- Date released: 6 May 2026
- Request number: 202604002
- Release of information under: FOIA
Information requested
In publicly available ESBWR materials, the rationale for chimney partitions is discussed in detail. In the published BWRX-300 Generic Design Assessment (GDA) Step 2 outputs, I have not been able to locate an explicit discussion of how potential radial/cross-flow effects in the partitionless chimney are addressed with respect to core stability. I would therefore be grateful for a high-level clarification. I appreciate that some details may be commercially sensitive; pointers to published references would be very helpful.
Could you please advise: (1) whether this topic was explicitly assessed/reviewed during Step 2; and (2) where it is addressed (report/section) or, if not, whether it was deferred to later site-specific and/or further assessment work.
Our response
We confirm that under S.1 of the FOIA, we do not hold the information you have requested.
The regulatory assessment of GE Vernova Hitachi’s detailed analysis of the BWRX-300 internal chimney flow distributions, associated modelling approaches, and its verification and validation were identified as matters for later regulatory assessment stages (should the projects proposing to deploy the BWRX-300 in Great Britain progress). This would be enabled by additional analysis from GE Vernova Hitachi which was not submitted during Step 2 of the BWRX-300 GDA to ONR.
Further information
The design, the demonstration of operational effectiveness, and the demonstration of safety of both the ESBWR and the BWRX-300 is a matter for the technology vendor, GE Vernova Hitachi (and any licensees operating the two reactor designs, noting that whilst the ESBWR has been certified in the US it has not been built).
ONR has not completed a GDA review of the ESBWR (a review commenced in circa 2008 but GE withdrew from the process). We have completed a two-step review of the BWRX-300 which had the objective, based on a review of submissions that were provided to ONR, to identify if there are any fundamental reasons why the BWRX-300 could not be deployed in Great Britain. If GE Vernova Hitachi chose to return to complete GDA Step 3, or a developer wished to deploy the BWRX-300 in Great Britain, a more detailed regulatory assessment of BWRX-300 supporting evidence would be undertaken. However, our role as the regulator would be to form a view on the completeness and credibility of the analysis put forward by vendor/licensee, and not to independently demonstrate the effectiveness or safety of the design ourselves.
Our regulatory approach could include taking confidence from work previously done by GE Vernova Hitachi for the ESBWR and assessed in detail by the US regulator (for example US Nuclear Regulatory Commission’s (NRC) views on the verification and validation of computer codes used to model natural circulation or the derivation of appropriate operating windows to manage reactor stability). We would also look to collaborate with Canadian Nuclear Safety Commission and US NRC on the assessment of BWRX-300 safety claims and detailed evidence (the US and Canadian regulators have ongoing regulatory reviews of the BWRX-300 as part of live construction projects in the two countries). We would also look to collaborate with the Polish regulator (PAA) if BWRX-300 projects are advanced in Poland.
The submissions provided by GE Vernova Hitachi to ONR for the BWRX-300 GDA two-step fundamental review can be found at GE Vernova Hitachi BWRX-300 Documents.
Our regulatory assessment reports are published at GE Vernova Hitachi BWRX-300 - Step 2 GDA statement and summary report | Office for Nuclear Regulation.
The fuel and core assessment report is probably of most relevance to your query, notably section 4.3.5.4: Generic Design Assessment of the BWRX-300 – Step 2 assessment of Fuel and Core Design. Our regulatory assessment focused on the key design features and parameters underpinning stability behaviour, including pressure drop characteristics in the chimney region and their influence on natural circulation performance and stability margins.
Exemptions applied
None
Public Interest Test (PIT)
N/A