The St.-Petersburg Neutron Research School arose in the prewar years from nuclear physics research at the Technical Physics Institute (afterwards the Ioffe Technical Physics Institute). The director of that research, Igor Vasil'evich Kurchatov, and his students soon attained outstanding results in this area new to them. Already in 1935, he and colleagues had used Ra-Be source neutrons to make artificial bromine isomers, and research on them led to major advances in understanding nuclear isomerism [1]. Later, after the discovery of fission, his students L.I.Rusinov and G.N.Flerov in April 1939 reported the measurement of the number of secondary neutrons in the fission of U by slow neutrons: = 31[2]. In 1940, G.N.Flerov and K.A.Petrzhak discovered spontaneous uranium fission [3].

   During the war, Kurchatov and many of his colleagues in the section moved to Moscow to work on nuclear weapons. However, the research on nuclear physics at the Technical Physics Institute continued after the war, particularly in Rusinov's nuclear isomerism laboratory. Physicists acutely needed high-power neutron sources. Then, when the government decided on an extensive program of research reactor construction initiated by Kurchatov, the Technical Physics Institute was included in the list. The scientific director of the Technical Physics Institute reactor was Kurchatov's student L.I.Rusinov.

WWR-M reactor

   The comparatively poor parameters of the WWR -S standard research reactor did not satisfy Rusinov, since they did not allow one to perform the most interesting neutron researches. He set about upgrading the reactor and raising the neutron flux within a very short period. At the start of 1956, members of the Technical Physics Institute were sent to L1RAN (now the Kurchatov Institute Russian Scientific Center): three physicists (G.V.Skornyakov, Yu.V.Petrov and V.A.Shuslov) and three designers. P.P.Moiscenko from LIPAN supervised the group. The upgrading affected mainly the reactor core. The EK-10 rod fuel elements were replaced by aluminum seamless WWR -M1 tubular elements containing a uranium-aluminum ceramics in the core. Three concentric tubes were assembled in a single fuel unit. Jackets and other constructional components became unnecessary (for example, the bearing plates in the standard American MTR assemblies). This made it possible to increase the specific heat-transfer surface by a factor of 4 and to increase the speed of the cooling water in the gap. The reactor design power was increased from 2 to 10 MW. A metallic beryllium reflector was used here for the first time in this country in order to render the core compact. The choice of metallic beryllium (Yu.V.Petrov) was successful: the reflector remains viable after 40 years. The beryllium oxide used later as reflector in the SM-2 reactor after irradiation for some years began to crumble and had to be replaced by metallic beryllium. In the new WWR -M reactor, the neutron flux was raised to 310 ¹⁴ n s^-1 cm^-2 in the central light-water trap.

   Without a wait for the completion of the design, the foundation pit began to be dug at Gatchina for the reactor in the summer of 1956. The design of the core was completed at the end of 1956. The All-Union Atomic Materials Institute designed the technology for making WWR-M1 ceramic fuel rods. All the engineering drawings for the WWR -M were formulated in the designs office at the Technical Physics Institute in 1957 (about 30% of the old drawings had to be revised). To implement the project, Rusinov organized a technological group led by K.A.Konoplev in the summer of 1957. The design was based on neutron-physics calculations performed under the direction of Skornyakov. The calculated critical mass agreed satisfactorily with the startup mass as measured will) a special assembly at Gatchina [4, 5].

   The WWR -M was commissioned in December 1959 and formed one of the ten best swimming-pool reactors of the time (Fig. I). The successful design of the fuel assemblies and reflector was used in other research reactors (in Kiev, Obninsk, Alma-Ata and also abroad in Hungary, the GDR, Poland, and Vietnam). In reactors of the IRT type, the WWR -M design was copied in those rods were replaced by tubular fuel elements (but square in shape, rather than hexagonal). Most of the pool research reactors made in this country have thus used the tubular fuel elements first developed for the WWR -M.

