# Modeling PWR Core With Spacer Grid

In summary, the conversation is about a PhD scholar named Ali who needs help with MCNP. They want to model a PWR core with a spacer grid but are unsure how to do so. They are asking for help on the Physics Forums website and have received responses about modeling techniques, the importance of getting the mass correct, and using MCNPX.
TL;DR Summary
MCNP Model
Hello everyone, i am Ali a PhD scholar i need some help regarding MCNP, i wand to model a pwr core with spacer grid but i don't know how to model spacer grid can anyone help?

Regards

Are you asking what a spacer grid looks like? or setting up the geometry in MCNP?

What work have you done so far?

watermelon
What part of modelling the spacer grid are you stuck on? If you have more specific question please feel free to ask.

If it's a complicated shape you have a lot of choices. The usual choice is to get the amount of material correct, and simplify the pattern a little. For example, if you had a sheet of metal with a lot of little holes, you would usually model that as a solid sheet of metal, but slightly thinner so it has the same total mass of metal. You would not model all the little holes. The typical example is a spring. You probably don't model it as a coil, but as a cylinder of metal, possibly a cylindrical tube, with the same mass and density as the spring. It is more important to get the total mass of each material correct. Putting in a lot of tiny details probably won't affect the results, probably won't make it more accurate. But it could well make the run time for your model much longer.

Tiny is defined in terms of the mean free path of neutrons in the material you are modeling. So if it was zircaloy, for example, you probably don't need to model anything below something like 1 cm. As long as you get the mass correct, and the mass is not very far from where it should be.

To refine your model, you run short cases with a manageable number of particles. Then you look in the output file for the mean free path. If you have a model with, for example, 1 cm detail, and the mean free path is 10 cm in that material, you are probably good to go.

If it's a repeated structure, MCNP has several built-in methods of modeling it. You can define one unit of a structure then duplicate that unit many times.

watermelon
rpp said:
Are you asking what a spacer grid looks like? or setting up the geometry in MCNP?

What work have you done so far?
want the mcnp model of the grid. i modeled the core but without grid

want the mcnp model of the grid. i modeled the core but without grid

Yes. You would need to give more detail to help. I can't see what information you have.

What part of creating the model of the grid are you having trouble with?

want the mcnp model of the grid. i modeled the core but without grid
Sir actually i am modeling the quarter core but i am not understanding the location and position of the grids where it is located and what is the shape to be modeled and where in the core.

DEvens said:
Yes. You would need to give more detail to help. I can't see what information you have.

What part of creating the model of the grid are you having trouble with?
Sir actually i am modeling the quarter core but i am not understanding the location and position of the grids where it is located and what is the shape to be modeled and where in the core.

Sir actually i am modeling the quarter core but i am not understanding the location and position of the grids where it is located and what is the shape to be modeled and where in the core.

Ok, as I said. I can't see the information you have. I can help you with MCNP. But the design of reactor you are looking at is not visible to me. I can not see your desk.

DEvens said:
Ok, as I said. I can't see the information you have. I can help you with MCNP. But the design of reactor you are looking at is not visible to me. I can not see your desk.
my design is PWR 300 MWe core 121 FA and 15x15 array.

DEvens said:
What part of modelling the spacer grid are you stuck on? If you have more specific question please feel free to ask.

If it's a complicated shape you have a lot of choices. The usual choice is to get the amount of material correct, and simplify the pattern a little. For example, if you had a sheet of metal with a lot of little holes, you would usually model that as a solid sheet of metal, but slightly thinner so it has the same total mass of metal. You would not model all the little holes. The typical example is a spring. You probably don't model it as a coil, but as a cylinder of metal, possibly a cylindrical tube, with the same mass and density as the spring. It is more important to get the total mass of each material correct. Putting in a lot of tiny details probably won't affect the results, probably won't make it more accurate. But it could well make the run time for your model much longer.

