Chapter 44: Nuclear Safety Backfits

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on December 18, 2012.


Backfit is defined as a design change in operating nuclear power plants required by safety-related regulatory decisions.  Here is another definition articulated by the United States Nuclear Regulatory Commission (NRC):  The modification of or addition to systems, structures, components, or designs of a facility, or design approval of a facility, or the procedures or organizations required to design, construct or operate a nuclear power reactor; any of which may result from a new or amended provision in the Commission’s rules or the imposition of a regulatory position interpreting the Commission’s rules that is either new or different from a previously established position.

Backfits are expected to occur as part of the regulatory process to ensure the safety of power reactors. It is important for sound and effective regulation, however, that backfitting be conducted by a controlled and defined process. The NRC backfitting process is intended to provide for a formal, systematic, and disciplined review of new or changed positions before imposing them.  The backfit process enhances regulatory stability by ensuring that changes in regulatory staff positions are justified and suitably defined. For example, even if not needed to meet the standard of adequate protection or to ensure compliance, backfitting is proper if a substantial safety benefit is realized and the costs are justified by the safety benefit.

It is now generally recognized that the nuclear power programmes of the Organization for Economic Co-Operation and Development (OECD) countries have reached a high level of maturity.  A great deal of information and lessons have been learned from the several thousand reactor years of operating experience and supporting research in the OECD countries, and these lessons have become institutionalized in various national laws, regulations, nuclear plant operating procedures and nuclear plant programmes such as training, security, quality assurance and emergency planning.

The fact of the matter is that while there are differences in the laws and regulations of each member country affiliated with the OECD, all regulatory bodies set a level of safety that must be achieved by nuclear power plants. In the past five decades of commercial power operation, regulators have often required safety backfits, for a number of reasons, including the following:

  • To maintain the required level of safety of a plant or plants;
  • To require compliance with existing regulations; and
  • To require substantial safety enhancements when new information or analyses show that such enhancements are necessary and practical to implement.

The term backfit for dealing with new safety issues is intended to include a range of regulatory approaches. In countries where the approach is less prescriptive, with only the objectives being set by regulations, a new backfit issue is resolved through a process that includes discussions between the regulator and the operator, without formal changes in the regulatory requirements. In countries with a more prescriptive approach, after discussions between the regulator and operator, a backfit means a new or changed requirement by the regulatory authority to modify the operating conditions of a plant, to modify the systems, structures or components of a plant, to modify the programmes or procedures used to support operation of a plant, to modify the organization used to support operation of the plant, or to modify the qualifications or training of safety workers at a plant.

Of course, there have been many instances where the nuclear plant operators, on their own initiative, have implemented backfits for the same reasons above. Some operators have adopted a policy of continuous improvement, which can also lead to operator-initiated backfits for improving safety.  This continuous improvement policy focuses on regular self-assessments of safety performance and includes programmes for corrective actions, operating experience evaluation, and benchmarking against best practices in the nuclear industry.  In some cases, new information has shown that safety margins were greater than believed, and that information has been used to relax some safety requirements.

It is no secret that the economic pressure of electricity market competition has led nuclear power plant operators to seek ways to increase electricity production and to reduce operating costs at their plants.  Just as market competition produces competitive pressures on nuclear operators, there will be corresponding pressures on the regulatory bodies that include the demand to reduce regulatory burdens perceived as unnecessary and a general resistance by operators to consider safety backfits sought by the regulator. A frequently voiced demand by nuclear operators is the need for regulatory stability – that is, a stable set of regulatory safety requirements that the operator must meet and that are not changed frequently by the regulator.  In other words, there is a growing pressure on regulators to reduce the number of safety backfits. This pressure presents a challenge to the regulator.

Some countries have adopted the concept of a Periodic Safety Review (PSR) for each nuclear power plant.  A PSR gives the operator the responsibility to review the overall safety of the plant against current standards and to evaluate and justify any deviations.  Experience in those countries has shown that operators generally have embraced PSRs because it allows potential backfit issues to be addressed in an integrated fashion and gives the operator the opportunity to put his safety case in an overall safety perspective.


While it is the responsibility of the operator to safely operate the nuclear power plant, the regulator has responsibility for independently assuring that all nuclear plants are operated safely.  It is in the interest of safety that the regulator and the operator work together in a professional manner to establish the basic facts in the situation and to agree on a plan to develop the additional data and information that is needed.Slide1The first question that must be asked in such situations is whether an immediate safety problem is present and whether urgent protective measures must be taken, such as the shutdown of the plant. This question of whether to take urgent protective measures may arise at any time during the backfit process.  Experience has shown that most often the safety issues do not require urgent protective measures.  If there is a difference of opinion on this matter, the regulator’s views must prevail.

Whether or not immediate protective actions are taken, the broader question remains whether a backfit is required to address the safety issue and, if so, what is the appropriate backfit? The regulatory approach to judging safety backfits is based on the model (Figure: 44-01) shown above.

After the new information raising a safety issue has come into clear focus and a decision has been made on immediate protective actions, a period of fact finding will be needed to assess the scope of the safety issue.

