Fuel and Poison Management

When the time comes to replace fuel in a reactor, either because of loss of reactivity or because of changes in its physical properties, the reactor operator is faced with a number of alternative choices. The operator must decide whether to remove all or part of the fuel in the reactor, and whether to move some of the fuel remaining in the reactor from one location to another, and he or she must choose the composition of new fuel to replace the fuel removed. 

The principal objectives of fuel and poison management are as follows  

To keep the reactor critical during long-term changes in fuel composition and reactivity 

To shape power density distribution to maximize power output 

Not all these objectives can be achieved simultaneously in a given reactor, and some compromises among them are usually necessary. Each objective will be described briefly in turn. 

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To point to the importance of using improved methods of fuel and poison management, we shall discuss qualitatively the multiple drawbacks of the simplest method, which is batch irradiation of fuel initially uniform in composition, with spatially uniform distribution of boron control poison and with complete replacement of fuel at the end of its operating life. An example of this would be a PWR charged with fuel of uniform enrichment containing 4 percent and 96 percent and controlled by adjusting the concentration of boric acid dissolved in the water coolant to keep the reactor just critical at the desired power level. When this reactor starts operation, the compositions of fuel and poison are uniform throughout the core, and the flux and power density distribution are very nonuniform. 

This figure illustrates immediately one of the disadvantages of batch fuel management. The power density, which is proportional to the product of the neutron flux and the fissile material concentration, is just as nonuniform as the neutron flux. If the local power density must be kept below some safe upper limit, to keep from overheating the fuel or cladding, only the fuel at the center of the reactor can be allowed to reach this power density, and fuel at all other points wiU be operating at much lower output. In a typical uniformly fueled and poisoned water-moderated reactor, the ratio of peak to average power density is over 3, so that the reactor puts out only one-third as much heat as it could if the power density were uniform. 

Each of these elements may be used for production of nuclear fuel or other purposes. The recovery efficiency for uranium is reported as 99.87% and for plutonium 99.36%-99.51% (NEA 2012). The extended PUREX includes separation of neptunium and technetium as well as recovery of americium and curium that are also separated from each other by additional extraction stages as given in detail in the flowsheet (NEA 2012). The advanced UREX-i-3 process generates six streams after separation uranium for re-enrichment Pu-U-Np for mixed oxide fuel c for managed disposal Am-Cm to be used as burnable poisons and for transmutation high-heat-generating products (Cs and Sr) and a composite vitrified waste with all other fission products. Some fuel types may require preliminary steps like grinding to enable their dissolution. 

The first two chapters serve as an introduction to the basic physics of the atom and the nucleus and to nuclear fission and the nuclear chain reaction. Chapter 3 deals with the fundamentals of nuclear reactor theory, covering neutron slowing down and the spatial dependence of the neutron flux in the reactor, based on the solution of the diffusion equations. The chapter includes a major section on reactor kinetics and control, including temperature and void coefficients and xenon poisoning effects in power reactors. Chapter 4 describes various aspects of fuel management and fuel cycles, while Chapter 5 considers materials problems for fuel and other constituents of the reactor. The processes of heat generation and removal are covered in Chapter 6. 

The main advantage of the PAFC is that the higher temperature eliminates or reduces two major problems with the PEFC, CO poisoning sensitivity and water management. The PAFC cannot accomplish internal reformation like the MCFC or SOFC, but because of the elevated temperature of 200°C compared to 80°C for the PEFC, the anode in the PAFC can tolerate a 1-2% CO in the feed stream. This allows operation on reformed natural gas and other fuel feedstocks with minimal CO filtering, greatly reducing reformer size and control requirements. 

A big challenge in the field of PEMFCs is to develop PEMs with high proton conductivity at elevated tanperatures (>100°C) and low RHs because of the merits of fast electrochemical kinetics, high tolerance of catalyst to CO poison, and easy management of water when the fuel cells are operated at elevated temperatmes. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Because of its lower temperature and special polymer electrolyte membrane, the proton exchange membrane fuel cell (PEMFC) is well-suited for transportation, portable, and micro fuel cell applications. But the performance of these fuel cells critically depends on the materials used for the various cell components. Durability, water management, and reducing catalyst poisoning are important factors when selecting PEMFC materials. 

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