Evaluating the operational dependability of a pulse research reactor
Authors: Boykova T. V. , Kochnov Yu. O. , Petrunin N. V. , Tutnov I. A. 
Published: August 2023

Aim. For the purpose of substantiating the safety of further operation of the IIN-1 pulsed solution research reactor beyond the design service life, its dependability has been evaluated for the future operation period. The paper aims to describe the criteria and show an example of IIN-1 vessel dependability evaluation.

Method. At the stage of IIN-1 design, no dependability criteria were defined, therefore, based on the NP-009-17 general norms of research nuclear reactor safety, an original dependability criterion, the vessel seal, has been chosen. A reactor vessel that, over the period of operation, is subject to cyclic thermomecanical and radiation loads at the moments of nuclear fuel fission pulse, corrosion damage at the moments of solution heating, dynamic forces of chemical microexplosion of the explosive mix during physical experiments, is a hazardous structural element of IIN-1 that is exposed to the highest loads in terms of emergency tolerance and a priority contributor to the overall nuclear and radiation safety of the research installation. The IIN-1 vessel seal and its general dependability define the efficiency of its safety barriers for the environment and personnel over the long operation of the research installation. IIN-1 vessel dependability is evaluated using experimental methods of non-destructive testing aimed at verifying the seal and state of the metal exposed to corroding media, such as metallographical observation of surveillance specimens, mechanical testing, etc. The strength and plastic properties of the vessel’s metal were tested by static tension.

Results. The ultimate stress limit, yield strength, impact resistance and percent elongation of the vessel’s material under temporal degradation of its service properties in the course of life ageing have been defined. Based on the results of specimen tests, tables have been drawn up and conclusions have been made regarding the emergency tolerance of the reactor vessel for the future operation period of IIN-1. Metallographic research in terms of the tendency to intercrystalline corrosion were conducted using the AM method according to GOST 6032-58. The key factors have been defined that affect the ageing of the vessel material of a pulsed reactor: fast neutron flux and their integral values in the reactor vessel’s most vulnerable elements and formation of explosive mix (consisting of hydrogen and oxygen) that causes immediate boiling of the fuel and, subsequently, significant cyclic stress in the vessel’s material that can cause permanent deformation. They can eventually disrupt the vessel seal and destroy the reactor. The dependability of the vessel of such nuclear reactor is evaluated through recurrent in-service inspections of the degradation of the metal’s properties, including in terms of corrosion resistance and mechanical strength by examining surveillance specimens. The paper describes the surveillance specimens and the procedure of their examination.

Conclusion. The approach suggested in the paper enables predictive assessment of the operational dependability of a solution nuclear reactor in the course of a long operation. The authors suggest key criteria for evaluating the characteristics of a vessel’s safety and dependability state that allow accurately defining the safe life of a research nuclear reactor and verifying the extendibility of its design life.