Revisiting Basics of Fertile-Free-Fuel Reactor Physics

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In recent years numerous R&D projects investigating potential of the Fertile Free Fuels (FFF) were carried out and reported. The main objective of these efforts is to investigate the potential of FFF to stabilize storage requirements of the long lived transuranium elements, to reduce the amounts and emissions of the spent fuel, as well as incinerating excess stockpile of Pu. The research effort is spread over a variety of reactor types, such as existing and evolutionary light water reactors (LWR) and emerging Generation IV reactor concepts.

The reactor physics challenges of core and fuel cycle designs based on FFF are well known, reported in several publications and may be summarized as follows:

•High concentration of thermal absorbers (Pu isotopes) degrades reactivity worth of control absorbers (soluble boron, burnable poisons, and control rods);

•Absence of fertile component of the fuel modifies significantly the reactivity letdown curve, especially towards the end of the cycle;

•Temperature reactivity coefficients are degraded, especially fuel temperature coefficient.

Possible design solution to mitigate the problems listed above is a judicious application of burnable poisons.

Various burnable poisons design and materials were investigated for a variety of fuel compositions and matrices. This work was aimed to perform a systematic evaluation of a potential of burnable poisons to address the design challenges related to FFF concepts for LWR's.

As a reference design, a standard PWR fuel assembly design was chosen loaded with the reactor grade Pu (composition typical for a 50 MW/kg burnup) within MgO-ZrO2 inert matrix. The BP designs investigated were based on WABA, IFBA, and Homogeneous poison/fuel mixtures. Main candidates of three BP materials: Gd,Hf, and Er were analyzed for each of the geometries listed above.

Potential of a specific fuel/BP design to produce an acceptable candidate for a LWR cycle within a safety-related envelope of existing power plants was evaluated based on calculations of fuel load required to assure 18-month inter-refueling interval, moderator and fuel temperature coefficients, and soluble boron reactivity worth. Analyses were carried out by an assembly level code – BOXER (1) and an application of a modified linear reactivity model for estimating full core performance parameters.

Results of this work present a comprehensive analysis of the potential of BP utilization to address the design challenges of the FFF fuel cycles. In addition, conclusions derived from these results may be used as a guideline for an optimal choice of acceptable core and fuel cycle concepts


1. Paratte J.M., Grimm P., and Hollard J.M., “User’s Manual for the Fuel Assembly Code BOXER”, PSI, CH-5232 Villingen PSI, (1995)

Acknowledgement: This research was carried out with partial support of Harry Reid Center for Environmental Studies, University of Nevada, Las Vegas


Nuclear fuels; Nuclear fuels – Design and construction; Nuclear physics; Plutonium; Radioactive wastes — Storage


Heat Transfer, Combustion | Nuclear | Nuclear Engineering | Oil, Gas, and Energy




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