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Metals items used in certain types of nuclear power station can become brittle over time due to exposure to radiation.

Given the amount of cosmic radiation, is embrittlement of metals on space craft an issue that needs to be considered?

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No. You can expect materials embrittlement from space radiation to be less than one-billionth of that taking place in a nuclear reactor.

Neutron bombardment is the primary (but not only) mechanism for metal embrittlement in nuclear reactors. Neutrons are heavy and fast enough to knock atoms out of place in their crystalline structure, which over time manifests as brittleness. The neutron flux in nuclear reactors is much, much, much more intense than it is in space. From Wikipedia's "Neutron Flux" article: A reactor vessel of a typical nuclear power plant (PWR) endures in 40 years (32 full reactor years) of operation approximately 3.5×10^19 neutrons per square centimeter.

In space, especially outside of the Earth's magnetic field, solar protons dominate the radiation spectrum. They have nearly the same mass as neutrons, although their charge and energy levels cause them to interact differently. Ignoring those differences and looking exclusively at the intensity, the GOES satellite shows an average proton flux well below 10 neutrons/second per square centimeter at earth's distance from the sun. That adds up to fewer than 1x10^10 protons in the same 40-year period.http://www.swpc.noaa.gov/products/goes-proton-flux That's roughly one-billionth the flux of a reactor. So from a gross materials-embrittlement perspective, space radiation will not cause the same damage as nuclear reactor radiation.

Over time, solar protons can damage sensitive spacecraft systems like solar panels, electronics, and crew. Humans are more fragile than steel reactor vessels.

A paywalled article http://link.springer.com/article/10.1007/BF00166645 indicates that the largest source of neutrons in the near-earth region is the Earth itself. Cosmic rays striking the upper atmosphere create particle cascades which includes the release of neutrons. It's interesting that some forms of radiation become less intense when you leave Earth. Overall, though space is a dangerously radioactive place for long duration missions.

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    $\begingroup$ Do you know over what timescales embrittlement of metals would become an issue? $\endgroup$
    – called2voyage
    Jul 14 '16 at 18:48
  • $\begingroup$ I don't. There is a lot of work being done now on modeling the space radiation environment and its effects on crew and possible mitigation methods. Someone better schooled than me might be able to take that data and models (focused more on the crew) and apply them to the spacecraft structure over long periods. $\endgroup$
    – Kengineer
    Jul 19 '16 at 18:22
  • $\begingroup$ Thanks, I was just curious. It seems like other effects will probably overwhelm the metals before radiation does--in most cases at least. $\endgroup$
    – called2voyage
    Jul 19 '16 at 18:23

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