Zircaloy is a problem

Fuel rods in many types of nuclear reactors consist of pellets of fissile material (uranium and plutonium) sheathed in a metal alloy called zircaloy, made unsurprisingly from zirconium.

Zircaloy seems to be one of the major reasons why the Fukushima crisis is scary. When the earthquake happened, the reactors automatically inserted neutron-absorbing control rods to stop the nuclear chain reaction. This process is called a SCRAM. Despite this, the fuel rods are still producing heat because of other ongoing nuclear reactions (decay heat). This is what makes Zircaloy so worrisome, because when it gets heated up it can oxidize or burn exothermically, producing even more heat. That adds to the risk that fissile material will end up in the wider environment. It also adds to the risk that the nuclear fuel will melt, dribble down to the bottom of the reactor containment vessel, and re-form a critical mass.

That’s the really scary possibility, as far as reactors one, two, and three at Fukushima are concerned. If the fissile material in the fuel rods forms a critical mass again, it could melt through the bottom of the containment vessel. It could also trigger a large steam and/or hydrogen explosion that could spread radiation further.

There are other risks with Zircaloy. When hot, it oxidizes in the presence of water, stripping oxygen from water molecules and producing explosive hydrogen. Zircaloy is also what makes spent storage pools so scary. If they lose cooling, the rods can heat up, burn, and release large amounts of radiation into the environment. Cooling ponds are not placed inside containment vessels in the same way reactor cores are.

So, what seem to be possible lessons learned here?

1) If it is possible to find something better than Zircaloy, we should. It needs to have a low neutron cross-section, so that neutrons from different fuel rods can induce fission in one another. At the same time, it would be really nice if it would not oxidize exothermically, generate hydrogen in the presence of water, or burn.

2) Perhaps spent fuel cooling ponds should be inside containment structures.

3) Perhaps containment pools should be embedded in solid rock, not perched atop buildings.

It’s possible there is no material that satisfies (1) and it is possible that (2) would make nuclear reactors impractically expensive. If so, perhaps the appropriate option is to pull back from nuclear power as an energy source and concentrate on reducing total energy demand, while deploying renewable forms of energy.

14 thoughts on “Zircaloy is a problem

  1. .

    When the water in a storage pool disappears, residual heat in the fuel rods’ uranium left over from their time in a nuclear reactor continues to heat the rods’ zirconium cladding. This causes the zirconium to oxidize, or rust, and even catch fire. This breaks the seal of the rods, and pressurized radioactive gases like iodine, which accumulated in the rods while they were in the reactor, suddenly spurt out, Mr. Albrecht said.

    Each rod inside the assembly holds a vertical stack of cylindrical uranium oxide pellets. These pellets sometimes become fused together while in the reactor, in which case they may stay standing up even as the cladding burns off. If the pellets stay standing up, then even with the water and zirconium gone, nuclear fission will not take place, Mr. Albrecht said.

    But Tokyo Electric said this week that there was a chance of “recriticality” in the storage ponds — that is to say, the uranium in the fuel rods could become critical in nuclear terms and resume the fission that previously took place inside the reactor, spewing out radioactive byproducts.

  2. .

    The nuclear crisis in Japan has laid bare an ever-growing problem for the United States _ the enormous amounts of still-hot radioactive waste accumulating at commercial nuclear reactors in more than 30 states.

    The U.S. has 71,862 tons of the waste, according to state-by-state numbers obtained by The Associated Press. But the nation has no place to permanently store the material, which stays dangerous for tens of thousands of years.

    Plans to store nuclear waste at Nevada’s Yucca Mountain have been abandoned, but even if a facility had been built there, America already has more waste than it could have handled.

    Three-quarters of the waste sits in water-filled cooling pools like those at the Fukushima Dai-ichi nuclear complex in Japan, outside the thick concrete-and-steel barriers meant to guard against a radioactive release from a nuclear reactor.

    Spent fuel at Dai-ichi overheated, possibly melting fuel-rod casings and spewing radiation into the air, after Japan’s tsunami knocked out power to cooling systems at the plant.

    The rest of the spent fuel from commercial U.S. reactors has been put into dry cask storage, but regulators only envision those as a solution for about a century and the waste would eventually have to be deposited into a Yucca-like facility.

    The U.S. nuclear industry says the waste is being stored safely at power-plant sites, though it has long pushed for a long-term storage facility. Meanwhile, the industry’s collective pile of waste is growing by about 2,200 tons a year; experts say some of the pools in the United States contain four times the amount of spent fuel that they were designed to handle.

  3. Milan Post author

    It is very hard to put that sort of story into context – to distinguish between technological tweaks that might be a bit helpful and those that have a really huge potential to change how the global energy system works.

