How water boils within the scale on a nuclear fuel rod

A model on the boiling phenomenon inside the scale, a hard material that covers the nuclear fuel rod, has been developed. Intrinsically, boiling water in a hot pot is a useful analogy for this phenomenon, except that the liquid supply is governed by the capillary force of the soil-like structure of the scale. Based on the boiling water analogy, this article suggests a model to predict the heat transfer characteristics of the scale.

Article  |  Spring 2018

Everything leaves a trace, even water. When the water in a cup evaporates, a scale is left behind. Someone washing that cup might labor to remove it. Scales are also a problem in nuclear power plants. In nuclear power plants, hot fuel rods are submerged in water, causing the water to boil. As the water boils, a scale is left on the fuel rod surface. Once, an emergency situation was caused in a U.S. nuclear power by this scale on fuel rods.

The insufficiently burnt fuel caused by the scale attached on the fuel rods was found to be the major cause of the incident. As a result, 20 million dollars of damage was done to the power plant.

 

The insufficient burning of fuel mentioned above, is closely related to the boiling phenomenon within the scale. Like the soil in a flowerpot, the scale absorbs the water around it, and the absorbed water boils at the bottom of the scale. When water is absorbed by the scale, the chemical additives in the water are also absorbed. However, when water boils, only the chemical additives remain within the scale and they inhibit the fuel from burning.

 

Professor Hee Cheon No and his research team have suggested a model that depicts how water boils within the scale. They explained that the boiling phenomenon within the scale is not very different from the boiling of water in a hot pot: the bubbles are nucleated at the heated surface, and they are detached from the wall as the bubbles grow to sufficient size. However, the researchers argued that the phenomenon is only different in the way liquid is supplied to the heated wall. In the case of a hot pot, the water supply is related to the bubble growth and detachment. On the other hand, in the case of the scale, the water is supplied as the scale absorbs it by capillary force. With the model suggested by Professor No and his team, the temperature and the boiling rate within the scale can be predicted with smaller error than with previously suggested models.

 

This model also found that the microstructures of the scale, such as pore size, are not very important for predicting the boiling behavior within the scale. Therefore, without knowing the detailed form of the scale, the operators can accurately predict the boiling behavior within the scale.

 

 

By the development of this model, the understanding on the nuclear power plant scale and its effects on fuel rod has moved one step forward. Along with the prediction of the boiling behavior by this model, how chemical additives move within the scale can be better understood. The amount of the chemical additives that remain after boiling will determined the effect of the scale on the fuel burning. Once the problem caused by the scale is predicted and understood, the problem will cease to be a problem.

Original paper: https://doi.org/10.1016/j.ijheatmasstransfer.2016.12.046