Hi Jim,
This is a great question. You're getting at the heart of why we moved forward with a "maximum credible accident" method, which has its roots in regulatory history at the beginning of the builds of the first nuclear plants. There wasn't a lot of operational data, so the analyses then were conservative, i.e., bounding. Similarly, we find ourselves in a place in history where new types of plants are being built. We see challenges to trying to build all accident analyses off of risk analysis which simply can't be built off of significant operational experience, so we wanted to be conservative too.
Long story short, the accident that we identified as the "MCA" and analyzed to was the complete loss of the entire secondary side of the plant, that is, the entire loss of everything above ground, as well as the simultaneous loss of one of three independent shutdown systems. This is far and away a worse event than existing nuclear plants analyze to. It "bounds" many events such as severe tornadoes, wind, earthquake, etc. We can do that because of the strong inherent safety characteristics.
What you are talking about, the complete loss of everything including all shutdown etc, we did analyze and present to the NRC early in our pre-application (in 2017). At the time, our work with the national labs showed that it would not result in any core damage. That's relatively to be expected because of the EBR-II results with the same fuel type.
As tools have improved since 2017 there are other accident types we can analyze and more nuance that has come to light. Generally speaking, the question of whether there could be an event within the realm of possibility that has a worse outcome than what we identified as the MCA is somewhat semantic, as the fundamental outcome as far as staying below dose limits to the public remains true, as is the NRC's mission. In other words, we even showed completely unprotected accident analysis to the NRC in our acceptance review, although we worry about how that sets an unrealistic precedence.
There's more to be said here. Without belaboring it, we can say we think we've learned more about what the NRC staff wants to see and how. We believed that what the NRC set as Step 1 goals was about coming to agreement on those 4 key concepts, while Step 2 was to analyze specific numbers, the details on how those were implemented, etc. What we conjecture now was that the NRC staff then seemed uncomfortable to come to agreement on concepts without diving into way more of the specifics. We will use this and other learnings as we move forward.
