PNPI experience with the development of cold and ultracold neutron sources at research reactors

The Petersburg Nuclear Physics Institute (PNPI) is located in Gatchina, the Russian Federation, and operates an 18 MW WWR-M research reactor. The WWR-M was commissioned in December 1959 and formed one of the ten best swimming pool reactors of the time. The successful design of the fuel assemblies and reflector was used in other research reactors (in the Russian Federation at Kiev, Obninsk, Alma-Ata; and also in Germany, Hungary, Poland and Vietnam). By the start of the 1970s, considerable experience had been accumulated in operating the WWR-M1 and WWR-M2 fuel assemblies. Comprehensive research had been done on their heat engineering and corrosion features, and the sealing level had been determined. The result of investigations has enabled the design of the next generation WWR-M5 fuel assemblies, which had half the wall thickness (1.25 mm) and double the fuel concentration: 125 g 235U/L. The surface per unit volume in the core attained the record value of 6.6 cm2/cm3, which provided a maximum specific power of 0.9 ± 0.1 MW/L in the pool reactor. The reactor was completely converted to WWR-M5 fuel assemblies in 1980. The power was raised to 18 MW. The assemblies contained more 235U (90% enrichment), which resulted in freeing more than half of the space in the core for experimental equipment. In particular, a source for cold neutrons and ultracold neutrons was located at the centre of the core, whose output was greater than that of analogous sources in the most powerful ILL research reactor at Grenoble, France. Since the middle of the 1970s, cold neutron and ultracold neutron generation has been in progress there. As intended, this programme has increased substantially the experimental capabilities of the research reactor as an instrument for fundamental physics research. It has allowed progress to be made in the investigations which require extremely high neutron beam intensity, such as the search for neutron electric dipole moment, neutron lifetime and correlation constant measurements, the search for P-odd effects in interactions with cold neutrons and so on. The layout of the WWR-M research reactor with cold neutron sources (CNSs) and ultracold neutron sources (UCNSs) located at different positions is shown