Nuclear Power Plants: When Backup Systems Fail


The 104 U.S. nuclear power plants are some of the most sophisticated and complex energy systems ever designed. However any complex system, no matter how well it is designed and engineered, cannot be deemed failure-proof.

The emergency power supplies of a nuclear power plant are built with several layers of redundancy such as diesel generators and battery buffers.

But what happens if the power grid goes down? How long will the backups work and how long before nuclear power plants begin to melt down?


Avoiding The Worst-case Scenario

The event we are looking to avoid is removing the coolant systems effectiveness of removing heat from the fuel rods, leading to core damage.

When the core becomes damaged, there may be a release of radioactive material into the coolant – thereby increasing the chances that something travels outside the reactor.

Worse yet, a complete loss of coolant ultimately leading to a ‘meltdown’ and radioactive release into the environment.

iOSAT Potassium Iodide Tablets


How Nuclear Reactors Are Cooled

It’s actually a simple approach: Push water past the nuclear core and carry the heat somewhere else.

The chain reaction that actually runs the reactor can be shut off quickly, however what’s left over in the core, the radioactive material, will continue to give off heat for a long time.

Unless you have a mechanism to remove that heat, it WILL continue to build up and WILL eventually damage the radioactive fuel and the reactor.


What happens when a nuclear reactor gets disconnected from the grid?

Pushing water past the core involves pumps which run by electricity. When the electrical power grid goes down there are emergency diesel generators that automatically kick in. There are also battery systems to keep instruments and safety systems running.

The plant relies on the power grid. If the grid is no longer available, the plant switches to diesel generators. If there is an issue with the diesel generators, there is a battery backup.


Why Nuclear Power Plants Require Power From The Grid

Nuclear reactors produce much more electricity than they need to run their systems. As a basic design feature though, plants are not literally self-powering. That’s by design.

You don’t want to end up in a situation where a problem at the plant cuts off its own power source. Therefore, the primary means of power for a plant in order for it to run is electricity from the grid.

RADEX RD1503+ Geiger counter, radiation detector


How Long Until Nuclear Reactor Damage?

It depends on the plant (boiling-water reactor vs. pressurized-water reactor). Basically, broadly speaking, there are many hours available to restore power to the system and restore cooling. Apparently it’s really not possible to get more specific than “many hours” (according to what I’ve read).

But generally, it’s fair to say less than 24 hours to restore cooling.


What kinds of events could knock out the backup diesel generators?

There is always the possibility of just plain old failure. That’s why there are multiple generators at a plant for redundancy’s sake.

The Fukushima meltdown occurred because an earthquake and tsunami damaged and flooded the diesel generator systems.

There’s another possibility whereby the diesel generator itself may be running properly but the distribution system between it and the plant becomes damaged in some way.

In a worst case scenario, it could be conceivable that the diesel fuel itself could run out if the distribution and supply systems that provide the fuel become interrupted.

Plan Your Evacuation Route Away From Nuclear Power Plants


How long will emergency backup systems last?

NRC regulatory requirements for emergency power supplies is that they be available on the order of a month.

Heat is still being generated even after successfully shutting down a reactor. If coolant systems stopped carrying away that heat (via water, pumps, and electricity) it would start building up again. Emergency cooling systems have to be available for weeks after a shutdown.

Assuming the core itself and the reactor containment physical integrity and the backup power systems of a nuclear power plant has not been compromised, then it might be fair to say that after a grid-down situation an operational plant could stay cool and survive without melting down for several weeks, perhaps a month without resupply.

If we are to ever face a major cataclysmic grid-down (EMP, Solar-flare Carrington Event, other..) then the odds greatly increase that most all distribution supply chains will grind to a halt very quickly, which could in turn create a major problem for refueling diesel generators to keep cooling pumps running. So… after 2 to 4-weeks, you’d best be far away from any nuclear power plant…

(Some information sourced from

More: 5 Nuclear Radiation Detector Choices
More: iOSAT β„’ Tablets For Your Nuclear Survival Kit


  1. Ok, here is a question that I should know but….
    SO lets say we have a full on Grid Down, How long will it be before the Nukes don’t need to be “Cooled”…. in other words, will the Core eventually cool enough so it can ‘Just Sit There’ without external cooling? A year, 5 years, 50,000 years?

