Spent Nuclear Fuel

Mina Bionta
December 14, 2009

Submitted as coursework for Physics 204, Stanford University, Fall 2009


Fig. 1: This figure shows the nuclear fuel cycle process. The uranium is first mined and then processed into nuclear fuel rods. After the rods have been used in a reactor they are stored in a spent fuel pool for at least a year. Then they can either be reprocessed and used again, or disposed of in more permanent storage.


What is Spent Nuclear Fuel?

Spent Nuclear Fuel (SNF) is nuclear fuel that has been irradiated in a nuclear reactor. This means that the nuclear fuel can no longer fission effectively to maintain the integrity of the reaction taking place in the nuclear reactor. Thus, that fuel must be removed, replaced with new fresh fuel, and then disposed of in some manner. [1]

Nuclear fuel rods are composed of pellets made of a uranium ceramic. After the fuel is spent, there are still radioactive materials present in the rods that need to be properly discarded. This SNF is made up of mostly unused U-238 and fission products from the U-235 and Pu-239 that are made during the nuclear fission process. Other potentially harmful elements present in SNF include transuranic elements created by neutron capture during the reaction process. Transuranic elements are elements that have a higher atomic number than Uranium. They are created when a U-238 atom absorbs a neutron emitted by a fissioned U-235. Several of these transuranic elements include fissionable products such as Plutonium isotopes.

Why is it Dangerous?

SNF is dangerous because if its high levels of radioactivity as well as the massive amounts of heat that are produced. The amount of heat produced just after the reactor is shut down is about 7% of the amount of heat produced in the reactor itself. [2] In fact, so much heat is produced that the rods need about a year of time to cool in a pool of water (see below for more details). The heat produced is not just limited to the immediate time after the SNF is removed from the reactor. Heat is also being produced by isotopes in the spent fuel itself, so the amount of heat in the fuel can increase as time is spent outside of the reactor depending on what isotopes are present in the spent fuel. The radiation is produced from the leftover uranium that is left in the rods as well as in the fissionable transuranic elements that are produced during the reaction process. Other radioactive products are also produced by the fission process itself and continue to be made because of the spontaneous fissionable nature of the uranium.

The harmfulness of SNF is categorized by its potential toxicity to humans. Radioactive products of particular risk in SNF have potentially biologically harmful effects. The fission products of Sr-90 and Cs-137 provide risks due to their thermal impact on the spent fuel. The heat produced by these two isotopes is the major source of heat emitted by the SNF for the first several years. Tc-99 and I-129 are more biologically harmful isotopes as they are long lived and a large source of the fission products. Both elements are easily absorbed in groundwater, thus contamination is a concern when dealing with these isotopes. The actinides produced by reactors (uranium, plutonium, neptunium, americium, and curium) are dangerous because of their long-lived half-lives as well as their tendency to fix to bones when ingested, thus irradiating blood producing cells. [2]

What to Do With SNF

Since SNF is by definition harmful to people and the environment, a proper waste technique must be established. The fuel must be prepared in such a way that it can be safely transferred and stored.

Since the fuel that comes out of the reactor is extremely hot, it must first be cooled to a temperature that can be easily handled. This involves storing the fuel in a pool of water to cool off the fuel. In a spent fuel pool system, the used fuel is stored under at least 20 feat of water. This much is needed in order to provide sufficient radiation shielding as well as to extract heat from the fuel rods. After 12-18 months, the fuel rods can be rotated out and replaced with new ones.

According to the Nuclear Regulatory Commission (NRC), at the current rate of consumption and disposal of nuclear fuel, all the Spent Fuel Pool systems will be full by 2015. This means an alternative, more permanent storage system must be built. After the fuel has spent time in a spent fuel pool, it can be moved to dry cask storage. In dry cask storage, the already cooled spent fuel is stored in leak-tight, sealed steel compartments surrounded by inert gases. Spent fuel can then be stored in these casks for an unlimited amount of time.

Other, more permanent storage spots are being developed. For example, in France nuclear waste is buried under the rocks in Normandy. [2] Even in the United Stated, the Yucca Mountain Complex would provide a place for the United States to permanently store all of its nuclear waste. However, as of early 2009, plans for the development of this site have been suspended. [3]

The final solution to nuclear waste would be reprocessing. Reprocessing is a form or recycling or reusing the spent nuclear fuel. By reprocessing fuel, one can gain nearly 25% more energy from the used fuel. The main idea with reprocessing is to access the fissionable long living materials such as leftover uranium and plutonium and make new fuel rods out of them. This would account for nearly 96% of the original uranium from the rods. Much reprocessing occurs in Europe, Russia and Japan, however none is being done in the United States. The waste from reprocessing is less radioactive than waste from a single use in a reactor and thus is easier to handle when being disposed of. This is because although the ratio of and quantities of U in the fuel are the same, the fuel is simply older than that in the original fuel rods, thus has had more time to decay [4]. The fuel cycle can be seen in Fig. 1.

© Mina Bionta. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Economic Assessment of Used Nuclear Fuel Management in the United States," The Boston Consulting Group, July 2006
(PDF 1.03 MB).

[2] National Research Council, Nuclear Wastes: Technologies for Separations and Transmutation (Nat. Acad. Press,1996).

[3] H. J. Herbert, "Nuclear Waste Won’t Be Going to Nevada’s Yucca Mountain, Obama’s Official Says," Chicago Tribune, 6 Mar 09.
http://www.chicagotribune.com/news/nationworld/chi-nuke-yucca_frimar06,0,2557502.story.

[4] "Management of Reprocessed Uranium - Current Status and Future Prospects," Intl. Atomic Energy Agency, IAEA-TECDOC-1529 - February 2007.
(PDF 1.28 MB).