An example of a logical approach to standardization of in-service inspection requirements is introduced, analyzing each measure according to risk reduction factors in order to develop a systematic configuration of safety measures. Comparison of a heavy pressure vessel section fabricated to resist pressure from the many nozzles in a light water reactor vessel with a thin-walled reactor vessel in a liquid-metal fast breeder reactor (LMFBR) operating at high temperatures and low pressure indicated no apparent difference in the failure probability of the reactor vessel. If both large failures and cracks are considered, 10−5/vessel⋅yr is to be assumed. In assessing the unreliability of the guard vessel, dominant factors are common cause failures and subordinate modes. The coupling factor of the common cause failure of the guard vessel with reactor vessel failure is assumed 10−3. The subordinate mode failure of the guard vessel when the sodium leak from the reactor vessel is left unattended should be considered in the rate of 10−1/reactor vessel failure. Especially in LMFBRs, visual tests are more practicable than volumetric tests for primary sodium boundaries, and the risk reduction factor by periodic in-service inspections is limited to only 5%. It is an advantage of LMFBRs that a primary coolant leak from a boundary defect can be so easily detected. Sodium leak monitors have been successfully developed. Philosophically, “integrity surveillance during operation” is far better than “in-service inspection,” not only for quality assurance and safety but also for improving plant availability. These instrumentation circuits can be designed and maintained at a level of unreliability of <10−1. Another fundamental advantage of LMFBRs is that thin wall boundaries with low pressure allow “leak before break,” which is far less thick than critical crack length. The safety of LMFBR vessel functions is adequately assured by a leak monitor and guard vessel without having periodic in-service inspections of the reactor vessel. Generally, quality assurance of plant facilities is achieved as follows: (a) Quality control of active components is maintained by in-service tests and inspections as well as preservice tests and inspections; (b) quality control of very passive components, such as buildings and their foundations, is maintained by infabrication and preservice inspections, because failures of these very passive components mostly originate from errors in fabrication. The roof slab of a pool-type LMFBR is also considered to be a very passive component whose quality is fundamentally assured by in-fabrication and preservice inspections.