XA04C1623 THE STATUS OF THE PIK REACTOR Yu. V. Petrov ACADEMY OF SCIENCES OF RUSSIA PETERSBURG NUCLEAR PHYSICS INSTITUTE on leave at: Technische Universitit Miinchen, Physik Department E18 D8046 Garching, Germany May 18,1992 Abstract This report describes the 100 MW research reactor PIK which is now under construction. The thermal neutron flux in the heavy water reflector exceeds 1015 CM-2S-1 ; in the light water trap, it is about 4 - 015 cm -23-1 . The replaceable core vessel allows to vary the parameters of the core over a wide range. The reactor provides sour- ces of hot, cold and ultracold eutrons for 10 horizontal, 6 inclined neutron beams, and neutron guides. At the ends of the beam tubes, the neutron flux is 101 _ loll -1-28-1. The flux of the long wave neutrons exceeds 109 M-2S-1 . To ensure precise measurements, the experimental hall is protected against vibrations. The poject meets all modern safety requirements. The calculated parameters of the reactor were verified using a full-scale mock-up. Seventy percent of the reactor construction ad installation were completed i te be- ginning of 1992. Version of the preprint: A.N.Erykalov, O.A.Kolesnicbenko, K.A.Konoplev, V.A.-Nazarenko, Yu.V.Petrov, S.L.Smolsky: PIK Reactor, PNPI-1784, St. Petersburg 1992 1 I The airns of the reactor 45 k7n south fom St. Petersburg and 4 m from te town of Gatchina, the high flux research reactor PIK [1 - 4 is being constructed. Fig.1 shows a drawing of the reactor building, and Fg.2 shows the building site in au- tumn 1991. The reactor is designed for a broad range of research in nuclear and solid state physics, for studying the basic properties of matter, e.g.. of newly developed materials, including high-temperature superconductors, for radiobiological research and also for solving many applied technical po- blerns. The envisaged hh flux of thermal., cold and hot neutrons permits to plan the investigation of basic neutron characteristics such as the electric and magnetic dipole moments, charge, life time and to study the fundamen- tal neutrons interactions, e.g., strong interactions in neutron collisions, and weak interactions after neutron capture. The reactor allows to develop independent sources of neutrinos and an- tineutrinos (- 10" Is) with known spectra: this can contribute to the development of neutrino physics. The high flux of thermal neutrons and the low background of fast neutrons and 7-quanta permit to continue the traditional research in nuclear physics, including and spectroscopy and various experiments with polarized neutrons and targets. Solid state physics will be represented by studies of the kinetics of non- systems, by neutronographic research on hgh-temperature su- perconductors, ceramics and new materials, and also y structural research on magnetic materials etc. The research in biology will include neutron and structural analysis of biological objects, studies of membranes etc. A more detailed description of the experimental program is gven in ref. [5]. The experimental program will be carried out in co-operation with the leading scientific research centers in Russia and abroad. The project of the reactor supervised by B.P.Konstantinov. Petersburg Nuclear Physics Institute of the Academy of Sciences of Russia, is being realized bv the Scientific Research Institute for Power Reactor Design and by other organizations of the State Committee for Atomic Energy. 2 2 Reactor design and neutron parameters The design and parameters of the PIK reactor were chosen so as to provide the maximum number and minimum cost of events in the experimental detectors 6,7]. The actual technological and heat transfer limitations were observed. The light water core with a volume of about 50 is placed in a heavy water reflector and serves as an intense source of fast neutrons with a power of 100 MW (see Figs. 3 - 5). The hea-vy water reflector n which the fast neutrons are slowed down ives the best ratio of thermal neutron flux to power as compared to other moderators [1]. Due to the large dffusion length in D20(LDO I at 02% of H20) and to the considerable dimensions of the heavy water tank diameter 2.5m; height from 25 to 2.0m), the thermal flux is rather high at large distances from the core where the background of fast neutrons as well as that of / quanta is small (Fig.6). A reflector of this type makes it possible to displace and to replace experimental channels either before or after the reactor starting-up. This reflector is safe against radiation darnaaes. Accumulated tritium and hydrogen are removed by a special isotopic purification circuit, and therefore the tritium activity in the D20 does not exceed 01 TBq11 [8]. The reflector has its own