According to Wikipedia,

Depleted uranium (DU) is the best radiation shielding by weight, due to the high atomic weight of the uranium atoms... DU is used as shielding in Industrial radiography cameras

According to https://www.nasa.gov/pdf/284275main_Radiation_HS_Mod3.pdf

Aboard the space station (which is in LEO), the use of hydrogen-rich shielding such as polyethylene in the most frequently occupied locations, such as the sleeping quarters and the galley, has reduced the crew’s exposure to space radiation … (beyond LEO) radiation shields would need to be very thick to prevent the primary cosmic rays (high-energy protons and heavy ions) from penetrating into habitation modules where astronauts will live.

Apparently, beyond LEO, mass is required to protect the crew. If DU “is the best radiation shielding by weight (mass)”, has it been considered for this application?

Is DU, in fact, the best shielding by mass? To a first approximation, a Kilo of hydrogen is basically a 1.0Kg of protons. A kilo of DU is basically 0.4Kg of protons and 0.6Kg of neutrons. Does this make much difference in stopping cosmic rays (ions) and gamma rays?

Does Depleted Uranium (DU) have a role in spacecraft shielding?

Crewed spacecraft require shielding to protect crew from hazards of space, particularly:

1. Micrometors. The chief defense is a Whipple Shield https://en.wikipedia.org/wiki/Whipple_shield consisting of “bumper” layers to break up the micro-meteor before it hits the main shielding layer. This strategy is similar to “air gap armor” used in tanks. DU is incorporated in Abrams tank armor plate due to its high resistance to penetration. DU could be used as the main, inner shielding layer for a Whipple Shield.

2. Primary cosmic rays are high speed positively charged atomic nuclei including protons. Unfortunately, when primary cosmic particles hit a spaceship hull (or shielding), they produce a spray of secondary particles. Hydrogen (in the form of fuel, water or hydrogen-rich plastic) is the most mass-efficient shielding for cosmic rays.

3. Gamma rays are high energy electromagnetic radiation. Heavy atomic nuclei (Tungsten, Gold, Lead, and Uranium) are the best shielding materials. Uranium is the most mass-efficient shielding for gamma rays.

Based on this information, I would expect mass-efficient integrated shielding (to protect against micro-meteors, cosmic rays and gamma rays) to consist of:

1. Spacecraft design utilizing water and fuel storage as shielding, when practical.
2. Multiple bumper layer Whipple shielding to protect from micro-meteors
3. Plastic between the Whipple layers to absorb cosmic rays.
4. An inner layer of depleted uranium to protect from gamma rays and micro-meteor fragments spallated by the Whipple shield.

Depleted uranium is the opposite of “enriched uranium”: it has a lower percentage of the fissile isotope U235, consisting of 99.7% U238 with a half life about the age of the Earth.

Due to its long half life, health hazards of DU are chiefly chemical rather than radiation. It has heavy metal toxicity (similar to lead) with renal, CNS and cardiac toxicity. It has a short elimination half life of 15 days, but can accumulate in internal organs.

Has depleted uranium been considered for radiation shielding in crewed spacecraft beyond LEO?