At present NRC «Kurchatov Institution» – PNPI has developed the project of a source of ultracold neutrons (UCN) at WWR-М reactor . The technique of obtaining ultracold neutrons employing super fluid He appears to be very promising. It is based on the effect of accumulating ultracold neutrons in super fluid He, due to some peculiar features of this quantum liquid. . This project is supposed to be a successful stage in the program on preserving and further progress of fundamental and applied investigations based on research reactors in Russia. The source will make use of superfluid He, thus making it possible to reach UCN density equal to 10 –10 n/cm , which has not yet been achieved anywhere in the world. Reaching such UCN production level is expected to become an essential breakthrough in the progress of fundamental and applied scientific research.
A number of successful experiments have been already performed on the beams of cold neutrons in Japan and France. The principle of obtaining UCN on super fluid He has been already confirmed experimentally, hence, the question arises on practical implementation of this effect and engineering solution of this task. Nowadays, UCN sources on superfluid He are primarily elaborated on output neutron beams in scientific centers of Europe, America, Japan and Canada. WWR-М reactor has a unique opportunity to considerably enhance UCN source performance, by placing a source chamber into a thermal reactor column at the distance of 30 cm from the active zone. Such location of a source chamber will provide increasing UCN flux density by 2 orders of magnitude in experimental installations, in comparison with sources at the outlet beams. However, at such a location of UCN source, one must solve a complicated task of taking away 30 W from superfluid He at temperature of 1.2 К. It was successfully done at a full scale model of UCN source .
Helium efficiently converts neutrons with wavelength of 9 Å into ultracold neutrons, however, those cold neutrons, which passed through helium without converting into ultracold state, will be released as beams of cold and very cold neutrons. For the source to operate effectively in producing ultracold neutrons, it must be placed into an intensive flow of neutrons with wavelength of 9 Å. Hence, an additional chamber with premoderator should be put into the fore part of UCN source.
Besides, a pre-moderator is a productive source of cold neutrons in wavelengths range over 4 Å, which will be emitted out of the thermal column and effectively used. As a working medium for a pre- moderator liquid deuterium at temperature of 20 К, traditionally applied for cold and ultracold neutrons sources, has been selected. A detailed analysis for choosing a pre-moderator for UCN source is given in the article .
2. UCN source neutron guide system layout at WWR-M reactor
UCN source is situated in the northern part of the main hall of WWR-M reactor complex. Fig.1 shows a location scheme of experimental installations for work with UCN source.
Application of a new UCN source is supposed to enhance precision in measurements of neutron EDM by two orders(1) and verify assumptions of supersymmetry theories, which considered to be versions of extension of the Standard Model. Within the framework of these theories, neutron EDM is predicted at a level available for experimental observation. At the same time, supersymmetric theories predict the Universe baryon asymmetry at the observed level, which points to possible validity of the proposed theoretical approach.
Two installations for measuring neutron lifetime are presented in addition to the installation for measuring neutron EDM: one - with a magnetic trap (2) and the other one - with a big gravitational trap (4). Neutron lifetime precise measurements are of significance for testing the model of Universe formation at its primordial stage, as well as for search of deviations from the Standard Model. Moreover, an installation for search of mirror dark matter (n–n') (3) is presented. All these installations have been developed and produced in PNPI, and now they are tested on UCN beams in ILL. They will be transported to a new UCN source in PNPI. Increase in UCN intensity by over two orders of magnitude will enable conducting principally new researches. Finally, for UCN sources of high intensity, one can discuss conducting of an experiment on search for neutron-antineutron oscillations (n-ñ) in order to verify baryon number violation, which is the second condition for Universe origin, according to A.D. Sakharov theorem. Thus, besides the experiment on search for neutron EDM, which is of great significance, there arise possibilities for conducting a series of experiments on physics of fundamental interactions.
