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I keep reading that final storage sites for nuclear waste are being built in salt domes. In the case of the German Gorleben project, the salt domes were determined to be unsuitable after prolonged investigation.

Regardless of the Gorleben case, why are salt domes preferred?

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  • $\begingroup$ Could you make at least some of that clear? When you keep reading whatever, why leave out the details about salt in storage sites? $\endgroup$
    – Robbie Goodwin
    Commented Jul 16, 2022 at 20:35

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Using salt domes in which to store nuclear waste has been considered in the USA and Germany. These two countries have potentially suitable salt domes.

Factors in favor of salt domes are (Sect 12 of the reference):

  • Salt is already mined from such domes in the USA. The voids left behind provide reduce the cost of nuclear waste storage. The cost of producing the voids is absorbed by corporations mining the deposit for salt. The nuclear waste disposing entities spend less money digging fewer holes.
  • Salt deposits are geologically stable, having existed for millions of years.
  • More so in the USA, the geological setting of the deposits is also secure. The salt domes are in regions with low seismicity.
  • In bedded salt deposits the overlying strata such as limestone and dolomite provide truss-like support to the overburden. The possibility of roof collapse causing the release of radioactive materials stored under these conditions appears very small but merits verification.
  • Salt is a good thermal conductor and will supposedly dissipate the heat from the nuclear waste. I would be more comfortable with this reason if the containers of nuclear waste were placed directly into specially sized holes within the salt domes, where the containers are in maximum direct contact with the salt. If the containers of nuclear waste are simply placed in within the air space of the underground chamber the waste must first heat the air which then heats the walls of the salt chamber. The dissipation of heat takes longer.
  • Salt domes are only secure if they are mined by mechanical means: drilling and blasting or grinding. Salt domes mined by aqueous methods - dissolving the salt with water and removing the salt as brine produces underground chambers with no internal support, such as pillars or human installed rock or cable bolts which reinforce the walls and ceiling of underground chambers. Such chambers would be unsuitable and could be prone to collapse.

Issues with using salt domes to store nuclear waste:

  • Salt is generally regarded as being self healing. Any small cracks that may develop within the salt deposit will close. This applies to small cracks, but may not apply to very large cracks.

  • Salt domes are generally considered to inhibit the flow of fluids through them. However, under conditions of high temperature and pressure, as exists deep underground, salt can become more porous allowing more fluids to pass through it. This can be problematic if in flowing water was to pick up radioactive material and start spreading it around the region of the salt dome.

  • The nature of the nuclear waste to be stored needs to be considered. Salt domes would be better at storing solid waste. Some waste is liquid which can more easily contaminate storage area via leaks and spills. Such waste would need to be thickened so it contained 35 percent solids.

  • If high level nuclear waste is to be stored, the amount of heat generated would be 1 - 3 Btu/h per US gallon (278.7 - 836.1 J/L). An acre-foot (1233.48 m3) of such waste would produce 1 MBtu per hour (1055 MJ per hour). The equivalent of burning 700 lb (317.5 kg) of coal.

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  • $\begingroup$ I'm not sure about the very last calculation: Energy density of coal=24 MJ/kg, so 44kg/hour to give 1055MJ/hr - an order of magnitude difference. I haven't checked the conversion from non-SI $\endgroup$
    – Chris H
    Commented Jul 15, 2022 at 13:28
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    $\begingroup$ Isn't "Salt domes are only secure if they are mined by mechanical means" an 'issue' rather than a factor in favour of salt domes? (Or at least neutral rather than a posivite?) $\endgroup$
    – Pharap
    Commented Jul 15, 2022 at 13:39
  • $\begingroup$ Seems like all that heat it generates would be ripe for energy generation - If you put a geothermal plant on top of your disposal site maybe? Allows the site to still be useful for more than just hazardous material storage. Any safe way to take advantage of that? $\endgroup$
    – Darrel Hoffman
    Commented Jul 15, 2022 at 13:52
  • $\begingroup$ It is unclear how the heat generation is specifically a problem for salt domes: Are they better heat insulators than other geological formations?-- Another issue I'm missing in the downsides is the corrosiveness of hot salt, at least in the presence of humidity. So if water seeps in somehow it will compromise the containers faster than other environments. Not sure about the chemistry of dry hot salt and metal/glass containers. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 16, 2022 at 14:55
  • $\begingroup$ After reading up some it seems that the plan for the German salt storage in Gorleben was to actually flood it intentionally ("we cannot keep it dry anyway") which would start corroding the barrels and lead in geological time, together with geological pressure, to their complete destruction and contamination of the site. The long-term containment would be purely by geological barriers. (I suppose that that's also the case in other geological formations, over geological times, so corrosion may be irrelevant as a specific salt dome issue.) $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 16, 2022 at 15:07
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Hi Salt domes have formed over a period of several million years. These salt domes are usually also very well sealed against water by a protective layer (Anhydrite) (salt dissolves in water and if the salt dome were not dry it would not exist). These drying conditions, in addition to an already existing infrastructure through salt mining, are the reason why salt domes are used as nuclear repositories.

Have a Look: https://www.sciencedaily.com/releases/2020/04/200429075857.htm

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Source: https://antisocialnetwork-2labz.blogspot.com/2017/04/rock-salt-for-dummies-freelance-files_15.html

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