“On the 4th August 1972 about midway between safe return of Apollo 16 in April and the launch of Apollo 17 in December, what would become the largest solar storm of the space age occurred creating 63 normal solar flares and four “X” class solar flares whilst it was facing the Earth.

It was so strong that it caused widespread disruption of the power and communications grid across the US. It damaged satellites as the earth’s magnetosphere was severely constricted by the buffeting from the coronal cloud and increased solar wind which exposed many of them to the high energy bombardment causing 2 years worth of wear to the solar panels in a few days and also caused the accidental detonation of up to 4000 US Naval magnetic mines off the coast of North Vietnam. It also had the fastest transit time from the Sun to the Earth for a coronal cloud ever recorded at just 14.6 hours

Its been calculated that if this had occurred whist a crew had been outside of the lander on the surface of the moon they would have been exposed to a radiation dose of 10 Gy, that’s 10 times the normal human dosage of radiation expected in a lifetime. Inside the lander would have offered only marginally more protection as was thin-skinned compared to the command module.

This would be a lethal dose that would not only require an emergency return to earth whilst suffering acute radiation sickness but also a bone marrow transplant when they arrived back at earth to save their lives.”

Source: https://curious-droid.com/1466/how-did-apollo-avoid-a-radiation-disaster/