Lasers

2.1 Specifics of Experiments on Pulsed Reactors

Virtually all of the experimental investigations to seek and study the characteristics of NPLs excited by the products of nuclear reactions were carried out on pulsed nuclear reactors. Pulsed nuclear reactors are distinguished for the composition and structure of their core, the duration and fluence of the neutron pulse, the volume and configuration of the space for the irradiation, and the pulse repetition frequency. Some aspects of the use of pulsed reactors for laser pumping were examined in the survey [1]. This section uses some of the data from study [1], supplemented by information from other sources.

By now—in Russia and elsewhere—more than 10 pulse reactors and a number of modifications have been developed and are in operation. More information about pulse reactors and their characteristics is contained in monograph [2]. The fuel for such reactors includes: metallic highly enriched 235U, uranium-molybdenum alloys, a triple alloy of uranium-zirconium-hydrogen, uranium-aluminum alloys, aqueous solutions of uranium salts, etc. Starting in the 1950s, pulsed reactors were inten­sively utilized in various fields of science and technology as powerful sources of neutron and у radiation. The duration of neutron pulses varied from 30 ps to ~0.1 s at neutron flux densities in experimental channels of 1017-1019 cm-2 x s-1 and neutron fluences of 1013-1017 cm-2.

The most complete information about pulsed reactors that operate (or operated) in various countries, the operating principles of these devices, and their character­istics and application, is contained in the monograph [2]. In Russia, pulsed reactors are developed and operated primarily at VNIIEF and VNIITF [2, 3]. At various times in Russia, the pulsed reactors VIR-1, VIR-2, TIBR, BR-1, BIGR (VNIIEF), IIN-3 (IAE), EBR-L (VNIITF), BARS-6 (FEI) have been used, and outside Russia—pulsed reactors such as the Godiva, SPR, TRIGA, ACRR, APRF, etc., which have similar characteristics. In 2003 reports were issued about experimental NPL investigations in China on the CFBR-II reactor [4]. Table 2.1 shows the

© Springer Science+Business Media New York 2015 S. P. Melnikov et al., Lasers with Nuclear Pumping, DOI 10.1007/978-3-319-08882-2_2

characteristics of certain of these reactors and experiments with NPLs conducted on them (the results of experiments are discussed in Chap. 3).

Initially the pulsed reactors were developed for testing various materials and electronic devices inside or close to the reactor core, so by no means were all of them adapted for conducting complex laser experiments. The reactor EBR-L [2,3,31], the dual-core reactor BARS-6 [3, 32], as well as the water reactor VIR-2 M [2, 26, 33], originally designed for general research uses and later adapted for experiments with NPLs, were perhaps the exceptions.

In experiments to find laser media for NPLs, fast neutron reactors were used as a general rule. Examples are the BR-1, EBR, Godiva, and SPR using small reactor cores (~30 cm) made of metallic 235U or its alloys, with a duration of reactor pulse of 50-100 qs. This selection may be explained by the fact that such reactors assure a maximal neutron flux density, and accordingly, maximal specific power depositions in laser media, thus facilitating attainment of the lasing threshold in NPLs. The reactor core usually is set at a height of 1.5-2 m from the floor of the casemate, the thick walls of which serve as biological shielding. The fluxes of neutrons and у quanta are maximum at the center of the reactor core and decrease roughly by an order of magnitude at its outside surface. Among the drawbacks of using such reactors include the large spatial non-uniformity of the neutron flux, which limits the length of the laser cells to ~50 cm. The exception is the two-core BARS-6 reactor, which can be used to uniformly irradiate laser cells up to 150 cm in length.

Different versions of the placement of NPLs in experiments using fast neutron — pulsed reactors are shown in Fig. 2.1. The version of Fig. 2.1a is the most widespread, because in this case the influence of NPLs on the parameters of the reactor is not significant. The maximal specific power deposition of the gas media

of up to 5 x 103 W/cm3 is implemented with the arrangement of the laser cells inside the reactor core (Fig. 2.1b, d).

The active length and volume of the NPLs can be increased using a multipass laser cell. Two versions of such devices are shown in Fig. 2.2. In the first case [18, 34], up to six ceramic tubes were placed inside a rectangular stainless-steel chamber; layers of 235U3O8 were deposited to the interior surfaces of these tubes having a diameter of 2.54 cm. The full active length of such a laser reached 240 cm [34]. Flat gold-coated mirrors were used as the deflecting mirrors. Another version

of the design [35], intended for excitation of gas media with nuclear reaction products 3He(n, p)3H, differs in that there are no tubes with uranium layers, and rectangular plates with a gold or silver coating were used as the deflecting mirrors. In this case, when the angle between the cavity mirrors is changed, it is possible to vary the number of passes in the cell and consequently the active length of the laser.

Laser cells usually are surrounded by a layer of moderator (polyethylene, Plexiglas, water) 3-5-cm thick for softening the neutron spectrum, making it possible to increase the laser power deposition by a factor of 10-100 when using the nuclear reactions given in Table 1.4 (Chap. 1, Sect. 1.2). When pulsed reactors with a short pulse duration are used (50-100 qs), the time dependencies of the flux of fast neutrons (reactor power) and the pumping power of the laser medium do not coincide, which is related to the process of thermalization of the neutrons inside the moderator. Figure 2.3 shows the results of calculations [36] of the time dependence of the power deposition (flux density of the slowed neutrons) inside the moderator under the conditions of experiments [21] (thickness of the cylindrical polyethylene moderator 5 cm, duration of reactor pulse t1/2 = 50 qs). The use of a moderator spreads the pumping pulse duration to ~150 qs, and shifts the pulse maximum position on ~30 qs. The results of calculations agree with experimental data.

It is more convenient to optimize the design of the NPLs (composition and parameters of laser medium, design of cavity, methods of removing surplus heat) for the purpose of selecting the optimal variation of the cell of the stationary or quasi-stationary nuclear-laser facility using pulsed nuclear reactors based on ther­mal or intermediate neutrons, with the moderator in the reactor core: the VIR-2 M reactor with reactor core from a solution of 235U salts in water, and the TRIGA pool-type reactor and its modification, the ACRR, the fuel elements of which are made from a uranium-zirconium-hydrogen alloy. These reactors are characterized by a long-duration neutron pulse (1-50 ms), with a maximal neutron fluence of ~5 x 1014 cm~2, as well as the possibility of experimentation with laser cells up to 200-cm long.

The above experimental layouts were used primarily to search for active NPL media and study their characteristics (laser spectrum, energy characteristics, and laser thresholds). To study multichannel NPLs, VNIIEF created the experimental complex LM-4/BIGR [15, 16] and the IKAR nuclear-laser facility, which consti­tutes a model of the reactor-laser [37]. VNIITF and FEI are developing multichannel LIRA [38] and Stand B [39] facilities. Among the studies outside of Russia with which we are familiar, one can note the experiments on the ACRR reactor (Sandia Laboratories, United States) using the large-scale laser setup ALEC (Advanced Laser Excitation Cavity) [19]. The designs of such complex multichannel facilities and the basic results obtained from studying them are cited in Chap. 6.