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IEC 62118:2000 — Nuclear Reactor Instrumentation — PWR of VVER Design — Monitoring Adequate Cooling During Shutdown
Standard Snapshot: IEC 62118 provides instrumentation requirements for monitoring core cooling in VVER-type pressurized water reactors during all shutdown states. It addresses the unique design features of VVER reactors — particularly horizontal steam generators, loop configuration, and reduced inventory operation — that affect core cooling monitoring.
1. Scope and Design Context
IEC 62118:2000 is the VVER-specific counterpart to IEC 62117 (which addresses Western-style PWRs). Developed by IEC SC 45A, this standard recognizes that the VVER design — featuring horizontal steam generators, specific loop configurations, and different reactor pressure vessel penetrations — presents unique challenges for core cooling monitoring that are not fully addressed by IEC 60911 or IEC 62117.
The VVER (Vodo-Vodyanoi Energetichesky Reaktor) design, developed in the former Soviet Union, includes two primary configurations: VVER-440 (with six primary loops and horizontal steam generators) and VVER-1000 (with four primary loops and larger horizontal steam generators). Both configurations have distinct instrumentation requirements due to their thermal-hydraulic characteristics during shutdown.
| Parameter | VVER-440 | VVER-1000 |
|---|---|---|
| Number of primary loops | 6 | 4 |
| Steam generator type | Horizontal (PGV-440) | Horizontal (PGV-1000) |
| RPV inlet/outlet nozzle configuration | Bottom-mounted inlet, top outlet | Nozzles above core |
| Primary coolant pumps | Horizontal, glanded | Vertical, canned motor |
| Thermal power (MWth) | 1,375 | 3,000 |
2. Shutdown Operating Conditions
The standard defines the full range of shutdown conditions that must be monitored, categorized by operating state:
- Normal power operation: Full coolant inventory, forced circulation, all instrumentation operational.
- Hot shutdown: Reactor subcritical, primary system at reduced temperature and pressure, natural or forced circulation depending on pump status.
- Cold shutdown: Primary system depressurized and cooled below 100 °C, reduced coolant inventory possible, residual heat removal via RHRS.
- Abnormal and accident conditions: Includes loss of RHRS, inadvertent draindown, primary-to-secondary leakage, and loss of off-site power.
Key VVER-Specific Consideration: In VVER plants, the horizontal steam generators contain a large inventory of secondary-side water. Under certain conditions, the steam generators can act as heat sinks that drive natural circulation differently than in Western PWR designs. This must be accounted for in the interpretation of core cooling measurements.
3. Measurement Methods
3.1 Differential Pressure Measurement
As in Western PWRs, differential pressure measurement is the primary method for RPV water level determination. However, the standard provides VVER-specific guidance on tap locations, accounting for the different nozzle configurations. For VVER-1000, where the hot leg nozzles are above the core, the differential pressure measurement must account for the thermal-hydraulic effects of reduced inventory and reverse flow (addressed in detail in Figures 4 and 5 of the standard).
3.2 Core Exit Temperature Monitoring
Core exit thermocouples (CETs) are required with sufficient coverage to detect local cooling anomalies. The standard specifies that CETs should be distributed across the core cross-section to cover all fuel assemblies, with particular attention to assemblies adjacent to control rod guide tubes where flow blockage is more likely.
| Measurement Method | VVER-Specific Adaptation | Key Requirement |
|---|---|---|
| Differential pressure | Accounts for VVER-440/1000 nozzle geometry | Tap locations per standard figures |
| Heated sensor | Applicable to both configurations | Vertical array at representative locations |
| Ultrasonic level | Applied to hot leg piping | Transducer mounting considerations |
| Core exit thermocouples | Distributed across core cross-section | Coverage for all fuel assemblies |
4. Instrumentation System Requirements
4.1 Safety Classification and Redundancy
The standard mandates that instrumentation for core cooling monitoring during shutdown meets the plant’s safety classification requirements, with redundancy and diversity provisions consistent with the single failure criterion. For VVER plants, the standard recognizes that the classification system may differ from Western plants (using Russian safety classification categories), and provides guidance on mapping between classification systems.
4.2 Thermal-Hydraulic Effects on Measurement Accuracy
The standard devotes significant attention to the thermal-hydraulic phenomena that affect measurement accuracy during reduced inventory conditions in VVER plants:
- Reverse flow: During certain shutdown conditions, flow in some loops may reverse direction, altering the pressure drop distribution and affecting DP measurements.
- Steam binding: Steam accumulation in instrument lines can cause significant measurement drift.
- Condensation effects: In the hot leg, steam condensation rates vary with pressure and temperature, affecting ultrasonic level measurements.
Critical Design Note: In VVER-1000 plants, the RPV outlet nozzles (hot leg connections) are located above the core elevation. During reduced inventory conditions, the hot legs may not remain water-filled, creating a “steam binding” condition that severely affects differential pressure measurements unless reference columns are properly designed and maintained.
5. Engineering Design Insights
- Diverse measurement philosophy: The VVER-specific standard reinforces the principle that at least two fundamentally different measurement methods must be available for core cooling assessment during all shutdown states.
- Reference column design: For VVER plants with horizontal steam generators, the reference column design must account for the possibility of steam generator heat transfer affecting the temperature distribution in connected instrument lines.
- Data processing and display: The standard includes requirements for data validation, signal selection algorithms (e.g., median selection from redundant channels), and operator display formatting to ensure that the plant operating staff can rapidly assess core cooling status.
Pro Tip: When retrofitting VVER plants with upgraded instrumentation, pay special attention to Figures 4 and 5 of the standard, which illustrate the thermal-hydraulic conditions during reduced inventory and reverse flow. These conditions are the primary challenge for reliable core cooling monitoring and should drive the selection and installation of replacement instrumentation.
Frequently Asked Questions
Q1: What is the difference between IEC 62118 and IEC 62117?
IEC 62117 addresses core cooling monitoring for Western-design PWRs during cold shutdown, while IEC 62118 specifically addresses VVER-type PWRs. The key differences arise from the VVER’s horizontal steam generators, different loop configurations, and specific thermal-hydraulic characteristics.
Q2: What are the typical instrumentation challenges in VVER cold shutdown monitoring?
The main challenges include steam binding in instrument lines, reverse flow in some primary loops, reduced differential pressures at low power, and the unique thermal-hydraulic behavior created by horizontal steam generators during natural circulation conditions.
Q3: Does the standard apply to both VVER-440 and VVER-1000?
Yes, the standard covers both configurations. Where requirements differ between the two designs, this is explicitly noted (e.g., different nozzle locations, different loop configurations, different reference column arrangements).
Q4: How does the standard address the issue of instrument line maintenance?
Clause 10 requires periodic in-service testing and maintenance of all core cooling instrumentation, including verification of instrument line integrity, reference column level checks, calibration verification, and response time testing. The standard emphasizes that instrument line plugging or gas accumulation is a significant concern that must be addressed through regular surveillance.
