Development of HfC Barrier Coatings for Accident-Tolerant Control Rods

The typical absorber material used in Pressurized Water Reactor (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 absorber could 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. 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 and dissolve the HfC, thereby resulting in a loss of shutdown margin. During the Phase I proof-of-concept laboratory work various coatings were developed to protect HfC absorber pellets from high temperature water. Both singlelayer and multi-layer barrier coatings were deposited onto HfC pellets that were fabricated during a previous research program. The coated test articles were extensively characterized and then subjected to thermal cycling tests, mechanical strength tests, and long-term corrosion tests. An ancillary issue that was identified is the short L/D ratio of HfC pellets that makes them difficult to handle and load into control rods. In Phase I we also developed prototype fixturing and processes to assemble multiple pellets into short mini-columns and showed that they can then be handled and installed as a unit. Sample HfC pellets were also produced using commercial sintering with higher density and better dimension control at substantially reduced cost. This is a critical step towards commercialization. In Phase II, this bench-scale fabrication, testing, and characterization work will be scaled up culminating in the fabrication and testing of full-scale partial-length prototype control rods. This testing will include long duration elevated temperature water exposure tests, flow tests, SCRAM tests, and life & wear tests. Coated HfC reactor test specimens will also be fabricated for long-term irradiation in Phase IIA.