The owner of the three reactors at the Palo Verde nuclear plant in Arizona recently informed the NRC about a mistake made a decade ago. The mistake involved protection against criticality of nuclear fuel stored in the spent fuel pool.
When sub-atomic particles called neutrons interact with certain uranium and plutonium atoms, these atoms can become unstable and split apart, or fission, releasing energy and neutrons. When the released neutrons cause more atoms to fission, a nuclear chain reaction, also called criticality, is achieved.
Protection against criticality of the fuel in the spent fuel pools at Palo Verde was provided by a combination of geometry (i.e., maintaining distance between adjacent fuel assemblies making less certain that neutrons interact with uranium and plutonium atoms) and dissolving boron into the spent fuel pool’s water. Boron acts like neutron glue, soaking up neutrons to preventing them from interacting with uranium and plutonium atoms.
In 2003, Palo Verde’s owner sought and the NRC later approved a power uprate for all three reactors. The three reactors were allowed to operate up to 3,990 megawatts thermal power from their prior maximum level of 3,876 megawatts thermal.
The higher power level of the reactor core would cause more plutonium atoms to be produced. Initially, the nuclear fuel contains no plutonium – only uranium atoms. During operation, the intense radiation inside the reactor core can transform some of the uranium atoms into plutonium atoms. Some, but not all, of these plutonium atoms interact with neutrons and fission, adding to the power produced by the reactor core. After a few years inside the reactor core, fuel assemblies are discharged to the spent fuel pool.
Because of the power uprate, the fuel assemblies discharged to the spent fuel pools at Palo Verde could contain more plutonium atoms than previously considered. These additional plutonium atoms reduced the protective margins against criticality. The spent fuel pools’ criticality calculations had not been updated to reflect the changes induced by the power uprate.
To restore the margins compromised by the power uprate, Palo Verde’s owner belated increased the minimum amount of boron dissolved into the spent fuel pools’ water from 900 parts per million to 935 parts per million.
Spent fuel pools pose undue risk to millions of Americans. They are accidents waiting to happen.
As in this case, nuclear fuel in spent fuel pools must be protected against damage caused by criticality.
As at Fukushima, nuclear fuel in spent fuel pools must be protected against damage caused by overheating.
There’s a ready solution just waiting to better protect against these hazards – dry storage of spent fuel. Five years after being removed from reactor cores, nuclear fuel can be transferred from spent fuel pools to dry storage. The remaining fuel can be spread out inside the spent fuel pool, increasing the margins available to both criticality and overheating.
If the nuclear fuel inside an overcrowded spent fuel pool is damaged by criticality or overheating, it will be nuclear negligence and not an accident.
The Congress is considering legislation to better manage the risk from interim storage of spent fuel until a repository opens. Millions of Americans would benefit if that legislation accelerates the transfer of spent fuel from pools into dry storage.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.
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