If Nuclear Waste Is Still Radioactive, Why Can't You Just Leave It In The Reactor?

Nuclear fuel comes out of a reactor with about 95% of its uranium unused because a reactor does not run on radioactivity; it runs on a chain reaction that needs one neutron from every split to go on and cause the next. As the fuel splits atoms, its small share of splittable uranium runs down while the leftover fragments, some of which swallow neutrons thousands of times more readily than uranium does, choke the reaction until the fuel can no longer keep it going, and the metal tubes holding the fuel wear out after a few years anyway. The waste is still intensely radioactive, but radioactive only means it is falling apart on its own schedule, which is a different thing from being able to power a reactor.

Somewhere online right now, someone is making a point that sounds unanswerable. A fuel assembly comes out of a reactor so radioactive that ten years later it can still deliver a fatal dose in minutes. It stays dangerous for tens of thousands of years. And yet about 95% of the uranium inside it was never used.

So why not leave it in and keep using it? It is the nuclear version of leaving a restaurant with most of your dinner still on the plate, except that this plate can kill you.

Both halves of that suspicion are true, and the mistake is the word "so." The answer turns on two words that sound the same and are not: radioactive and reactive. Tell them apart and the whole thing makes sense.

What Is The Difference Between Radioactive And Reactive?

Picture a campfire, and notice that two things are happening in it. The logs are burning, which you can feed, starve, or smother. And the embers are glowing, which is hot material giving off heat on its own. You cannot make an ember glow harder by adding logs.

Radioactivity is the ember: an unstable atom falls apart on its own timetable, and nothing you do changes it. The pace is set by a fixed number, the half-life, the time for half of a batch of atoms to decay. Some atoms decay in minutes, some take thousands of years. Uranium does not check whether anyone is watching.

A chain reaction is the fire, and it starts when a slow neutron hits a uranium-235 nucleus. The nucleus splits, releases energy, and throws out fresh neutrons. If those hit more uranium-235, the process feeds itself. If they do not, it goes out. The fire has to be fed, while the ember glows whether you feed it or not.

A reactor runs on the fire, not on the ember. The glow in a spent fuel pool is the ember. That radioactivity is a side effect of running a reactor, rather than the thing that runs it.

What Does It Mean For A Nuclear Reactor To Go Critical?

"Critical" sounds like a warning, but in a reactor it means the machine is working.

Each split of a uranium-235 nucleus releases 2.42 neutrons on average. That figure is from the US Department of Energy's reactor handbook. The reactor needs one of those, on average, to split another nucleus. The handbook calls this k, the multiplication factor. It is the neutrons in one generation divided by the neutrons in the one before.

1. k = 1: each fission funds one more and power holds steady, a state called "critical."
2. k < 1: fewer than one, so each generation is smaller and the fire dies.
3. k > 1: more than one, so power climbs, and control rods exist to stop this.

So of every 2.42 neutrons born, about 1.42 must be lost on purpose. Uranium-238, the control rods and other parts of the core absorb them, or they leak out. A reactor is a bank that has to end every day with the same balance. Think of the neutrons as a budget, where anything that eats a neutron is a cost and uranium-235 is the income.

Fuel goes in with a healthy budget. Over the next few years the income falls and the costs rise. When the budget can no longer balance at k = 1 with every control rod pulled out, the fuel is done. How radioactive it is plays no part in that.

How Much Of The Uranium In Nuclear Fuel Actually Gets Used?

Fresh fuel is not even mostly uranium-235. Natural uranium is about 0.7% uranium-235, and it is enriched to between 3% and 5% before it goes in. The rest is uranium-238, which tends to catch a neutron instead of splitting. So "95% unused" starts to look less like a scandal and more like arithmetic. Most of the uranium was never going to burn in this type of reactor.

How hard the fuel has been worked is called burnup. It is the energy each ton of fuel delivered before it came out. The unit is gigawatt-days per metric ton of uranium, written GWd/MTU. A training module from Lawrence Livermore National Laboratory gives the rule of thumb. Fuel stays in a pressurized water reactor for three to five years. It comes out at about 10 to 12 times its enrichment. So 4% fuel typically leaves at 40 to 48 GWd/MTU. The NRC says the US average was around 35 two decades ago and is over 45 today.

As for what comes out, a federal review board gives the makeup at 50 GWd/MTU. About 93.4% is uranium, 5.2% fission products, 1.2% plutonium and 0.2% other heavy elements. Of that uranium, only 0.8% is still uranium-235. It went in at 4%, and that drop is the income cut.

