MARIA is a reactor with an unusual design. It is a channel reactor and a pool reactor at the same time. The fuel has been placed in special tubular fuel channels, ensuring that it is effectively cooled by water at a pressure of 1.7 MPa. The fuel channels, together with other reactor core components such as the beryllium moderator and graphite reflector blocks, absorber rods, and target material containers, are located in the so-called core basket in the reactor pool under a seven-metre layer of water. The reactor pool is a concrete structure with a steel lining. The pool is connected by a sluice to the adjacent storage pool, which acts as a short-term repository for spent fuel and irradiated reactor structural and experimental components.

The core of the MARIA reactor has a modular design, adapted each time to the production or research programme. In addition to fuel elements, it contains beryllium moderator blocks, channels with absorber rods and isotope channels of various types for irradiation of target materials. The outer part of the basket contains graphite reflector blocks, devices for irradiating high-volume materials such as silicon or minerals, and other experimental equipment such as an epithermal neutron beam converter.

Immediately behind the core basket, the ionisation chamber channels are arranged for reactor control and monitoring. The concrete basin structure provides (along with the water in the vessel) a lateral biological cover for the reactor. At core height, heavy concrete with a density of 3.6 g/cm3 was used. There are eight openings in the concrete casing. In six of these, horizontal channels are located through which the neutron beams used for physical tests are routed out.

The configuration, shown in the figure includes most of the typical core components. In addition to those mentioned earlier, the uranium plate irradiation channels for the production of ⁹⁹Mo and the hydraulic mail channels for short duration irradiations are also shown. Arabic numerals and lowercase letters indicate the positions of the fuel channel slots, Roman numerals and uppercase letters indicate the positions of the beryllium blocks. 

Compensation bars (PK) are used to compensate for the effects of ¹³⁵Xe and ¹⁴⁹Sm reactor poisoning, fuel burnout and temperature effects. The safety rods (PB) are extended above the core during reactor operation and are used to shut down the reactor. A single automatic control rod (PAR) works in conjunction with the automatic power control system and allows the reactor to automatically maintain a constant power level.

The control and protection of the reactor is achieved by means of absorber rods filled with boron carbide (neutron absorption occurs in the ¹⁰B isotope).