Tiny is defined in terms of the mean free path of neutrons in the material you are modeling. So if it was zircaloy, for example, you probably don't need to model anything below something like 1 cm. As long as you get the mass correct, and the mass is not very far from where it should be.

To refine your model, you run short cases with a manageable number of particles. Then you look in the output file for the mean free path. If you have a model with, for example, 1 cm detail, and the mean free path is 10 cm in that material, you are probably good to go.

If it's a repeated structure, MCNP has several built-in methods of modeling it. You can define one unit of a structure then duplicate that unit many times.
Hello
i am using MCNPX i have this warning message in output. Can you please tell me what dose this message mean and what is the reason of this warning message.
1299 fission cell elements had no neutron tracks entering:
1299 fission cell elements had no neutron collisions
1299 fission cell elements had no fission source points

Hello
i am using MCNPX i have this warning message in output. Can you please tell me what dose this message mean and what is the reason of this warning message.
1299 fission cell elements had no neutron tracks entering:
1299 fission cell elements had no neutron collisions
1299 fission cell elements had no fission source points

It can mean one of two things.

First, it might mean you have too few particles in your run. If you have many cells with fissionable material, and you are doing a KCODE calculation, you need to be sure there are enough particles to produce at least a few fissions in every cell with fissionable material. Otherwise, you have a situation where the analysis is not testing the impact of those cells. You can check this by running a case with more particles in the KCODE. Probably the particles per cycle is the number to increase.

Second, it could mean you are using a universe to fill another universe. If part of the used-to-fill universe has cells with fissionable material, and those cells don't get used in the filled universe, you can get this kind of message. Like so. The blue squares are a region with fissionable material in universe 1. That's used to fill universe 0. The blue cells outside the circle have fissionable material in them, but never become part of the calculation. So the warning message.

You could check if this is the case by examining your model to find a possible place like that. Then, in your definition, change one of those universe 1 cells to a void and rerun. The number of cells warned about should go down. You got 1299. If you void a cell the number should drop.

If this last is the case, then you can ignore the warning message.

watermelon
DEvens said:
It can mean one of two things.

First, it might mean you have too few particles in your run. If you have many cells with fissionable material, and you are doing a KCODE calculation, you need to be sure there are enough particles to produce at least a few fissions in every cell with fissionable material. Otherwise, you have a situation where the analysis is not testing the impact of those cells. You can check this by running a case with more particles in the KCODE. Probably the particles per cycle is the number to increase.

Second, it could mean you are using a universe to fill another universe. If part of the used-to-fill universe has cells with fissionable material, and those cells don't get used in the filled universe, you can get this kind of message. Like so. The blue squares are a region with fissionable material in universe 1. That's used to fill universe 0. The blue cells outside the circle have fissionable material in them, but never become part of the calculation. So the warning message.

View attachment 252372

You could check if this is the case by examining your model to find a possible place like that. Then, in your definition, change one of those universe 1 cells to a void and rerun. The number of cells warned about should go down. You got 1299. If you void a cell the number should drop.

If this last is the case, then you can ignore the warning message.
big thumb!

my design is PWR 300 MWe core 121 FA and 15x15 array.
What is the core height?

300 MWe from 121 FA would be relatively low power density, unless the core height is about 1.83 m (~6 ft). Plants with 121 FA with 14x14 fuel of 3.66 m (12 ft) height produce about 625 MWe from ~1800 MWt. A 14x14 lattice would have 16 guide tubes for control rods and possibly one instrument tube for in-core instrumentation (neutron detector and thermocouple). A 15x15 lattice may have 16 or 20 guide tubes, and one instrument tube (usually central lattice position).

