How a phased, documentation-driven reconstruction progressed from a single fuel assembly to the complete reactor module, and why the HCSG was far more than a complex spiral.
Nine months is long enough to almost forget how small the first step was. But not why it was chosen.
The reconstruction of the complete NuScale Power Module was conceived from the outset as a multiphase project. Its scope was not defined by a single final image, but by a sequence of spatial levels that first had to be understood and then connected.
The first phase was the reconstruction of a fuel assembly. The second covered the assembly of the core with 37 fuel assemblies. The third was the reconstruction of the reactor pressure vessel, its internals and integral systems. The fourth phase covered the steel containment vessel and the assembly of the final cutaway of the complete module.
The fuel assembly was not an arbitrary or modest starting point. It was the logical first unit of the system. First, I had to understand how it was assembled, how the fuel rods, guide tubes and instrumentation location were arranged, and how the 17 × 17 geometry continued through the full axial height of the assembly. Only then could the core be assembled reliably and the reconstruction proceed toward the structures that support, guide, measure and surround it.
The documentary basis was not limited to one chapter or a few characteristic cross-sections. I reviewed practically the entire, extensive FSAR Revision 5 and connected written descriptions, system diagrams, tables, cross-sections, elevations, azimuths and interface data. No single drawing contained the complete module. The whole was dispersed throughout the documentation and had to be assembled component by component.
The phases were defined in advance. Within them, however, the project did not progress through a simple component list, but through a chain of questions. The core required the reactor vessel internals. The positions of the control rod assemblies required guides, drive mechanisms and their own penetrations in the reactor pressure vessel head. The instrumentation positions required a different spatial chain: guide tubes, passages through internal structures and terminations at the instrument seal assemblies. The reactor vessel required the steam generators, the pressurizer and every connection that ultimately had to terminate correctly within the containment vessel.
When a fuel assembly becomes a system
A single fuel assembly can be treated as a self-contained object. Once 37 are arranged in the core, none remains self-contained.
The assembly layout determines the positions of the control rod assemblies and the twelve measurement locations of the in-core instrumentation system (ICI). What appears on a core map as a mark in a single square continues upwards in the three-dimensional model. It becomes a mechanical path through several consecutive structures to a penetration through the pressure boundary.
A small error at core level therefore does not remain in the core. It appears in the upper reactor vessel internals, in the guides, in the position of a control rod drive mechanism or at a penetration through the pressure boundary. A component may look locally correct while being unable to exist within the system that surrounds it.
This changed the way I worked. The question was no longer only whether an individual part had been modeled accurately enough. It became whether all the parts belonged to the same spatial logic at the same time.
The core was followed by the lower and upper reactor vessel internals, support plates, reflector blocks, reactor vessel surveillance capsules, guides and the control rod drive mechanism region. Each new assembly had to confirm the preceding one. If a connection did not work, moving a part to a visually convenient position was not enough. I had to return to the documentation and determine which assumption was wrong.
Integral design does not mean simple geometry
The NuScale design is an integral pressurized water reactor. The core, reactor vessel internals, two helical-coil steam generators and the pressurizer region are integrated within the reactor pressure vessel. The vessel is then installed inside a steel containment vessel, and the module is designed for placement in the reactor pool.
The configuration examined here is the 160 MWt design documented in the NuScale DCA/FSAR Revision 5. The later uprated US460 design, rated at 250 MWt per module, is outside the scope of this reconstruction.
This arrangement removes some external connections, but it does not remove the relationships between systems. It compresses them into a limited volume.
Within the same space, the primary coolant flow path, the tubes of both steam generators, the upper riser, the ICI guide tubes, the drive mechanisms, pressure-boundary penetrations, feedwater inlet, steam outlet, support structures and access for inspection and maintenance must all coexist.
The reconstruction therefore could not progress only from large shapes toward small details. It progressed from internal relationships toward external boundaries. The reactor pressure vessel was not first treated as a shell into which components would later be inserted. Its shape acquired meaning only together with what it had to contain, support and connect through its walls.
HCSG: three weeks inside one component
The helical-coil steam generator (HCSG) was the most demanding part of the entire reconstruction.
