Using closed thermosiphons for cooling the spent-fuel pools of nuclear power plants requires calculation tools for simulating the thermal performance of long thermosiphons. The ability of the computer program THERESA to simulate the thermal performance of a vertical thermosiphon with water as the working fluid has been investigated. THERESA was used for simulating a cylindrical thermosiphon with a one-meter-long evaporator and a 32-mm inside diameter. The condenser section had the same inside diameter and a length of 0.5 m. Modelling the long thermosiphons of the type required to span the distance from the spent-fuel pool to the atmosphere requires that the effects of hydrostatic pressure in the evaporator and the pressure drop in the vapor between the evaporator and condenser sections be correctly simulated.

The input data to THERESA are the outside wall temperatures of the evaporator and condenser. The thermal resistances between these two surfaces consist of conduction through the walls, the thermal resistance due to vapor-pressure drop in the adiabatic section between the evaporator and the condenser, and the various modes of heat transfer in the liquid phase of the working fluid. The heat transfer in the liquid of the condenser was simulated by a mixture of drop-wise and film-wise condensation. The heat transfer in the evaporator was simulated by a mixture of natural convection, nucleate boiling, and film boiling. After simulating the thermal resistances, THERESA solves for the heat-transfer rate of the thermosiphon.

The simulation results were compared with data from experiments with steady-state boundary conditions. The trends in the experimentally derived heat-transfer coefficients as a function of heat-transfer rate were used to identify the type of phenomena that dominate the processes in the evaporator and the condenser. Additional insight into the instabilities occurring during these experiments led to new methods for calculating the heat-transfer coefficients in THERESA. The number of various heat-transfer phenomena simulated in THERESA was reduced to those with the trends that matched the trends in the experimental data. The heat-transfer coefficients for those dominant phenomena were modified in order to represent a time-averaged value over an entire instability cycle. Empirical constants for the modifications were obtained from the experimentally derived values.

使用封闭的热虹吸管冷却核电站的乏燃料池需要使用计算工具来模拟长热虹吸管的热性能。已经研究了计算机程序THERESA模拟以水为工作流体的垂直热虹吸管的热性能的能力。THERESA用于模拟一米长的蒸发器和32毫米内径的圆柱形热虹吸管。冷凝器部分的内径相同，长度为0.5 m。对跨越从乏燃料池到大气的距离所需的长型热虹吸管进行建模，需要正确模拟蒸发器中的静水压力以及蒸发器和冷凝器段之间的蒸汽压降的影响。

THERESA的输入数据是蒸发器和冷凝器的外壁温度。这两个表面之间的热阻包括通过壁的传导，由于蒸发器和冷凝器之间的绝热部分中的蒸汽压下降引起的热阻以及工作流体液相中的各种传热模式。冷凝器液体中的传热是通过逐滴和薄膜冷凝的混合物来模拟的。蒸发器中的热传递是通过自然对流，成核沸腾和薄膜沸腾的混合物模拟的。模拟热阻后，THERESA求解热虹吸管的传热速率。

将仿真结果与稳态边界条件下的实验数据进行了比较。通过实验得出的传热系数随传热速率的变化趋势可用于确定主导蒸发器和冷凝器过程的现象类型。对这些实验中发生的不稳定性的进一步了解导致了在THERESA中计算传热系数的新方法。THERESA中模拟的各种传热现象的数量减少到与实验数据趋势相匹配的趋势。修改了那些主要现象的传热系数，以表示整个不稳定周期的时间平均值。