Argonne National Laboratory puts it this way: "Imagine the mess if we mined one ton of coal, burned five percent of it for energy, and then threw away the rest." It is the thing an accountant notices before a physicist does. The Department of Energy agrees that "more than 90% of its potential energy still remains in the fuel". But "potential" is doing a lot of work there. The energy is present, but the budget to release it is not.

What Is A Neutron Poison, And Why Does Fission Make Its Own?

On the cost side, when uranium splits, it leaves two smaller fragments behind, the fission products. That is the ash of the fire, and some of that ash eats neutrons.

Physicists measure how likely a nucleus is to catch a passing neutron as a target size, called a cross section. The unit is, with a straight face, called the barn. One barn is a trillionth of a trillionth of a square centimeter. For a slow neutron, uranium-235 has a fission cross section of 582 barns, while uranium-238 absorbs at 2.71 barns.

Then there is xenon-135. The same handbook lists its cross section at 2.6 million barns. That is roughly 4,500 times the target uranium-235 presents. Samarium-149, the second worst, sits at 41,000 barns. Beyond the famous two there is the crowd. Each minor fission product matters little on its own. Together they pile up at an average of 50 barns for every fission, and every split adds more ash.

These are the neutron poisons, and poison is the real technical term for them. A poison does not have to be radioactive to do its damage. Samarium-149 is stable, so it will sit in the fuel forever, eating the budget. The most reactive thing in the ash is not the hot stuff.

Is Xenon-135 The Reason Nuclear Fuel Is Removed?

No, and this is where most popular explanations go wrong. Xenon-135 is the most dramatic poison, so it gets the blame, but it only passes through.

Most xenon-135 comes from the decay of iodine-135, which has a half-life of 6.57 hours. Xenon-135 itself has a half-life of 9.10 hours, and the neutrons it swallows destroy it faster still. While the reactor runs, xenon settles at a steady level in about 40 to 50 hours. Then it does something strange: shut the reactor down and the xenon level rises. The iodine keeps decaying into xenon, and there are no neutrons left to burn it off. The handbook says it peaks about 10 hours after shutdown. The curve below, computed from those two half-lives, peaks at about 9. The core is "xenon-free" only after about 3 days. Restart too soon and the poison can hold the reactor shut, which operators call xenon dead time.

In September 1944 the B Reactor at Hanford, Washington, was switched on. It was the first full-scale reactor ever built. It promptly turned itself off, even though nobody had touched anything. Physicists Leona Woods Marshall and John Wheeler diagnosed xenon poisoning. The fix was to load extra fuel tubes: enough spare budget to outspend the poison. Forty-two years later it played a part at Chernobyl. Xenon built up during a botched low-power test, one ingredient of the disaster.

So xenon matters for operating a reactor, but it is a transient that builds and burns off in hours. Fuel comes out because of the slow things. The uranium-235 runs down, and the stable poisons stack up over years. Blaming xenon for spent fuel is like blaming the smoke for the fire going out.

Why Can't Fuel Rods Physically Stay In The Reactor Longer?

Even if the physics were kind and the budget still balanced, the metal would give up anyway.

Reactor fuel is a stack of ceramic uranium pellets, each about the size of a fingertip. They are sealed in zirconium alloy tubes 3.7 to 4.6 m (12 to 15 ft) long. That tube, the cladding, is all that stands between the ash and the cooling water. It spends years in hot water under a neutron bombardment, which it does not enjoy.

The NRC's backgrounder on high burnup fuel explains what happens. The cladding reacts with the water, forming an oxide layer like rust and releasing hydrogen. Those processes "begin slowly, then start to accelerate as the fuel reaches burnup of 45 GWd/MTU." Inside the tube, some fission products are gases. Under accident conditions, fuel pellets can fracture from expanding gas bubbles.

So regulators cap how far fuel can be pushed. The NRC says the current limit for a fuel rod "corresponds to roughly 62 GWd/MTU when averaged over the rod." Fuel vendors are working on 75 or 80. Even that would leave most of the uranium untouched. The physics says the fuel cannot keep the fire going. The metallurgy says the tube would fail before you found out.

How Long Does Nuclear Waste Remain Radioactive?

Once the fire is out, the ember comes back into the story, because the ash glows for a long time.