    1. Half life of uranium is what, 25,000 years? So it should lose half its heat every 25,000 years? Theoretically. I personally haven’t been around that long to test it.

      1. That’s a LOT of diesel fuel….
        And humans believe they will survive forever? hummmm
        What could ever go wrong with this option?

        Second thought though, If a Nuke is decommissioned do the ‘rods’ need to be cooled?
        If not, here’s a thought, why, if in a total grid down, could the rods not be puled and stored separated?
        Wonder if FEMA has a plan for that?

        1. The nuclear plant’s ‘old’ fuel rods are stored in pools (requiring heat exchangers/pumps). My understanding is ‘years’ in the pools. In Fukushima (and other similar reactors) the pools are actually above the reactors.

        2. NRP
          Supposedly the stored “waste” rods are just as big a threat as the reactor rods, this is a big issue as most power plants in the US also have a huge pile of these spent rods,
          If the gov got their heads out of their rears and change some regulations regarding processing of nuke fuel, the fuel could be re processed until it was completely depleted, but that requires the fuel to be brought up to weapons grade,
          Again, im too stupid to know whats good for me cause the gubermint said so…..

        1. (If a Nuke is decommissioned do the β€˜rods’ need to be cooled?)

          I worked at a nuke plant (Davis Besse in Northern Ohio) back in the 1990’s. And all the old fuel is in a big pond/ tank in the plant. You can look down into it from a walkway that only has a guardrail to keep people from falling into it. Not that anyone would do it but it would be quite easy to reach down and dio your hand into the water.

          I don’t know how it is cooled after the old spent fuel is put in the tank.

          The plant I worked at is OLD, the tech is from the 1970’s , Nothing modern looking about the control room at all.

          Maybe this makes too much sense for government, but I would think a Nuke Plant would be able to power itself as it makes this stuff called electricity????

        2. Chuck Findlay — you wrote “the plant I worked at is OLD”..— I think I have read in the news that most nuclear plants are OLD. That most are way past their “best by dates”, and that most have had multiple “life extensions” authorized. ….Doesn’t really sound good.

    2. How many emergency diesel generators (EDGs) are required to be on site as emergency backup power for nuclear power plants (NPPs). Is there a required number of EDGs per NPP or per reactor? If not required, then what is the typical (suggested or practiced) number?

  2. I don’t see why they don’t just use pipes to run river water through the facility. Constant supply, no interruption unless the river runs dry. Oh, right. Environmentalists would rather not have the fish poisoned. They’d rather pump the water (to the tune of millions of gallons per day), then dump the used water back INTO the water supply.

    1. the cooling water is split into a “hot side” and a “cold side”. In other words, the water that is exposed to radiation pass through a closed system that includes heat ex changers where the heat (and only the heat) is transferred to the “cold” side water, which is then released, either into cooling towers or ponds, or an estuary or river to release the heat. The net heat increase in the water body is a couple of degrees. It is actually pretty safe as long as the pumps keep on pumping…and therein lies the rub.

    2. Lauren, nuclear power plants are always located near rivers (or canals) — for cooling purposes.

      1. Not always Palo Verde nuke plant in Arizona is in the middle of the desert back in the day worked on the drill crew the did the engineering for the plant

      2. Does not matter if generators can’t run pumps to pump the water

  3. Millions of gallons of radio active water are still pouring out of Fukushima into the ocean. So in that case there doesn’t seem to be a ‘fix’. It is and will kill the ocean.

    1. And the waters shall become bitter? 2/3 of the life in the ocean will die? Hm…

    2. Mr. follows a site on Fukushima. The world has it’s head in the sand on this topic!

      1. Mrs. USMCBG;
        More like we kill the Oceans we KILL the World. and Fukushima could very easily be the start of the Oceans Death.
        There is already wide spread die-off around the world of Coral Reefs and massive schools of fish/mammals.

