Blind calculation of the aerosol experiment K187 of the OECD-CSNI international standard problem ISP 44 using MELCOR 1.8.5

In one of the 4 experiments of ISP 44 - the blind one - the behavior of aerosols in the atmosphere of the containment in saturated steam conditions was modelled. The containment was represented by a tank. Different soluble (CsI, CsOH) and insoluble ()Ag) aerosol materials were used. In the K187 test, all the 3 materials were used simultaneously. Except for problems with fixing the initial thermal hydraulic conditions, extremely fast aerosol removal from the atmosphere was observed when using the hygroscopic aerosol model. Such a fast removal was not in agreement with the results of the open tests within this ISP. Therefore, the hygroscopic model was switched off during the calculation in spite of the existence of soluble aerosol particles in the test. Later, when the results of the K187 test were published, it was found that all the participants using MELCOR encountered the same problem and that the hygroscopic model is presumably wrong in MELCOR. The agreement with the experimental results were also much better when the hygroscopic model was omitted from the calculations