The NRC's waste backgrounder splits the hazard in two. Fission products such as cesium-137 and strontium-90 come first. They "account for most of the heat and penetrating radiation." Both have half-lives of about 30 years. Half is gone in 30 years, three-quarters in 60, and so on. Then come the heavy elements made when uranium-238 caught a neutron. The main one is plutonium-239, with a half-life of 24,000 years. They give off far less radiation but last far longer. They "account for most of the radioactive hazard remaining in high-level waste after 1,000 years."

Ten years after removal, the NRC says, a typical spent fuel assembly still gives off more than 10,000 rem per hour at its surface. That works out to 100 sieverts per hour. A fatal whole-body dose is about 500 rem (5 sieverts). So the fuel goes into a deep pool first, typically for at least five years, then into steel-and-concrete dry casks. The United States holds over 90,000 metric tons of it. We cover what happens next in what nuclear waste is and how we deal with it.

None of this radioactivity helps a reactor. The 30-year embers are fission products, which are ash. The 24,000-year embers are plutonium, which a reactor can burn, and does. Over a typical three-year stay it supplies about one-third of the energy. But the Livermore module is clear that this new fuel is "not enough to outweigh" the losses. In budget terms, the plutonium income never catches up with the cost of the ash.

Why Is Hiroshima Not Radioactive But Chernobyl Is?

The same two ideas answer a question people ask all the time. Both events involved uranium fission, and the difference between them is the ash.

The Little Boy bomb held a uranium core of 64 kg (141 lb). Less than 2% of that uranium-235 fissioned, which is under 1.3 kg. It went off 580 m (1,900 ft) above the city. Hiroshima's own account says roughly 80% of the residual radiation was emitted within 24 hours. Radiation in the city today is "on a par with" the natural background found anywhere on Earth. A bomb makes a small amount of ash, fast, and high up.

Chernobyl's Unit 4 held a 190.3-tonne fuel load, and about 135 tonnes of it melted. That fuel had been running for years, so it was full of the long-lived ash this article is about. How much of that ash got out of the reactor? "About half of the iodine and caesium," the World Nuclear Association estimates. The exclusion zone now covers 4,300 square kilometers. That is a large amount of ash, made over years, and dropped where people live. With a 30-year half-life, the cesium that fell in 1986 is just over one half-life old today.

It was the same physics and the same element. One left a kilogram of ash in the sky, and the other left tonnes of it in the soil.

Why Don't We Dump Nuclear Waste In The Ocean Or Just Recycle It?

The ocean is large, but it was still not considered large enough. The London Convention is the treaty on dumping at sea. Its 1993 amendments banned dumping even low-level radioactive waste in the ocean. They took effect in 1994. (Firing it into the Sun has its own problems.)

Recycling is the more serious question, and it is where this article hands off. Spent fuel can be taken apart, the uranium and plutonium pulled out, and new fuel made from them. France does this, and it gets about 70% of its electricity from nuclear power, and about 17% from recycled fuel. The United States does not recycle spent fuel. Argonne's own answer to "why not?" begins with "Lack of financial incentive. Raw uranium is cheap."

That is a live policy dispute and we will not settle it here. The technical side lives in what makes it so difficult to reuse radioactive waste. The short version is that recycling gets the ash out of the fuel. It is not a way to burn fuel with the ash still in it, which is what "leave it in the reactor" would need.

So, Why Can't You Just Leave Nuclear Waste In The Reactor?

Because a reactor does not run on radioactivity. It runs on a budget of neutrons, and the fuel goes in with enough to balance it and not much more.

Every year in the core, the income falls. The uranium-235 that was 4% of the fuel drops toward 0.8%. Every year the costs rise as the ash builds up, and some of it presents a target thousands of times bigger than uranium does. Plutonium covers about a third of the energy, but never closes the gap. At some point, with every control rod pulled out, the reactor cannot hold k = 1. The NRC's phrase for that moment is that the fuel "can no longer economically keep a chain reaction going." At that point the fire goes out. It would go out even if the physics held, because the tubes are cooked at about 62 GWd/MTU.

What is left is a battery that reads 95% full and cannot light a bulb. It holds uranium-238 this type of reactor cannot burn, and some plutonium. The few percent of ash is glowing hard enough to kill. Radioactive, then, is not a measure of how much energy is left. It is a measure of how fast the leftovers are falling apart.

Nature ran this experiment first. Two billion years ago in Gabon, a uranium deposit went critical on its own. It ran on and off for hundreds of thousands of years, switching itself off each time it boiled away its own groundwater. The chain reaction stopped long ago, but the deposit has stayed faintly radioactive ever since. The xenon it made is still in the rock, and it is how we know.