CNN reports it was 2,750 metric tons of ammonium nitrate. Let’s say AN and TNT are equivalent in energy/weight (they aren’t). So 2750 metric tons is 2.750 kilotons.
AN by itself has an r value of about .9 ( TNT=1.0).
So if we assume that some decomposed iot create a
deflagration situation, assume 2.5 MT of AN at .9 would equate to around 2.2 kt yeild.
Of course, perhaps after 6 years of poor storage much more may have been contaminated etc.