Nuclear Power: Nuclear Waste

Jun 4th, 2008 | By | Category: Nukes

We’ve got our reactor up and humming. Our fuel is fissioning, splitting into smaller atoms and releasing neutrons. Our moderator is slowing down the neutrons, keeping them around long enough to fission the next fuel molecule. Our control rods are absorbing enough neutrons to keep the chain reaction in check. The coolant is transferring the heat out of the reactor.

Over time we have to pull our neutron-absorbing control rods out farther and farther just to keep the chain reaction going. Why?

When we loaded our reactor, the fuel was chemically fairly pure. Recall, however, that nuclear decay typically results in new chemicals being created–whether by alpha or beta decay or by fissioning. As our reactor operates, these new atoms build up. Most are radioactive themselves, also undergoing various decays. Most of these atoms are neutron hoarders–gleefully absorbing our precious neutrons, while offering up few when they themselves decay. So, as these new atoms build up, we lose more and more neutrons. Eventually there are too few free neutrons left to keep the chain reaction going, even if we completely remove the control rods. Such fuel, still containing a bunch of Uranium but now contaminated various highly radioactive but non-chain reacting atoms, is called spent. It’s hideously radioactive, more radioactive than when we put the fuel in the reactor, but useless as fuel.

Welcome to the trickiest problem of nuclear power, the waste. What can we do?

Pull the spent fuel rods out of the reactor and replace them with fresh ones. We’ve only used a tiny percent of the Uranium up, but on the positive our reactor will start working again. On the negative, we have a whole bunch of really radioactive former-fuel that is useless.

Consider these unwelcome new atoms. The more radioactive the atom–the more often the atom decays–the quicker it uses itself up and becomes something else. So, the most dangerous to health atoms disappear relatively quickly–within years or maybe decades. The problem is, even the less radioactive waste–that will last for hundreds of thousands of years–is still radioactive enough to be a threat to health.

We could just store the spent fuel rods, for millions of years, until it is only minimally radioactive. Water is a pretty fantastic shield against radiation. So, we build a whole bunch of swimming pools in the reactor containment buildings and then sink the radioactive waste to the bottom, waiting for the most radioactive atoms to decay into other more manageable things. That will be fine for a few years at least. When the pools get filled, we’ll next build some metal and concrete casks outside of the reactor building, next to the plant and store the rods there. Finally, we’ll hope that someone, somewhere, finds a suitable mountain to carve out and drop the waste into, just to get it off our property.

But wait, you say. The new atoms, ruining our chain reaction, are only a teeny percentage of the spent rods. We could chemically break down the fuel, purify for Uranium and create new fuel from the old. Great! We’ve now reduced the freakishly radioactive waste we cannot use to a much smaller mass and recycled the fuel. Neat. We stopped doing this in the 1970’s. Why? While the leftover atoms are useless for commercial power plants, they would make great starting material for a dirty bomb. Ouch.

So, we’re back to the pool, cask, prayer plan for the waste. While there are proposals for more clever ways of dealing with the waste, right now nearly three hundred thousand tons of highly radioactive spent fuel rods are scattered all over the world. Typically in casks next to the power plant, they represent by far the biggest environmental and health risk from nuclear power. And we really don’t have anything better planned in the near future, right as we’re embarking upon a plant building binge.

Unlike waste from chemical plants, or carbon pouring out of smokestacks and tailpipes everywhere, this waste is a uniquely human creation. While sitting in a reading of The World Without Us, about what a world after the end of humanity would look like, I realized that radioactive waste would be the longest lasting legacy of humanity.

After all the bridges and towers crumble, after all the seawalls give up, after all the dams burst, after all the plastic garbage is eaten by some newly evolved bug, after our bones are recycled into every imaginable different thing, our radioactive waste–made of atoms not seen on the planet since its birth–will persist.