IEEE 323 – Qualifying electrical equipment to the harsh environments of nuclear power plants

Cherenkov Radiation in the Advanced Test Reactor core, Idaho National Laboratory

IEEE 323 is the standard used to qualify electrical equipment for safety-related use in nuclear power plants. Here, we explore that standard and its requirements.

Does a certain piece of electric equipment installed in a nuclear plant perform a safety-related function? If so, then it must be qualified for its environment. This qualification must be documented.

Nuclear power plants have two unique properties that set them apart from other power plants:

  1. Particularly harsh environments created by the presence of radioactive materials
  2. An important category of critical systems, structures, and components (SSCs) whose primary function is to prevent or mitigate the release of radioactivity to the environment. This class of SSCs is called “safety-related.” That’s because they are directly related to protecting the health and safety of the public.

Certain types of electric equipment perform safety-related functions. They are classified as Class 1E equipment. Certain motors and valves, for example, are required to be installed in radioactive environments. The environments are radioactive either normally or following a design basis event. Environments can change substantially between normal operation, where conditions may be rather standard, and post-accident conditions, where temperatures, moisture, and radiation may become severe. A high-energy line break can produce this kind of drastic change.

Because electric equipment that performs safety-related functions can be subjected to harsh conditions, the NRC requires proof that the equipment won’t fail under those conditions. Safety-related equipment is most critical during and after an accident, but since accidents produce the harshest environmental conditions, it is more likely that important equipment will fail under those elevated stresses — the time when it is needed most.


The NRC codifies into law the requirement that safety-related electric equipment (i.e. Class 1E equipment) be qualified for its environment. This qualification must be documented. The general design criteria (GDC) in Appendix A of 10CFR50 establish these requirements. The fourth GDC, for example, requires general environmental qualification:

Criterion 4—Environmental and dynamic effects design bases. Structures, systems, and components important to safety shall be designed to accommodate the effects of and to be compatible with the environmental conditions associated with normal operation, maintenance, testing, and postulated accidents, including loss-of-coolant accidents

Similarly, Criterion 1 requires documentation be kept to ensure quality assurance.

Specific requirements for environmental qualification of Class 1E equipment are codified in 10CFR50.49. Three types of electrical equipment are covered:

  1. Safety-related electric equipment
  2. Non-safety-related electric equipment whose failure under postulated environmental conditions could prevent satisfactory accomplishment of safety functions
  3. Certain post-accident monitoring equipment.

The NRC issued Regulatory Guide 1.89 to endorse IEEE 323-1974 as a satisfactory means of implementing 10CFR50.49.

RG 1.89 was first issued in 1974, then revised in 1984. The Reg Guide also contained several clarifications that needed to be combined with the IEEE standard to make its use acceptable to them. The NRC reviewed it in 2014 and affirmed its content. It also noted that it should be updated soon to incorporate the later revisions to IEEE 323 that have been issued since 1974.


The standard is divided into eight sections:

  1. Scope
  2. Purpose
  3. Definitions
  4. Introduction
  5. Principles of Qualification
  6. Qualification Procedures and Method
  7. Simulated Service Condition Test Profile
  8. Documentation

Section 6 is by far the longest section and the bulk of the document.

The standard opens up by explaining that it contains the requirements for qualifying Class 1E equipment. When the standard is followed, the equipment design will be adequate under “normal, abnormal, design basis event, post design basis event, and containment test conditions for the performance of Class 1E functions.”

The standard affirms that using its methods to qualify equipment will fulfill the requirements of IEEE 279 and IEEE 308. The NRC requires that plants adhere to IEEE 279 (in 10CFR55).

Several terms are defined, including Class 1E and design basis event. A particularly helpful definition is that of a type test, which is a test “made on one or more sample equipments to verify adequacy of design and the manufacturing processes.” In other words, you test a representative sample from the production line, and if the representative passes then they all pass.

