Several papers [16,17] contain information on the characteristics of a stationary RL with longitudinal gas circulation relative to the axes of the laser channels. Calcu­lations for this design were made by employees of the IOFAN and MIFI.

A schematic of one RL channel (cell) is shown in Fig. 10.3. In this case, an RL is considered with a cylindrical core consisting of neutron moderators and a pene­trated system of parallel channels, which are filled with a laser gas medium. A critical mass of uranium is accumulated from the uranium layers deposited on the internal surfaces of the channels. According to one study [16], the optimal diameter of the laser channel, which is determined by the path length of the fission fragments in the gas medium and the profile of the energy deposition function along the channel radius, should be approximately 30-40 % of the path length of the fission fragments in gas. For example, the path length of fragments for helium at room temperature and 3 atm is 3.4 cm (see Table 7.1), and the optimal diameter of the channel according to [16] is 10-14 mm. At a reactor core temperature of ~1,000 K, the fragment path length and the optimal channel diameter for helium at 3 atm are equal to 11 cm and 30-45 mm, respectively.

Neutron calculations of the critical RL parameters as functions of the volume concentration of the moderators in the core, the diameter of laser channels, and the thickness of the uranium layer were performed in a diffuse multigroup approxima­tion. BeO, Be2O, and graphite were examined as neutron moderators. As shown by the calculations, each moderator has a certain minimum of critical R and H (radius and height of the cylindrical core), which is a function of the volume concentration of the moderators in the core. With the laser channel diameter at 20 mm, a core with a BeO moderator has the minimal dimensions R = H = 80 cm at an 80 % volume concentration for the moderators (the volume concentration of the gas medium, therefore, is 20 %). The base version for further examination was chosen as a core with a BeO moderator surrounded by side and end neutron reflectors made of 15-cm

Fig. 10.3 Schematic of RL channels [16]: (1) neutron moderator; (2) the metal tube of a laser channel with a layer of fuel; (3) laser gas medium; (4) channel with coolant

thick beryllium. The results of neutron calculations for this variant of the RL are given in Table 10.1.

The output performance of the base version RL using He-Xe as the laser medium is given in Table 10.2. To operate this RL in stationary mode, gas flowing must be provided along the closed path at a sufficiently high velocity ~150 m/s. This requires significant power consumption.

As the authors note [16], this variant of the RL is not definitive, because choosing the optimal design for the RL is a difficult problem with a large number of constraints for different parameters, many of which are known to a poor degree of accuracy. It should be pointed out here that at a gas velocity of U = 150 m/s, the Reynolds number, Re = U/dv ~105, significantly exceeds the critical Recr = 2,300, therefore the gas flow will be substantially turbulent. This is a serious obstacle to realizing the laser operation (see also Chap. 9, Sects. 9.1 and 9.2).