New technique can tell apart spent nuclear fuel from six reactor types

The technique can also differentiate between fuel from BWRs and PWRs, a historically challenging problem.

Published - March 26, 2023 10:30 am IST - Chennai

A view of the Kudankulam Nuclear Power Plant, which is expected to have six VVER-1000 reactors. VVER-1000 is a type of PWR developed in the erstwhile Soviet Union.

A view of the Kudankulam Nuclear Power Plant, which is expected to have six VVER-1000 reactors. VVER-1000 is a type of PWR developed in the erstwhile Soviet Union. | Photo Credit: PTI

Scientists in China have developed a technique to identify whether some nuclear fuel originated in one of six types of nuclear reactors.

Importantly, their technique can reliably distinguish between spent fuel from two common kinds of reactors that have historically presented a challenge to scientists.

Their work was published in Physical Review Applied on March 9.

Nuclear fuel is a highly regulated material because of its destructive potential. Governments, regulators, and militaries maintain detailed inventories to safeguard it.

Nuclear forensics uses analytical methods to identify the origins of nuclear materials and whether they were used for military applications. In the current study, the scientists used experimental data and machine-learning (ML).

Fuel from which reactors is hard to identify uniquely?

A view of the 80 MWe indigenous PWR operating at Kalpakkam. In the foreground is the pressure hull and behind is the shield tank containing water.  An identical reactor of the same size powers India’s indigenous nuclear-powered submarine INS Arihant, launched on July 26, 2009.

A view of the 80 MWe indigenous PWR operating at Kalpakkam. In the foreground is the pressure hull and behind is the shield tank containing water. An identical reactor of the same size powers India’s indigenous nuclear-powered submarine INS Arihant, launched on July 26, 2009. | Photo Credit: Special Arrangement

Spent fuel from boiling water reactors (BWRs) is hard to differentiate from that from pressurised water reactors (PWRs).

This is because both “use water as moderator and have similar thermal neutron spectra, so they are quite similar in neutron reaction mechanism,” Shengli Chen, assistant professor at the Sun Yat-Sen University, Guangdong, and a coauthor of the study, told The Hindu by email.

The group used experimental data instead of simulations, whose accuracy is unknown.

How do is nuclear fuel identified?

The reactor type, the fuel’s exposure time inside the reactor, and the extent of the fuel’s enrichment can uniquely identify spent nuclear fuel.

For nuclear fuels, ‘enrichment’ refers to the amount of the fissile isotope, such as uranium-235, that the fuel compound, like uranium dioxide, contains.


File photo released by the Atomic Energy Organization of Iran shows centrifuge machines in the Natanz uranium enrichment facility in central Iran.

File photo released by the Atomic Energy Organization of Iran shows centrifuge machines in the Natanz uranium enrichment facility in central Iran. | Photo Credit: AP

Using a database that contains the composition of different isotopes in spent fuel from reactors over 50 years, the researchers developed equations that related these quantities to each other.

They assumed based on past work by other scientists that the relationship between exposure time and the ratio of isotopes, for example, could be represented by a linear equation.

Once they had an equation, they applied it to different isotope ratios in the database. Where their equation’s solution differed from the table value, they tweaked the equation until it fit.

If one quantity can be determined through other measurements, like gamma-ray emissions from the spent fuel, the team could calculate the values of the other two quantities using the equation.

What is the issue with BWRs v. PWRs?

They also trained data from the database to develop three ML models to distinguish fuel from BWRs from that from PWRs.

In BWRs, the fuel rods are submerged in water. When nuclear fuel undergoes fission, it releases heat, boiling the water. The resulting steam drives a turbine. In PWRs, the fuel rods aren’t exposed to the water; only the heat is exchanged.

A material called the moderator slows neutrons down so that they have just the right energy to trigger fission. Such neutrons are called thermal neutrons.

It’s hard to say if spent fuel is from a BWR or a PWR because the energies of the thermal neutrons are similar in both reactors. So the resulting nuclear reactions similarly affect and transform the nuclear fuel.

What did the ML model find?

The group’s best ML models were able to correctly identify 91% of fuel from BWRs and 95% of fuel from PWRs.

Among others, the model used a common technique called logistic regression: based on studying a dataset of independent variables, the model estimated the chance that an outcome that depends on these variables will occur.

“There was no explicit information about the accuracy of discriminating between PWRs and BWRs” in older papers, Dr. Chen said.

A 2014 study, to which he pointed, used a simulation plus ML technique with a low classification-error rate, yet still reported difficulty separating BWRs from PWRs.

“Previous works, based on simulation data,” could differentiate between the reactors whereas “our work, based on experimental data, explicitly concludes which reactor type has been used to irradiate the sample.”

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