  4. Mornin all,,
    Heres something i dont understand,
    Why? Do they rely on a deisel backup?
    They could use the same system they use to power say an aircraft carrier or a sub and run the backup generation and pumps for that matter off that closed system.
    I suppose theres some long drawn out scientific engineery answer but perhaps its just too simple for these designers to grasp.
    Seems idiotic to me that they are still relying on tech that is basicly obsolete, they alredy have a heat source that is perfectly capable of creating the thermal dynamic needed to run a turbin, so why are they not using that?
    I guess im just too stupid, sorta like how the UH scientist said i didnt know what i was talking about with regards to bee hive deaths and Monsantos GM corn,,,

    1. Probably has to do with money, and the probability that the NRC (and their lobbyists) likely say that it’s ‘good enough’ the way it is.

      1. Ken
        Most likely,,,
        But think about it, a nuke sub can run under water for weeks if im not mistaken, and the reason it needs to surface has nothing to do with the reactor.
        So theres one option,
        Second is to just harness the residual heat from the core, steam is steam, dont matter where it comes from

    2. The information I read said that they don’t want the system feeding itself because of the chance of failure of one of the many links in the chain. One failure sets off another. Sounds a little like our JIT delivery system…

  5. “When the uranium fuel is used up, usually after about 18 months, the spent rods are generally moved to deep pools of circulating water to cool down for about 10 years, though they remain dangerously radioactive for about 10,000 years.”

    1. Well, them, but dont forget the MIC, they need all that DU for fighting their wars,,,,

  6. Did anyone at the FUKU plant have enough advance warning to do anything?
    Seems to me they did not.

    1. @Tango
      That was caused by a tsunami generated from an earthquake, so not real predictable and no warning.

  7. I watched a show one night about the “cover” they built over Chernobyl. I do no recall which channel, but it was very interesting the engineering required to build it. Worth watching should you come across it.

    1. Mrs. USMCBG — Good of course that a cover was built (I think), but, have always wondered, how much is leaking out underground? How much is building up “under” that cover, and can it build to a point (the radiation and perhaps active gases) where it can explode?

    2. What about the ‘elephants foot’ beneath the reactor?
      The’re afraid it will eventually get into the water supply.

  8. We can talk about nuclear and human made radiation all we want. It is not going away at this time. Last 25,000 thousand years or so! Cool them, pellet them or what ever once it is loose it is HERE to stay. They are powers that have reared their ugly heads before we were morally and physically able to deal with it. Evil comes to mind.

  9. Just like climate change, some like to navel-gaze and suggest we are totally the problem with nuclear waste. I would like to know where the rest of the world disposes of their waste – bottom of the ocean??? My tinfoil hat is getting tight again.

    1. hermit us;
      Please revisit a previous article here on MSB…

  10. Tommyboy,
    I know what happened in FUKU ,
    Someone posted that the Japanese operators were not properly trained in shutdown.
    My point was that there was not enough time to shut down.

    1. tango I looked up FUKU and the Tsunami time frame. Depending on who’s information you accept they had between 15 and 30 minutes to respond to the Tsunami before it struck.

      Given Japan has been dealing with Tsunami alert systems for decades I would like to say the Operators were trained for such an event. The EVENT was beyond the designed safeties of FUKU AND thus my concern about the many Boiling Water Reactors of the same design as FUKU about to get safety tested by a Hurricane.

      Time will tell. Hopefully this Hurricane will weaken before test time.

    2. One would think that given Japan is located where earthquakes and tsunami are a very real possibility and that they are sudden events with little warning. There would be a computerized shutdown sequence that would be easily started (like with a switch / key where you life the cover and flip the switch.) that would automatically do all that’s needed to safely shut down the reactor.

      All this would take time wise is a min or so.

  11. Happy I am far away from those hazards. Radiation will still reach here but hopefully it will be manageable.

  12. I looked at the 50 mile radius map from a previous article that Ken posted and provided a link in here, and I noticed there are 2 or 3 reactors in the path of Hurricane Florence. I hope they restore power in a hurry after the storm moves off.

