It's leaking between the washer and the volute, both parts supplied by RO. I'm outside of the return period so...sealant it is.You should not be applying any sealant to the threads. You should have a washer in between the volute and the union. This will provide the seal needed to prevent the leak.
What broke, the Union or the Volute? And maybe you're at the point you could use PVC cement, because it doesn't matter on these cracked pieces. (buying new ones anyways, and maybe cement will seal the crack - Lemonade out of Lemons?!?)

