Difference between system subchannel and CFD codes

• Meb15aa
In summary, the three main methods used for nuclear reactor simulations are 1D codes, system codes which simulates the entire reactor core, and CFD which provides detailed flow physics in 3D.
Meb15aa
Hi everyone, I am researching how numerical simulations have evolved over the years in nuclear reactors for assessing the thermal hydraulics inside the reactor core.

I have found vague information in regards to the three different main numerical techniques but want to learn more.

So far, system codes are very basic and simulate the entire reactor core as a single node, where macroscopic parameters like total pressure can be extracted. Sub channel codes take this one step further by simulating codes per each sub channel providing better information about the physics inside the reactor. Finally CFD provides detailed flow physics in 3D as opposed to the latter two simulating in 1D but at the expense of computational time.
I would like to learn more about these three methods particularly the reason for evolving the precision of numerical technique. would anyone be interested in sharing their experience, or providing literature around this subject.
Thank you,
Meb15aa

The simplest 1D codes were developed when computational resources were limited - limitations on processing and memory. As computers evolved, so did the numerical methods.

One should look into the difference between Lagrangian and Eulerian methods. Basically has to maintain mass, momentum and energy in either single phase (liquid or gas) or two phase (liquid with vapor).

One should look into thermal hydraulic codes, RELAP, COBRA, and RETRAN

Thank you for the reply Astronuc, very useful information.

I should have also mentioned VIPRE, which was derived from COBRA, hence the reference to a snake, and TRAC. All three codes have been adapted by different organizations.

Two major CFD codes in the industry are CFX and STAR-CD or STAR-CCM+

This is where we are now - https://www.osti.gov/pages/servlets/purl/1332948

okay cool, thanks once again.

There is also work being done is coarse grid CFD as as a means to fill the gap between the existing but limited sub channel codes and highly detailed but time consuming 3D CFD models. It generally works by extracting a representative part of the mesh model from a highly resolved CFD model and overlapping that information with a coarse grid model to re calibrate the data an basically correct it.

https://www.researchgate.net/publication/283684011_Coarse-Grid-CFD_for_the_Thermal_Hydraulic_Investigation_of_Rod-Bundles

I have also read some papers declaring current methodology (subchannel codes) cannot meet the safety requirements of future generation reactors, therefore explaining one of the reasons why the industry is looking towards other means like the link you sent and coarse grid methodology to get relatively quick but informative data.

Moreover, going back in time, the reason why simulations were introduced were due to experiments being very time consuming, expensive and dangerous especially when considering accident scenarios. Chernobyl, 86 was an experiments that had gone disastrously wrong. Simulations need to be validated through experiments but due to the vast stored data over the decades, simulations are becoming increasingly dominate in the industry (Also due to rise in computational capabilities).

Very interesting area of work I believe :)

1. What is the difference between system subchannel and CFD codes?

System subchannel codes are used to model the fluid flow and heat transfer in nuclear reactor systems, while CFD (Computational Fluid Dynamics) codes are used for general fluid flow and heat transfer simulations.

2. How do system subchannel and CFD codes differ in their modeling approach?

System subchannel codes use a lumped-parameter approach, where the system is divided into subchannels and the governing equations are solved for each subchannel. CFD codes, on the other hand, use a continuum approach where the fluid flow and heat transfer equations are solved for each individual point in the domain.

3. Which code is more accurate for nuclear reactor simulations?

System subchannel codes are more accurate for nuclear reactor simulations because they are specifically designed for this purpose and take into account the unique characteristics of nuclear systems, such as the presence of fuel rods and control rods.

4. Can CFD codes be used for nuclear reactor simulations?

Yes, CFD codes can be used for nuclear reactor simulations, but they may not be as accurate as system subchannel codes. CFD codes require additional modifications and validation to accurately model nuclear systems.

5. What are the advantages and disadvantages of using system subchannel and CFD codes?

The advantages of using system subchannel codes include their accuracy for nuclear reactor simulations, their ability to handle complex geometries, and their faster computation times. CFD codes, on the other hand, have the advantage of being more versatile and can be used for a wide range of fluid flow and heat transfer simulations. However, they may require more computational resources and may not be as accurate for nuclear reactor simulations.

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