It's pronounced "crism."

XRISM, a collaboration between NASA and @JAXA_en, will soon lift off to give us a new look at some of the most energetic objects in the universe—and the X-rays they send through space: go.nasa.gov/44vyeQW
#NASA

XRISM Mission To Study ‘Rainbow’ of X-rays

A new satellite called XRISM (X-ray Imaging and Spectroscopy Mission, pronounced “crism”) aims to pry apart high-energy light into the equivalent of an X-ray rainbow.

^{Jeanette Kazmierczak (NASA)}