   After the reactor was commissioned [6], further work was done to extend its experimental facilities and to improve the safety. This research reactor was a factory for scientific information, especially on account of its neutron, economic, and purely design characteristics [7]. The approach provided for the best design for the reactor, the best core composition, and the best burnup, as well as physical experiments at minimum cost. In the 1960s, when we did not have the current computers, a physically adequate simple model for computing light-water research reactors was developed (Yu.V.Petrov and others). The classical Gurevich-Pomeranchuk formula for resonant capture in fuel rods was extended to the case of the closely spaced arrays in research reactors [8]. A few-group diffusion model for neutron moderation was formulated for the IR water-metal cores [9].

   When there was a considerable neutron leak into the reflector, it provided a reasonably accurate prediction of the critical mass, the thermal-neutron fluxes, the power production, and so on. A comparison was made with exact critical experiments on annular cores by K.A.Konoplev and others [10], which showed that the error in that model was less than 6.2% in reactivity [11]. The slight deviation from cylindrical shape in the reactor led to corrections of the second order of smallness to the reactivity [12], and this explains the good agreement between theory and experiment.

   The fuel pin parameters were again optimized (metal-water ratio, fuel concentration, and so on) on the basis of highly developed calculation methods that employed the principle of minimizing the reactor costs [7]. By the start of the 1970s, considerable experience had been accumulated in operating the WWR -Ml and -M2 fuel pins. Comprehensive research had been done on their heat-engineering and corrosion features, and the sealing level had been determined. This led to the design of tubular pins in the next generation: WWR -M5, which had the wall thickness halved (1.25 mm) and doubled fuel concentration: 125 g of ²³⁵U/liter [13]. The specific surface per unit volume in the core attained the record value of 6.6 cm²/cm³, which provided a maximum specific power of 0.90.1 MW/liter in the pool reactor [14]. Although the amount of fission products in an assembly was increased by a factor of 1.5, the sheaths contained them reliably. The reactor was completely converted to thin-walled WWR -M5 fuel pins in 1980 (Table 1). The power was raised to 18 MW. The pins contained more ²³⁵U (90% enrichment), which resulted in freeing more than half of the space in the core for experimental equipment.[15] In particular, a source of cold and ultracold neutrons was located at the center of the core, whose output was greater than that of analogous sources in the most powerful ILL reactor at Grenoble (Fig. 2) [16].

   The Institute initiated the industrial production of thin-walled pins for other types of research reactor (IRT-8 and IVV-2). By 1999, the energy production by the WWR -M5 pins attained 52% of the total energy production by the reactor (72.1 GW day). For the 40 years of operation, there has been no case of environmental contamination because of failure of these pins. The consumption of WWR -M5 assemblies has been reduced by a factor of 1.7 for identical power production which has led to a saving, and during recent difficult years this has enabled one to operate without additional fuel pin purchases.

   The transfer to the WWR -M5 pins was the most important step in extending the reactor facilities but not the only one; others were the reconstruction of the thermal column, the drilling of new horizontal channels, the installation of a chamber above the reactor, an extended range of experiments in the vertical channels, and a new servo system, which provided access to the entire core volume including the center. Improved working methods gradually reduced the doses to staff and lowered the radioactive discharges to the atmosphere and the production of effluents for chemical treatment. The discharges even at that time were lower than the contemporary strict standards, but nevertheless they were reduced by a further factor of 10 [17], which was obtained as a result of detailed and laborious researches on the gas and water working conditions in the reactor together with the introduction of a new technology for operation without deaeration in the water-treatment system for the first loop by the use of electrophoresis filters.

   Reliability and safety in all the units are particularly important for research reactors. The fast-neutron fluence for some of them exceeded 10²² n cm^-2.

   Tests were done on the SAV1 material from which they were made, and research was done on test specimens, where surface examination showed dial there was a large safety margin for the further viability of the reactor.

   The WWR -M has operated without an accident for almost 40 years. The level of the discharges (90% of them being ⁴¹Ar) is far below the permissible values and leads to a dose for the inhabitants of Gatchina of 0.2 mber/yr, which is less than the natural background by about a factor of 1000.[19] During the existence of the reactor, there has been no case of a staff member suffering from radiation disease or excessive irradiation. To a considerable extent, this was due to the provision of highly qualified staff continuously directed by R. G. Pikulik. However, the WWR -M has become out of date for the most interesting neutron research, and the future hopes of the Institute are attached to the PIK reactor.

