Thermal Hydraulics of Wire Wrapped Nuclear Fuel Rods

Figure 1: Structured multi-block grid for a wire-wrapped 19 nuclear fuel rods bundle.

1700 words / 8 minutes read

Intro

Nuclear power plants generate about 16% of the world’s electricity. Nuclear fuels are an incredibly compact source of energy that liberates a tremendous amount of energy, which is used to generate electricity. One nuclear fission reaction generates 200 million units of electricity when compared to a chemical reaction which liberates only one unit of electricity. It is for this reason, nuclear energy is very attractive from an engineering perspective. Though, the nuclear energy industry has been marred with accidents and environmental concerns it has managed to stand the test of time and continues to contribute energy for human consumption.

Need for 4th Generation Nuclear Reactors

Nuclear energy does outweigh the other forms of energy generation, however, the current reactors ( 3rd generation) worldwide are far behind in terms of fuel efficiency. The conventional pressurized water reactors (PWR) use only about 2-3 percent of the uranium atoms in the nuclear fuel. This is an astoundingly low utilization ratio. On top of that, they use the open once-through type of fuel cycle, i.e, fuel enters a reactor, 2-3 percent of the fuel is used, it is taken out, and disposed of. The fuel is grossly under-utilized and hence, there is a strong need to come up with ways to use it more efficiently if we want to make the process sustainable.

**Figure 2: a.** Fuel is used once in a reactor and discarder. b. Actinides are separated from the used fuel and burnt in, appropriate reactor types, while waste contains only fission products.

For this reason, nuclear scientists and engineers have been investigating building a new type of reactor called Generation 4 nuclear reactors. Firstly, these fast breeder reactors that come in this category apply a fully closed cycle and utilize 70-80 percent of the uranium fuel before they are disposed of. This leads to a 20-30 times increase in the efficiency of the use of uranium.

Secondly, the nuclear wastes produced by the current once-through cycle need 300,000 years to reach the mine radiotoxicity level. However, in the fully-closed cycle, if we can segregate the elements called actinides which are generated around plutonium and uranium, and then store the nuclear waste geologically for 300 years, the radiotoxicity levels will return to that in the mine from where it was taken out in the first place.

a. Relative Radiotoxicity. b. The number of years of energy at our disposal. — **Figure 3: a.** Relative Radiotoxicity. b. The number of years of energy at our disposal.

Lastly, as shown in Figure 3b, if we were to use uranium in a fast breeder reactor, we increase the number of years they can be used from 200 to 800 years and if we were to add minerals like thorium in the fuel cycle, we have around another 2000 years of energy at our disposal.

The above reasons make the 4th generation fast breeder reactors the hot research topic today and many companies both government and privately funded, are aggressively pursuing it to make it a reality.

Need for Thermal-Hydraulic Analysis in FBR

The central core of a nuclear plant consists of a few hundred fuel assemblies consisting of a large number of fuel rods. Fast breeder reactors utilize ducted fuel assemblies with helically wire-wrapped fuel pins. Coolants flow around these rods/pins and absorb the heat liberated during the nuclear fission reactions. In these reactors, liquid metals like sodium or lead are envisaged as coolants instead of water. This is because, they have 100 times more thermal conductivity than water, higher boiling temperature, and lesser neutron interactive property. The coolant moving out of the core rotates a set of turbine blades and generates electricity.

Safety studies are mandated by the safety authorities in order to license a nuclear power plant, thus ensuring the prevention of nuclear catastrophe like core melt-down, etc. This fundamental requirement necessitates designing all the components to meet safety requirements. Detailed thermal-hydraulic investigations of the core, fuel assemblies, and sub-channel are one such requirement.

The technical challenges in the core include the pressure drop and heat transport efficiency under nominal, transient, and incidental conditions. As for the safety issue, the limitation is on the clad temperature. There are other issues like fuel rod vibration due to coolant flow which leads to gradual fretting wear and fatigue at contact surfaces.

**Figure 4:** a. Bill Gates explaining the wire-wrapped fuel pins at TerraPower. b. A closer look at the wire-spacer pin bundle. Image source Ref [9,7].

At the fuel assembly level, thermal-hydraulic accident analysis concentrates on blockage scenarios and thermal fatigue evaluation. Lastly, at the sub-channel level, the focus is on the detection of hot spots.

These thermal investigations have larger significance especially in fast breeder reactors due to the large heat flux of about 1.5MW per square meter. Interestingly, it is only very recently that CFD tools have become advanced enough to model core coolant flow with high details and resolution. Traditionally, the core design was performed entirely using what are called sub-channel codes. Lately, CFD has become increasingly relevant to the core design. The increased resolution and fidelity CFD provides are very beneficial especially for complex geometries like wire-wrapped pins.

