In the field of nuclear fuel, there are many occasions when researchers are engaged in such specialised work that they have no written references, no textbook to guide them. The only way to master the particular methods needed for an experiment is to learn from fellow researchers. Such was the goal for the experimentalists of the In-pile Creep Studies of ATF Cladding (INCA) project, a Joint ExpErimental Programme (JEEP) in the Framework for Irradiation Experiments (FIDES) on the subject of a sodium–potassium alloy (NaK).
In the INCA experiments, fuel cladding materials coated with chromium (a leading idea for improving nuclear fuel’s accident tolerance) are irradiated. The creep properties of the coated cladding are then compared to the creep properties of standard claddings.
NaK is an alkali metal with unique chemical and thermodynamic properties that make it challenging to work with but also advantageous in nuclear fuel irradiation experiments. NaK has a high chemical reactivity with oxygen and can even react explosively with water. For this reason, the NaK alloy must be handled with extreme care. However, NaK also has unique thermodynamic properties. The NaK alloy is liquid at room temperature, has high thermal conductivity, which enables easy heat flow through it, and low specific heat capacity. These properties make NaK an excellent material for heat transfer applications in the nuclear industry.
In an irradiation experiment, using NaK as a coolant allows for uniform temperatures along a test segment and a steady, predictable cooldown period after the reactor is turned off. In the INCA project, the beneficial attributes of NaK are significant enough to motivate researchers to learn how to manage this challenging and sometimes dangerous material.
To learn how to handle NaK, a group of experts at the Research Centre Rez (CVR) in the Czech Republic travelled to Cadarache in France in November 2021 to learn from experts at the Commissariat for Atomic Energy and Alternative Energies (CEA) who had used NaK in the now closed OSIRIS Reactor. The training covered safe handling, storage and disposal of this hazardous substance, applying many years of French experience. The training also included transport container inspection, sampling and disposal of small quantities of NaK.
The training was designed to transfer practical experience, which is critical as many of the experts at CEA are preparing for retirement.
Following the CEA-led training, experts from the LVR-15 reactor continued their studies of NaK with a series of small-scale, controlled experiments to gain experience with the volatility of this unique material.
To preserve the knowledge and ensure it is transferred to the next generation of experts, NaK handling procedures and protocols are well documented. However, writing procedures are not sufficient and a deep, practical understanding of the properties of NaK is needed. For this, peer-to-peer knowledge exchange is crucial.
The NEA Framework for Irradiation Experiments (FIDES) supports the fuel and material experimental needs of nuclear safety regulators, technical support organisations, research institutions and industry and safeguards experimental knowledge for future generations. FIDES connects a global network of research facilities to perform high-priority experiments through Joint ExpErimental Programmes (JEEPs).