Train-World: Storing RHUs for decades to centuries

...no, I mean, supposedly, I believe there is more about it than mere kans on our part. Installations like this one are found all along the spoorwegs, and yes, around some you should tread lightly. Yet most are safe, and held, maybe still hold, functional warmers! What was th-

Ade, 2nd Class Engineer of Brug Gibraltar before getting shot

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Welcome to a future. Mankind has brought doom upon themselves, their cities have been flattened by war and weather, and most of the northern hemisphere is radioactive badlands.

They put their differences aside to fix keep the planet, reaching an unprecedented scale of globalization, making major breakthroughs as a unified people - just to bomb themselves to smithereens once that immediate fear was off the table.

In fact, the fragile peace lasted less than a decade without the external pressure of an extinction event providing something to be worked on.

Once that fate became clear, governments hastily began restoring and expanding abandoned and repurposed cold war era bunkers. To little avail. Only a fraction of the populace managed to find shelter when the bombs started falling.

Months to years, and in some cases decades, later survivors start emerging again. They are quick to start establishing new communities. Unfortunately, the electro-magnetic-fallout from the bombs fried a majority of the world's power- and communication-grids, preventing these places from easily connecting with each other.

Yet, sturdier tech survived relatively unscathed. Most places are still connected by rails and a fair number of pre-war train-engines are found in rail-depots in various states of disrepair - ready for their new job as the lifeline humanity 2.0.

The trains are powered by huge Radioisotope Heater Units (RHU), using Radium, as described in this question on the feasibility of such engines.

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For the purposes of my story I am aiming to have RHU storage facilities in rail-depots and/or other secure locations (e.g. tunnels), where RHUs are stored in pits in the floors and hoisted up by overhead cranes to be transported or installed into engines.

I was originally thinking about having them submerged in a heat-conducting liquid and using the heat to drive turbines to power workshops and machinery around the facility. But to my understanding there are at least 2 major issues with this approach considering I need these facilities to last while there are no people to maintain them for up to multiple decades:

1. Breakdown of moving parts: when (not if) the heat exchangers or secondary cycles start breaking down the heat will accumulate in the main 'pit' and eventually ignite things.

2. Decay of the submerging fluid: Water would amass algae and other biomatter, oil would degrade over time, car coolants seem to last about 5 years tops - that is nowhere near long enough...

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Q: How can these RHUs be securely stored for decades to even centuries?

Solutions will be judged by the following criteria weighted from top to bottom:

• engineering complexity: The fewer moving parts the better
• recoverability of RHUs: The less work is necessary to get an RHU back into working condition before it can be reinstalled the better
• deterioration in storage: The less RHUs deteriorate in storage in regards to time spent in storage the better
• footprint of installations: The less space is necessary per stored unit the better
• Availability of used materials: The easier to come by the materials are all around the world the better

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This question is about:

• finding a working storage solution for the RHUs over the time of decades to optimally centuries

This question is not about:

• what element the RHUs are made of. Please refer to this question if you want to propose alternatives to the described radium RHUs
• finding processes to generate power from the RHUs, unless it is part of a storage-solution