Correct me if I'm wrong, but from what I see NaNs are not supported by Cassandra Decimal type.
From the python driver I tried to INSERT values that are python Decimal, some of them are NaN. An exception is thrown in cassandra.cqltypes.DecimalType.serialize (cassandra/cqltypes.py line 454 (line 14 in code block below))
From Python documentation :
Decimals also include special values such as Infinity, -Infinity, and NaN. The standard also differentiates -0 from +0.
If I'm correct, Cassandra python driver should support Decimal('NaN') when converting to CQL types in cassandra.cqltypes.DecimalType.serialize.
Linux (Ubuntu 18.04 & Manjaro 5.7)