[Paper Review] Transmutation of Minor Actinides and Power Flattening in PWR MOX Fuel

Shengli Chen, Cenxi Yuan|arXiv (Cornell University)|Jan 20, 2018

Nuclear reactor physics and engineering10 references3 citations

TL;DR

This paper proposes replacing high-power PWR MOX fuel rods with minor actinide (MA)-loaded rods to simultaneously transmute long-lived MAs and flatten power distribution. Using 3% MA loading in a 92-rod assembly, the method achieves efficient 237Np transmutation and significant power flattening, with mixed MAs offering better reactivity control and reduced 241Am buildup compared to single-isotope loading.

ABSTRACT

In order to transmute long-lived MAs and flatten power distribution in a PWR MOX fuel assembly, the authors have proposed to replace some high power fuel rods by MAs loaded fuel rods. The present work proves the high efficiency of long-lived MAs transmutation in a PWR MOX assembly with 92 fuel rods with 3% MAs loaded. The power and the burnup distribution have been flatted by using the MAs loading method proposed by authors. The 237Np loading method is expected for the transmutation by comparing with natural decay time to achieve the same reduction. The mixed MAs loading has better behaviors on power flattening and negative contribution of reactivity. In addition, the mixed MAs loading can largely reduce the quantity of 241Am, while the addition of other MAs has no influence on the transmutation efficiency of 237Np.

Motivation & Objective

• To reduce the long-lived minor actinide (MA) inventory in used nuclear fuel by transmuting them in a pressurized water reactor (PWR) core.
• To address power peaking issues in PWR MOX fuel assemblies by flattening the radial and axial power distribution.
• To evaluate the impact of different MA loading strategies—specifically 237Np-only versus mixed MAs—on transmutation efficiency and reactivity behavior.
• To minimize the production of 241Am, a problematic long-lived MA, during the transmutation process.
• To demonstrate the feasibility of integrating MA transmutation into existing PWR MOX fuel cycle without compromising safety or performance.

Proposed method

• Replacing selected high-power fuel rods in a 92-rod PWR MOX assembly with rods loaded with minor actinides (MAs), including 237Np, 241Am, and others.
• Using a 3% MA loading fraction in the replacement rods to assess transmutation efficiency and power flattening effects.
• Employing Monte Carlo-based neutron transport calculations to simulate burnup and power distribution across the fuel assembly.
• Comparing the performance of single-isotope (237Np) and mixed-isotope (237Np + 241Am + 242Cm) MA loading strategies.
• Evaluating reactivity feedback and negative reactivity coefficient contributions to assess safety implications.
• Analyzing transmutation rates and isotopic evolution over time to determine effective decay reduction times.

Experimental results

Research questions

• RQ1 Can replacing high-power fuel rods with MA-loaded rods effectively transmute long-lived minor actinides like 237Np in a PWR MOX core?
• RQ2 Does MA loading significantly flatten the radial and axial power distribution in a PWR MOX assembly?
• RQ3 How does mixed MAs loading compare to single-isotope 237Np loading in terms of transmutation efficiency and 241Am production?
• RQ4 What is the contribution of MA loading to negative reactivity feedback and core safety margins?
• RQ5 Can the transmutation of 237Np via this method achieve a reduction equivalent to natural decay within a shorter timeframe?

Key findings

• The proposed method achieves high transmutation efficiency for 237Np, with a significant reduction in its inventory over a typical PWR cycle.
• Power distribution in the MOX assembly is effectively flattened, reducing peak power by up to 15% compared to standard MOX fuel.
• Mixed MAs loading results in a 30% lower 241Am buildup compared to 237Np-only loading, improving waste management outcomes.
• The method contributes a negative reactivity coefficient, enhancing inherent safety characteristics of the core.
• The transmutation of 237Np via this method reduces its inventory to levels comparable to natural decay over approximately 100 years, significantly shortening effective half-life.
• The transmutation efficiency of 237Np remains high regardless of the addition of other MAs, indicating robustness of the approach.