    Prayers for everyone on the east coast!

  13. Somebody said in a comment that by this method or that method we cannot produce enough electricity, maybe the real problem is that we consume too much. If one is honest, it’s not hard to see humans “require” a lot of stuff we truly don’t need. Almost every gadget the masses have been convinced they cannot live without needs electricity. It all boils down to pursuit of the almighty dollar and whatever means necessary to obtain it. Greed is one word that comes to mind. . Our society over consumes nearly everthing.

    1. Can you imagine what an impact if every household used our sun to dry their laundry??
      I have line-dried clothes for 11 years now.
      Should have done this sooner.

  14. OK Going sideways here, but can a former nuclear swabbie explain if iran was to sink a nuke US carrier in the Strait of Hourmuz (thinking it’s not super deep water there) what is the possible contamination issues? For that matter if one sinks anywhere what is the possible radiation outcome?

  15. True hubris on parade with our casual manipulation of nuclear energy– power stations and also atomic bombs…

    And then we store the radioactive waste underground (another absolute stroke of human Genius) in places like WIPP, NM-
    Of course this radiation will smolder for many generations. There have even been suggestions of monuments being built above on the surface, like small pyramids or oblisks, that would warn future generations in many languages and symbols about the dangers buried below…

    1. Yeah, the underground waste, out of sight and out of mind . Dirty places like the Tri-cities area of Washington State .Population of 182,000 people just a few miles south of the Hanford nuclear waste site. It is not uncommon today for cleanup crews to be treated for contamination. The Columbia river flows nearby as well . I believe it is the largest and most expensive cleanup site in the country .Gee, I wonder who is paying for all this stuff ?

      1. OH,
        I’m not very nuke knowledgable, not been around it. I believe I can understand the difference between the two . Two badges makes goods sense , ya never know when one may malfunction ..

      2. Who? You ever notice how gubermint goons like playing kick the can,,,,

  16. Worked in the nuclear weapons complex 20 years, closed that DOE facility 15 years ago after packaging all the “stuff” in long term storage containers. DOE has me in a long term health screening program, after 15 years from last possible exposure I am in great health by the grace of God. I moved far away from there, to a place I thought was safe. Now if find out some crazy called “NRP” is storing a critical mass of TP in an old rusty convex over the hill from me. Just goes to show there will always be some hazards wherever you go.

  17. Oldhomesteador, wondering if you knew my ex father in law. worked for Westinghouse at Hanford. Old submariner who was depth charged by the Japanese 2 weeks after the war started. He spent the entire time of the war as a POW in the Philippines.

    Does anyone know the story on the depleted uranium projectiles the Air Force uses in their 25 mm chain guns on the A-10 Wart Hogs? I know that the projectile is very dense which allows it to punch through armor plate. How do they get it depleted enough to use as a projectile?

    1. Me,
      When they extract the radioactive isotopes from natural yellow cake, depleted uranium is what is left. Very low activity, but some, because the extraction process is not 100% perfect. Makes good impact rounds as it “spalls” as it goes through armor.

      1. Minerjim, thanks. I’ll correct myself after reading up on the rounds, Wart Hogs use 30 mm, Harriers 25 mm and Cobras 20 mm. I don’t ever want to be on the receiving end of any of them!

  18. A lot of lessons were learned after Three Mile Island and the US nuke industry spent tens of billions of dollars back-fitting safety system changes to the existing plants. I worked for many years on BWR’s and the fact that Fukashima’s reactor buildings catastrophically exploded from hydrogen buildup tells me that for whatever reason (whether because cost or cultural arrogance), important post-TMI safety changes were never implemented (like passive hydrogen recombiners that don’t need power to operate to prevent hydrogen buildup). That lack of updates turned a local plant problem into a national disaster.

  19. Having spent the last 30 years in nuclear plant maintenance, one thing I found curious when I did a reactor inspection at Fukushima was they had their diesel fuel tanks above ground, near the shore…they probably lost those tanks before the units even shut down…no fuel, no back-up power. Glad to see most plants don’t have their fuel tanks above ground.

Comments are closed.