It is proposed to equip the PIK and WWR-M research reactors at the Petersburg Nuclear Physics Institute (PNPI) with high-density ultracold neutron (UCN) sources, where UCNs will be obtained based on the effect of their accumulation in superfluid helium (due to the specific features of this quantum fluid). The maximum UCN storage time in superfluid helium is obtained at temperatures on the order of 1 K. These sources are expected to yield UCN densities of 103–104 cm–3, i.e., approximately three orders of magnitude higher than the density from existing UCN sources throughout the world. The development of highest intensity UCN sources will make PNPI an international center of fundamental UCN research.

The WWR-M reactor of the Petersburg Nuclear Physics Institute provides a unique opportunity for creating conditions of low radiative heat release (∼4 × 10 −3 W/g) at a sufficiently high neutron flux (∼3 × 10 12 neutrons/(cm 2 s)). This opportunity can be implemented in the reactor thermal column, which represents a 1-m-diameter channel adjacent to the reactor core. This diameter of the channel allows the arrangement of the core gamma shielding made of bismuth (15 cm thick), a graphite premoderator (300 dm 3 ) at a temperature of 20 K, and a converter with superfluid helium (35 dm 3 ) at a temperature of 1.2 K. Calculations show that the heat release in the source (20 W) can be removed by pumping helium vapor, and the density of ultracold neutrons in an experimental trap will be ∼10 4 neutrons/cm 3 , which is higher than that of existing sources of ultracold neutrons by two to three orders of magnitude.

The WWR-M reactor of PNPI offers a unique opportunity to prepare a source for ultracold neutrons (UCN) in an environment of high neutron flux (about 3 � 10 12 n/cm 2 /s) but still acceptable radiation heat release (about 4 � 10 � 3 W/g). It can be realized within the thermal column situated close to the reactor core. With its large diameter of 1 m, this channel allows to install a 15-cm-thick bismuth shielding, a graphite premoderator (300 dm 3 at 20 K), and a superfluid helium converter (35 dm 3 ). At a temperature of 1.2 K it is possible to remove the heat release power of about 20 W. Using 4p flux of cold neutrons within the reactor column can bring more than a factor 100 of cold neutron flux incident on the superfluid helium with respect to the present cold neutron beam conditions at the ILL reactor. The storage lifetime for UCN in superfluid He at 1.2 K is about 30 s, which is sufficient when feeding experiments requiring a similar filling time. The calculated density of UCN with energy between 50 and 230 neV in an experimental volume of 40 l is about 10 4 n/cm 3 . Technical solutions for realization of the project are discussed.

The WWR-M reactor of PNPI offers a unique opportunity to prepare a source for ultracold neutrons (UCN) in an environment of high neutron flux (about 3*10^12 n/cm^2/s) at still acceptable radiation heat release (about 4*10^-3 W/g). It can be realized within the reactor thermal column situated close to the reactor core. With its large diameter of 1 m, this channel allows to install a 15 cm thick bismuth shielding, a graphite premoderator (300 dm^3 at 20 K), and a superfluid helium converter (35 dm^3). At a temperature of 1.2 K it is possible to remove the heat release power of about 20 W. Using the 4pi flux of cold neutrons within the reactor column can bring more than a factor 100 of cold neutron flux incident on the superfluid helium with respect to the present cold neutron beam conditions at the ILL reactor. The storage lifetime for UCN in superfluid He at 1.2 K is about 30 s, which is sufficient when feeding experiments requiring a similar filling time. The calculated density of UCN with energy between 50 neV and 250 neV in an experimental volume of 40 liters is about 10^4 n/cm^3. Technical solutions for realization of the project are discussed.

The solutions of a system of equations governing the accumulation of tritium and helium in the heavy water of a reactor are presented for arbitrary initial conditions. The solutions are presented as functions of heavy-water exchange and helium extraction. The concentration change is illustrated for typical values of the thermal-neutron flux density.

Calculations of the possibility of switching the matrix of the fuel-element cores and the vessel with the reactor envelope to weakly neutron-absorbing aluminum and lowering at the same time the fuel content in the fuel elements by 40% are performed for the high-flux PIK reactor which is under construction in Gatchina. As a result, the flux density of the thermal neutrons in the heavy-water reflector increases by a factor of 1.4–1.5 and the maximum unperturbed flux reaches a world record value – 1.9·10 15 sec –1 ·cm –2 . The nonuniformity of the volume energy-release decreases to 2.3. The weight of the control rods and reactor safety increase. As a result of the decrease of the fuel content in the fuel elements and the increase of the run duration, about 60 kg of high-enrichment uranium 235 U are saved annually, decreasing annual fuel costs by one third.

In this report we give the detailed description of the mock-up of reactor PIK and also the results of experiments of core poisoning with boron acid (H 3 BO 3 ). The core of mock-up consists of 18FA of reactor PIK. They have cross shaped fuel elements with active height 500 mm in triangle lattice with spacing 5.23 mm. The mean concentration of uranium-235 (90 % enrichment) in core is 540g 235 U/l. Metal ratio (UO 2 in copper matrix and in steel gladding) is 0.41. In reflector of mock-up are placed the same horizontal, inclined and vertical channels as in reactor PIK. The set of 6 experimental series of critical situations (60 measurements), differs by boron poisoning of light water of the core and by the position of the central control shutters. In two series the outer reflector was a light water, in four other it was heavy water. The total uncertainty due to material and geometry leads to the accuracy of reactivity: Δσ exp = ±0,2 %.