Wire-Wrapped Fuel Rods

The hexagonal array of wire-wrapped fuel pins is the trademark fuel arrangement system in sodium-cooled fast reactors. The wires serve as a support grid between fuel rods and they also help to maintain the gap between rods.

The helically wound wires enhance the mixing of coolants by redirecting the coolant to neighbouring sub-channels. This increased mixing is beneficial as it aids in better heat transfer and also prevents temperature peaking in hot channels.

Furthermore, the wire wrappers acting as spacers, separate the rods and minimize flow-induced vibrations which may induce reactivity fluctuations possibly leading to mechanical failure of the fuel cladding. However, wire wrappers cause a drop in pressure through the core compared to bare rods. The pressure drop is observed to be marginal at low Reynolds number but becomes quite significant at high Reynolds number.

**Figure 6:** Variation of flow pattern with wire angle. The red box shows the swirl flow in the outer channels. Image source Ref [1].

Flow Field Characteristics

The flow inside a fuel bundle can be divided into two regions, a peripheral region where large swirl flow exists and the inner region, where the complex transverse flow exists. Figure 6 shows the variation in contours of axial velocity and streamlines of transverse flow with a change in wire angle. What can be observed is that the axial velocity is higher in the edge sub-channel compared to that in the interior sub-channel. Also, the interior sub channel’s axial velocity and streamline pattern tend to be similar irrespective of the position of the interior channel. However, the flow near the outermost region in the edge channel has large swirl flows which tend to rotate with the wire.

In the interior sub-channels, the wrapped wires make the flow inside the fuel bundles complicated by generating sweeping flow and vortex flow. What happens is that a portion of the axial flow sweeps along the wire and transforms itself into a transverse flow. In addition, another segment of the axial flow creates a vortex structure by tripping over the wire.

**Figure 7:** Generation and destruction of vortices around the wire in the interior sub-channel. Image source Ref [1].

Figure 7 shows the generation and destruction of the vortices in the interior sub-channel. Vortices are periodically created in the interior sub-channel at a frequency of 3 times for every wire rotation. The vortices affect the flow field and the heat transfer inside the sub-channels and hence understanding their flow characteristics is essential. The main flow in the interior sub-channel is axial and when the flow gets blocked by the wire, the pressure on the windward side of the flow increases relative to the leeward side. As the wire passes the interior sub-channel ( station P1 to P5), part of the main flow which has traveled over the wire gets converted to a large center vortex (V1). It rotates in a direction opposite to that of the wire rotation and its length scale depends on the width of the sub-channel and transverse flow. Another vortex created in the sub-channel is the back vortex (V2), which is formed due to the transverse flow behind the wire. This vortex is fairly small in nature and is mostly confined between the surface of the pin and the wire.

It is observed that the largest of vortices occur in the edge sub-channel, which tends to block the swirling flow in the peripheral region. The vortices formed in the corner sub-channel are relatively small.

One thing to note is that the occurrence of these vortices is directly related to the position of the wire and does not depend on any geometric variables like the number of pins or pin pitch to diameter ratio, etc.

The transversal flow developed due to helical wire bring in many benefits. One, the coolant outlet temperature is now more uniform leading to lower levels of fluctuation in the readings of the core monitoring thermocouples which is essential for safer reactor control operations. The second advantage is that the clad temperature becomes more uniform in the circumferential direction due to the gyratory flow created by the helical wire. The coolant is made to impinge and sweep the corners formed by the junction of the pin and the spacer wire, thereby preventing a possible hot spot beneath the wire wrap. Lastly, it allows the FSA to be designed to generate a larger power without exceeding the temperature limits of the clad and sodium.

Video 1: Coolant Flow in Sodium Reactor Subassemblies.

Parting Thoughts

A better understanding of these intricate flow fields is extremely essential for the proper design of these critical nuclear core components considering the high levels of safety requirements. CFD has emerged lately as a reliable computational technique used extensively for design and safety evaluation purposes. Particularly for wire-wrapped fuel bundles, CFD has been pivotal in understanding and appreciating the complex flow physics and thermal-hydraulics.

With this, we have come to the end of Part 1 of the series on Nuclear Fuel Rods. This is a 3 Part series, starting with this article on flow physics.

Part 1 – Flow Field Inside a Wire Wrapped Nuclear Fuel Rod Bundle
Part 2 – Role of Structured and Unstructured Meshes in Nuclear Fuel Rods CFD
Part 3 – Meshing Wire Wrapped Fuel Rods in GridPro

In the next article, Part 2 – Role of Structured and Unstructured Meshes in Nuclear Fuel Rods CFD, we try to cover aspects of CFD simulation of these wire-wrapped fuel pins – the challenges, the geometric approximations, gridding requirements, etc. In the last part, Part 3 – Meshing Wire-Wrapped Fuel Rods in GridPro, we cover, how to generate high-quality structured multi-block grids for various geometric variants of wire-wrapped fuel rod assemblies, automation, etc, using GridPro.