Here, some general aspects of radiation effects are summarized.

• Mechanical Properties

a) Hardness and strength increase (radiation hardening) due to increased defects mainly dislocations, precipitates, and so on.

b) Ductility decreases (radiation embrittlement).

c) Strain hardening exponent decreases (i. e., uniform ductility decreases).

d) Ductile-brittle transition temperature increases (radiation embrittlement).

e) Fracture toughness decreases (i. e., upper shelf energy decreases).

f) Creep enhancement occurs (radiation-induced and radiation — enhanced) because of increased defect concentration and diffusivity.

g) Low cycle fatigue life decreases and high cycle fatigue life increases due to embrittlement and hardening, respectively.

• Physical Properties

a) Density decreases, that is, volume increases (radiation swelling) due to the formation of voids, bubbles, and depleted zones.

b) Electrical resistivity increases (or conductivity decreases).

c) Magnetic susceptibility decreases.

d) Thermal conductivity decreases due to the increased defect concentration.

• Corrosion Properties

a) Corrosion is enhanced by radiolytic dissociation of the environment.

Detailed discussion on radiation effects on materials is impossible in a single chapter. To have more detailed information on this vast topic, readers can refer to the excellent texts mentioned in the reference list [11, 24] or numerous papers pub­lished in nuclear materials specific journals.

Problems

6.1 A pressure vessel steel specimens (A533B, low-alloy ferritic steel) of diame­ter 1 mm and grain size 0.2 mm exhibited a yield point with the lower yield stress equal to 520 MPa. The source hardening term was found to be

150 MPa. For this steel, ky decreases with temperature by 2MNm—3/2 for 50 °C increment. Assume that this linear dependence is valid and also the yield stress is temperature insensitive. The surveillance capsule tests revealed that the DBTT increased from —20 to +90 ° C following radiation exposure in an LWR environment.

a) Estimate the increase in friction hardening (s) required for this change.

b) Assuming that all this increase in friction hardening is due to dislocations, determine the dislocation density of the irradiated steel.

c) During radiation exposure in the reactor, the steel specimens were at around 425 °C that resulted in an increase in grain size from 0.2 to 0.4 mm. Determine the yield stress ofthe irradiated steel.

6.2 What are the effects of radiation on the following: (a) dislocation density,

(b) vacancies, (c) diffusion, (d) corrosion, (e) strength (hardness), (f) ductility,

(g) toughness, (h) DBTT, (i) upper shelf energy, (j) strain hardening, (k) creep (low temperature versus high temperature), (l) low and high cycle fatigue,

(m) burnup, (n) density, and (o) thermal conductivity.

6.3 Describe the source and friction hardening in BCC and FCC alloys and the effects of neutron radiation exposure on them.

6.4 Describe the pellet-cladding interaction in LWR fuels and possible solutions to mitigate PCI.

6.5 In a fusion reactor blanket coolant channel, a copper (fcc) coolant tube (exposed to 14 MeV neutron radiation to a fluence of 5 x 1022 n cm—2) exhibited radiation hardening and embrittlement with twice the tensile strength accom­panied by reduced elongation to fracture (by 1/2 of that before irradiation) with no necking. The following properties were reported on the unirradiated copper: Young’s modulus = 30 x 106psi, nominal tensile strength = 50ksi, fracture strain = 35%, and uniform strain = 25%.

a) Estimate the change in the toughness of the material following radiation exposure.

This material is known to follow the universal slopes equation relating the fatigue life to the applied strain: De = (Sf/E)(Nf)—0,12 + e—0 6, where Sf is the fracture strength, ef is the fracture strain, and Nf is the number of cycles to fatigue failure.

b) What are the effects of radiation exposure on fatigue life in LCF (Nf <

50 000) and HCF (Nf> 106)? Does neutron irradiation always decrease fatigue life (explain your answers)?

c) Calculate the endurance limits before and after radiation exposure?

d) The radiation exposure resulted in 0.15 dpa. If one vacancy survived per million atomic displacements due to recombination and so on, calculate the density of Frenkel defects following radiation exposure? What is the probability (%) that a unit cell contains a vacant lattice site?

e) How does radiation exposure influence self-diffusion?

f) Show the effect on an Arrhenius plot indicating the relevant parameters.

6.6 Both zircaloys and stainless steels exhibit dimensional changes in-service (in-reactor) known as radiation growth and radiation swelling, respectively.

Describe these two phenomena with emphasis on distinctions between them.

6.7 A copper specimen is exposed to 2 MeV monoenergetic neutrons of flux, ф of 2 x 1016 n cm-2 s-1 for 10 h.

a) Determine the number of atomic displacements per atom (dpa).

b) If PKAs were produced with 80keV, calculate the number of atoms dis­placed by a PKA (assume Kinchin-Pease model)

c) The steady-state creep rate (e) of Cu follows a power law: e — ADlS3, with A — 5 x 10-7 (e in h-1, D in cm2 s-1, and s in MPa). If the creep rupture life of the unirradiated Cu at 400 °C at 20MPa is 100 days, estimate the rupture life of the irradiated material at 300 °C under the same stress (assume that the same equation is valid for the irradiated material also).

1 Nuclear Energy, 48, 325-359.