From HMP/HTP spacer grids to a NuScale SMR 17×17 fuel assembly
A fuel assembly is often described as a repeated element inside the reactor core.
A square lattice.
Fuel rods.
Guide tubes.
Spacer grids.
Top and bottom nozzles.
From a distance, it can look like a standardized component, almost like a pattern that simply repeats across the core.
But nothing in nuclear engineering is only a pattern.
Every pitch, every rod position, every guide tube, every spacer grid and every structural interface exists because it answers a set of constraints. Power density, moderation, coolant flow, thermal limits, vibration behaviour, manufacturability, emergency cooling performance, control rod insertion and long-term fuel integrity all meet inside this single component.
That is why my documentation-driven NuScale SMR fuel assembly reconstruction did not begin with the complete reactor module, the reactor pressure vessel or the full core. It began at the grid.
Starting with the smallest functional structure
The first technical threshold was the spacer grid.
At first, the HMP and HTP spacer grids appeared to be a localized modelling task. In reality, they became the entry point into the wider logic of the fuel assembly.
Spacer grids are not only structural separators between fuel rods.
They support alignment, help restrain vibration, maintain coolant channels and influence local flow behaviour. Their geometry belongs to both mechanical design and thermal-hydraulic performance.
For this reconstruction, the distinction between HMP and HTP grids was treated as an important indicator of how different vertical regions of the assembly serve different priorities.
HMP, or High Mechanical Performance grids, are associated with stronger mechanical support and rod restraint. Their role can be understood through alignment, stiffness and vibration control in regions where mechanical stability is especially important.
HTP, or High Thermal Performance grids, are associated more closely with coolant flow behaviour. Their geometry points toward mixing, heat transfer support and hydraulic performance as coolant moves through the fuel rod bundle.
This difference matters.
It shows that a fuel assembly is not a uniform stack of repeated elements. Different vertical regions of the assembly have different priorities. Some regions require stronger mechanical restraint. Others are more closely linked to flow mixing, thermal performance and pressure drop management.
In other words, the spacer grid is not a small detail. It is a compressed expression of system logic.
Why the 17×17 lattice matters
The 17×17 fuel assembly is often recognized as a mature PWR configuration, but it should not be treated as a simple template.
Inside the reactor core, the lattice is a negotiation.
- It balances power density and moderation.
- It shapes coolant flow and fuel rod spacing.
- It defines guide tube positions and control rod insertion paths.
- It influences thermal margins, structural stability, hydraulic behaviour and the operating envelope.
Changing the lattice would not only change the drawing. It would affect the whole system.
- The neutron spectrum would shift.
- Coolant channels would change.
- Thermal-hydraulic margins would need to be reassessed.
- Control rod geometry and structural behaviour would be affected.
- The relationship between reactor physics and mechanical design would have to be reconsidered.
This is why the 17×17 pattern should not be understood as a visual grid alone.
It is a mature engineering compromise, where many competing requirements converge into a structure that must remain stable, predictable and verifiable cycle after cycle.
From grid logic to fuel assembly logic
Once the spacer grid logic was established, the reconstruction expanded into the complete NuScale SMR 17×17 fuel assembly.
The model developed into a full 17×17 assembly including the major structural elements:
fuel rod array, guide tubes, central instrumentation tube, HMP and HTP spacer grids, bottom nozzle with coarse-mesh debris filter, and upper nozzle with control rod assembly insertion interface.
At this stage, the objective was not only to produce a finished render. The objective was to validate whether the geometry could read correctly as a coherent engineered structure. That meant checking internal clearances, rod alignment, grid positioning, coolant flow paths, nozzle relationships and the way the assembly behaves as a load-bearing component inside the core.
The bottom nozzle is not only an end piece. It contributes to alignment, load transfer, coolant entrance conditions and debris mitigation.
The upper nozzle is not only a cap. It is an interface region connected to control rod insertion logic, upper core geometry and structural positioning.
The guide tubes and central instrumentation tube are not empty spaces. They define how the fuel assembly connects to control, measurement and core monitoring functions.
