Long-term Containment Cooling System for VVER‑1000 Nuclear Power Units. Updated Concept and Preliminary Calculation Results

Severe nuclear accidents (SA) require continuous cooling of nuclear fuel, regardless of its condition, and thus generate large volumes of radioactive water. At Fukushima Daiichi NPP, storing this water in tanks proved problematic due to limited capacity, leakage risks, and environmental contamination. A more effective approach is to manage radioactive water within the containment (CTMT) during SA ex-vessel phase. Cooling corium with design systems, however, washes out highly active particles that accumulate in equipment. Since VVER‑1000 heat removal systems are not designed for such conditions, this can lead to system failure, loss of repair capability, and release of radioactive coolant. Following the Fukushima accident, Ukrainian NPPs introduced safety upgrades to stabilize CTMT conditions during the first 72 hours after an initiating event. However, for long-term CTMT integrity, additional technical measures for CTMT and corium cooling are required. This paper proposes a Long-Term Containment Cooling System (LTCCS) that condenses steam formed after ex-vessel corium flooding. This concept was created and patented through a partnership between Westinghouse and its subcontractor, ES Group. The system ensures efficient heat removal through boiling and condensation, prevents radioactive coolant leakage and spread of corium particles outside the CTMT, and avoids excessive accumulation of radioactive water by returning condensate to the CTMT. It also secures long-term CTMT integrity under SA conditions. For VVER‑1000 reactors, the LTCCS mitigates the impact of corium on the CTMT base plate and preserves containment integrity by maintaining pressure and temperature within safe limits. The concept uses a heat exchanger (condenser unit) and, if necessary, a turbo-driven pump (so-called RTS Pump™ by KSB) both located outside the CTMT. When possible, condensate flows back to the CTMT by gravity. Comparable systems, based on Westinghouse patents, are being implemented at Rivne NPP VVER‑440 Units 1 and 2. Preliminary calculations confirm the effectiveness of the proposed LTCCS concept for VVER‑1000 reactors.