MW heavy water cooling circuit which allows to maintain its temperature within the range from 50 to 601C. Light water is used as a cheap coolant in the core of the reactor. Light water as in-core moderator provides a small neutron migration length which permiits to design a compact core. The core with high pressure (up to MPa) and high energy release (about MW11 on the average) is separated by a double core vessel from the reflector where the thermal neutron flux is formed and where the pressure is low 0-3 Wa). Precautions are taken in case of damage of experimental channels. Thin membranes through which the neutrons can easily pass are installed at the output of te heavy water tank to prevent the penetration of radioactivity into the experimental hall. A water pool of 12 -M depth protects of the staff from possible damages of the circuits. To prevent the contamination of the hall, ventilation is provided above the water surface. The vertical cylindrical core vessel serves as internal wall of the reflector tank and is connected to the water supply tubes that it can be replaced without affecting the tank itself. Owing to these measures, the PIK will 3 be a versatile unit permitting to change the arrangement and dimensions of the core even after the reactor is put in operation. Every to years when te core vessel is replaced because of the radiation dairiages, it is possible f necessary) to change the type of the fuel elements or even to install special experimental facilities inside the core. In the beginning of the PIK exploitation, the core vessel will be made of austenitic steel which has sufficient viscosity to prevent the vessel from cracking. Later on, this vessel w'II be replaced by an aluminum or zconium vessel. In order to ensure the high -flux of neutrons in the reflector and the light water trap, the fuel elements of the reactor should provide a high neutron multiplication factor . The fuel elements should also ensure a high spe- 'fic power to obtain a hgh absolute neutron flux. Tese requirements are met oing to the high density of fuel 90% enriched 115U with average den- sity 40 g1l) and owing to the increase of the specific heat transfer surface (6.5 m,/cm'). The fuel elements of the PIK reactor are twisted rods with crosslike section and an external diameter of 5.15 mm (see Fig. 5a). The twisting around the axis with a thread-spacing of 300 mm ensures a fixed distance between the fuel elements inside the bundle. The steel cladding of the fuel element is 0.15 'mm, the fuel loading is 714 ... U, and the meat density is - 22 `Ulcm'. The fuel elements are placed into a triangular lattice with a spacing of 523 mm iside the fuel assembly. The core is for- med by 12 hexagonal and 6 square fuel cassettes (Fig.5). The hexagonal fuel cassette contains 241 fuel elements, the square cassette contains 61 elements. The fuel elements were tested at 75 MW/1 core power density (see also 9 The neutron parameters of the reactor with the above fuel elements are shown in Fg.6 and are tabulated in Table . For longer terms it is planned to use aluminum fuel elements providing a higher neutron flux [10]. The control and safety system consists of a central control unit and rods in the reflector. The central unit (so-called "shutter") is made of two 'de rngs embracing the central light water trap and absorbing neutrons. In order to aoid asymmetrical distortion of the neutron flux. the rings are simultaneously moved apart from the central reactor plane. This system will compensate the burn-up and provide automatic control and emergency protection. Both halves of the central control unit normally have a clearance permitting quick emergency input of not less than 05 Of f 0.35 s. The absorber rods a-re realized as rectangular cassettes located in 4 the heavv water reflector ad containing europium oxide. Some of the rods serve as safety rods, the others are used as starting absorbents. The lateral shielding of the reactor is divided into biological and "expe- rimental' shielding. The bological shielding consists of iron, water (0.5 m), and heavv concrete 0-9 ro., 3.6 g/c7773). It diminishes the radiation to a level permitting attendance of the equipn-ient when te reactor is stopped. The experimental shielding I.Orn thick is part of the physical instruments and consists of movable units. The shielding reduces the reactor radiation to 14 vlh, i.e., half of the limit permitted b te existing standards. The reflector tank is located 9 m deep in the pool. The pool com- municates with the operating hall situated on the top floor of the reactor building. The experimental hall, into wich the neutron eams are guided, is separated from the operating hall (Fig.7).