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  6. .

    The accident at Fukushima Daiichi in Japan, following the magnitude 9.0 Tohoku-oki earthquake and subsequent tsunami on 11 March 2011, occurred while only three (units 1, 2, and 3) of six boiling water reactors were in operation. Most of the fuel in these reactors was UO2, although there were also 32 mixed-oxide fuel assemblies containing ~6% Pu in unit 3, corresponding to ~4% of the core loading. The operating units shut down promptly in response to the earthquake; however, when the tsunami inundated the site about 40 min later, electrical power was lost, followed by the loss of on-site backup power, resulting in a station blackout and a loss of reactor coolant. A partial core-melt event ensued in units 1, 2, and 3. In a preliminary analysis, the Japanese operator TEPCO has surmised that there was a nearly immediate loss of core cooling in unit 1 and almost all of the fuel assemblies melted and accumulated in the bottom of the pressure vessel. Partial melting of the cores in units 2 and 3, damaging approximately one-third of the fuel assemblies in each, occurred over the following days. Reaction of the zirconium alloy (Zircaloy) fuel cladding with water at high temperatures generated hydrogen gas that accumulated and exploded in four of the units. Seawater was injected into the three active reactors and sprayed onto fuel storage pools (e.g., near unit 4) to cool them. With boiling and evaporation of seawater, large amounts of salt may have deposited in the reactor cores. The release of radioactivity other than gaseous and volatile fission products at Fukushima Daiichi, unlike at Chernobyl, was dominated by the many metric tons of seawater used to cool the cores and storage pools. An unknown fraction of this water was released to the environment, together with accumulation in the basements and trenches of the reactors. Direct discharge of contaminated water to the ocean and groundwater occurred through approximately 8 April 2011 (7). Estimates of the amount of radioactivity released differ by a factor of about 20, with one of the higher estimates indicating that 27,000 terabecquerels of 137Cs was discharged to the ocean .

  7. Kismet

    Zirconium does produce hydrogen at high heat – however, it will not burn, the solid metal does not burn, only the powder can burn. It is not possible for a reactor fire to start from zirconium.
    It is possible for them to melt, and in this way the rods would become exposed to air. But it would not happen merely because they oxidize, and rust – it would take continuous intense heat, (like in a LOCA at Fukushima)

    Zircalloy is chosen because of its high resistance to heat – there are few alternatives. The hydrogen formation that occurred at Three Mile island, showed how hydrogen formation could be dangerous. Plants have venting to deal with this kind of emergency (which Fukushima Dai-ichi, built in 1967-71 did not have) A reactor which uses ceramic instead of zircalloy is being experimented with, the ceramic pebble bed reactor

    Also, your narration says there was no cooling after the accident — the reactors did not lose all cooling until 2-3 days after the incident (as this system gradually failed). One of the emergency cooling systems remained on, providing (inadequate) cooling


    You say Yucca Mountain could not have handled all that waste — Yucca was designed to handle more waste then 72 000 tonnes. 72 000 of radioactive waste will fit in one football field. Yucca Mountain can handle 140 000 tonnes of waste, and could be enlarged (which is why it was chosen)

  8. Benjamin

    The fuel for commercial reactors CONNOT sustain criticality in the absence of water or some other moderating material. This makes the following claim, at least as far as criticality is concerned, very dubious. “It also adds to the risk that the nuclear fuel will melt, dribble down to the bottom of the reactor containment vessel, and re-form a critical mass.

    That’s the really scary possibility, as far as reactors one, two, and three at Fukushima are concerned. If the fissile material in the fuel rods forms a critical mass again, it could melt through the bottom of the containment vessel.” Molten or melted and resolidified commercial fuel (3-4% enrichment) CANNOT sustain a chain reaction, regardless of mass or geometry, i.e., has infinite critical mass for neutrons of any energy.

  9. Carlo V. Santarelli

    A storage site for most of the world’s nuclear waste could be built in Antarctica, where
    only a few research stations are now present.
    The fuel rods could be transported there in containers that fit in the launch tubes of
    decommissioned nuclear missile submarines.
    The site would be several hundred miles inland and reached by train from a port where
    the nuclear submarines would dock and off load the containers.
    This I think would be a very good solution to the problem of where to store nuclear
    waste for the next ten thousand years.

  10. Anonymous

    Hmm one of the two worlds largest reserves of stored water (ice)
    And you think storing materials under that is a good idea?
    Not to mention security, Antarctica is pretty remote, unless you want a permanent military presence there how do you find it even feasable to prevent the theft/misuse of such materials as a weaponised threat.

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