Reactor Core Damage: Meltdown

, director, Nuclear Safety Project | April 5, 2016, 6:00 am EDT
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Disaster by Design/Safety by Intent #26

Disaster by Design

We often talk and write about equipment failures and/or worker mistakes that increased the chances of reactor core damage. And much is reported about damaged reactor cores, such as during the five years since three reactor cores at the Fukushima Daiichi nuclear plant in Japan became damaged. This commentary explains how a reactor core overheats and melts down.

Fig. 1 BWR core (Source: NRC)

Fig. 1 BWR core (Source: NRC)

Meltdowns are the best known way for reactor cores to become damaged. Next week’s post will describe a lesser known way to damage a reactor core.

Two types of nuclear power reactors have comprised the U.S. fleet over the past quarter century—pressurized water reactors (PWRs) and boiling water reactors (BWRs). While there are many differences between these two designs, those differences have very little distinction when it comes to how reactor cores melt down. The reactor cores in both PWRs and BWRs reside in the lower half of a pill-shaped reactor vessel made from steel (Figs. 1 and 2). Feedwater systems supply water to the reactor vessels. The water gets warmed by the heat produced by the reactor core. The warmed water in PWRs or steam in BWRs leaves the reactor vessel.

A reactor produces heat from the fissioning, or splitting, of uranium and plutonium atoms when it is running. The reactor core continues to generate heat after it is shut down. Many of the smaller atoms created by fissioning atoms are unstable. Unstable fission byproducts seek stability by emitting radiation in the form of neutrons, alpha particles, beta particles, and gamma rays. Thermal energy, called decay heat, is produced by these radioactive emissions. Decay heat continues to be generated after the reactor shuts down. Water is continually needed to cool the reactor core long after it is shut down.

Fig. 2. PWR core (Source: NRC)

Fig. 2. PWR core (Source: NRC)

Loss of cooling water

Adequate cooling of a reactor core can be jeopardized when water drains or boils away from the reactor vessel, preventing sufficient cooling water flow though the reactor core to remove its heat. Emergency systems are installed to provide many diverse ways of getting makeup water into the reactor vessel. If these systems fail, the water level inside the reactor vessel will drop until the top of the reactor core is uncovered.

Initially, the exposed top portion of the reactor core continues to be adequately cooled by the steam flowing upward past it. The steam has the ability to absorb some heat—doing so removes the decay heat emitted from the exposed reactor core region.

As the water level continues dropping, the amount of decay heat emitted from the exposed reactor core region exceeds the amount of heat that can be absorbed by the steam. The heat that cannot be carried away by the steam warms the exposed region of the reactor core—the fuel pellets along with the metal rods containing them. The fuel rods in U.S. reactors are made from zirconium metal. As the fuel rods heat up to 1,500 to 1,800°F (compared to less than 700°F when the reactor is operating at full power), a chemical reaction between the metal rods and the steam adds more heat to hasten the progress towards meltdown. The reaction also produces large amounts of hydrogen gas.

The rising temperature causes the fuel pellets and the metal rods to expand. Stresses caused by the expansion can cause the fuel rods to balloon and stretch, causing the fuel rods to break open (Fig. 3). The fuel pellets are stacked inside the fuel rods like peas in a pod—openings in the fuel rods release radioactive gases that collected in the spaces between fuel pellet and the fuel rods. This phenomenon is termed the “gap release” as radioactive gases filling the gaps get discharged from the damaged fuel.

Fig. 3

Fig. 3 (Source: NRC)

As temperature increases past the melting point for the metal fuel rods and for other components inside the reactor core (such as control rods), these materials become molten and fall towards the bottom of the reactor vessel. Noble gases and high volatile fission byproducts such as iodine, krypton, xenon and cesium get released into the reactor vessel as the extent of core damage increases.

The uranium dioxide fuel pellets are in a ceramic form also melts at high temperatures. The melting of the fuel pellets releases low volatile fission byproducts like strontium. The molten core region will fall, or slump, towards the bottom of the reactor vessel.

