RBMK Basic Design and Safety Improvements
Major safety improvements have been implemented in RBMKs since the Chernobyl accident in April 1986. They address both the direct causes of the accident and other safety shortcomings which have been identified in various analyses.
Reactor Core. Safety modifications directly related to the Chernobyl accident focus on reducing the void reactivity coefficient and improving the control rod design. These modifications have been implemented in all RBMKs. Main measures taken to reduce the void coefficient include:
- loading additional absorbers. The number of additional
absorbers varies from 85 to 103 depending on the reactor.
Technical specifications require at least 81 additional
absorbers.
- increasing the fuel enrichment from 2.0% to 2.4%.
- controlling the operational reactivity margin (ORM).
The ORM fully inserted value is controlled between 43 and 48
equivalent control rods.
Emergency Protection System (EPS). Three safety improvements were carried out to improve the EPS efficiency and speed of response. The manual control rods were replaced by rods of an improved design. This includes elimination of the water column at the bottom end of the reactor control and protection system channels, and increasing the neutron absorbing section. The rod drives were also modified, reducing the time required to insert the rods fully into the core from 19 to 12 seconds. These two measures have improved the EPS response efficiency during the first few seconds of rod insertion. As a third measure, a fast-acting EPS was developed and installed in all operating reactors. This system can fully insert 24 control rods in less than 2.5 seconds, or in 7 seconds depending on the emergency signal activated.
Control and Monitoring System. Other measures were taken to improve the control and monitoring system. They include manual reactor trip when the power falls below 700 MW(th); and manual trip if the ORM is less than 30 equivalent control rods.
Pressure Boundary. Two independent loops provide cooling for each half of the reactor core. Each loop contains four main coolant pumps and associated piping. The pressure in the system is 7 MPa.
Emergency Core Cooling System (ECCS). For Smolensk-3, the design basis accident for the ECCS is a double-ended guillotine break of a 900 mm tube and loss of off-site electric power. This corresponds to a break in the main circulation pump pressure headers or suction header. In the event of such an accident, the ECCS makes provision for both fast-acting cooling of the core and long-term decay heat removal. The long-term cooling system comprises six emergency core cooling pumps taking suction from the accident localization system (ALS) for cooling the damaged half of reactor and three pumps taking suction from the tanks for pure condensate for cooling the non-damaged half of the reactor. Both sets of pumps are electrically driven with their power supplies backed up by diesel generators.
Design modifications not related to the causes of the Chernobyl accident are being introduced at first and second generations RBMK plants. Among other steps, these include: increasing the number of emergency feedwater pumps from three to five and the number of ECCS lines from one to two; installing additional ECCS pumps (three for cooling the damaged core side and three for cooling the undamaged side) and the associated three divisions of piping; installing check valves between the distribution group headers and the main coolant pump discharge header; and installing large capacity accumulators.
Accident Localization System. RBMKs are protected by an Accident Localization System (ALS). This pressure suppression system encloses part of the main circulation circuit and consists of leak-tight compartments. All main pipelines, headers, and components carrying cooling water are part of the ALS. The ALS differs considerably in design from one plant to another. The reactor coolant system of first-generation RBMKs is not enclosed in a leak-tight ALS, as is the case at the other RBMKs. Even in these other RBMKs, however, only part of the reactor coolant circuit is confined by an ALS of pressure compartments.
Reactor Cavity Overpressure Protection System (RCOPS). This is an important part of the RBMK's safety system. The cause of overpressurization is postulated to be a failure of pressure tubes inside the reactor cavity. Relief is provided by tubes which connect the reactor cavity to the ALS via a water lock. The design basis accident of RBMK safety analyses is the rupture of one tube. The system has the capacity for two or three channel tube ruptures (for first and second generation units, respectively) which reflects a safety margin over the design basis accident. For first-generation units, the steam discharge system vents the steam/gas mixture from the cavity to a condenser, with the gas subsequently held up and released through the stack.
To improve the capacity of the RCOPS, work is being conducted in stages for all units. At Smolensk-3, the existing system already has the capacity for the simultaneous rupture of up to nine pressure tubes under conservative assumptions of simultaneous ruptures.