Post-implementation review of inadequate core cooling instrumentation
Summary (3 min read)
Instrumentation Needs for Detection of Inadequate Core Cooling
- Studies of the Three Mile Island (TMI) accident identified the need for additional instrumentation to detect inadequate core cooling (ICC) in nuclear power plants.
- Industry studies by plant owners and reactor vendors 1 supported the conclusion that improvements were needed to help operators diagnose the approach to or existence of ICC and to provide more complete information for operator control of safety injection flow to minimize the consequences of such an accident.
- In 1980, the U.S. Nuclear Regulatory Commission (NRC) required further studies by the industry 2 and described ICC instrumentation design requirements that included human factors and environmental considerations.
Review of RCITS
- The NRC, with assistance from Oak Ridge National Laboratory, reviewed generic RCITSs proposed by reactor vendors, instrument manufacturers, and individual utilities.
- Both of these systems were tested extensively under simulated accident conditions, including extremes of temperature and twophase flow, and the results have been reviewed.
- In addition to the generic reviews, the plant-specific installations at 66 PWRs were reviewed for conformance to NUREG-0737 requirements.
- Of these, 48 plants use one of the generic vessel water level measurement systems, 18 plants have unique designs, and 2 plants (3 units) use a gamma-thermometer (GT) level measurement scheme very similar in principle to the HJTC system.
- The remainder of the unique designs use dp measurement schemes with various configuration differences.
ICC Instrument Performance
- Subcooling margin monitoring provides an early indication of potential voiding but does not of itself provide any additional information about the possible approach of ICC.
- This situation has occurred in steam generator tube rupture events and could happen in any overcooling event that results in loss of pressurizer water level.
- Some loss-of-fluid tests (LOFT) 7 suggest that during reflood or coolant injection, CETs may be subcooled while the core remains voided.
- Some of those problems and the current status of industry efforts to achieve a reliable and unambiguous indication of water level were discussed recently in another paper.
- The possible effects of level measurement uncertainty for a typical small-break transient are shown in Fig. 1 .
- The Westinghouse Reactor Vessel Level Instrumentation System uses redundant sets of 3 dp cells to measure pressure drop from the bottom to the top of the reactor vessel and from the hot legs to the top of the vessel.
- A narrow-range transducer is calibrated to indicate full-scale with the static head across the vessel and pumps off, and the output is conditioned to display the equivalent collapsed liquid level in the vessel.
- The head-tohot-leg measurement is used for head venting operations during long-term recovery.
- For plants with upper head injection, the measurement from the bottom to the top of the vessel is omitted.
- The Combustion Engineering HJTC system measures reactor coolant liquid inventory using discrete HJTC sensors located at different levels within a separator tube that extends from the reactor vessel head to the top of the core.
RCITS Installation Experience
- Fifty operating power plant units have installed one of the two generic RCITS.
- The operating experience obtained so far has been generally satisfactory, although a number of early failures and problems occurred in both system types, some of which necessitated field design changes.
- Eleven units have chosen to design their own dp measurement systems or use other vendors.
- B&W type reactors, because of their geometry, were required by NRC to have level measurement from head to hot leg and additional measurement of hot-leg (candy cane) level.
- Thirteen additional units included ICC instrumentation in their initial licensing submittals and were reviewed separately within NRC.
- All but a few of the PWRs have completed installation of the ICC instrumentation systems and have acquired operating experience with them over 1 to 3 fuel cycles.
- While ICC instrumentation was intended primarily for accident monitoring, expanded uses have been found to improve routine operation as operators develop confidence in the indications.
- The SMM and CET system requirements were rather explicit in NUREG-0737.
- Conformance to these requirements has been good, and both performance and acceptance have also been good.
- As a result, there has been much more variation in design, performance, conformance, and acceptance.
Accuracy and Procedures
- NRC requirements have not included an absolute accuracy specification for water level asasurements.
- Analyses based on SfiLOCA scenarios suggest that ±6% is acceptable for procedural action requirements dealing with ICC.
- Typically the operator is not instructed to take action on the basis of level measurement system indication alone.
- Procedures were developed for some units with "conservative" decision points which, coupled with appropriate operator training, purportedly would not mislead operators into initiating inappropriate action.
- This procedure may cause problems in the case of events that call for a strategy of keeping the core covered while maintaining the water level below the vessel nozzles to reduce the flow of water out of the nozzles where a break may exist in the hot leg.
- Most utilities have done a good job of integrating ICC information displays into already full control rooms.
- An appropriate choice taken by many is to incorporate the primary ICC display in the safety parameter display system (SPDS) or other advanced information display systems.
- The addition of inadequate core cooling accident monitoring instrumentation to all PWRs is nearing completion after more than eight years.
- Indirect costs for items such as procedures and maintenance may add another $2H to $411.
- Cost effectiveness cannot be measured directly, but must be judged subjectively in relation to the socioeconomic impact of another accident similar to the one at TMI.
- The response and acceptance of the utilities has been largely positive, with only a few holdouts.
- The systems developed, while not providing the precision that might be desired, are sufficient to provide vital information for accident avoidance and recovery.
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"Post-implementation review of inade..." refers background in this paper
...Some of those problems and the current status of industry efforts to achieve a reliable and unambiguous indication of water level were discussed recently in another paper.(8) The possible effects of level measurement uncertainty for a typical small-break transient are shown in Fig....