[24gttghh]

A very basic nuclear reactor can be explained pretty simply I think. You enrich a bunch of let's say uranium. Pack it together in a rod, and put a bunch of those rods in a pond. Those rods have controlled (ideally) nuclear decay from their being in close proximity to other rods which generates a lot of heat, which is transferred to a separate cooling loop that boils water to make steam which drives an electric turbine.

Now I'm no nuclear scientist so please be forgiving with that description, but that's how I understand them to work :)

I can't even begin to explain how an SSD works, but I know there are no moving parts besides electrons.

edit: moved the "(ideally)"

[dr_zoidberg]

Ok, I've studied the Flash storage (most SSDs these days) technolgy and can be understood like this:

* At the "lowest" level, there's a little cell that it's very much an EEPROM (but better, because newer tech). This little cell can hold 1, 2, 3 or 4 bits, depending on gen/tech.

* You group a bunch on those cells together and they form a page. Usually it's 1024 cells a page.

* You group a bunch of pages together and they form a block (don't confuse with "block" as in "block oriented device"). Blocks are usually made of 128 pages.

* You group a bunch (1024 usually) of blocks together and you get a plane.

* You get your massive storage by grouping a lot of planes together. Think of it as small (16-64 MB) storage devices that you connect in a RAID-like manner.

* Operations are restricted because of technology. On an individual level, cells can only be "programmed", that is, a 1 can bit flipped into a 0, but a 0 cannot be made a 1.

* If you need to turn a 0 into a 1, then you must do it on a block level (yep, 128 pages at a time).

* That's where the Flash Translation Layer kicks in: it's a mapping between the (logical) sectors (512b or 4096b) and the underlying mess. The FTL tells you how you form the sectors (which would be the blocks of a "block oriented device", but I'm trying to avoid that word).

* You also have "overprovisioning" at work - that is, if your SSD is 120gb, it's actually 128GB inside, but there's 8GB you don't get access (not even at the OS level), that the device uses to move things around.

* Wear Leveling/Garbage Collection mechanisms work to prevent individual cells from being used too much. Garbage Collection makes sure (or tries) that there are always enough "ready to program" cells around.

* The firmware makes everything work transparently to the world above it.

That would be a very (very very) simple explanation of how Flash storage works. Things like memory cards and thumb drives usually don't get overprovisioning nor wear leveling.

[wtallis]

Your quantities are way off if you're trying to describe the kind of NAND flash that goes into SSDs. Typical page sizes are ~16kB plus room for ECC, so a page is several thousand physical memory cells, not just one thousand. Erase blocks are several MB, so at least a thousand pages per erase block. A single die of NAND typically has just 2 or 4 planes, each of which is at least 16GB.

[dr_zoidberg]

Largest sizes I'm finding for erase blocks are 128 and 256KB, not several MB. I am finding larger plane sizes, that probably comes from grouping more blocks together. In general, it's not massively different from what I described, it's just a difference in sizes involved at the higher levels.

[wtallis]

It still sounds like you're looking at tiny (≤4Gb) flash chips (or NOR?) for embedded devices, not 256Gb+ 3D NAND as used in SSDs, memory cards and USB flash drives. Micron 32L 3D NAND (released 2016) had 16MB blocks for 2-bit MLC, ~27MB blocks for 3-bit TLC. SK Hynix current 96L TLC has 18MB blocks, and even their last two generations of planar NAND had 4MB and 6MB blocks.

Having only 2 or 4 planes per die with per-die capacities of 32GB or more is a big part of why current SSDs need to be at least 512GB or 1TB in order to make full use of the performance offered by their controllers. 265GB SSDs are now all significantly slower than larger models from the same product line.

[cattlemansgold]

I guess what I meant is that the overall concepts are understandable. Nuclear fuel gets hot, boils water, drives a turbine. For transmissions, different sized gears allow things to turn at different rates.

But as soon as I dive into the details, I get lost. How exactly can you control the nuclear decay? How exactly does do the gears in the transmission move around and combine with eachother to create a specific gear ratio? These concepts probably are probably pretty simple for a lot of people, but they just make my head spin.

[toast0]

> How exactly can you control the nuclear decay?

That's what the control rods are for. The uranium in one fuel rod in isolation decays at whatever natural rate, which would warm water but not boil it, and placing the rods near each other allows for the decay products (high energy particles) to interact with other fuel rods and induce more rapid decay.

The control rods slot in between the fuel rods, and absorb the decay products without inducing further nuclear decay. Usually these are graphite rods.

> How exactly does do the gears in the transmission move around and combine with eachother to create a specific gear ratio?

It really depends on the specific transmission, a manual transmission is using the shift lever movement to move the gears into place. An automatic transmission most likely uses solenoids to move things (a solenoid is basically a coil of wires around a tube with a moveable metal rod inside, when you put current through the wire, the metal rod is pulled into the tube, you attach the larger thing you want to move to the end of the rod (sometimes with a pivot or what not), and use a spring, another solenoid, or gravity, etc to make the reverse movement. A solenoid by itself gives you linear movement, if you need rotational movement, one way to do that is have a pivot on the end of the solenoid rod, then a rod from there to one end of a clamp on a shaft, then when the solenoid pulls in its rod, the shaft will rotate (this is the basic mechanism for pinball flippers).

[labawi]

> The control rods slot in between the fuel rods, and absorb the decay products without inducing further nuclear decay. Usually these are graphite rods.

AFAIU graphite rods increase fission by slowing (not capturing) neutrons which in turn have a better chance of propagating further fission, because .. physics.

Quite nifty actually - without the moderator, the fuel wont burn.

[toast0]

I think you're right; I misinterpreted the term 'graphite-moderated reactor' to mean something it doesn't. Graphite will slow the neutrons so they react more. Also, the Chernobyl reactor design has graphite tips on its control rods, which I misremembered as the primary substance of the rod.

The primary substance of the control rods is (usually) a neutron absorber, and most reactors with control rods have a passive safety system, so gravity and springs will force the control rods in to significantly slow the reaction unless actively opposed by the control system.

The Chernobyl rods had graphite ends so that when fully retracted, the reactor output was higher than if there was simply no neutron absorber present; unfortunately, this also meant that going from fully retracted to fully inserted would increase the reactivity in the bottom of the reactor before it reduced it, and in the disaster, this process overheated the bottom of the reactor, damaging the structure and the control rods got stuck, and then really bad things happened.

Long story short, most control rods don't have graphite. ;)

[geocrasher]

Start here: https://www.youtube.com/watch?v=pWWjbnAVFKA

Scott Manley explains things so well. Highly recommended channel.

[sq_]

For anyone who hasn't seen it, Scott's video on Chernobyl is also well worth a watch: https://www.youtube.com/watch?v=q3d3rzFTrLg

[m4rtink]

What you describe is basically a radioisotope thermal generator, like those used by space probes. In such a device, you use natural decay heat of unstable radioisotopes.

In a nucler reactor, things are a bit different. You start with uranium that is only slightly rsdioactive and does not produce any usable quantities of heat. You place in ib the corrrct geometry and start a controlled nuclear chain reaction. You jave neurons split uranium, which produces heat and more neutrons.

Controlling this reaction so that it actually runs runs, but not so much as to melt your reactor is what, as far as I understand it, makes nuclear reactor design hard.

[labawi]

I think the harder part is making the reactor safe, idiot proof, bomb material production semi-incapable, issues of dealing with remaining irradiated materials..

Also politics, corruption etc.

[zzzcpan]

You can explain SSD as a controller you send commands to that writes data to and reads from a log structured storage on top of raw flash.