Lava-like debris from the molten core at Three Mile Island Unit 2 (TMI-2) reached the domed lower surface of the reactor vessel and began melting through six to seven inches of carbon steel. Before it breached the reactor vessel and fell onto the containment floor below, the burn-through stopped (Fig. 4).

Fig. 4 (Source: NRC)

Fig. 4 (Source: NRC)

A reactor core meltdown destroys two of the barriers between radioactive material and the environment: the fuel pellets and the fuel rods. Consequently, a vast amount of radioactive material gets released into the reactor vessel. The molten core debris can breach the reactor vessel, as almost happened at Three Mile Island, eliminating another barrier. The large amount of hydrogen gas produced by the overheated reactor core en route to meltdown can explode, challenging or even breaching the containment structure, the final barrier.

Safety by Intent

It took longer than a decade to clean up the reactor core meltdown at Three Mile Island (Fig. 5). Workers were no longer discharging fuel assemblies from the reactor vessel—they were dealing with a debris field. They lacked procedures, equipment, and experience cleaning up after a meltdown. They had to develop the tools and techniques for doing the job. It has been five years since three reactor cores melted down at Fukushima and workers have not yet ascertained the final location and condition of the cores, let alone begun removing the debris fields.

Fig. 5 (Source:

Fig. 5 (Source: U.S. Department of Energy)

Nearly 300,000 pounds of core debris were removed from Three Mile Island and shipped to Idaho for long term storage. A stitch in time may save nine, but the value of better management and oversight that could have avoided turning a reactor core into 300,000 of highly radioactive debris is calculable. The cost of cleaning up the core meltdown at Three Mile Island exceeded $1,000,000,000.

Postscript: This description of a reactor core meltdown progression was based largely on a report titled “Molten Material Relocation into the Lower Plenum: A Status Report,” issued in September 1998 by the Nuclear Energy Agency in France.


UCS’s Disaster by Design/ Safety by Intent series of blog posts is intended to help readers understand how a seemingly unrelated assortment of minor problems can coalesce to cause disaster and how effective defense-in-depth can lessen both the number of pre-existing problems and the chances they team up.

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  • n_coast

    The power output from a light water cooled power reactor is typically about 94 percent from fission and 6 percent from decay of radioactive fission products. Reactor shutdown stops the fission, but there is no way to prevent the continued decay of the fission products. The heating from this radioactive decay drops rapidly as short-lived fission products are consumed, but cooling is critical in the hours after shutdown.

    At elevated temperature the zirconium reacts with available water or steam to produce free hydrogen and zirconium oxide. The lava like mixture produced by melted fuel and other core materials is called corium. Wikipedia and other internet sites have color representations of the disposition of corium and oxidized metal in the TMI reactor vessel. There was a hydrogen explosion at TMI, but it was contained by the reactor containment vessel.

    The TMI reactor debris was (mostly) removed from the reactor vessel with tools designed for that purpose. I would expect the eventual removal of Fukushima debris to be built on the TMI experience.

    The disposition of corium from the three Fukushima reactors has not been determined. It seems to be accepted that corium melted through the bottom of the reactor vessels, but available means have not verified this melt through or determined the extent.

    • nikkkom

      TMI experience probably tells Japanese that removal of that shit will be so difficult and costly, I’d ask myself, “why are we doing this, again? How about cocooning it in place? Americans did it in Hanford.”

      • jharragi

        >How about cocooning?
        It might not help. Radiation is probably escaping more in water flow more than anything else. Also as far as Tepco is concerned, the more radiation that escapes is the less they have to treat. You are talking about an industry that can rationalize all kinds of things so they can continue to make money. One of the favored ideas is ‘the solution to pollution is dilution’.

        • nikkkom

          Radiation wouldn’t be able to escape via water flow if there will be no water flow.
          Decommissioned Hanford reactors are dry.
          TEPCO can stop pouring water into theirs.

