Application of a two-phase thermosyphon loop calculation method to a cold neutron source

A new method for thermodynamic calculations of a two-phase cryogenic hydrogen thermosyphon loop is verified by means of process responses and neutronic performance measurements on the JEEP-II cold neutron source. The method is based on a one-dimensional finite element division, where each time-step is split up in a change of enthalpy and a change in entropy. The method is used to simulate the pressure during cooling from room temperature down to cryogenic temperatures. The simulated void fraction is compared with the neutronic performance of the moderator during the cool down process. The method is also applied for the simulation of the process step responses when the temperature set point is varied and when the applied heat load is changed. Comparisons of the simulated results with actual measurements show that the method can give an accurate representation of the pressure response and resulting void fractions. The method and results presented here should be useful for the optimization of cold sources where accurate control of pressure and knowledge of void fraction are important for efficient operation.