A simpler variant of an RL with minimal critical mass parameters does not use coolant and excess heat is absorbed inside the core. In this case, the operating time of the RL is limited by the permissible temperature of the core.

An RL with energy release in the core <10 MW/m3 has a simple construction (Fig. 10.5) if the laser cell (1) is carried out according to the schematic in Fig. 10.2a. Beryllium (or beryllium oxide) is used as the neutron moderator (2) as it is the

substrate for the uranium layer. With the output laser power at ~200 kW, the RL core contains almost 300 cells that are 2-m long (the core is cylindrical with a 2-m diameter). The central channel contains reactor control devices (5). The inside of the core may be supplied with auxiliary mechanisms and gas-intake devices. In addition, the cavity of the central channel may have an initiating small-scale pulsed reactor intended to accelerate output of the device to the given power level and to simply operations with control devices at this output. A neutron reflector (3) is used (the beryllium thickness ~100 mm) to vary the neutron flux round core volume. The entire structure is placed in a cylindrical metal enclosure (4) with optical elements

(6) . To reduce the flowing velocity of the laser medium, the core may be formed in sections in the axial direction as shown in Fig. 10.5.

The energy release in one startup (that is the operation time at the given power) of the core is limited by the thermal capacity of the core. If the permissible rise in the core temperature is accepted to be at most 500 °C, then stationary operation is ~100 s (with other power levels, this time changes accordingly). After this time, downtime of approximately 1 h is needed for forced cooling of the core. The startup frequency may be ~5 min with forced cooling of the core.