Longevity of fuel rods mapped out

​Much of the fuel in a nuclear reactor is wasted due to fuel rods that need to be replaced when they for some reason start to swell and bend due to pick-up of hydrogen. Professor Itai Panas and his PhD-student Mikaela Lindgren has now found out the mechanism for this hydrogen pick-up by mapping out the corresponding chemical reactions.

​Inside a nuclear reactor the fuel rods, immersed in water, are surrounded by control rods which are there to control the flux of neutrons into and out of the nuclear fuel. It is estimated that only about 3 percent of the energy in the fuel is actually used. A major reason for this is that the rods that contain the fuel start to corrode and after some time they even get bent. Since these rods are situated very close to the control rods there is the risk of getting stuck, which very likely would result in a very costly reactor shutdown simply because the control of the neutron flow would stop working properly.


Today this aging process is under strict surveillance and the rods are being changed long before putting the plant at risk, with lots of nuclear energy wasted as a result.


In Mikaela Lindgren’s PhD thesis she goes through earlier experiments and creates a theory for relevant aspects of the complex interactions between the fuel rod and water. 


The fuel containing rods are made of a zirconium alloy, called Zircalloy. Since the rod is surrounded by water oxidation of the alloy occurs together with evolution of hydrogen gas. This, in itself, doesn’t cause the bending of the rod. It is, instead, inside the alloy the problem starts since all of the hydrogen doesn’t turn to gas. Sometimes up to 40 percent makes its way through the oxide scale, first as hydroxide ions and, when almost through, also as hydride ions.


The thesis also shows how this penetration depends on additives in the alloy, for example iron or nickel. Moreover, after having described how the oxidation takes place and how it is affected by hydrogen in the alloy, Mikaela Lindgren went through the periodic table and discovered that the nobler the additive metal the better it binds to the hydrogen in the cathodic part of the oxidation reaction. This accumulation of hydride ions in the oxide leads to an influx of hydrogen through the oxide, as protons and hydride ions, that become absorbed by the Zircalloy. Depending on the nobility of the additive either hydrogen pick-up or hydrogen gas evolution is preferred. The nobler the additive the greater is the risk for the hydrogen pick-up, which I turn is the cause of the kind of alloy swelling that eventually leads to bent fuel rods.


The study explains results from earlier autoclave experiments. Itai Panas emphasizes that the mechanism may not be exactly the same when taking radiation into account, but this study gives a new and important tool that might result in better used fuel rods.


In addition, because of the fundamental nature of the study regarding oxidation, the mechanism has opened up for new possibilities in oxidation research also in other areas than the nuclear, including biofuelled powerplants.

 

Published: Tue 18 Dec 2018.