In a simplified cutaway, the HCSG can appear as a densely drawn texture around the upper riser. The documentation describes a far more precise arrangement. Each steam generator consists of interlacing helical tube columns connected to two feedwater inlet plena and two steam outlet plena. Together, the two steam generators form an intertwined tube bundle, with four feedwater and four steam plena arranged around the circumference of the reactor pressure vessel. Primary coolant flows outside the tubes, while feedwater and the resulting steam flow inside them.
For the complete NPM, FSAR Revision 5 specifies 1,380 tubes arranged in 21 helical tube columns. An individual tube is not merely a helix. At both ends, it must pass through a transition bend into a straight section and terminate correctly at the tubesheet of the corresponding plenum. The tube bundle is supported by dedicated support assemblies that must simultaneously limit unsupported spans, accommodate thermal expansion and preserve the primary coolant flow path through the bundle.
Creating one helix is not difficult. Maintaining system-level correctness across more than a thousand tubes with different radii, transitions, supports and end connections is. A small error in pitch or radial position grows with every repetition. Eventually, a tube no longer reaches the correct tubesheet, interferes with an adjacent column or occupies space required by another component.
The HCSG therefore required approximately three weeks of almost continuous checking. I joked that it had taken ten years off my life. In reality, this part demonstrated better than any other the difference between geometry that merely resembles a technical object and geometry that must preserve the internal logic of a system.
The correction that appeared only after assembly
The most important correction did not occur at the beginning, but almost at the end.
The twelve ICI positions were known at core level. The FSAR also confirms twelve ICI guide tubes, their separate passages through the baffle plate of the integral steam plenum and four instrument seal assemblies in the upper part of the reactor pressure vessel. It does not, however, provide a single detailed routing that unambiguously connects each core location to one of the four seal assemblies. Only after the HCSG and the surrounding structures had been completed did it become clear that these paths could not be treated in the model as twelve simple, straight axial extensions.
In the transition region toward the four instrument seal assemblies, I therefore had to abandon the original assumption of completely straight routing and use spatially coordinated study geometry. The public documentation did not contain a single detailed representation of the complete paths, so the local geometry was not presented as a final design solution. What could be supported had to be preserved: the starting positions, documented intermediate passages, the target region and the space through which the connection was possible.
This correction was more important than many visible details. It showed that the final cutaway was not merely the sum of the modeled components. It was a test of whether all documented connections could exist at the same time.
A probe passing through the helix does not yet demonstrate full inspection capability
When I published the final cutaway, the question of HCSG inspectability arose among specialists. It was one of the best questions the reconstruction could have prompted.
In its publicly available research and development information, NuScale describes a feasibility study in which a conventional eddy current testing (ECT) probe traversed helical coils with prototypic lengths, diameters, curvatures and inclination angles. This demonstrates that the probe can mechanically traverse such geometry.
Mechanical passage alone, however, does not demonstrate that a reliable, comprehensive inspection can be performed.
Comprehensive inspection capability includes signal quality, detectability of the expected forms of degradation, the effects of tube curvature and material condition, repeatability, reference standards, procedures, equipment and personnel qualification. The FSAR therefore goes beyond physical access. For each steam generator, it requires a full-length volumetric preservice inspection of 100 percent of the tubing to establish an initial baseline record for subsequent inspections. The Steam Generator Program forms part of the broader inservice inspection program. It implements the applicable provisions of ASME BPVC Section XI and follows NEI 97-06 and the associated EPRI guidance.
The documentation also describes access openings at the feedwater and steam plena, the ability to remove an individual tube from service by plugging it, and removable flow restrictors at the tube inlets. Their removal provides access for inspection, cleaning, tube plugging and other maintenance and repair activities.
Experience with conventional vertical U-tube steam generators cannot simply be transferred to an integral HCSG. In the NuScale design, the secondary fluid is inside the tubes and the primary coolant is outside them. The FSAR states that the once-through design has no bulk secondary-side water reservoir or low-flow regions in which hard sludge piles could accumulate. Conventional blowdown and water-lancing practices therefore do not apply directly. Periodic cleaning of the tube inner surfaces is performed during outages through the feedwater and main steam disconnect flanges.
The documentation also addresses flow-induced vibration (FIV) through the arrangement of the tube support assemblies and a comprehensive vibration assessment program. The model cannot verify these analyses. It can, however, show the locations of the tubes, supports, transitions and access openings at the feedwater and steam plena, and therefore why inspectability and maintainability are not additions that can be considered only after the geometry has been completed.
