Salient features of Chernobyl RBMK Reactor

Part 3: Salient features of Chernobyl RBMK Reactor

Chernobyl Nuclear Power Plant Unit 4: Soviet RBMK Reactor: Its salient features

Reactor (RBMK): 

Reaktor Bolshoy Moshchnosty Kanalny in Russian or High Power Channel Reactor (RBMK) in English, is a graphite moderated boiling water (light water, H2O) reactor.   

Statistics about RBMK: 

RBMK Reactors were designed, constructed and operated only in the USSR. There were 15 RBMK reactors were operational in the USSR at different point of time between 1974-1996.

Unit 4 of Chernobyl Nuclear Power Plant (NPP): 

29 September 1967: The design specifications were approved by the USSR Ministry of Power.  

April 1979: The construction started and 

March 1984: Commercial operation started.

Fuel: 

Enriched uranium as fuel (2% 235U), 114.7kg U. 

Peak fuel temp.: 2100 oC. 

Peak clad temp. In/out: 325 oC/ 295 oC.

Sub assembly and fuel element

18 fuel clad tube/ sub assembly. 2 Sub assembly/ fuel assembly. 

RBMK 1000 Reactor

Moderator: 

Graphite

Coolant: 

Light water. The coolant enters the fuel channels from the bottom of the core. The inlet pressure and temperature are 8.2 MPa and 270 oC respectively. Pressure tubes is made up of Zr, 2.5% Nb alloy.

Control and protection system

Boron carbide. 

Absorbing rods had special graphite displacers.  These displacers remain in the core by full withdrawal of absorbing fractions of rods. 

The use of graphite displacers improves significantly the neutron economy of the RBMK reactor because graphite absorbs neutrons much less than the light water (Thermal neutron absorption cross section: 12C: 0.0035 barn, 1H: 0.3326barn, 10B: 3835 barn).  

No. of control rod: 179

Power

3200 MWth

1000 MWe 

Plates

The upper and the bottom plates of it were made from steel (10CrNi1Mo) of 40 mm thick

Clad material

Zirconium, 1% Niobium alloy

External diameter : 13.5 mm
Wall thickness : 0.9 m
Length : 3.644 m

Specialty of RMBK reactor core:  

Large graphite blocks were used as the neutron moderator. These blocks were arranged into a 7 m tall, 12.2 m diameter, cylindrical shape to form the vertical reactor core.  

Active core height x diameter = 7m x 12m

Design origin: USSR

Total amount of Ziconium in core

Pressure tubes (Zr, 2.5% Nb) : 100 t

Fuel cladding (Zr, l% Nb) : 58 t

Total: 158 tons (6 times more than PWR plants)