See below. It was tested for 9 years and didn't show significant corrosion. The leak happened not in the Hastelloy but in a ceramic seal between the alloy components. An appropriate seal or coating on the seal may be able to fix this issue. Overall, I'd say the containment issue is as you say mostly solved.
Here's the report:http://moltensalt.org/references/static/downloads/pdf/ORNL-T...