Ukraine's nuclear plants: Chernobyl off diesel power, explosions explained If you've been wondering about the fate of Ukraine's nuclear power stations amid Russia's full-on invasion of the nation, you're not alone. Here's an update from the International Atomic Energy Agency (IAEA). The Chernobyl plant, which is being held by Russian troops, lost its connection to Ukraine's power grid on March 9, and …
"The radioactive fuel removed from Chernobyl following the 1986 disaster is set in large pools of water onsite to dissipate the heat it emits."
The spent fuel pools (SFP) at Chernobyl Nuclear Power Plant still holding water contain used fuel rod assemblies from reactors No. 1, No. 2 and No.3, which kept running until 1996, 1991 and 2000 respectively. Their latent heat will by now have dropped to levels where simple natural convection within the SFP will keep them cool enough to prevent boiling or melting from occuring. IIRC the spent fuel rods from Reactor No.4 (the one that went kaboom) were in the South Cooling Pond which is inside the building now covered by the New Safe Confinement dome and which has boiled/evaporated/drained dry following the incident (Here too, latent heat has dropped to levels where now natural air convection avoids meltdown.)
(Even the SFP at Fukushima during the disaster and blackout never actually boiled with MUCH fresher and hotter spent fuel rod assemblies in the pool, it's estimated it reached about 60 to 70 degrees C at max)
At Fukushima, water in one of the SPFs did boil, as this article recounts. Only a fortuitous leak from an adjacent pool prevented a much worse accident.
It did not exceed 90 degrees according to this article: https://atomicinsights.com/oak-ridge-researchers-prove-fukushima-unit-4-spent-fuel-pool-never-a-danger/ (Thus NOT BOILING) and I've read other articles stating that the temperature likely never exceeded 70 degrees.
Yes, the water level dropped for a multitude of reasons, evaporation being one of them but, I will repeat, there was no boiling!
Mostly depends on the age of the spent fuel relative to when they were removed from the reactor. Once they have been removed from the reactor and no longer subjected to the neutron chain reaction, the heat they produce is from the transuranics and other actinides that were formed as the uranium-235 did its fission thing.
The most energetic isotopes are for example iodine, with a half-life of 8 days, so presumably at Fukushima Daiichi there would still have been quite an amount of such short-lived (and thus energetic) isotopes present, which meant that their SFP required active circulation to keep the fuel rods from getting too warm.
In the case of Fukushima Daiichi's SFPs, they did heat up the cooling water to the point where the zirconium cladding of the fuel rods were no longer submerged, but instead got exposed to the hot steam. Steam + hot zirconium is an excellent catalyst to create hydrogen. And since TEPCO had not done the installation of hydrogen venting (which was performed at TMI), eventually this hydrogen found an ignition source and went boom, destroying the roofs of multiple of the reactor buildings. This did make it really easy for fresh cooling water to be added to the SFPs afterwards, however.
To get a proper meltdown with corium forming and everything, you pretty much need to be running the fuel in an active reactor. When Chornobyl NPP had its meltdown, it was running the reactivity in the bottom of the reactor at roughly 10x its rated output, causing an intense formation of heat. That not only literally flipped the lid on the reactor vessel with a steam blast, but also caused the surrounding material to start melting.
SFPs are significantly less exciting, generally being just fancy self-heated swimming pools with blue bottom lighting.
" the case of Fukushima Daiichi's SFPs, they did heat up the cooling water to the point where the zirconium cladding of the fuel rods were no longer submerged, but instead got exposed to the hot steam."
The explosions in reactors 1 and 3 were directly from hydrogen created inside the reactor vessel itself. The explosion in the reactor building of unit 4 was likely only partially from hydrogen created in the SFP (since, as I linked above, the water level did not actually get low enough to expose the cladding, nor did the bundels get hot enough to cause extensive steam+zirconium hydrogen generation. More likely most of the hydrogen came from unit 3 through interconnects.
You need to read up on the "black start" issue. The problem is that before you can start up the main turbines that actually generate electricity, you need electricity - that's to say, you need a 50Hz supply synchronised precisely with the national grid, because when you bring up the turbines, they must be precisely synchronised with the grid. Kind of, you need a small chicken to hatch a large egg.
So that small generator has to be powered by something - that'll be a tank of diesel.
Come to think of it, I bet the Ukrainian grid is still synchronised to the Russian grid... [Google]... no, they switched to the European synch yesterday: Statement by Commissioner for Energy Kadri Simson on Synchronisation of the Continental European Electricity Grid with Ukraine and Moldova.
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