          • jharragi

            Unfortunately TEPCO cannot stop the earth from pouring water in. Water has been leaking into the facility for years – probably since it was constructed.

          • nikkkom

            Yes it can. Drainage wells.

          • jharragi

            I’ll give up on this thread (with the exception of posting a formal concession in some far distant future when these wells are operational).

            I do however want to correct my earlier statement ‘Radiation is probably escaping more in water flow…’. It should read ‘Radioactive materials are probably escaping more in water flow…’

  • Build a Better?

    The failure to the MOX fuel containment system seems to have produced a exponentially more severe catastrophe. I believe this fact was considered in the decision of the Obama administration to scuttle the SRS MOX facility, even after wasting $24B. The question is what possible containment or remediation could be used so late in the game to stop this spread of death in the Pacific?

    • nikkkom

      The failure to the MOX fuel containment system seems to have produced a exponentially more severe catastrophe? Where? Fukushima?

      In meltdowns, there are exactly two dangerous long-lived volatile isotopes: Cs-137 and Sr-90. With half-lives of 30 years, these two can’t be “waited out”. (There are a bunch of shorter lived ones too, I’m not talking about those, I’m focusing on long term hazards here).

      Plutonium is much less volatile than they, and any location which gets significant amounts of Pu is bound to get lots and lots of Sr-90 and Cs-137 anyway.

      In Fukushima, Pu was detected only in barely measurable quantities very close to the station.

      In Chernobyl, it is worse, but Sr-90 and Cs-137 contamination dwarfs Pu contamination there.

    • n_coast

      Woods Hole Oceanographic Institute has posted extensive information about radioactive contamination in the oceans and the effects of the Fukushima disaster. The scare mongers have hyped this subject far into the twilight zone.

      Tepco seems to finally be mastering the groundwater problems.

      • jharragi

        While Woods Hole is spouting rosy reports that the media dutifully churns out for
        the California coast, the Japanese waters and the fish they catch are
        likely not to be so good. A matter of weeks after the meltdowns, Hillary
        Clinton (remember her?) announced that the United States would continue
        importing fish from Japan. Is it just me – or is she not qualified to even
        run the FDA?

        > Tepco seems to finally be mastering the groundwater problems.

        …that is the mass-media for you. Since they don’t know where the core is – let alone have it contained – and until they do, the disaster there will continues to unfold – perhaps to ways worse than it already has. So for now, mastery is just an illusion presented by the nuclear industry – of course the continuation of their business depends on this illusion. Indian Point has not even ‘mastered’ the groundwater problem – and their problem is miniscule in comparison.

        To highlight the situation, for WHUD (or as they refer to themselves, The Entergy Workplace Radio Station) Fukushima ceased to exist about 5 years ago. That is to say that shortly after the disaster and when it was still huge news, they abruptly stopped talking about it.

        Since the inception of the industry, the narrative has been controlled. Risk is presented as minimal, and notions like ‘electricity to cheap to meter’ keep getting tossed around. I think this site has covered the safety facts well. The other fact is that the economics of nuclear suck. Even after 50 years of all kinds of subsidies, from construction grants & low finance to free liability insurance, it is still the most expensive means of electric power production.

        For the cost of every kilowatt of nuclear capacity we don’t build, as much as 2 or 3 times the capacity of renewables can be installed – and that is after you take capacity factors into consideration. And the trend is that this ratio will favor renewables even more.

        Another issue is that renewable deployment is making nuclear even less profitable due to the additional power on the grid. Thus the industry seeks to slow deployment of more green technologies – which is in effect making society respond even more slowly to climate change.

        Anyway, nobody had an edge over the nuclear industry on hype mongering…

        • n_coast

          Are you implying that we shouldn’t trust information from Woods Hole?

          The cores are certainly contained, and they haven’t moved since the corium solidified in 2011. Admittedly the exact locations are unknown, but the corium in each plant must be divided between a portion welded to the lower end of the reactor pressure vessel and a portion embedded in the concrete below the reactor vessel. In either case the cores are harmless to anyone outside the reactor buildings.