Here, the reconstruction reached its most useful boundary. It showed enough to raise the right question, but not enough to pretend that it had already answered it.
The containment vessel was not merely the final shell
Once the interior had been assembled, the steel containment vessel (CNV) followed. At first glance, it is the outer envelope around the reactor pressure vessel. In the reconstruction, however, it became the place where all the preceding relationships had to terminate.
The CNV includes the main closure flange, large access openings, process, electrical and instrumentation penetrations, the support skirt and three lateral support lugs. The upper region carries most of the visible complexity. According to the documentation, the lower shell and bottom head have no penetrations. Their apparent emptiness is therefore not a missing detail, but a feature of the design. The access covers are not decorative elements either.
The different openings serve different system interfaces and provide access for inspection and maintenance activities. Their size, position and visual hierarchy explain how the module can be assembled, opened, inspected and closed again. The containment vessel therefore did more than complete the outer silhouette. It completed the spatial chain that began with a single fuel assembly.
What was actually completed after nine months
By a rough estimate, the project required more than 1,100 hours of work. Much of that time was not spent on visible modeling, but on searching, comparing, revisiting earlier decisions and correcting connections that became problematic only several components later. The final result is an integrated 3D study cutaway of the reactor module, created from publicly available documentation. But the most important result is not one final image.
More important was the transition from object to system.
At the beginning, I asked what each component was. Towards the end, I asked what had to pass through it, what had to align with it, which boundary it crossed, how it could be accessed and what would happen to every other part if I moved it.
For nine months, I was not simply creating a rendering. For nine months, I was determining what had to be in it and why. If, after all this, the model raises a question about inspectability, maintenance or the life cycle, then it has not strayed beyond its purpose. It has fulfilled it.
The Boundary of the Reconstruction
The reconstruction is based on the publicly available NuScale Design Certification Application, Final Safety Analysis Report, Revision 5. It is an independent study model for technical communication and spatial analysis. It is not an official NuScale model, a manufacturing or design model, a licensing document, a safety analysis or a substitute for engineering documentation.
The model is tied to the revision of the documentation used. Later versions of the design must not automatically be assumed to match every detail shown. Where public sources did not allow an unambiguous reconstruction, the geometry was simplified or left unresolved.
Note
This reconstruction is an independent technical interpretation created from publicly available documentation. It does not represent an official model of NuScale Power, supplier or fabrication drawings, engineering documentation or a proprietary representation of the system.
It was created exclusively for educational purposes, technical communication and analysis. It is not intended for design, engineering calculations, licensing, operation or any safety-related use.
All visual material, the 3D reconstruction, renderings, diagrams and written analysis are the intellectual property of Elite Studio 3D / By the Protocol, unless otherwise stated.
Primary technical sources
- NuScale Standard Plant Design Certification Application, FSAR Revision 5, Chapter 1: Introduction and General Description of the Plant, Sections 1.1 and 1.2
- NuScale Standard Plant Design Certification Application, FSAR Revision 5, Chapter 3: Design of Structures, Systems, Components and Equipment, Section 3.8.2
- NuScale Standard Plant Design Certification Application, FSAR Revision 5, Chapter 4: Reactor, Sections 4.2, 4.3.2, 4.5 and 4.6; Figure 4.3-18
- NuScale Standard Plant Design Certification Application, FSAR Revision 5, Chapter 5: Reactor Coolant System and Connecting Systems, Sections 5.2.3, 5.3 and 5.4.1; Table 5.4-2
- NuScale Standard Plant Design Certification Application, FSAR Revision 5, Chapter 7: Instrumentation and Controls, Section 7.0.4.7
- NuScale Power, US460 Design and SDAA Overview Update, 2023
- NuScale Power, Research and Development: Steam Generator Tube Inspection Feasibility Study
NuScale and NuScale Power Module are trademarks of NuScale Power, LLC. Their use is descriptive and does not imply affiliation with, approval of or endorsement of this independent project.
3D Reconstruction Containment Vessel CRDM FSAR fuel assemblies HCSG helical-coil steam generator ICI nuclear energy NuScale reactor core reactor module Reactor Pressure Vessel Small Modular Reactor SMR Technical Visualization
Last modified: September 3, 2026