  • Are the physical phenomena understood?
  • Is more research or analysis needed?
  • How many plants are affected?
  • Do the plants meet the current regulations?
  • Will new regulations, license conditions or regulatory guides be needed?
  • Will safety be substantially improved by a backfit?
  • Are there legal, regulatory credibility or other public policy issues that must be considered?

The regulator may request the operator to perform special safety analyses, including probabilistic safety analyses (PSA).  This phase of the process may take time, perhaps several months or a year or more, and of course there will be frequent discussions with plant management during this process.

As fact finding proceeds, the regulator and the operator will begin to form preliminary views on what is needed to resolve the safety issue.  In some instances the operator may propose a voluntary backfit that is completely satisfactory to the regulator.  In such cases, the regulator need do little more than formally document the backfit commitment and monitor its implementation through the regulatory inspection programme.

A more likely situation is that the regulator and operator do not initially have congruent views on the remedy for the safety issue. This situation will require discussions with plant management, and it is usually best for the regulator to request the operator to propose a remedy for the issue. If multiple options are under consideration, the operator may perform comparative analyses of the options.

After fully discussing the backfit options with plant management, the regulator must come to a decision on the type of safety backfit (if any) and the allowed time to implement the backfit.  If the regulator concludes that a safety backfit is necessary to ensure the required level of safety, then the major remaining question is the time to implement the backfit. This judgment can usually be aided by insights from PSA as well as operating experience and an assessment of the robustness and effectiveness of the backfit.

In those cases where the required level of safety is not challenged, but where safety improvements are thought to be possible through a backfit, the regulator will balance the advantages and drawbacks of the proposed backfit.  Some regulators may use a largely qualitative approach, considering such factors as the degree of improved safety, improved public confidence, or other factors. There may be considerations other than health and safety in deciding on backfits – for example, environmental protection, nuclear material security and compliance with international obligations such as non-proliferation objectives.

Other regulators may choose to use a more quantitative approach to judging backfits, such as the following two-part test:

  • The backfit must provide a substantial increase in safety; and
  • The direct and indirect costs of implementing the backfit must be justified in view of the substantial increase in safety.

In judging whether a proposed backfit provides substantial additional safety protection, the analysis should follow established regulatory guidelines.  One may use PSA for insights into quantitative benefits, for example the incremental reduction in core damage frequency resulting from implementation of the backfit. If the proposed backfit meets the first test of substantial increase in safety, the regulator may request cost-benefit information from the operator or may develop its own cost-benefit analysis. In evaluating whether the backfit benefits outweigh the costs, the analysis should include all costs associated with implementing the backfit – for example, design, procurement, installation, worker radiation exposure, procedure revision, training and costs for any plant shutdown time. Likewise, the benefits should include the reduced likelihood of accidents and their consequences (i.e. all averted costs including averted radiation exposure).

Whether the regulator uses a qualitative or quantitative approach, or some mixture of the two approaches, after reviewing the pros and cons of backfit options with the plant management the regulator must decide whether to impose the safety backfit.

If the decision is for a backfit, as mentioned above the regulator must also specify a time for completion of the backfit implementation, after discussions with the plant management.  In some cases (e.g. steam generator tube inspections and tube plugging which are normally done during plant outages) the timing will be clear.  In other cases involving design modifications and hardware changes, the practicalities of the design, procurement and installation processes will be major considerations in the implementation time, along with judgments of the safety importance of the backfit. A factor in the timing decision may be the use of compensatory actions while the backfit is being implemented.

After the decisions on the backfit and implementation time have been communicated to the operator, the regulator should maintain a dialogue with the operator as the planning for the backfit progresses.  In some cases, the regulator may want to review and approve any design changes to be sure that there are no unintended systems interactions or negative safety effects of the backfit.

The operator must review the design changes and bring the safety analysis reports, operating and maintenance procedures, training programme and other programmes into conformance with the backfit before it is actually implemented.  Likewise, the regulator should ensure that the relevant regulatory guidance is revised, if necessary, to conform to the backfit decision.  The regulator should plan to inspect the actual backfit implementation through the regulatory inspection programme.

As a conclusion to this regulatory approach for judging safety backfits, and in the spirit of improving regulatory performance, the regulator should consider conducting a retrospective self-assessment.  Some of the questions that such a self-assessment could address are:

  • Could the process for identifying new safety problems be improved?
  • Was the fact-finding process concerning the new safety problem thorough?
  • Were the interactions with the operator conducted professionally?
  • Were communications with the public adequate?


While the operational safety performance of nuclear power plants in OECD countries has improved in recent years, neither operators nor regulators should allow that performance to be a cause for complacency.  Regulatory authorities can still expect to be confronted with challenging safety backfit decisions from time to time.  There will continue to be situations where operating experience or new information will give rise to safety issues and questions concerning the need for safety backfits.  In this regard, regulatory bodies should continue to share essential safety information with their international colleagues.


  1. Nuclear Power Com;
  2. The United States Regulatory Nuclear Commission;
  3. US Nuclear Regulatory Commission;
  4. NEA Improving Nuclear Regulatory Effectiveness; and
  5. The Nuclear Regulatory Challenge of Judging Safety Backfits.

Chapter 45 …