10.1 Main Types of Stationary Reactor Lasers

Chapter 6 gives a brief overview of designs for nuclear laser setups with a pulse duration of ~10 ms (OKUYAN and LIRA setups). Their operation is based on the “initiating reactor-subcritical laser block” scheme. The literature has repeatedly suggested different design schemes for stationary nuclear laser facilities or reactor — laser (RL) based on gas laser media. The term RL was apparently first proposed in article [1], where the potential of removing energy in the form of laser radiation from the reactor core during the relaxation stage was noted in principle. The most interesting version of the RL is a device where, as a consequence of spatial overlapping of the nuclear fuel and laser medium, a significant portion of the nuclear energy is converted into laser radiation bypassing the intermediate thermal energy stage.

The idea of creating a stationary nuclear-laser facility based on using a gas medium was expressed in 1964 before the appearance of the first nuclear-pumped lasers (NPLs). In this study [2], a reactor core was examined that consisted of 1,500 aluminum tubes surrounded by moderators. These tubes had layers of 235U depos­ited on their inner surfaces and were filled with a He-Ne mixture at ~10 Torr of pressure. There are some schemes of stationary RLs that are differentiated by the nuclear fuel used for reactor core operation and pumping of the laser medium (Fig. 10.1): (a) a gas compound UF6, uranium vapor, or uranium aerosol particles; (b) thin layers of uranium or its compounds on metal substrates.

Interest arose in UF6, because it is practically the only gas-phase compound of uranium at low temperatures, and the energy deposition efficiency for fission fragments in the “UF6 + laser gas” mixture approaches 100 %. In addition, a series experiments [3, 4] were carried out that show the potential for creating gas-core nuclear reactors with comparatively low critical masses of UF6 (~6 kg).

Thus, there is currently no doubt of the potential for creating gas-core reactors based on UF6, although these devices are rather complicated and dangerous.

© Springer Science+Business Media New York 2015 373

S. P. Melnikov et al., Lasers with Nuclear Pumping,

DOI 10.1007/978-3-319-08882-2_10

Laser beams

Fig. 10.1 Schematics of the main variants for a stationary RL [6]: (a) a gas-core reactor whose core is a mixture of uranium fuel (UF6, uranium aerosol, or vapor) and laser gas; (b) an RL using thin-layer uranium fuel

However, when choosing laser gas media together with UF6, a number of difficul­ties arise, which are caused by the chemical aggressiveness of UF6 and its radiolysis products, high losses in the active medium due to absorption of the laser radiation by UF6 molecules, and the high quenching rate of the excited atoms and molecules in collisions with UF6 molecules.

As laser media for gas-core RL based on UF6, these mixtures have been suggested: UF6-CO2-N2-He [6], UF6-He, UF6-TlF-F2 [7], UF6-He-Xe [8], UF6- Ar-Xe [9], etc. However, there are currently no laser media using UF6, for which the feasibility of pumping by nuclear radiation would have been established experimentally. Experimental [10] and theoretical [11] studies have shown that, at the present time, the most effective laser media with nuclear pumping at the infrared transition of atoms M = Xe, Kr, and Ar may not operate at UF6 concen­trations higher than 1 % due to a reduction in the ionized atom M+ concentrations and the high rate of collisional quenching of the excited atoms M* by the UF6 molecules. One paper [9] concludes that designs of gas-core RLs using UF6 are apparently unrealistic because it is difficult to provide the conditions for laser operation with sufficiently high concentrations of UF6, which are necessary to attain reactor criticality. The authors of study [9] came to the same conclusion when analyzing the possibility for creating an RL using aerosol fuel with uranium particles several micrometers in diameter.

The design of an RL using uranium vapors was not examined in detail. In the authors’ opinion [12], the main difficulty for creating such facilities is the necessity to maintain the active medium at a high temperature (~4,000 K). This substantially complicates the construction of the device and severely limits the choice of laser medium.

At present, the most realistic designs are heterogeneous RLs using thin-film uranium fuel. The core of this RL is, in essence, a specific quantity of laser cells with uranium layers appropriately placed in a neutron moderator matrix. Calcula­tions show that, with the appropriate selection of components, this system has a sufficient neutron multiplication factor when only uranium layers inside laser cells are used, that is, the conditions for the operation of the RL are provided without utilizing additional fuel (uranium). The number of laser cells may vary from a hundred to several thousand. The total amount of uranium may vary from 5.7 to 40­70 kg, and characteristic linear dimensions are 2.5 m.

Figure 10.2 shows the two main alternatives for laser cells: (a) a cylindrical cell capable of longitudinal gas circulation relative to the axis of the cell; (b) a rectan­gular cell with transverse gas circulation relative to the axis of the cell (in the direction indicated by the arrows). Longitudinal gas circulation is possible with low power deposition and slow (<10 m/s) gas velocities. At high power, transverse circulation with the energy transfer to internal radiators is more effective (Fig. 10.2b). Some characteristics of laser cells with transverse gas flow are examined in Sect. 9.3 of Chap. 9. This laser cell design was proposed at the All-Union Scientific Research Institute for Experimental Physics [VNIIEF] [13, 14] and verified experimentally on the LM4/BIGR complex (see Chap. 6, Sect. 6.1).

Stationary RLs using thin-layer uranium fuel have been considered in Russia [15—18] and abroad [2, 6, 19]. The main characteristics of these systems and the issues that arise with their design are considered below in examples of the RLs that are discussed in studies [15—18].