All of the structures surrounding the core region contribute to some extent to biological shielding. The principal structural components have been described in the previous section, they are complemented where required by additional material. The principal structures serving the shielding function include the graphite reflectors, the internal spaces of the metal structures, the gap between the concrete vault and the outer surface of the core support metal structures. With respect to the center of the core, the biological shields can be divided into three parts: top shield (in the direction of the refueling hall), bottom shield (in the direction of the lower coolant channel banks), and radial shielding.
Composition and dimensions (in meters) of principal biological
shield components
Shielding Direction
Material Top Bottom Radial
Graphite (reflector) 0.5 0.5 0.88
Steel (shield plates and metal structures) 0.29 0.24 0.045
Serpentinite filling (1700 kg/m3) 2.8 1.8 -
Water (annular tank) - - 1.140
Steel (metal structure) 0.04 0.04 0.03
Sand (1300 kg/m3) - - 1.3
Heavy concrete (4000 kg/m3) 0.89 - -
Construction concrete (2200 kg/m3) - - 2.0
Biological shielding in the direction of the refueling hall encompasses the 0.5 m thick upper graphite reflector, 0.25 m high steel shielding blocks. the upper metal structure which is filled with a mixture of serpentinite chips and gallium (weight ratio of 3:2), and the top shield cover. The density of the fill material is 1700 kg/m3, its height is 2.8 m, and the thickness of the steel foundation plates of the structure is 40 mm.
A number of special design features are incorporated into these structures in order to reduce direct streaming of radiation along the gas-filled channels (temperature, neutron flux instrumentation and ion chamber channels) and the fuel channels which in the upper region of the core are filled with a steam-water mixture. The fuel channels are capped with special steel-graphite plugs which incorporate spiral passages for the low of the two-phase coolant. The ring-shaped gaps between the channels and the guide tubes are covered with shielding sleeves. Graphite followers are employed in the control channels to reduce direct neutron and gamma streaming into the spaces underneath the reactor. Whenever possible, the gas and coolant pipes which penetrate the shielding structures are bent so that direct streaming is reduced.
The radial shield consists of the radial graphite reflector (average thickness 0.88 m). the shell of the core, the annular water-filled steel tank. sand filling between the tank and the walls of the reactor vault, and the 2 m thick concrete walls of the vault. The walls of the vault are made from ordinary construction concrete with a density of 2200 kg/m3.
Design criteria for shielding below the core include the requirement to reduce gamma radiation during shutdown in order to allow personnel access for maintenance, and the necessity to minimize activation of the metal structures and coolant feeder pipes. The bottom shield consists of the 0.5 m thick graphite reflector, 0.2 m of steel blocks, and the bottom biological shield, filled with a mixture of serpentinite chips and gallium. The density of the mixture is 1700 kg/m3. There are O.1 m thick steel screens under the annular water tank (above the bottom water piping) and between the reactor vault and the water tank.
During reactor operation the biological shielding limits the radiation dose rate in the refueling hall and in the areas adjacent to the reactor to levels not exceeding 2.8 x 10-s Sv/h. During refueling operations the gamma dose in selected locations close to the refueling machine can range up to 1.E-03 Sv/h.
It is reported that the tests of the biological shielding effectiveness, conducted in the first unit of the Ignalina NPP with the RBMK-1500 reactor operating at nominal power, confirmed that the radiation field in the reactor service areas meets health standard requirements.' For example, the average equivalent dose rate in the central refueling hall was found to be (11-18)E-06 Sv/h, while in the access chamber to the coolant flow control valves it did not exceed 4 Sv/h, with the reactor operating at thermal power 3850 MW. These tests were performed by the Moscow Research and Development Institute of Power Engineering (RDIPE).
During nuclear fission 95 % of the generated energy is released in the fuel element and an additional 5 % is released in the graphite during neutron moderation and gamma absorption. A helium-nitrogen mixture circulates around the fuel channels and between the graphite blocks. This gas retards the oxidation process, and the humidity and temperature readings of the gas are monitored to indicate leaks of the fuel channels.
Under normal reactor operating conditions the biological shielding makes it possible to perform certain repair and maintenance tasks. This applies to piping, which serves the various channels and is located below the bottom and above the top biological shields. The non-service compartments are accessible only during the reactor shutdown.