The spacer grids are not repeated decoration. They are part of the mechanical and hydraulic architecture of the assembly.
This is where the reconstruction became more than component modelling. It became a way of reading the system.
Same lattice, different reactor logic
The next important step was the comparison with a standard Westinghouse 17×17 PWR fuel assembly. At first glance, the comparison is tempting because both assemblies share the same general 17×17 lattice logic. But sharing a lattice does not mean sharing the same operating environment.
A conventional large PWR fuel assembly operates inside a large reactor core, connected to external steam generators and driven by reactor coolant pumps. It belongs to a system with high coolant flow, high power density and a long active fuel region.
The NuScale SMR fuel assembly belongs to a different system context.
In an integral PWR SMR, the core, steam generators and primary coolant inventory are arranged within a much more compact reactor vessel configuration. Natural circulation and passive safety features play a central role in the system concept.
This changes how the fuel assembly must be understood.
The NuScale SMR fuel assembly is significantly shorter than a standard Westinghouse 17×17 PWR assembly, despite sharing the same rod lattice. In the comparison model, the Westinghouse reference assembly is approximately 4.0 m, while the NuScale SMR fuel assembly uses the 2.44 m FSAR value.
That difference is not only dimensional. It reflects a different system philosophy.
A large PWR assembly belongs to a high-flow, high-power, pump-driven primary system.
An SMR assembly belongs to a compact core geometry, where natural circulation, reduced hydraulic load and tight system integration become defining design conditions.
This is the key lesson of the comparison:
the same lattice does not mean the same reactor logic.
Modelling as technical interpretation
For documentation-driven 3D reconstruction, this distinction is essential. The goal is not to create a visually convincing object in isolation. The goal is to understand why the object has the structure it has, and how that structure belongs to the wider system.
In this phase, the model helped clarify several relationships:
- how spacer grid architecture supports, guides and restrains fuel rods,
- how mechanical and thermal performance requirements shape grid geometry,
- how grid position influences coolant flow and mixing,
- how nozzles contribute to load distribution, alignment and debris mitigation,
- how SMR-scale geometry differs from large-PWR fuel assembly architecture,
- and how a fuel assembly becomes a coherent structural, hydraulic and neutronic interface inside the core.
This is why the reconstruction began at the grid.
A grid is small enough to appear secondary, but technical enough to reveal the system.
Once its logic becomes visible, the rest of the fuel assembly can no longer be seen as a simple collection of rods and plates.
It becomes an engineered structure where every feature exists for a reason.
Documentation discipline
This model is an independent study reconstruction based on publicly available FSAR information and vendor-released technical documentation. No proprietary or confidential CAD data was used or referenced.
The reconstruction is intended for educational, analytical and communication purposes. It is not an official NuScale Power model, not an official Framatome model and should not be interpreted as an exact replication of any commercial design.
Where geometry could be supported by public documentation, it was interpreted with dimensional discipline. Where public sources did not clearly support specific details, those areas were simplified or treated conservatively.
That discipline matters.
In nuclear visualization, credibility does not come from adding as much detail as possible. It comes from knowing where the available evidence ends.
Conclusion
The completed NuScale SMR 17×17 fuel assembly was not the beginning of the work. It was the visible result of a slower process: reading, comparing, modelling, checking and gradually connecting small structures into a larger system logic.
The reconstruction began with HMP and HTP spacer grids. It expanded into the full fuel assembly. It was then placed beside a standard Westinghouse 17×17 PWR assembly to understand what scale changes actually mean.
The final lesson was simple, but important:
SMR compactness does not remove complexity. It compresses it.
And sometimes, the best way to understand a reactor system is not to start with the vessel, the module or the final render.
Sometimes, the system begins to reveal itself at the grid.
This article describes an independent documentation-driven reconstruction based on publicly available sources. It is not an official vendor model and does not reproduce proprietary CAD geometry. The visual material is intended for educational, analytical and technical communication purposes only.
17x17 Engineering Visualization Framatome FSAR Fuel Assembly HMP HTP NuScale PWR SMR spacer grids technical reconstruction Westinghouse
Last modified: August 21, 2026
