Having looked at the article I am not sure there is a need to change anything. The possibilities given are full of ifs and the chance of producing a viable nuclear bomb seems incredibly small.

There is a small theoretical possibility of making a bomb with 10% enriched U235. Weapons are usually made with 80% or more enriched U235.

To start with you would need to divert around 1 ton of HALEU. At a suggested cost of around USD 20 000 per kg this would have a value of around USD 20 million I think that people would be looking after it quite carefully anyway.

https://www.nuclearinnovationalliance.org/sites/default/files/2023-12/NIA%20HALEU%20Cost%20Report%20%2812_15_23%29_1.pdf

You would then need the ability to convert this to a suitable form and phase to make the core of a bomb. Uranium metal is pyrophoric to add to the challenge of working with it.

Next you would need to be able to make an explosive implosion system to very  symmetrically compress the core to a supercritical mass and then inject a stream of neutrons to start the chain reaction. You will probably need a source of Tritium for this.

If you manage to get this far, and as the paper suggests this is something that would require the resources of a state not a small terror group, there is still a high possibility that the neutrons from the U238 will cause a premature weak explosion, a fizzle.

As the paper also states if you have the resources to put a bomb together from HALEU you would do better to follow a different route and make a smaller much more reliable weapon.

Actually it would be a better idea if the spent radio active material was placed on the moon  for long term storage.

It is very difficult to send a rocket directly to the sun- I suggest you look at the path they had to use for the Parker probe.

Peter Brooks

Palm Bay 

given the fraction of space rockets that blow up on the pad, a safer place for the spent radioactive material is probably in  the car park. So long as you don't loiter near it it will be quite happy there. If you want a bit more shielding so you can stand  closer, surround it with water or concrete blocks.

This is, oddly enough all well understood, and nothing special for the SMR.

Mike

Hello Mike:

If your worry about the high number of space rockets that blow up on the pad, then that's a very good reason for launching them all from North Scotland.

Peter Brooks

Palm Bay  

Looks like my first option of dropping the radio active  container in middle of the ocean is the safest because even if it leaked in a thousand years time it will do little damage.

Hello Clive:

If dumping of radio active material in the water is an acceptable option then may I suggest it be placed in the Beaufort's Dyke, which is a trench between Northern Scotland and Ireland.

It was used after WW1 to dump million tons of munitions and chemical weapons.

If that is not acceptable, then one of the other sites quoted in the following report "Overview of Past dumping at sea of chemical weapons and munitions in the OSPAR Maritime area" published in 2005 (page 9 shows a map of other areas) by the OSPAR Commission (see www.ospar.org), could be used.

I still like my idea of using the Moon for storage of this material.

Peter Brooks

Palm Bay 

A big question here is what we actually need to dispose of?

Used fuel rods are not waste, they can be reprocessed to recover usable fuel. They will contain varying amounts of uranium and plutonium isotopes as well as trans uranic elements that are all fissionable with appropriate reactor physics. The fission products are often highly active but have short half lives so they can be allowed to decay.

The reactors in use today were generally designed to produce weapons grade plutonium 239 which requires a fairly short burn up time of around 3 months otherwise there are too many other plutonium isotopes present which tend to cause the weapons to fizzle. Low enriched uranium tended to be used as the fuel rods would be exchanged on a fairly short cycle. Some reactor designs such as the UK Magnox allowed online exchange of fuel rods. One of the key anti proliferation actions is to ensure that the fuel is kept in the reactor for long enough  to ‘spoil’ the plutonium.

If a higher enriched uranium is used a much longer burn up time is possible which improves the uptime and efficiency of the reactor, hence the op regarding HALEU for modern reactor designs.

Hello Roger:-

This brings up the next big question - The need to replace the HALEU fuel rods. 

Do you happen to know how long the HALEU fuel rods are expected to last, before they need to be replaced in the SMR?

Peter Brooks

Palm Bay 

This is dependant on the degree of enrichment, as that affects the 'burn rate'  - and how far down on it's initial state a rod is allowed to get before being considered 'due for replacement'- which is very far from it being radioactively  'cold' !!

Also spent fuel may be re-used in multi-stage processes, the used fuel, or it least a significant traction of it, may live to light another day.

However, generally single  pass fuel cycles of 18 months to a few years are in the right sort of time frame.
Pros and Cons Analysis of HALEU Utilization in Example Fuel Cycles describes for more about this than I know.

Mike.

Hello Mike:

That reference you supplied appears to be excellent, and is a "must" read.

It raises the big questions about general maintenance of the SNR unit and (for example) the need for Hazmat suits, switch gear, transformers and the connection to the power grid.

Maybe these maintenance issues are enough good reasons to require high security around their chosen locations. 

Peter Brooks

Palm Bay 

HALEU contains more fissile product than LEU but how this is used depends on the end user requirements and economics. Designs of SMRs exist for life spans of up to 10 years. This may be economic for remote locations but will have a lower output and hence a lower return on investment. The overall economics of power supply in remote locations  may justify this. Typical SMR fuel cycles appear to be, as Mike said, a few years. This is better than conventional LEU fueled reactors where the cycle is one to two years.

I would suggest that all large energy infrastructure  should have a fairly high level of security, conventional power stations, large switching stations, the termination point of subsea cables etc. These also contain as significant number of hazards as well as an opportunity to cause major disruption to the energy supply.

HALEU contains more fissile product than LEU but how this is used depends on the end user requirements and economics. Designs of SMRs exist for life spans of up to 10 years. This may be economic for remote locations but will have a lower output and hence a lower return on investment. The overall economics of power supply in remote locations  may justify this. Typical SMR fuel cycles appear to be, as Mike said, a few years. This is better than conventional LEU fueled reactors where the cycle is one to two years.

I would suggest that all large energy infrastructure  should have a fairly high level of security, conventional power stations, large switching stations, the termination point of subsea cables etc. These also contain as significant number of hazards as well as an opportunity to cause major disruption to the energy supply.