Manufacturers and power plants are required to provide assurance that Class 1E equipment will meet or exceed its performance requirements throughout its installed life. One aspect of the entire quality assurance program is qualification, which this standard covers. Qualification can be achieved through three methods:

  1. Type testing
  2. Operating experience
  3. Analysis

Type testing is the preferred method. Representative samples are subjected to the simulated harsh environments that they will be installed in. Margin is added to cover uncertainties. Operating experience is limited in its ability to demonstrate full qualification but is used to supplement testing and fill in the gaps that tests can’t cover; it is more appropriately applied to the qualification of equipment installed outside containment. Analysis can be used if its methods, theories, and assumptions can be justified, but it is generally recognized that electrical equipment is too complex to qualify by analysis alone. However, analysis can be used to extrapolate and extend testing qualification when minor design changes have been made to previously tested equipment.

Whatever methods used, the ultimate results must be documented in a step-by-step report that can be followed and reviewed by someone who is reasonably skilled.


Section 5 provides the six principles of qualification that will be fulfilled by the methods supplied in Section 6. The principles include:

  • Assuring that the severity of the qualification methods meets or exceeds the expected installed conditions
  • Requiring that potential failure modes and mechanisms be considered
  • Granting provisions for on-going testing of equipment that will be in service longer than it’s qualified for
  • Keeping documentation of the methods and bases employed in the qualification process

Section 6 goes into details about the methodology for each of the qualification methods: type testing, operating experience, and analysis. All methods begin with describing the equipment’s specifications. The spec should include:

  • Performance requirements under normal, abnormal, containment test, design basis event, and post-design-basis-event conditions
  • Range of electrical characteristics (voltage, frequency, etc.)
  • Mounting method
  • Preventative maintenance schedule
  • Design life
  • Auxiliary devices within — or connected to — the equipment required for proper operation (indicating lights, etc.)
  • Range, type, and duration of environmental conditions
  • Operating cycles
  • Qualified life

The bulk of Section 6 focuses on the testing procedure. Testing begins with a test plan that is detailed enough to justify that the proposed conditions and test sequence are sufficient for the target environment.

During the test, equipment must be mounted according to the manner and position in which it will be installed. Similarly, electrical connections must be simulated, including conduit and cable connections. External conditions and inputs, and the equipment’s electrical and mechanical characteristics, must be monitored during the test by calibrated instruments. Suggested margin parameters are provided for various characteristics (voltage, frequency, radiation, etc).

A general worst-case test sequence is supplied:

  1. Inspection performed to verify the equipment is in good condition
  2. Base-line operation conducted at normal conditions
  3. Operation conducted at the worst-case conditions described in the product specifications (but not at design basis event or post-accident conditions)
  4. Aging performed to end-of-life conditions
  5. Vibration testing performed on aged sample
  6. The aged equipment is then operated under design basis event conditions
  7. The equipment is then operated under post-accident conditions
  8. The unit is inspected and disassembled as necessary to evaluate and documents its condition

Technical guidance is provided on how to conduct aging, radiation, vibration, DBE, and post-accident testing, as well as what details to be concerned about during the final inspection.

Qualification based on operating experience requires detailed documentation of the equipment’s installed configuration, operating history, and failures. Emphasis is placed on the need to attain measured data.

Qualification by analysis must include the construction of a reliable mathematical model using established principles of physics and engineering. Extrapolation is used to extend type testing.

Two ways to perform on-going qualification are offered. If the methods fail, then a program of periodic replacement must be instituted for equipment with insufficient qualified life.

The standard wraps up by requiring the plant to furnish a design basis event profile for conducting simulation testing. It also details the documentation requirements.

There are three appendices. Appendix A contains general accident profiles and testing parameters for simulating design basis events in boiling water reactors (BWRs) and pressurized water reactors (PWRs). Appendix B contains the same, but for high-temperature gas-cooled reactors; there are none in operation for commercial power production in the US. Finally, Appendix C contains a suggested method for measuring test chamber environments at high temperatures and moisture content.


Class 1E equipment must be qualified to the requirements of IEEE 323. There could theoretically be other ways to qualify it, but the NRC endorsed IEEE 323 in Regulatory Guide 1.89. Consequently, it’s become the de-facto industry standard. The NRC endorsed the 1974 edition of the standard, but it has been revised since then. The NRC acknowledges this, so we shouldn’t be surprised to see a new revision of RG 1.89 in the future that updates its endorsement to include the later versions of the standard.

The 1974 edition of IEEE 323 can be downloaded by clicking here.

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