Shielding Fuel Assembly Design Optimization

SUMMARY

The aging nuclear fleet is facing demand of lifetime extension to ensure reliable and carbon-free energy production. A major limiting parameter for lifetime extension is the neutron fluence of the reactor presure vessel. Furthermore, new requirments on flanges may require extensive material investigations.
Framatome’s shielding fuel assemblies reduce the neutron flux in critical locations, while reactor power is kept at full level or even increased. Several years of experience and the evolution of shielding fuel assembly design allow customization and optimization of shielding factor and operation flexibility. Modifications in radial and axial design as well as different levels of enrichment are key features of this solution.

INTRODUCTION

Energy demands and the target of low carbon energy sources raises the question of lifetime extension of nuclear power plants. Beside topics of preservation of know-how, modernization, and security of supply, the extension of lifetime is linked with the aging of material. Primary focus is on components which cannot be replaced e.g., reactor pressure vessel and its associated parts. Framatome offers various designs of Shielding Fuel Assemblies (SFA) to increase the remaining lifetime without the loss of operational power level and with minimal impact on neutron economy and operation flexibility. The present paper shows the evolution of design during continuous application and improvement. It summarizes various designs, the shielding factors and expected effects on core design.

RESULTS

Theoretical Background

The neutron fluence E > 1 MeV causes embrittlement in the material structure of the reactor pressure vessel and its welds. For this reason, the neutron fluence has a defined design fluence limit or the neutron fluence limit determined by the applied neutron fluence of material surveillance capsules and its material samples. These measurement validations for fast neutron fluence requires time effort regarding preparation, irradiation, and evaluation period, which might be higher than the remaining lifetime of the plant. The concept of the shielding fuel assemblies is another approach to reduce the neutron flux on critical locations. [1] already showed the difference of neutron pads and shielding fuel assemblies. The shielding factor is the ratio of neutron flux with shielding fuel assemblies to the neutron flux without shielding fuel assemblies:

This factor is the upscaling factor of the remaining plant lifetime in terms of neutron fluence. Framatome has various design solutions available to protect dedicated locations of the reactor pressure vessel or its welds. This includes the use of steel bars which replace parts of fuel rods to shield the reactor pressure vessel. These steel bars can form an axially homogeneous shield or can consist of axial segments of steel and fuel materials. Position and size of the shield part can be adapted for the required protection.
This design optimization enables an efficient lifetime extension without losing neutron economy and keeping the operation flexibility. Table 1 gives an overview of three different shielding fuel assembly designs, the achieved shielding factor (SF) and the remaining fuel in the shielding fuel assembly. As the shielding fuel assembly still acts as a fuel assembly the reactor core power can remain unchanged. A slight change in the power distribution has only small effects on the core design. Operation experience has demonstrated the feasibility of a reactor power uprating parallel to the introduction of Framatome’s shielding fuel assemblies.

Operation Experience

Shielding fuel assemblies are applied in the Vattenfall plant Ringhals 3 since 2009, as well as in Ringhals 4 from 2009 to 2015. [2] summarizes the experience in operation and the validation of the shielding factor through measurements for the two different designs. Figure 1 illustrates the neutron fluence of the plant at the beltline weld. The change in reactor loading strategy already reduced the increase in neutron fluence. However, applying shielding fuel assemblies was a game changer. The insertion of the first design, containing 3 pin rows of steel and two different enrichment levels, results in only 10% neutron fluence in 5 equivalent full power years (efpy) compared to the accumulated neutron fluence in the first 21 efpy. Advanced shielding fuel assemblies, with shielding factor above 7, yield approximatly 15% of this neutron fluence in 24 efpy.
The shielding factor was kept on the same level when applying the newest design in 2023. The three-segemented design optimized axially the shield part in the beltline weld area but the increase in fissile material improved operational flexibility. This design optimization demonstrates again the effectivness of Framatome’s shielding fuel assemblies. Parallel to the introduction of SFA in RH-3 and RH-4, a power uprating project had been performed to further increase the nominal reactor power. The positive feedback of the operator and the validiation of the design is a significant difference to other theoretical solutions.

Design adaptation

As described above, the design can be optimized in number of shield rods, enrichment of fuel, and the axial location of the shielding part. Figure 2 illustrate the location for an axial centered shield segment. The yellow areas mark the fuel parts. The shielding segment can be shifted and extend to gain the required fluence reduction while optimizing the amount of fissile material.

Detailed investigations ensure compatibility to ordinary fuel, operational flexibility, detector efficiency and maximum lifetime extension. State-of-the-art codes and methodology are applied for short term licensing and manufacture.

CONCLUSION

Framatome has know-how in designing, licensing, and manufacturing of shielding fuel assemblies and has several years of operation experience with different shielding fuel assembly designs. Figure 3 gives an overview of a typical plant lifetime based on design concepts with 40 equivalent full power years of operation and the effect of switching from out-in-loading pattern to low leakage loading pattern. The figure also illustrates the effect of implementing shielding fuel assemblies with different shielding factors to ensure lifetime extension. Framatome’s shielding fuel assemblies can be customized regarding shielding factor and impact on operation based on the available simulation tools and its large experience. Furthermore, we can offer an experience in the licensing and manufacturing of shielding fuel assemblies in short period.

REFERENCES

[1] Lars Ackermann, Christian Möllmer, Jörg Peucker, “The use of neutron fluence analyses as verification of reactor pressure vessel shielding design,” 46th Annual Meeting on Nuclear Technology (AMNT), May 5 – 7, 2015, Berlin, Germany.

[2] Lars Ackermann, Christian Möllmer, Jörg Peucker, Dominik Streit, Jenny Roudén, Henrik Nylén, Malin Löwe, David Schrire
“Framatome Shielding Fuel Assemblies ensure Plant Lifetime Extension” ENS webinar, Location, 10th October2019. https://www.youtube.com/watch?v=Nrf3qhLfxD4&ab_channel=EuropeanNuclearSociety

AUTHORS

Lars Ackermann
Framatome GmbH
Lars.Ackermann@framatome.com

Christian Möllmer, Jörg Peucker
Framatome GmbH
Christian.Moellmer@framatome.com, Joerg.Peucker@framatome.com