20.08.2015   Handbook Nuclear Terms

Materials

The conventional control rod for water cooled reactors is made of stainless steel with neutron absorbing material either supported and clad by the rod structure or alloyed with the rod material The neutron absorbing material absorbs thermal neutrons, which reduces the effective multiplication constant, k, below unity The most com monly used neutron absorbing or poison materials in control rods are cadmium, boron, and hafnium Other less commonly used materials are silver, europium, and indium Factors determining the usefulness of a control rod material include not only thermal-neutron absorption properties but also availability, cost, and structural and machinabihty properties The material must be fabricated into various shapes and must not be affected appreciably by the temperature or pressure of the particular environment in which it is to be used Its susceptibility to corrosion and nuclear-radiation damage must be low In some instances

Table 7 2—Control Worth for Various Materials*

Relative effectiveness

Material in a water-cooled reactor

3 0 wt % 1 0 В in stainless steel (dis persion of minus 100 mesh particles

of 90% enriched 1 °B)

і

12

Dispersiont containing 10 vol %

B4C (90% enriched 10B)

і

06

Hafnium

і

00

0 97 wt % 1 °B in stainless steel (alloy)

0

98

Ag—22 wt % In alloy

0

96

15 wt % Eu203 in stainless steel

(dispersion)

0

96

Indium

0

93

Silver

0

88

Cadmium

0

80

8 7 wt % gadolinium—titanium

0

77

Tantalum

0

71

2 7 wt % Sm2 03 in stainless steel

(dispersion)

0

70

Haynes Stellite 25(Co—20 wt % Cr—

15 wt % W-10 wt % Ni)

0

68

Titanium

0

24

Zircaloy 3

0

05

2S aluminum

0

02

‘Based on data from C R Tipton, Jr (Ed ), Reactor Handbook Vo! 1, Materials, 2nd ed p 779 Interscience Publishers Inc, New York, 1960, and W К Anderson and J S Theilacker (Eds), Neutron Absorber Materials for Reactor Control p 117 Superintendent of Documents, U S Government Printing Office Washington, D C, 1962

tDispersion assumed to have a nonabsorbing matrix and to be clad with 0 02 in of nonabsorbing materials

alloys of the poison materials with other materials improve their suitability for control-rod application

Table 7 2 indicates the relative worth of commonly used control-rod absorber materials

6- 3.4 Rod Shape

The shape, dimensions, and number of reactor control rods are dependent on the core mechanical design and the amount of negative reactivity needed for shutdown To function efficiently as a neutron poison, a control-rod material must have sufficient thickness to absorb most of the flux at the rod surface In light-water reactors the slowing-down length, і e, the distance required for a fission neutron to be reduced to thermal energy, is short Therefore, to be effective in absorbing thermal neutrons, the poison material must be physically close to the fuel surface and have a high surface-to-volume ratio A cruci form shape with thin wide blades of poison material fulfills these requirements Figure 7 2 shows typical control-rod configurations for power reactors Since a light-water reactor has a neutron spectrum with an appreciable fraction of epithermal neutrons, materials with large absorption cross sections for these energies, such as hafnium and indium, are used in addition to the usual thermal neutron absorbers, such as cadmium or boron