PHATsakk43 t1_jc27opb wrote
It’s “cheaper” not in the sense of the control rods themselves, but the reactor.
The graphite tips were key to getting an RBMK to operate without either enriched uranium (expensive) or a fancy moderator (heavy water, likewise expensive.)
By putting a couple inches of positive reactivity on the control rod, you can create a localized higher reactivity which can get the neutron flux high enough to “jump start” the reactor. This is necessary in natural uranium reactors, as the amount of fissile U-235 is very low.
While Russia absolutely had the capability to enrich uranium, there are other “benefits” from the RBMK design which make natural uranium reactors preferable, specifically their ability to produce plutonium for weapons. This seemed to have been left out of the discussion in the miniseries as well.
Accelerator231 t1_jc2t4di wrote
You know the surprising thing I learned today is that reactors can work without enriched fuel. I thought all reactors needed enriched fuel before they can work.
Crizznik t1_jc30n4k wrote
There are natural nuclear reactors in the world. Pits of natural uranium that get super hot from fission reactions.
echawkes t1_jc3uari wrote
There aren't been any natural nuclear reactors any more. Over a billion years ago, the natural enrichment of uranium was much higher, because U-235 and U-238 have different half-lives.
The only place a natural reactor was ever thought to have operated was at Oklo, and it can't happen anywhere now.
Accelerator231 t1_jc2txj0 wrote
>The graphite tips were key to getting an RBMK to operate without either enriched uranium (expensive) or a fancy moderator (heavy water, likewise expensive.)
Wait a moment. Where's the source on this?
VorAbaddon t1_jc38zt6 wrote
Pretty much the design of the reactor. It doesnt use enriched uranium nor heavy water. So it has to have another source of moderation to get the reaction going from a less fissile fuel. Hence, graphite.
Hiddencamper t1_jc3b51m wrote
The graphite blocks are the key.
The tips are there to help levelize axial flux tilt (get power more uniform across the core) in a safe manner… when done correctly (and by safe, I mean in a way that when executed as intended allows you to get enough power from the bottom 1/3rd of the reactor without risking other transient conditions causing core damage).
Accelerator231 t1_jc3bgem wrote
So... You don't need to refine material to get nuclear reactors?
Wow. I did not know that. So by adding graphite, the hurdles with using normal uranium can be overcome?
insta t1_jc3gufu wrote
This was a major benefit to RBMK. You can get limitless, carbon-free power from clean water and rocks you dug out of the ground. There's still 8 of them kicking around today, the design works well if you don't intentionally disable every single safety system at once.
Accelerator231 t1_jc3mrcy wrote
This changes my understanding. I always thought part of the reason why nuclear wasn't used was because of difficulty of refining fuel.
echawkes t1_jc3vk15 wrote
Not at all. In fact, unlike power plants that use fossil fuels (like coal), fuel costs aren't a huge part of the cost of running a nuclear power plant anywhere, regardless of enrichment. (Caveat: nuclear power plants use relatively low fuel enrichments, like 5% or less. If you had an NPP with a very high enrichment, the cost could change, but NPPs don't need high enrichments.)
[deleted] t1_jc5h9rx wrote
[removed]
[deleted] t1_jc3qzxy wrote
[removed]
jadebenn t1_jcao7ll wrote
Fuel enrichment was originally an extremely expensive service: Part of the advantage to the CANDU and RBMK designs was they didn't need it (CANDU still doesn't, modern RBMK does). Back then, uranium enrichment was primarily accomplished through an extremely energy-intensive process called 'gaseous diffusion' that required large facilities and infrastructure. Then the gas centrifuges arrived, and cut enrichment costs by an order of magnitude.
The last US gaseous diffusion plant was shut down in 2013, but it was uneconomic far prior (IIRC, it was kept around for DoE weapons purposes since that uranium can't be civilian-procured). Modern enrichment is relatively cheap now, which is part of the reason the nuclear industry is interested in boosting enrichment rates (which would have been prohibitively expensive originally).
Hiddencamper t1_jc3d3bq wrote
Correct. Graphite is a stronger moderator than water.
So is deuterium, which is why CANDU reactors can use natural uranium.
Accelerator231 t1_jc3mv3g wrote
Actually come to think of it you got a paper or anything similar?
saluksic t1_jc2m1ci wrote
So a regular reactor with uranium enriched beyond natural levels can’t make plutonium?
Hiddencamper t1_jc2nwl9 wrote
All uranium based reactors produce plutonium.
It’s a feature! We use U-238 as the filler material in the fuel, knowing we will get some breeding and use that plutonium to extend the fuel cycle.
When you pull fuel out of a LWR after three cycles, it’s running on about as much Pu-239 as it is U-235.
We have to account for that in fuel cycle analysis, hot excess reactivity / shutdown margin, and the Beta factor (fast/thermal fission ratio). It also can impact moderator temperature coefficient and cause it to shift to zero or even slightly positive.
Gunnarz699 t1_jc2nula wrote
It can and does, but makes less of it.
It would be fine in normal times, but the Soviets were stockpiling warheads in the tens of thousands.
PHATsakk43 t1_jc31lte wrote
While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.
Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)
So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.
This is a gross simplification and there are other things that can affect this, but it’s part of the equation.
UnamedStreamNumber9 t1_jc2o0o9 wrote
It can, but by enriching the 235 relative to the 238, you reduce fraction of the fuel that can be jumped from U 238 up to Pu 239 vs the fraction of U 235 that breaks down into barium, krypton and 3 neutrons. There’s still U238 in the fuel rods but with enriched uranium, there’s less of it available to be transmuted
PHATsakk43 t1_jc32sb1 wrote
While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.
Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)
So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.
This is a gross simplification and there are other things that can affect this, but it’s part of the equation.
Viewing a single comment thread. View all comments