The research program on beams of cold neutrons is supposed to be performed on five experimental stations. Four of them have been already constructed: (6), polarimeter (7), powder diffractometer (8) and spin-echo spectrometer (9). Using of the reserved VCN beam (CN3) is also planned for performing further experiments.
Most part of the northern side of the main hall is used for UCN source cryogenic equipment and UCN experimental stations. Thus, CN and VCN installations will be located outside the main hall of the WWR-M reactor complex. The floor level in these premises is higher than that in the main hall by 750 mm. At the height of cold neutron beam of 1000 mm, relative to the main hall floor level, it is necessary to raise the beam by 200 mm at the end point for installing a spin-echo spectrometer, a polarimeter and a reflectometer. Besides all other things, the beam, directed at a spin-echo spectrometer, must bypass a technological aperture for getting into hot chambers. For this purpose, a neutron guide for spin-echo should be deflected by at least 17 degrees relatively to the original beam.
In designing of neutron guides, it is necessary to cut off the total spectrum of gamma quanta and rapid neutrons, going directly from the reactor, by deflecting neutron guides at some angle.
The problem of great importance is concerned with considerable radiation background produced by neutron guides in premises of the main hall of WWR-M reactor. Therefore, all the neutron guides should be provided with sufficient biological shielding for creating favorable conditions for engineering and scientific personal working at experimental stations.
Based on the elaborated scheme, the neutron guide system of ultracold, cold and very cold neutrons has been designed (fig. 2).
3. Neutron flux densities calculations at the outlet of neutron guides
Estimation and optimization of output of ultracold neutrons from the source have been performed with the Monte Carlo method, applying the program  developed for neutron calculations in gravitational potential. In modeling, different parameters have been tested with the aim of optimization of UCN output.
As a result of modeling, UCN density has been obtained for traps with 35 l and 350 l volumes. The chosen trap dimensions are typical for experiments on measuring neutron electric dipole moment (35 l)  and neutron lifetime (350 l) . Fig. 3 shows UCN density depending on HeII temperature in the source chamber.
Thus, as a result of optimization of source parameters, UCN density in the major trap has been obtained equal to ρ = 1.3·10 n/cm (for the trap with volume of 35 l) and equal to ρ = 8.4·10 n/cm (for the trap with volume of 350 l) .
The cold neutron guide (CN2) is a direct neutron guide with cross-section of 30х200 mm for releasing cold neutrons. CN2 neutron guide needs to be lifted by 200 mm higher, because of the difference in the floor level. Fig. 4 (left side) presents the neutron flux density d /d , estimated at the outlet of a direct neutron guide of CN 2, elevated by 200 mm at the outlet point, with 7500 mm length and with cross-section of 30х200 mm .
Very cold neutron guide (CN3) is diverged at 16 degrees from the initial beam of CN, by four sections of 400 mm. Hence, it will provide space for installing a scientific station on very cold neutrons in the main hall of WWR-M reactor. Neutron flux density d /d , estimated at the outlet of CN3 neutron guide of 7500 mm length and cross section of 30х200 mm , is shown in Fig. 4 (right side) as a wavelength function.
Comparative characteristics of UCN sources are shown in table 1.
Thus, the ultracold neutron source of the highest intensity for scientific investigations in fundamental and applied physics will be created in NRC «Kurchatov Institute» - PNPI on the basis of the operating research reactor WWR-M. UCN production will be based on the effect of obtaining UCN in superfluid He owing to peculiarities of this quantum liquid. Implementation of the project on superfluid He at WWR-M reactor will provide an opportunity to create an ultracold neutron source with intensity by 2 orders of magnitude higher than that at the UCN source in the international neutron center at ILL reactor in Grenoble. Moreover, beams of cold and very cold neutrons are supposed to be created for solid state physics installations, working under low background conditions, in separate premises outside the main hall of WWR-M reactor.
The research has been carried out by NRC «Kurchatov institute» – PNPI and supported by RSF (project No. 14-22-00105).