Materials for nuclear RPVs

Materials for nuclear RPVs developed to meet the advances in RPV technology and attain the safety and reliability are discussed in this section. The designation of the materials has been standardized in the pressure vessel codes and regulations of many countries. The evolution of the ASME Code is described as a typical example. 

As mentioned earlier, the first LWR in Japan was the JAERIJPDR which started operating in 1963. The reactor is of the plate construction type and the material was SA302B modified. The first commercial nuclear power plant was JAPCO s Tokai 1 which went into operation in 1966. The RPV was the Calder HaU-type made of JIS SB46 modified (Coltuf 26 equivalent) plate steel. After that, JAPCO constructed the Tsuruga 1 BWR plant in 1965. The Tokyo Electric Power Company (TEPCO) and Kansai Electric Power Company (KEPCO) also decided to construct the Fukushima 1 

BWR plant and Mihama 1 PWR plant. Next, the BWR, ABWR and PWR plants were constructed and operated in Japan. 

ASME Code Section III at first used the fracture analysis diagram (FAD) for the prevention of brittle fracture. Linear elastic fracture mechanics was introduced in 1972 Summer Addenda, Appendix G. ASME Code Section XI Appendix A and NRC Federal Register lOCFR Part 50 were issued in 1973. In these codes and regulations, RTndt was introduced as an important index temperature to characterize the transition curve of fracture toughness. 

In addition, the codes specify that the toughness degradation of RPV steels during service due to neutron irradiation embrittlement must be considered. In 1975, NRC issued Regulatory Guide 1.99 in which the prediction procedure of toughness degradation based on neutron fluence and P and Cu content of materials was introduced. In 1984, the ASME Code decreased the permissible content of P, Cu and S contents in the materials. 

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RPV shell plate material specifications have evolved since the beginning of the commercial nuclear power industry in the mid-1950s. Due to safety concerns for nuclear RPVs, cautious steel-making was paramount, as well as careful RPV fabrication and welding practices. Nevertheless, the need for familiarity in steel-making and fabrication processes initially led to the selection and use of old steels typical of prior (non-nuclear) pressure vessel service. Table 1.1 lists the principal steel plate materials used in construction of nuclear RPV components. All of the plate materials used... 

In Japan, the Japan Electric Association Code, JEAC 4201, Method of Surveillance Tests for Structural Materials of Nuclear Reactors (JEAC 4201, 2007), specifies the design for a surveillance program to monitor radiation-induced changes in mechanical properties of beltline materials in light-water moderated nuclear power reactor vessels testing and evaluation of the test results. The JEAC 4201 was developed based on the ASTM E 185 for nuclear power RPVs for which the predicted maximum neutron fluence at the end of the operating period (usually 32 EFPY unless otherwise identified) exceeds 1 x 10 n/cm" E > IMeV) at the inside surface of the reactor vessels. 

Many of the early designed RPVs were made from carbon mild steels, usually in the normalized and tempered condition. The A212 plate material was only used in very early nuclear plants, which are now decommissioned. A more widely used material, SA-302, Grade B, is a Mn-Mo steel for plate materials. This type of steel was used in the quenched and tempered condition for a number of RPVs fabricated through the mid-1960s. 

A. Amyev, A. Krynkov, M. Sokolov, Recovery of Transition Temperature of VVER RPV by Annealing, Steele L. E. (ed.). Radiation Embrittlement of Nuclear Reactor Pressure Vessel Steels An International Review (Fourth Volume), ASTM STP1170, American Society for Testing and Materials, Philadelphia, PA, pp. 369-379,1993. 

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