          “Anyway, nobody had an edge over the nuclear industry on hype mongering…” Have you looked at

          • jharragi

            >Are you implying that we shouldn’t trust information from Woods Hole?

            …no I am stating that we shouldn’t trust the fish we are buying in the supermarket.

            Contained? Very hot metals in contact with water – that is certainly a
            recipe for a lot of chemical reactions, corrosion and very hazardous
            substances leeching into groundwater – which likely finds it’s way to
            the ocean – but it is possible that it may come to contaminate aquifers
            that are important to the Japanese.

            Solidified? What makes you think it (or parts of it) is not still
            fusing? The point is there are many unknowns. As time goes by we can
            gain confidence that what is left of the core has solidified and is not
            migrating – but as to the core being a quarter inch into the the
            concrete – or a quarter inch from being through (and traveling) who’s to

            I’ll take a look at your link. I agree that there are those who claim
            the worst has happened – that clearly was not the case – because it was a
            very real possibility that several other reactors melted down that day.
            That said, we don’t know how bad the current disaster is. Sadly, many
            residents of that region are finding out…

          • n_coast

            Wikipedia has a post for Decay Heat with a graph showing decay heating rate vs. time after reactor shutdown: I estimate 6.5% at 1 sec; 4 % at 1 min; 1.5% at 1 hour; 0.5% at 1 day; 0.3% at 10 days. (Here 100% would be full reactor power.) Meltdown is driven by the high heating rates in the hours after reactor shutdown, but it is limited by the consumption of the isotopes with short half lives. That illustrates what makes me know that the corium solidified years ago.

            MIT has taken down its site that showed their calculated decay heating rates, but you can find their graph by searching reactor decay heat images.

          • jharragi

            You might recall that the reactor did not ‘shutdown’. What was once the core is a likely a non-homogenous mass of unknown geometry. So we have no relevant information to project its current condition or know if some regions are still generating heat. There are many questions like what fraction of moderating materials and melted portions of the steel vessel dissolved into the core vs what floated on top due to low density – some of which would solidify in contact with water, condense on the reactor walls… Also how widely would the core disperse by steam & it’s own vapor, what is it’s surface area, how much comes into contact with water or steam… It would be quite interesting to know exactly what is happening – but there is far more that is unknown than what is known. It is probably safe to assume that there is a lot of fusible material there. It is also probable that some fusion was occurring immediately after the meltdown – how long that would continue, who’s to know?

          • n_coast

            Dai-ichi plants 1, 2, and 3 were shut down after the earthquake. The only heating was the fission product decay heating, and that is reduced by orders of magnitude. Not much is happening in the reactor’s remnants, but it will take a while before we know it all.


            But what about the burned spent fuel pools at Unit 4 or the new emergencies at Tokuhama and the leaking and severely cracked Sendai plant starting back up? With Japan’s criminally insane State Secrecy Act, we get nothing informationwise other than the overhead passing plumes. Sadly, if it did happen here, there would be crickets from the news media and not one evacuation. Even worse, there would be nobody in government except maybe seven employees at the NRC even looking into the matter. I would love it if someone proved me wrong, but I have noticed a pattern of shut off radiation monitors coinciding with reactor shutdowns and high citizen readings, so why are taxpayers funding agencies that don’t protect us and don’t regulate the industries that kill us all slowly?

          • n_coast

            Did you miss that the plant 4 spent fuel pool was emptied about a year ago?


        Mastering their water problems by deliberately draining radioactive water from those tanks into channels leading to buried pipes by a wastewater treatment facility into the Pacific Ocean? Thanks, TEPCO! I love that radioactive sea spray bombaring me here on the West Coast ! It’s done incredible things to the pine trees! They now produce cesium pollen excreting deformed and mutated cones. Thanks for kicking your problems down wind to me. I especially love that strange haze that settles over Oceanside from you guys burning all your trash bags full of radioactive wase. Gotta spiff things up for your 2020 Tokyo Olympics, do ya now? Thanks for everything.