Coupled calculation of the influence of the para to ortho conversion in a cryogenic hydrogen moderator

We will analyze the dynamics of ortho and para-hydrogen in a moderator. The energy transferred by neutrons during moderation tends to turn para into ortho, and the opposite reaction, ortho decaying to para at low temperatures, is very slow if not accelerated. Thus, there is a chance that ortho-hydrogen represents actually a significant fraction of the total hydrogen. Besides the moderator needs to be recirculating, and the Fluid Dynamics must be modeled, as radiation heat could accumulate in some spots, and raise the hydrogen temperature. This, in turn, would affect the moderating ability of the hydrogen, since ortho has a much greater cross section than para.A Computer Fluid Dynamics (CFD) model has been used to perform the thermohydraulic calculations for a cylindrical moderator, and obtain an estimation of the temperatures in it. Using those temperatures, the ortho-para balance is estimated, taking into account the different properties of the two forms, and the neutronic calculations have been redone, as a first iteration of the coupled calculations. A comparison of the results between pure para and a more realistic H2 moderator is given and its consequences are discusse.