In PWR assemblies, there is typically an odd number of spacer grids so that one is near the core midplane in order to support the core where maximum lateral deflection would occur during a seismic event, or if there was excessive bowing (lateral distortion) of the fuel assembly due to differential growth. In a 3.66 m (12 ft) core, one would have 7 spacer grid with the top and bottom grid typical Inconel 718 and the five intermediate grids composed of a Zr-alloy, e.g., ZIRLO (Zr-1Sn-1Nb-0.1 Fe) or M5 (Zr-1Nb). Zr-alloy strip in spacer grids is on the order of twice the thickness of Inconel grid strip. The spacer grid pitch is typically on the order of 0.52-0.6 m. Spans can have different lengths so as to avoid a periodicity that could establish harmonic vibration; lower spans may be slightly longer.

In addition, the fuel may have enriched blankets (15-20 cm (6-8 inches) length within the 3.66 m), i.e., portions of the fuel pellet column with reduced enrichment, which serve as neutron reflectors at the top and bottom of the core where the neutron flux decreases rapidly. In addition, the top and bottom nozzles, made of forged and cast stainless steel (e.g., AISI 304), serve as neutron reflectors outside the core.

See Table 3.2-1 NUCLEAR DESIGN DATA (page 51 of 132) and Table 3.2-5 CORE MECHANICAL DESIGN PARAMETERS (page 59 of 132)
in https://www.nrc.gov/docs/ML1625/ML16251A153.pdf

Last edited:
Astronuc said:
What is the core height?

300 MWe from 121 FA would be relatively low power density, unless the core height is about 1.83 m (~6 ft). Plants with 121 FA with 14x14 fuel of 3.66 m (12 ft) height produce about 625 MWe from ~1800 MWt. A 14x14 lattice would have 16 guide tubes for control rods and possibly one instrument tube for in-core instrumentation (neutron detector and thermocouple). A 15x15 lattice may have 16 or 20 guide tubes, and one instrument tube (usually central lattice position).

In PWR assemblies, there is typically an odd number of spacer grids so that one is near the core midplane in order to support the core where maximum lateral deflection would occur during a seismic event, or if there was excessive bowing (lateral distortion) of the fuel assembly due to differential growth. In a 3.66 m (12 ft) core, one would have 7 spacer grid with the top and bottom grid typical Inconel 718 and the five intermediate grids composed of a Zr-alloy, e.g., ZIRLO (Zr-1Sn-1Nb-0.1 Fe) or M5 (Zr-1Nb). Zr-alloy strip in spacer grids is on the order of twice the thickness of Inconel grid strip. The spacer grid pitch is typically on the order of 0.52-0.6 m. Spans can have different lengths so as to avoid a periodicity that could establish harmonic vibration; lower spans may be slightly longer.

In addition, the fuel may have enriched blankets (15-20 cm (6-8 inches) length within the 3.66 m), i.e., portions of the fuel pellet column with reduced enrichment, which serve as neutron reflectors at the top and bottom of the core where the neutron flux decreases rapidly. In addition, the top and bottom nozzles, made of forged and cast stainless steel (e.g., AISI 304), serve as neutron reflectors outside the core.

See Table 3.2-1 NUCLEAR DESIGN DATA (page 51 of 132) and Table 3.2-5 CORE MECHANICAL DESIGN PARAMETERS (page 59 of 132)
in https://www.nrc.gov/docs/ML1625/ML16251A153.pdf
Thank you sir for your kind reply, actually my model is for PWR core with 121 FA
15x15 lattice have 20 guide tubes, and one instrument tube. The core height is 290 cm the material for grid is GH-169A, There are 8 grids for each fuel assembly, of which 7 grids are within the core.
I hope this information will help
Hope to hear from you soon sir
Regard

Thank you sir for your kind reply, actually my model is for PWR core with 121 FA
15x15 lattice have 20 guide tubes, and one instrument tube. The core height is 290 cm the material for grid is GH-169A, There are 8 grids for each fuel assembly, of which 7 grids are within the core.
I hope this information will help
Hope to hear from you soon sir
Regard
Please describe the lattice geometry, or specifically, provide the fuel column (pellet) outer diameter, cladding outer diameter and fuel rod pitch. Is the fuel material typical UO2?

