Spent nuclear fuel

[1] Nuclear fuel rods become progressively more radioactive (and less thermally useful) due to neutron activation as they are fissioned, or "burnt", in the reactor.

A fresh rod of low enriched uranium pellets (which can be safely handled with gloved hands) will become a highly lethal gamma emitter after 1–2 years of core irradiation, unsafe to approach unless under many feet of water shielding.

This makes their invariable accumulation and safe temporary storage in spent fuel pools a prime source of high level radioactive waste and a major ongoing issue for future permanent disposal.

A paper describing a method of making a non-radioactive "uranium active" simulation of spent oxide fuel exists.

[2] Spent nuclear fuel contains 3% by mass of 235U and 239Pu (also indirect products in the decay chain); these are considered radioactive waste or may be separated further for various industrial and medical uses.

If compared with MOX fuel, the activity around one million years in the cycles with thorium will be higher due to the presence of the not fully decayed 233U.

Some natural uranium fuels use chemically active cladding, such as Magnox, and need to be reprocessed because long-term storage and disposal is difficult.

Practical spent fuel pool designs generally do not rely on passive cooling but rather require that the water be actively pumped through heat exchangers.

If there is a prolonged interruption of active cooling due to emergency situations, the water in the spent fuel pools may therefore boil off, possibly resulting in radioactive elements being released into the atmosphere.

When looking at long-term radioactive decay, the actinides in the SNF have a significant influence due to their characteristically long half-lives.

Th-232 is a fertile material that can undergo a neutron capture reaction and two beta minus decays, resulting in the production of fissile U-233.

The initial absence of U-233 and its daughter products in the MOX fuel results in a lower activity in region 3 of the figure on the bottom right, whereas for RGPu and WGPu the curve is maintained higher due to the presence of U-233 that has not fully decayed.

The United States has planned disposal in deep geological formations, such as the Yucca Mountain nuclear waste repository, where it has to be shielded and packaged to prevent its migration to humans' immediate environment for thousands of years.

[1][10] On March 5, 2009, however, Energy Secretary Steven Chu told a Senate hearing that "the Yucca Mountain site no longer was viewed as an option for storing reactor waste.

[14] Spent nuclear fuel stays a radiation hazard for extended periods of time with half-lifes as high as 24,000 years.

[16] There is debate over whether spent fuel stored in a pool is susceptible to incidents such as earthquakes[17] or terrorist attacks[18] that could potentially result in a release of radiation.

As a result, used fuel pools are encased in a steel liner and thick concrete, and are regularly inspected to ensure resilience to earthquakes, tornadoes, hurricanes, and seiches.

Spent fuel pool at a nuclear power plant
Spent nuclear fuel stored underwater and uncapped at the Hanford site in Washington , US
Decay heat as fraction of full power for a reactor SCRAMed from full power at time 0, using two different correlations
Activity of U-233 for three fuel types. In the case of MOX, the U-233 increases for the first 650,000 years as it is produced by decay of Np-237 that was created in the reactor by absorption of neutrons by U-235.
Total activity for three fuel types. In region 1 we have radiation from short-lived nuclides, and in region 2 from Sr-90 and Cs-137 . On the far right we see the decay of Np-237 and U-233.
Spent fuel pool at TEPCO 's Fukushima Daiichi Nuclear Power Plant on 27 November 2013