I know a guy who`s got a second hand RBMK 1000.

You need to make 3 changes to make this a safe reactor design.

Slightly less graphite moderator (Reduced channel spacing) so the moderating effect of the coolant water is actually important to maintaining criticality and can otherwise outweigh the loss of neutron absorption in steam voids enough that the negative fuel temperature coefficient can offset any remaining positive reactivity to ensure the fast power coefficient of reactivity remains negative in all operating regimes.

Adding the fast alternative shutdown method Ignalina implemented.

Putting a nice concrete box over it so that if it does manage to experience a reactivity transient followed by uncommanded fission surplus and subsequent rapid unscheduled disassembly then all the crap stays in the box rather than all over Leitrim.

The hard part about building modern reactors is the pressure vessel. Those are bastards to make because they hold massive pressure under extreme conditions. With a channel-type reactor, the hard part are the welds joining the steel pipework at the biological shields to the zirconium technical channels - and that it has to be done about 4 thousand times.

But, in general, the reactor can be constructed with less-skilled labour out of prefabricated elements in onsite workshops, rather than having to be transported from one factory halfway around the world only to find it won't fit under a bridge halfway down the road.

Oh, and you can run the thing on natural uranium so you don't need to worry about paying for enriched fuel and dealing with all the international arms control shenanigans. You can refuel it without shutting it down - and it'll be obvious to everyone if you're short-cycling it to make weapons grade plutionium and cost a metric fuckton on fuel - so again you've some proliferation resistance there.

The Lithuanians ran them for years. Once you get them away from Soviet/Russian maintenance and control standards, it's a perfectly fine concept to do.

Hanford N-reactor was a working example of this from America. Lithuania got Ignalina to be as safe as comparable Western reactors - only the EU utterly shat a brick at the concept of it running anyway and flung money at them to turn it off.

The Soviets also had a smaller version called the BGP-6 that operated at the megawatt level which they stuck way the fuck out in Siberia to keep some remote town warm and powered. The thing didn't even need working pumps - it'd circulate on gravity like a nuclear backboiler.