For example, a typical 15x15 fuel design for Westinghouse 15x15. Note dimensions in old British units.
https://www.nrc.gov/docs/ML1633/ML16336A535.pdf

Fuel rod (cladding) outer diameter = 0.422 inch (10.72 mm), fuel rod pitch = 0.563 inch (14.3 mm), and fuel assembly envelope = 8.426 inches (214 mm), which is about the same envelope as 17x17 fuel.
I assume one is considering a square lattice by virtue of 15x15.

Last edited:
Astronuc said:
Please describe the lattice geometry, or specifically, provide the fuel column (pellet) outer diameter, cladding outer diameter and fuel rod pitch. Is the fuel material typical UO2?

For example, a typical 15x15 fuel design for Westinghouse 15x15. Note dimensions in old British units.
https://www.nrc.gov/docs/ML1633/ML16336A535.pdf

Fuel rod (cladding) outer diameter = 0.422 inch (10.72 mm), fuel rod pitch = 0.563 inch (14.3 mm), and fuel assembly envelope = 8.426 inches (214 mm), which is about the same envelope as 17x17 fuel.
I assume one is considering a square lattice by virtue of 15x15.
FR Outside clad diameter is 1.0 cm, clad thickness is 0.07 cm, UO2 pellet diameter is 0.843 cm rod array is 15x15, lattice pitch is 1.33 cm, assembly pitch is 20.03 cm. I THINK YOU NEED THIS INFORMATION SIR.

Regards

## 1. What is the purpose of modeling a PWR core with spacer grid?

The purpose of modeling a PWR core with spacer grid is to simulate the behavior and performance of the nuclear reactor core under various operating conditions. This allows scientists and engineers to analyze and optimize the design of the spacer grid, which plays a critical role in maintaining the stability and efficiency of the reactor core.

## 2. What factors are considered in the modeling process?

The modeling process takes into account various factors such as the geometry and material properties of the spacer grid, the thermal and hydraulic behavior of the coolant flow, and the neutronic behavior of the nuclear fuel. Other factors may include the effects of radiation, temperature, and pressure on the spacer grid and surrounding components.

## 3. How accurate are the results obtained from modeling a PWR core with spacer grid?

The accuracy of the results depends on the complexity and fidelity of the model used. Generally, the more detailed and realistic the model, the more accurate the results will be. However, it is important to note that modeling is a simplification of the real-world system and there may be discrepancies between the simulated and actual behavior of the PWR core.

## 4. What are the benefits of using modeling for PWR core design?

Using modeling for PWR core design allows for a more cost-effective and efficient approach to optimizing the design. By simulating different scenarios and parameters, scientists and engineers can identify potential issues and make necessary adjustments before physically constructing the reactor. This can save time and resources, as well as improve the overall safety and performance of the PWR core.

## 5. Are there any limitations to modeling a PWR core with spacer grid?

While modeling is a valuable tool for PWR core design, there are some limitations to consider. Models can only simulate what is programmed into them, so unexpected events or phenomena may not be accurately represented. Additionally, the accuracy of the results is dependent on the accuracy of the input data and assumptions made during the modeling process. Therefore, it is important to validate and verify the results with experimental data and other methods.

• Nuclear Engineering
Replies
8
Views
3K
• Nuclear Engineering
Replies
4
Views
1K
• Nuclear Engineering
Replies
1
Views
2K
• Nuclear Engineering
Replies
11
Views
2K
• Nuclear Engineering
Replies
2
Views
1K
• Nuclear Engineering
Replies
2
Views
1K
• Nuclear Engineering
Replies
1
Views
305
• Nuclear Engineering
Replies
3
Views
2K
• Nuclear Engineering
Replies
2
Views
2K
• Nuclear Engineering
Replies
2
Views
2K