，很难在合理的时间内给出可靠的结果；数值方法如离散纵标方法则不适合复杂几何和源项，且不适合于快速计算；点核估算法虽然快速，但在源项周围结果不准确。因而我们有必要开发一种耦合方案来适应全空间快速计算的需求。本项目采用的蒙卡-点核耦合计算方法，在复杂几何和源项区域采用蒙卡方法进行模拟，发挥蒙卡方法不受复杂几何结构限制的优点，在大空间厚屏蔽区域采取点核积分进行快速计算，极大地缩短计算时间。本项目将研究耦合计算中的核心算法，开发三维辐射场快速计算程序，并选取基准算例对新开发程序进行基准校验。

基于蒙卡-点核耦合的计算方案，本项目开发了一套可以用于大型核设施三维辐射场快速计算的程序系统。耦合计算程序结合了蒙卡方法和点核积分方法各自的优点，解决了在处理复杂源项和几何的同时，进行大尺度厚屏蔽体剂量评估的问题。本项目选取源自典型废物处理设施的简化模型对耦合程序进行了基准校验和对比分析。和全蒙卡计算相比，耦合程序在不影响计算精度的前提下，计算时间比单纯蒙卡小一个量级左右。耦合程序可以在核设施的退役过程中，伴随着拆除工作的进行，快速地给出新的三维辐射场分布。使用耦合程序进行快速计算可以防止在实际的拆除工作中，在辐射源项和屏蔽结构已经发生变化的情况下，仍然采用旧的剂量数据而产生不必要的照射。因而，本项目开发的耦合计算程序系统满足了退役场所三维辐射场快速计算的需求。