Energy & Climate Change

Spent fuel is replaced with fresh normally during annual maintenance. This hot fuel cools down quite quickly. In 40 years, the activity it had has decreased by a factor of one thousand. Illustration: Ferry

Spent Nuclear Fuel is Hot Stuff

Nuclear waste. Especially in western culture, the mere mention of those two words arouse many feelings and summons many images, especially fear.

In previous articles, we dived into what uranium actually is, and how it is made into nuclear fuel. Now its time to move on to figuring out what is left of the fuel when it is removed from the reactor. Is there reason for all that fear and loathing we feel towards spent nuclear fuel?

Hot stuff!

When removed from the reactor, spent nuclear fuel is really, really hot. And even though the chain reaction has stopped, the many fission products left in the fuel continue to decay, producing more heat. Fresh spent fuel is also extremely radioactive. But this also means that it is losing its radioactivity fast.

When a reactor is shut down, the fuel still produces heat at around seven percent of the thermal power the reactor had while it was running. In one hour this heat production drops to 1.5 percent, and in a week it drops to 0.2 percent. A reactor with 3000 MW of thermal power (thats equal to about 1000 MW of electrical power) has a heat output of 60 megawatts one week after shutdown. Thats in the ballpark with some older nuclear powered submarine-reactors running at full power.

 

The activity of the spent fuel decreases rapidly when it is removed from the reactor. In the picture it is compared with natural uranium. After about one thousand years (blue dotted line), the spent fuel is dangerous to humans mainly because it is a somewhat toxic heavy metal, not because its radioactive.

After removal from the reactor, spent fuel is placed in a large water pool to cool down for a few years (normally between 2 and 10 years). After cooling, the spent fuel will be moved to an interim storage, which can be another pool of water or a so-called dry-cask storage, where it can safely be stored for decades. At this point, air cooling is enough to keep the spent fuel from heating up, and it poses no danger for people being near it.

It is the plutonium in the waste that causes concern for many. Isn't plutonium used to make nuclear weapons? Yes. But the story is not so simple[iii].

First, separating the plutonium from the rest of the spent fuel is rather expensive, hard to do and requires a host of special equipment and advanced facilities[iv]. Second, when the reactors are run normally, the plutonium formed in the fuel starts to transform into plutonium isotope Pu240, while nuclear weapons require the isotope Pu239[v].

The plutonium in spent fuel is called reactor grade, and it has Pu239 content of less than 80 percent. Nuclear weapons can be made with weapons grade plutonium, which has Pu239 content of around 93 percent. It is not entirely impossible to build a nuclear bomb with reactor grade plutonium, if someone was to get his hands on enough of it. But it is much, much harder than using weapons grade plutonium, which in itself is an undertaking fit for a nation. 

20,000 Russian nuclear warheads have been transformed into nuclear fuel


Weapons grade material[vi] can, however, be used to make nuclear fuel for civilian reactors[vii], and this has also been done in the case of weapons grade Uranium. Around 20,000 Russian nuclear warheads have been transformed into nuclear fuel in the Megatons to Megawatts -project that lasted for 20 years[viii]. If (and hopefully, when) the world wants to get rid of our total of around 15,000 nuclear weapons for good, using them as fuel in nuclear reactors is one of few reasonable ways of doing that[ix].

Between three and four percent of the used fuel is fission products that are very radioactive. As with the heat, this activity decreases the faster the stronger it is (radioactivity quite literally means the speed at which the radioactive matter is transformed to other minerals). In forty years, the average activity of the spent fuel has decreased to around one thousandth of the original. In a few centuries, most of the originally highly radioactive and dangerous fission products have disappeared from the fuel[x]. In roughly 1,000 years, the external radiation dose of the spent nuclear fuel has decreased to a similar level that the fresh fuel had when it was first placed in the reactor. It is relatively safe to physically be near the fuel at this point[xi].

So why on earth do we need to store the spent fuel for hundreds of thousands of years? Or could we perhaps do something else with it? These questions are answered in my next article. You can read it here. 

Notes:

[i] In this context, I mean all of the fuel in the reactor. Normally only 20 to 33 percent is replaced during refuelling. Older nuclear submarines like the first one, called Nautilus, which was eventually scaled up to represent most of our civilian reactors as well, had small reactors of several tens of megawatts of thermal power.

[ii] The actual content of spent nuclear fuel differs amongst various reactor types. In this article, I am referring to the fuel of the most commonly used light water reactors (LWR). 

[iii] It is also possible to build nuclear weapons using highly enriched Uranium. 

[iv] The reprocessing is explained a bit more thoroughly in my next article. 

[v] When the fuel is in the reactor, Pu239-isotope is formed from U238. With time, some of this Pu239 catches another neutron in the intense neutron bombardment of the reactor core, and it turns into Pu240. 

[vi] Whether it was Plutonium or highly enriched Uranium. 

[vii] If Plutonium is mixed with uranium, the result is called MOX-fuel (Mixed-Oxide).

[viii] The official name for the program was United States-Russia Highly Enriched Uranium Purchase Agreement

[ix] http://www.ploughshares.org/world-nuclear-stockpile-report

[x] These include Strontium-90 and Cesium-137 which both have half-lives of around 30 years. 

[xi] Activity is not the same thing are external radiation dose. Radiation dose is used to describe the health effects of radiation, while activity describes the amount of fissioning cores per second.

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