Development of HfC Barrier Coatings for Accident-Tolerant Control Rods

The typical absorber material used in Pressurized Water Reactors (PWR) control rods is a silver-indium-cadmium alloy with a low melting point. Hence, in a loss-of-coolant accident (LOCA) during which the control rods could overheat and fail, the potential exists for the absorber to melt and drain out of the core leading to a potential re-criticality. Accident-tolerant control rods seek to avoid this problem by substituting a high-melting-point material for the absorber. Hafnium carbide (HfC) is often proposed for this application because hafnium has excellent neutron absorbing properties and hafnium carbide has a melting point of 3900°C, making it one of the most refractory binary compounds known. However, HfC is thermodynamically unstable in water at PWR operating conditions. Therefore, should a cladding breach occur during normal operation, high temperature water could enter the control rod, dissolve the HfC, and transport it out of the reactor core via the primary reactor cooling system piping, thereby resulting in a loss of shutdown margin. To address this, NovaTech proposes to develop a coating to protect the HfC from high temperature water at PWR operating conditions. The Phase I proof-of-concept laboratory work will involve developing a coating to protect HfC absorber pellets from high temperature water at PWR operating conditions. Both single-layer and multi-layer protective barrier coatings will be deposited onto HfC pellets that were fabricated during the previous research program. The coated test articles will then be subjected to long-term, high-temperature, water-corrosion tests; thermal cycling tests; and mechanical strength tests. The test articles will also be extensively characterized dimensionally and their key physical properties will be established. An ancillary issue that was identified with HfC pellets fabricated using spark plasma sintering is their short L/D ratio which makes them difficult to handle and load into control rods. In Phase I we will also propose to investigate utilizing a variation of the coating process should it prove successful to assemble multiple pellets into short mini-columns that can then be handled and installed as units.