Nuclear Power Plant Safety Engineering countermeasures for radioactive material release

In order to prevent the release of radioactive material into the environment, nuclear power plants are designed to be resistant to a number of natural destructive forces, such as hurricanes, tornados, and earthquakes (U.S. Nuclear Regulatory Commission [U.S. NRC], 2011a). Within the plant, critical systems are designed to limit and control radioactive material release should they fail. These design considerations include the following (U.S. NRC, 2011a):

fire prevention, detection, and suppression strategies and technologies redundancies, compartmentalization, and automation in critical instrumentation and controls independent protection systems and controls a control room designed so that it can be occupied safely during an accident

reactor power oscillation suppression and reactivity limit controls a closed loop coolant system that can exceed normal operational conditions and is amenable to regular inspections and testing during reactor operation reactor coolant leak detection system

an emergency core cooling system amenable to periodic inspection and testing leak-tight reactor and coolant system containment barriers designed to control the release of both contaminated liquids and gas (atmosphere).

auxiliary and redundant power sources for critical systems designed to default to 'safe' mode should they fail auxiliary system (heat sink) to rapidly remove heat from the reactor in the event of coolant system failure both reactor and coolant components should behave in a non-brittle manner should design specifications for normal operation be exceeded

Fuel designs that minimizes cladding corrosion (U.S. NRC, 2008, p. 23)

Periodic non-destructive evaluation of reactor components and coolant systems help anticipate component failure before it happens (U.S. NRC, 2008, p. 35)

To mitigate environmentally assisted cracking, materials resistant to mechanical, chemical, radiation, and temperature stresses have been incorporated, and water chemistry is monitored and modified if necessary (U.S. NRC, 2008, p. 35)

What is a Quality Assurance Program?

A Quality Assurance Program (QAP) represents written criteria for ensuring procedures, materials, and services meet specific standards of quality (Total Quality Assurance Services, 2011a). A QAP ensures the performance, durability, and/or safety of a product or service satisfies the customer's needs and expectations. For example, this approach is used to minimize mistakes in a surgical wing of a hospital or to prevent sports stadium roofs from succumbing to tornadic wind shear forces.

Quality assurance typically involves establishing criteria for material specifications, in addition to inspection and testing procedures that ensure materials and components meet the quality standards (Total Quality Assurance Services, 2011b). Periodic audits are used to maintain the desired level of quality. Developing and implementing a QAP for a large project can therefore require a significant commitment of time and resources, but can also result in cost savings by minimizing resource and time investments in defective materials.

What is the NQA-1 structure?

The Nuclear Regulatory Commission has established minimum standards of quality assurance in the design, construction, and operation of nuclear facilities. These standards are described in the document 10 CFR 50, Appendix B, and are also called NQA-1 (Nuclear Quality Assurance-1) or just Appendix B (U.S. NRC, 2011b). Appendix B is structured so that a nuclear QAP is established at the earliest stages of the design process and propagated through all subsequent phases of design, construction, and operation of a nuclear facility. This ensures that safety is of primary concern when designing, manufacturing, and assembling the various components that make up a nuclear power plant.

All government agencies, research universities, and commercial contractors responsible for the design and safe operation of nuclear power plants and fuel reprocessing plants will establish their own QAP, but the baseline quality assurance criteria required to obtain a license to design, build, test, and operate nuclear facilities within the United States is Appendix B. Several versions of the original NQA-1 have been published, the most recent one in 2007.

10 CFR 50, Appendix B requirements

The quality standards covered in Appendix B include the following (Atomic Energy of Canada Limited, 2003):

Organization -- establishes responsibility for the development and implementation of the QAP, which is ultimately the applicant/licensee

Quality Assurance Program -- states that the QAP should be written and implemented at the earliest possible stage in the design process

Design Control -- implementation of the QAP in the design stage

Procurement Document Control -- all safety aspects should be documented during the procurement of material, equipment, and services

Instructions, Procedures, and Drawings -- all design materials should contain explicit criteria and instructions for quality standards

Document Control -- all design documents containing quality standards will be controlled by authorized personnel

Control of Purchased Items and Services...

NRC, 2010). These regulations require workers who are likely to receive a dose of radiation in excess of 100 mrem per year to first go through appropriate training prior to working with the material. Maximum annual exposures are limited to 5,000 mrem. A worker's history of exposure is maintained at a central repository, called the Radiation Exposure Information and Reporting System (REIRS). Should a worker desire a copy of their exposure history, they can submit a request to REIRS.
The NRC also requires licensees to post notices of regulations, licenses, violations, and operating procedures in convenient locations for viewing by workers (U.S. NRC, 2011c). Nuclear workers are required to be informed about their responsibility in the proper storage, use, and transfer of radioactive materials. Workers also have the right to request an inspection and speak to inspectors privately, should they be concerned about potential safety issues.

The diminishing nuclear workforce

In 2006 the chairman of the NRC, Dale Klein, spoke to female nuclear workers about the coming staffing crisis for nuclear power plant operations (Klein, 2006). Half of the workforce were over the age of 47 in 2006 and close to 40% were predicted to retire by 2011. The nuclear services and support industry is facing a similar situation and by 2009 nearly 32% of that workforce was expected to retire.

Klein further related that by 2006, 13 companies had announced intentions to apply for operating licenses for 27 more nuclear reactors. This is addition to the 104 nuclear power plants still in operation in the United States, which have no plans for decommissioning. To make matters worse, 35% of university nuclear engineering programs have been closed over the years since the 1970's. The nuclear industry staffing crisis raises important safety concerns when it comes to the proper operation and maintenance of extant power plants.

Nuclear plant fire protection strategies

Today, nuclear power plants rely on redundant fire protection systems to ensure fires can't interfere with procedures required to operate or shut a plant down safely (U.S. NRC, 2011d). These strategies include fire barriers, detection, and suppression systems. If such strategies aren't appropriate in some situations, for example at older power plants, then manned fire watches are established. Other strategies include allowing power plants to focus on areas of greatest fire risk, eliminating or protecting fire-susceptible electrical circuits, establishing fire response procedures, and modifying or redesigning fire-susceptible materials and components.

Goals of fire protection at nuclear power plants

A fire in 1970 at the Browns Ferry Nuclear Power Plant in Decatur, AL brought fire safety at nuclear power plants to the forefront, primarily because multiple redundant systems failed due to the fire (Nuclear Energy Institute, 2010). This made it difficult to shut the reactor down safely. As a result, the NRC established regulations designed to minimize the risk of fire at power plants.

The primary goal of fire protection strategies at nuclear power plants is the safety of plant workers and any communities that could potentially be affected should a catastrophe occur. In order to achieve this goal, fire protection strategies have been designed and implemented to protect critical systems required for safe operation of power plants, or a safe shut down should a large fire occur.

Notes

Atomic Energy of Canada Limited (2003). Comparison of 10 CFR 50…