plant turbines. Thermal efficiency of thermal power plants is about 50%, whereas that of NPPs is about 34%. In other words, about 66% of the heat generated in the reactor is thrown out as warm waste water. NPPs, which have very high heat generation density, however have a merit that larger power can be generated per unit plant in comparison with thermal power plants with limited capacity of their boilers.

Fast breeder reactors, which use sodium as coolant, may generate super-critical steam and realize thermal efficiency as high as that of thermal power plants.

2. Safety Design Principles for npPs

As described in Section 1.3, the specific feature of nuclear power generation is use of nuclear fission of uranium and plutonium. On the other hand, FPs (radioactive materials) are accumulated in the reactor during the operation. How to control and manage the potential risks of the radioactive materials is the principal matter for safety assurance at all stages of design, construction, operation and maintenance of NPPs. The safe design principle connoted in the safety design to assure safety by achieving high reliability, taking into consideration human errors and immaturity of human consideration, etc., is “Defense-In-Depth” philosophy. That implies preparation of defense measures assured by depth of consideration to achieve ultimate objectives of safety. The philosophy is embedded not only in system and equipment design, but also basically in all the stages of construction, operation and maintenance, etc. as mentioned above. “Defense-In-Depth” philosophy is implemented by various measures, such as fail-safe systems, prevention of event occurrence in principle and provisions of multiple defense measures. The Defense-In-Depth philosophy originated from a national defense strategy followed in the US. There may be some who think winning or losing a war forms the terminology of national defense; but the principal objective of “Defense-In-Depth” is how to prevent occurrence of a war. The idea resembles the strategy which was adopted by a Japanese shogun in the age of civil wars; he constructed a castle surrounded by moats and a number of fortresses for physical defenses, and then held hostages and

had relationships by marriage with adjacent feudal lords for emotional defenses. One typical and widely used measure of the Defense-In-Depth is provision of multiple layers of defense that is “a series of levels of defenses”. This is also called a philosophy of negation of previous defense measures (ijijTx lT je ), which is a strategy that assumes the measures to prevent occurrence and to prevent expansion of abnormal events fails to function as intended and prepares the next measure. Therefore just to say “Multiple Defenses” for ”Defense-In-Depth” is not necessarily adequate. How the Defense-In-Depth philosophy is used in safety design of NPPs is looked at next*¹

NPP safety should have three levels of safety measures; firstly “prevent occurrence of abnormal event”, assuming abnormal event occurs, “prevent abnormal event from expanding to accident”, and assuming accident resulted “prevent radioactive material from released abnormally to the environment”. The level 2 and 3 require assurance of basic safety functions of “shutdown”, “cooling” and “confinement”. Based on the Defense-In-Depth philosophy in Figure 1.4.1, examples of measures considered in design and operation for each defense level are shown below.

1. Prevention of Occurrence of Abnormal Conditions (Level 1)

To prevent occurrences of accidents, firstly prevention of occurrences of abnormal conditions is important, which could cause accidents, such as failures, damages of individual systems, components. For the purposes, it is necessary to design with sufficient margin and to prevention of failures and damages by elaborate activities of maintenance, inspection. Quality control is important to assure high reliability. Following are implicated measures in this level.

1. Design with safety margin

⁽*¹⁵ Some references define “Defense-In-Depth" and “Multiple Defenses” are synonyms. However, “Multiple Defenses” seems to emphasize provision of countermeasures for each process leading to accidents, but “Defense-In-Depth" used here is a philosophy, which differs from “Multiple Defenses", to prepare diverse measures including application to institutional framework or prevention of human errors in a broad sense.

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NSRA, Japan