PIK High-Flux Reactor

   By the early 1960s, it had already become clear that future progress in neutron research after 20 years required higher neutron fluxes. A scheme was drafted for a high-power high-flux research reactor denoted by PIK (this stands for beam research). The core had a volume of 50 liters and was cooled by light water under pressure, and had a light-water central trap surrounded by a heavy-water reflector of thickness 1 m and height 2 m. The parameters of the trap, core, and геfleсtor were optimized in accordance with the principle for minimizing reactor costs. Calculations for cores with volumes of some tens of liters showed that heavy water rather than beryllium provided for the best ratio оf thermoneutron flux density to power. As the thermal-neutron diffusion length is large (about 1 m), that flux is quite high far from the core, where the background from fast neutrons and rays is particularly low. With a power of 100 MW in the heavy-water reflector, the flux exceeds 10¹⁵ n s^-1 cm^-2, or 410¹⁵ n s^-1 cm^-2 in the central light-water trap. The heavy-water reflector made it possible to replace the experimental channels after the reactor had been commissioned. The accumulating tritium and protium were eliminated by isotopic fractionation. The interchangeable bodies enabled one to vary the core parameters widely. The coolant and moderator (light water) has a short neutron moderation length, so the core is compact. The intermediate cooling circuits protect the third circuits in contact with the atmosphere from the leakage of radionuclides. The PIK has also a full-scale physical model, on which all the neutron-physics parameters were checked by experiment.

   The first calculations on the reactor were performed by Petrov and Erykalov in 1963-1966, which were accompanied by studies on the engineering aspects directed by Konoplev. The first publications on the new reactor [1] appeared at the same time as the French ones on the ILL high-flux reactor in Grenoble, but construction was begun only in 1976. The delay in construction allowed not only careful calculations using the latest Monte-Carlo methods but also a comparison with benchmark experiments on annular cores to be made. Use was made of the latest nuclear data libraries, which involved no approximations as regards the geometry or physical processes, and this gave agreement between the calculated reactivity and the experimental value better than 0.2%. After 30 years of detailed design development and refined calculations supported by measurements on a full-scale copy or physical model (commissioned in 1981), the original estimates of the basic parameters for the PIK have not been altered [1]:

   It is planned to install 20-25 equipment [2] for research on elementaryparticle physics, nuclear physics, and applications based on nuclearphysics methods; these will be located mainly in the main hall of the reactor and in the inclined-beam hall, and partially also in the neutron guide hall. Much space will be given to fundamental research in physics of elementary particles and nuclear physics. To extend these researches, it is planned to set up a universal source of cold and ultracold neutrons in the GEK-4 horizontal channel. That source should greatly improve the experimental facilities compared to the analogus ones in the WWR -M. It is planned to conduct a very important experiment on the neutron electric dipole moment, as well as experiments involving precision measurements on the -decay of the neutron: lifetime and correlation constants. The purpose of the precision measurements on the neutron -decay is to check the standard model at a new level of accuracy and thus detect possible deviations. Much space will also be given to researches on weak nucleon-nucleon interactions by use of an intense beam of polarized cold neutrons. It is planned to perform experiments to detect the violation of T parity in the neutron -decay, and also for polarized and oriented nuclides. Preparations are also being made for neutron-optical and neutron-interferometric methods.

   About 25 installations are planned for the PIK for research in condensed-state physics [3]. They will be located partially in the main hall on the horizontal thermal and hot neutron beams, but mostly they will be in the neutron-guide hall, which has four thermal-spectrum neutron guides and four guides from the cold neutron source. The solid-state instruments include powder and single-crystal diffractometers, three-axis inelastic-scattering spectrometers, time-of-flight spectrometers, and instruments having no analogs on other reactors in Russia or abroad, or which exist as unique instruments. These instruments include a back-scattering spectrometer, a Sphinx diffractometer (d/d ~10^-3 - 510^-4), a modified spin-echo spectrometer involving modulation of the neutron spin phase precession spectrum in a magnetic field, a high-luminosity diffractometer for researching magnetic correlation tensors and involving the analysis of polarization in the region of small scattering angles, which involves simultaneous measurement of diffraction at large angles, а diffractometer employing high-intensity -ray sources and with angular resolution better than one second of arc, a neutron interferometer based on diffraction gratings for the long-wave region of the neutron spectrum, and so on. All these installations are equipped with the latest readout and control facilities for the angular and linear displacements, along with reliable computer support, and equipment for small-angle scattering: two-coordinate position-sensitive neutron detectors.

   Decisive contributions have been made to realizing the PIK project by the team at the Institute, which at various stages have been headed by B. P. Konstantinov, D. M. Kaminker, O.I. Sumbaev, A. A. Vorob'ev, A. A. Ansel'm, and V. A. Nazarenko. There has been active participation from institutes and other organizations in the Ministry of the Atomic Industry under the scientific direction of the St. Petersburg Nuclear Physics Institute: the main designer for the PIK has been NIKIET, with the general design responsibility of VNIPIET, NIKIMT and so on. The PIK facilities have been planned as the national center for neutron research, and requests to participate in its work have been received from leading world neutron laboratories [4].

   Although the reactor has been under construction for more than 20 years, its essential scheme is a compact light-water core surrounded by a heavy-water reflector, which was proposed at the beginning of the 1960s and still remains very up-to-date. The first reactor of that type, the Orpheus at Saclei, went critical in 1981; in Germany, that scheme is being used in the FRM-II reactor under construction in Munich. The parts of the core can be exchanged, and in future the PIK fuel pins can be replaced by aluminum ones (of WWR -M type), and the steel body can be replaced by aluminum, after which the neutron fluxes in the reflector will be increased by at least a factor of 1.5. In 1999 the PIK suite will be 90 % completed, with 75% of the equipment installed. It is hoped to verify that the theorem "it is always 5 years to PIK commissioning" will finally be violated, and that the reactor will be commissioned at the start of the next millennium.

REFERENCES

1. A. N. Erykalov, D. M. Kaminker, K. A. Konoplev and Yu. V. Petrov, "The PlK reactor for physics research." in: The Physics of Nuclear Reactors. Vol. 3 [in Russian], Melekess (1966), pp. 273-280; "Choice of the basic parameters for the PIK physics research reactor," Preprint FTI-153 (1968).
2. M. S. Onegin and Yu. V. Petrov, "Calculations on PIK critical assemblies. Part 1," Preprint LIYaF-2169 (1997).
3. A. N. Erykalov, O. A. Kolesnichenko, K. A. Konoplev, et al., "The PIK reactor," Preprint PI YaF-1784 (1992).
4. A. P. Serebrov, "High flux reactor PIK and the associated research program," Nucl. Instrum. Meth. Res., A284, 212-215(1989).
5. A. I. Okorokov, "Research program at LNPI high-flux reactor PIK," Physica B, 174, 443-450 (1991).
6. V. A. Nazarenko, "The PIK high-flux research reactor: Status and prospects," Preprint PIYaF (1995).