The function is_stable() is misnamed.

[karnigen]

I believe there is a naming inconsistency with the function is_stable(). Currently, it returns true for unstable nuclides, which is counterintuitive. A more logical behavior would be to return true for stable nuclides and false for unstable ones.

I suggest renaming the function to is_unstable() to better reflect its current behavior. Alternatively, we could keep the name is_stable() but invert the return values.

Python is_stable() wraps c++ isStable() that simply returns STA of MF=1 MT=451.
Meaning of STA from ENDF documentation 1.1.

STA Target stability flag:

• STA=0, stable nucleus;
• STA=1 unstable nucleus. If the target is unstable, radioactive decay data should be given in the decay data sub-library (NSUB=4).

The isStable() function returns true for stable nuclids for MF=8 MT=457, as expected (returns NST).
It's counterproductive to receive inconsistent results from the same function in different environments.