Radiation embrittlement of western PWR pressure vessels

The degree of embrittlement and hardening induced in ferritic steels after exposure to fast neutron radiation is an issue of the utmost importance in the design and operation of NPPs.

The area of the RPV surrounding the core (called the beltline region) is the most critical region of the primary pressure boundary system because it is subjected to significant fast neutron bombardment. The overall effect of fast neutron exposure is that ferritic steels experience an increase in hardness and tensile properties and a decrease in ductility and toughness, under certain conditions of radiation.

For example:

(1) Effect of neutron fluence on radiation hardening and embrittlement has been reported to be significant at fluences above 10²² n/m² (E >1 MeV). Unless a steady state or saturation condition is reached, an increase in neutron fluence results in an increase in RTNDT, yield strength and hardness, and a decrease in the Charpy toughness, also in the upper shelf

temperature region. There are significant variations in the fluence and radiation damage around the circumference and in the longitudinal direction of RPVs.

(2) Alloy composition (especially when consideration is given to impurity copper and phosphorus and alloying element nickel) is known to have a strong effect on radiation sensitivity. Data have been generated on both commercial and model alloys to show the effects of alloy composition.

(3) Radiation temperature has long been recognized to have an effect on the extent of the radiation damage. Data from the early 1960s demonstrated that the maximum embrittlement occurred during radiation at temperatures below 120°C (250°F). Recent studies have reported a decrease in radiation embrittlement at higher temperatures (>310°C), which is attributed to the dynamic in-situ "annealing" of the damage.

(4) Microstructural characteristics, such as grain size and metallurgical phases (lower or upper bainite, ferrite), can influence the severity of radiation damage associated with a given fluence.

(5) The neutron flux energy spectrum contributions to the embrittlement behaviour of ferritic steels are secondary effects. However, recent reactor experience has suggested that, under certain conditions, the flux spectrum may influence the degree of radiation embrittlement caused in ferritic steels.

The most important parameters listed above are fluence and alloy and impurity content. The deleterious effect of copper (Cu) as an impurity element on radiation embrittlement and hardening of pressure vessel steels and welds was recognized nearly 20 years ago. The dramatic increase in ductile-to-brittle transition temperature and reduction in upper shelf energy (USE) observed in a variety of pressure vessel welds after neutron radiation at ~288°C was broadly correlated with the nominal impurity Cu content in the steels. The increased sensitivity to embrittlement was more pronounced for welds because Cu-coated welding rods had frequently been used in the fabrication of the reactor vessels leading to Cu levels of ~0.3 weight %. For an equivalent copper level, the cast structure of weld metals is more sensitive to neutron radiation damage than the base metal.

Early methods used to quantify the effect of impurity elements on radiation sensitivity in western RPV materials indicated that both copper and phosphorus played a role [47, 48].

Later on, USNRC Regulatory Guide 1.99, Revision 2 omitted the effect of phosphorus but included nickel as a factor [49]. As a result, it is often assumed that copper and nickel play the dominant role in creating sensitivity to neutron radiation in low phosphorus steels. However, there are variations in alloying content and impurity element ranges in the various countries in which the RPV materials were produced and it is still necessary to consider the contribution of phosphorus, particularly when low levels of copper and nickel are present.

The radiation embrittling mechanism attributed to copper impurity level is well understood in terms of small copper-rich clusters or precipitates formed under the creation of minute matrix damage caused by fast neutron bombardment. Such precipitates can act as blocks to dislocation movement and cause hardening and embrittlement. Hawthorne and coworkers [50]

examined the action of Cu and P in a variety of A533B and A302B steels. Phosphorus contents greater than 0.014 weight% exerted a strong effect on the sensitivity of A302B steels to radiation embrittlement.

Unlike Cu and P, the role of nickel (Ni) in radiation hardening/embrittlement has been unclear.

The contradictory reports concerning the influence of Ni in the embrittlement behaviour of RPV steels indicated that its effect was a subtle one. The effect of Ni can be demonstrated qualitatively by studying the HY and A350LF steels (~3 weight % Ni). Although studies by Lucas et al. [51] and Igata et al. [52] showed no effect of Ni on radiation embrittlement of RPV steels, several other studies show a significant effect. In 1981, Guionnet et al. [53]

concluded that Ni was deleterious to the behaviour of A508 irradiated to a fluence of 5 × 10²³ n/m² at 290°C. A pronounced Ni effect in increasing the radiation sensitivity of high Ni (0.7 weight %) welds was reported by Hawthorne [54]. Similarly, Fisher and Buswell [55] noted that high Ni steels (i.e., those containing >1% Ni) were much more sensitive to neutron radiation than steels containing <0.85% Ni. Soviet experience with chromium (Cr) and Ni bearing RPV steels also indicated that Ni exerted a pronounced effect on embrittlement behaviour [56].

Odette and Lucas [57] examined the effect of Ni (0 to 1.7 weight %) on the hardening behaviour of A 533-B type steels as a function of neutron fluence, flux, temperature and manganese and copper content. Irradiations at fluxes of 5 × 10¹⁵ and 5 × 10¹⁶ n/m² / s gave final fluences of 9 × 10²² to 1.5 × 10²³ n/m² (E>1 MeV). Low fluence irradiations were done at 306°C and 326°C; the higher fluences were accumulated at 271°C to 288°C. The results indicated that Ni increased the sensitivity to radiation embrittlement in these materials, with increasing fluence, lower flux levels, lower irradiation temperature and increased manganese (Mn) levels causing more damage. The synergisms and complex nature of the response of the alloys examined makes a complete interpretation of the mechanisms difficult.

Although the roles of Cu, P and Ni as promoters of radiation hardening and embrittlement are well-recognized, the contribution of other elements such as manganese (Mn), molybdenum (Mo), Cr, arsenic (As) and tin (Sn), to the radiation induced behaviour of RPV steels has not been unambiguously identified.

Significance for western PWR pressure vessels

A fluence value of 1 × 10²² n/m² (E >1 MeV) is approximately the threshold for neutron induced embrittlement of the ferritic steels used in western PWRs. Therefore, the beltline region is the region most likely to undergo significant changes in mechanical properties due to neutron radiation. Components made of materials such as Alloy 600 or Alloy 182 are less susceptible to neutron embrittlement. The following components are subjected to lifetime fluences less than 1 × 10²² n/m² (E >1 MeV) or are made of materials not susceptible to

Therefore, neutron embrittlement is potentially significant only for that part of the RPV shell beltline region which is located in a high flux region.