serious error (re. radiation shielding) in 1975 Summer Study?

> Passive shielding would need to increase 10x but carries a weight penalty.

And volume penalty, too... we're talking about water 50 m deep (or
slightly less of something more dense). That's a lot!

> NASA has a active solution with 1/10th the weight of the passive solution.

Can you point me to a reference about this?

Thanks,
- Joe