The general physical and mechanical effects of the irradiation of metals by fast neutrons and other high-energy particles are summarized in Table 1.

For fast neutrons, the changes are usually undetectable below certain radiation levels (fluences below 10²² neutrons/m²). With increasing radiation levels, the magnitude of the effects increases and may reach a limit at very large fluences. The effects listed in Table 1 are generally less significant at elevated temperatures for a given fluence and some defects can be removed by heating (annealing).

Both the yield strength and the tensile strength of a metal are increased by irradiation, as shown in Table 2, but the increase in yield strength is generally greater than the increase in tensile strength. At the same time, ductility is decreased by irradiation as shown in Figure 4, which is representative of the behavior of many metals, including steel and zircaloy. The accelerated decrease in the ductility of reactor vessels is due to the residual copper (Cu), phosphorous (P), and nickel (Ni) content in the vessel steel.

Figure 4 Qualitative Representation of Neutron Irradiation Effect on Many Metals

For stainless steel exposed to a thermal reactor fluence of 10²¹ neutrons/cm², the tensile properties show some increase in ultimate strength (tensile strength), an almost threefold gain in the yield strength, and a drop of about one third in ductility (elongation), as shown in Table 2.

The Nil-Ductility Transition (NDT) temperature, which is the temperature at which a given metal changes from ductile to brittle fracture, is often markedly increased by neutron irradiation. The increase in the NDT temperature is one of the most important effects of irradiation from the standpoint of nuclear power system design. For economic reasons, the large core pressure vessels of large power reactors have been constructed of low carbon steels.

The loss of ductility and increase in the NDT temperature of these vessels is a primary concern to reactor designers because of the increased chance of brittle fracture. Brittle fracture of a material is a failure occurring by crystal cleavage and accompanied by essentially no yielding. A brittle fracture of a pressure vessel resembles the shattering of glass. Since such a failure would be disastrous, it is necessary to understand the brittle fracture mechanism. During normal reactor operation, the pressure-vessel steel is subject to increasing fluence of fast neutrons and, as a result, the NDT temperature increases steadily. The NDT temperature is not likely to increase sufficiently to approach the temperature of the steel in the pressure vessel. However, as the reactor is being cooled down, the temperature of the vessel may drop below the NDT value while the reactor vessel is still pressurized. Brittle fracture might then occur.

One of the areas of the reactor vessel that is of most concern is the beltline region. The Nuclear Regulatory Commission requires that a reactor vessel material surveillance program be conducted (in accordance with ASTM standards) in water-cooled power reactors. Specimens of steel used in the pressure vessel must be placed inside the vessel located near the inside vessel wall in the beltline region, so that the neutron flux received by the specimens approximates that received by the vessel inner surface, and the thermal environment is as close as possible to that of the vessel inner surface. The specimens are withdrawn at prescribed intervals during the reactor lifetime and are subjected to impact tests to determine new NDT temperatures. Figure 5 shows the increase in NDT temperature for a representative group of low carbon steel alloys irradiated at temperatures below 232C. Many current reactors have core pressure vessel wall temperatures in the range of 200C to 290C, so that an increase in NDT is of very real concern.

Irradiation frequently decreases the density of a metal over a certain temperature range, so that a specimen exhibits an increase in volume or swelling. The swelling of stainless steel structural components and fuel rod cladding, resulting from fast neutron irradiation at the temperatures existing in fast reactors, is a matter of great concern in fast reactors. The swelling can cause changes in the dimensions of the coolant channels and also interfere with the free movement of control elements.

Figure 5 Increase in NDT Temperatures of Steels from Irradiation Below 232C

The generally accepted explanation of irradiation-induced swelling is based on the characteristics of interstitial loops and voids or vacancy loops. If the temperature is high enough to permit interstitials and vacancies, but not high enough to allow recombination, a relatively large (supersaturated) concentration of defects can be maintained under irradiation. Under these circumstances, the interstitials tend to agglomerate, or cluster, to form roughly circular twodimensional disks, or platelets, commonly called interstitial loops. A dislocation loop is formed when the collapse (or readjustment) of adjacent atomic planes takes place. On the other hand, vacancies can agglomerate to form two-dimensional vacancy loops, which collapse into dislocation loops, or three-dimensional clusters called voids. This difference in behavior between interstitials and vacancies has an important effect on determining the swelling that many metals suffer as a result of exposure to fast neutrons and other particle radiation over a certain temperature range. When irradiation-induced swelling occurs, it is usually significant only in the temperature range of roughly 0.3 T_m to 0.5 T_m,where T_m is the melting point of the metal in Kelvin degrees.

Swelling can also result from gases produced in materials, such as helium formed by (n,) reactions and other gaseous impurities present in the metals. These traces of gas increase the concentration of voids formed upon exposure to radiation. For example, the (n, ) and (n,2n) reactions between fast neutrons and beryllium form helium and tritium gases that create swelling.

Under certain conditions, embrittlement can be enhanced by the presence of the helium bubbles (helium embrittlement). The accepted view is that this embrittlement is the result of stressinduced growth of helium gas bubbles at the grain boundaries. The bubbles eventually link up and cause intergranular failure.

Fissionable metals suffer from radiation damage in a manner similar to that encountered in structural alloys. Additional problems are introduced by the high energy fission fragments and the heavy gases xenon and krypton, which appear among the fission products. Two fragments that share 167 MeV of kinetic energy, in inverse proportion to their atomic masses, are produced from each fission. Each fragment will have a range of several hundred angstroms as it produces a displacement spike. A core of vacancies is surrounded by a shell of interstitials, producing growth and distortion. Figure 6 shows the growth in a uranium rod upon irradiation.

Figure 6 (a) Growth of Uranium Rod; (b) Uranium Rod Size Dummy

The gas formation produces eventual swelling of the fuel and may place the cladding under considerable pressure as well. One of the major challenges in alloying metallic uranium is the attainment of better stability under irradiation. Small additions of zirconium have shown marked improvement in reducing growth and distortion.

The physical effects of ionizing radiation in metals is a uniform heating of the metal. Ions are produced by the passage of gamma rays or charged particles through the metal, causing sufficient electrical interaction to remove an external (or orbital) electron from the atom. Metals with shared electrons, which are relatively free to wander through the crystal lattice, are effected very little by ionization.

Summary

The important information in this chapter is summarized below.

Effect Due To Neutron Capture Summary

Dislocation of an atom due to emission of radiation

Highly energetic recoil nuclei are produced indirectly by the absorption of a neutron and subsequent emission of a --ray . When the --ray is emitted, the atom recoils due to the reaction of the nucleus to the --ray's momentum (conservation of momentum).