REFERENCES

1. В. Kourchatov, I. Kourchalov, L. Myssuwsky and L. Roussinov, Comptes Rendus Acad. Sci., 200 (1935). In: I. V. Kurchatov, Nuclear Power for the Good of Humanity, Atomizdat, Moscow (1978). pp. 191-193; I. V. Kurchatov and L. I. Rusinov, "Nuclide isomerism," ibid., pp. 287-304.
2. L. I. Rusinov and G. N. Flerov, "Experiments on uranium fission," Izv. AN SSSR, 4, Issue 2, 310-314 (1940).
3. К. A. Petrzhak and G. N. Flerov, "Spontaneous fission in uranium," Dokl. AN, 8, Issue 6, 500-501 (1940).
4. К. A. Konoplev, R. G. Pikulik. L. I. Rusinov, et al., "Critical experiments on the WWR -M tester," in: Papers at the International Conference on Reactor Physics and Engineering in Bucharest [in Russian], OIYaI (1961), 112 pp.
5. D. M. Kaminker, K. A. Konoplev, Yu. V. Petrov, and R. G. Pikulik, "Operation of the WWR -M critical assembly," in: Exponential and Critical Experiments, Vol. II, IAEA, Vienna (1964), p. 197.
6. D. M. Kaminker and K. A. Konoplev, "The WWR -M reactor at Galchina has operated for 10 years," At. Energ., 27, Issue 6, 583-584 (1969).
7. A. N. Erykalov and Yu. V. Petrov, "Parameters characterizing reactors for physics research," ibid., 25, Issue 1. 52 (1968);
8. Yu. V. Petrov, "Choosing the parameters of a reactor for physics research," Preprint LiYaF-802 (1982).
9. Yu. V. Petrov, "Resonant absorption in closely spaced small blocks," At. Energ., 2, Issue 4, 357 (1957).
10. E. A. Garusov and Yu. V. Petrov, "A few-group model for moderation in water-aluminum cores," ibid., 32, Issue 3. 225 (1972); "Moderation-function moments and few-group models for water-metal mixtures," 36, Issue 2, 143 (1974).
11. A. G. Ashrapov, E. A. Garusov, V.V. Gostev, et al., "A few-group model for reactors of WWR -M type," Preprint FTI-152 (1986);
12. V. I. Gudkov, V. I.  Didenko, G. R. Dik, et al., "Critical-mass measurement for WWR -M fuel-pin assemblies," At. Energ., 72, Issue 4, 400-404 (1992).
13. G. R. Dik, A. N. Erykalov, and Yu. V. Petrov, "Accuracy of a few-group model for calculating the critical mass for the WWR -M," ibid., 75, Issue 2, 83-87 (1993).
14. Yu. V. Petrov and E. G. Sakhnovsky, "On the boundary perturbation theory as applied to nuclear reactors," Nucl. Sci. Eng., 90, 1-12(1985).
15. A. N. Erykalov and Yu. V. Petrov, "Optimizing the fuel pins for the WWR -M reactor," Preprint LiYaF-435 (1978); Heat-Producing Assemblies in a Physical Research Reactor, Authors' Certificate No. 743452, 28.02.1980, with priority 10.08.1978.
16. A. N. Erykalov, V. S. Zvezdkin, G. A. Kirsanov, et al., "Thin-walled WWR -M5 fuel pins for research reactors," At. Energ., 60, Issue 2, 103 (1986).
17. A. A. Enin, A. N. Erykalov, G. A. Kirsanov, et al., "Design and experience of HEU and LEU fuel for WWR-M reactors," Nucl. Eng. Design, 182, 233-240 (1998).
18. I. S. Altarev, N. V. Borovikova, A. P. Bulkin et al., "A universal liquid-hydrogen source of polarized cold and ultracold neutrons on the WWR -M reactor at the Leningrad Nuclear Physics Institute," Pis'ma Zh. Eksp. Teor. Fiz., 44, Issue 6, 269(1986).
19. D. M. Kaminker, K. A. Konoplev, Yu. P. Semenov and V. D. Trenin, "Reducing radioactive discharges to the atmosphere from the WWR -M reactor," At. Energ., 19, Issue 6, 517 (1965).