26.08.2015   An Introduction to Nuclear Materials

Nuclear Fuels

“We believe the substance we have extracted from pitch-blende contains a metal not yet observed, related to bismuth by its analytical properties. If the existence of this new metal is confirmed we propose to call it polonium, from the name of the original country of one of us.”

—Marie Curie

7.1

Introduction

The heart of a nuclear reactor is the “reactor core” that contains nuclear fuels among other components/materials. Nuclear fuel forms consist of radioactive materials that may create the fission chain reaction under suitable conditions creat­ing a large amount of heat that is then utilized for producing the electrical power. The following are the basic requirements of a nuclear fuel:

a) The capital installation costs for nuclear power plants are substantial. In order to maintain profitability in the power production, the fuel costs must be minimal.

b) Adequate thermal conductivity of nuclear fuels is necessary to ensure that they can withstand the thermal gradients generated between the fuel center and periphery.

c) The fuel should be able to resist repeated thermal cycling due to the reactor shutdowns and start-ups.

d) It should have adequate corrosion resistance against the reactor fluids.

e) It should transmit heat quickly out of the fuel center.

f)The fuel should be relatively free from the constituent elements or impurities with high neutron capture cross section in order to maintain adequate neutron economy.

g) It must be able to sustain mechanical stresses.

h) The fuels should be amenable for reprocessing or disposal.

Nuclear fuel materials developed over decades include metals/alloys (uranium, pluto­nium, and thorium) and ceramics (oxides, carbides, nitrides, and silicide compounds containing the former radioactive elements). Nuclear fuels are fabricated in a wide

An Introduction to Nuclear Materials: Fundamentals and Applications, First Edition.

K. Linga Murty and Indrajit Charit.

© 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.

variety of configurations, such as cylindrical pellets, long extruded rods (for metal/alloy fuels only), spherical particles, coated particles, dispersion fuels (such as cermets), and fluid forms (for aqueous homogeneous reactors and molten salt cooled reactors).

 

Some Basic Terms Regarding Nuclear Fuels

Burnup: Fuel burnup is an important property of nuclear fuels. Burnup is gener­ally defined as the amount of heavy metal (in the form of uranium and higher actinides) in the fuel that has been fissioned. This term can be expressed either as a percentage of heavy metal atoms that have fissioned (atom%) or in units of fission energy (gigawatt-day or GWd; 1 GWD = 8.64 x 1013 MWd) produced per metric ton of the heavy metal (MTHM), that is, GWd/MTHM or MWd/kgHM. One atom% burnup corresponds to approximately 9.4 GWd per MTHM. However, the fuel burnup is often limited by the fuel cladding per­formance. Superior cladding performance allows higher burnups in fuels. Blanket fuel: Nuclear reactor fuel that contains the fertile isotopes that are bred into fissile isotopes

Driver fuel: Nuclear reactor fuel that contains the fissile isotopes along with fer­tile isotopes that are bred into fissile isotopes Reproduction factor: It is generally represented by g, which is the number of neu­trons created per neutron absorbed in fuel. If v neutrons are produced per fission reaction, the number ratio of fission to absorption in fuel is of/oa and the number of neutrons per absorption is

sf

g = — v. (7.1)

sa

The value of g is higher in fast reactor compared to that in thermal reactors. Conversion ratio: The ability to convert fertile isotopes into fissile isotopes can be measured by the conversion ratio (CR) defined as

CR = Fissile atoms produced/fissile atoms consumed. (7.2)

If CR is >1 such as in a fast reactor, it is called the breeding ratio (BR). The breeding gain (BG) is given by (BR — 1), which represents the additional pluto­nium produced per atom burned.

Fission products (Fp): According to Kleycamp [1], the fission products can be classified as follows — (1) fission gases (fg) and other volatile elements — Br, Kr, Rb, I, Xe, Cs, and Te; (2) fission product forming precipitates — Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Se, Te; (3) Fp forming oxide precipitates — Rb, Sr, Zr, Nb, Mo, Se, Te, Cs, and Ba; and (4) Fp dissolved as oxides in fuel matrix — Rb, Sr, Zr, Nb, La, Ce, Pr, Nd, Pm, Sm, and Eu.

Fissium (Fs) or fizzium (Fz): Fissium is nominally 2.4wt% Mo, 1.9wt% Ru, 0.3 wt% Rh, 0.2wt% Pd, 0.1 wt% Zr, and 0.01 wt% Nb and is designed in such a way that it can mimic noble metal fission products remaining after a simple reprocessing technique based on melt refinement.

 

The history of metallic nuclear fuels dates back to the first developmental stages of nuclear reactors. U — and Pu-based fuels were used in the Experimental Breeder Reactor-1 (i. e., EBR-1) that produced useful electricity for the first time in December 1951. In addition, EBR-2, the first-generation Magnox reactors (such as Calder Hall in the United Kingdom), and many other subsequent fast reactors have used metallic fuels. When water-cooled reactors were being devel­oped, the metallic fuels were not chosen mainly because of the compatibility issues between water and metallic fuel at elevated temperatures arising during the event of a cladding breach resulting in the formation of metal hydrides or oxides. However, in the mid-1960s, when the fast reactor development was gain­ing ground, designers chose oxide fuels in place of metallic fuels as they envi­sioned that the metallic fuels would have only limited burnups because of the presumed swelling problems and anticipated creation of liquid phases in fuels during the higher temperature operations. During that time, oxide fuels were recommended for power reactors even though limited information was available on them. Later simple design changes for the fuel elements, widening the gap between the fuels and cladding materials and providing a plenum volume for accumulating fission gases, have shown marked improvement (1% versus 20% burnup) over the earlier designs where there was no gap or very little gap between the fuel and the cladding materials. However, it is important to note that research and test reactors have traditionally used metallic fuels mainly because of the lower temperature operations involved.

Here we highlight three main metallic nuclear fuel materials. Metallic fuels have a number of advantages as well as disadvantages often specific to the fuel types. However, the metallic fuels generally have higher thermal conductivity, high fissile atom density (improved neutron economy), and fabricability as their prime advan­tages, whereas lower melting points, various irradiation instabilities, poor corro­sion resistance in reactor fluids, and various compatibility issues with the fuel cladding materials are some prominent disadvantages. Metallic fuels can also be used in alloy forms to improve corrosion resistance and irradiation performance among others.

7.2.1