Chapter 5: Decommissioning of Nuclear Facilities and Radioactive Waste Management

Decommissioning is the set of technical and administrative actions taken to manage the radiological hazards of a nuclear facility that has ceased operation and to bring the facility and its site to a defined end state. It covers radiological characterization of the facility, dismantling, the processing, storage and disposal of waste, the assessment of residual contamination, and site release. Permanent shutdown of a reactor, removal of spent nuclear fuel, dismantling of the facility and site release each have their own completion conditions, and completion of each stage is confirmed separately.

The current IAEA safety guide on the decommissioning of nuclear power plants, research reactors and other nuclear fuel cycle facilities is SSG-47, Decommissioning of Nuclear Power Plants, Research Reactors and Other Nuclear Fuel Cycle Facilities (2018), which superseded WS-G-2.1 (1999). This chapter treats the general engineering concepts of decommissioning separately from the data requirements defined by WIA-ENE-026 v1.0. The WIA data model and API are described on the basis of §4 and §10 of the standard, and Section 5.5 sets out how records are kept when the code scheme of PHASE-1 is used alongside them.

5.1 Decommissioning Strategy and Decision-Making

A decommissioning strategy is a plan that links the timing of dismantling with the residual radioactivity, the safety functions that must be maintained, spent nuclear fuel management, the availability of waste acceptance, funding and staffing, and the conditions for the final use of the site. Options that satisfy the applicable safety and licensing conditions are identified first; only then are they compared for workload, cost and schedule variability.

5.1.1 Immediate and Deferred Dismantling

IAEA SSG-47 presents two decommissioning strategies as applicable in principle, immediate dismantling and deferred dismantling, with both defined as in IAEA General Safety Requirements GSR Part 6. Because it avoids transferring the burden of decommissioning to future generations, SSG-47 generally regards immediate dismantling as the preferred strategy, while noting that there may be situations in which deferred dismantling is more appropriate once all relevant factors are considered. An example is a multi-unit site with shared systems, where dismantling of one unit has to wait until the other units cease operation.

Immediate dismantling is a strategy in which dismantling proceeds once the necessary preparations and approvals following the end of operation are in place. It makes it easier to draw on staff with operating experience and on facility records, and it reduces the burden of maintaining the facility over a long period. On the other hand, demand for processing, packaging and storage, and expenditure, may be concentrated in the early phase of dismantling, so waste management routes and radiation protection measures for workers must be secured at the same time. “Immediate” means that dismantling proceeds in a timely manner without deferral; the date by which site release is completed is set separately in each project plan.

Deferred dismantling is a strategy in which the necessary safety functions and monitoring arrangements are maintained for a period before dismantling. The decay of some radionuclides can reduce the radiological burden of later work, but in the meantime the degradation of structures and equipment, the cost of monitoring and maintenance, and the possible loss of records and expertise must be managed. Changes in total worker dose and total cost are judged by weighing the reduction in radioactivity against these management burdens.

In the United States, the federal regulation 10 CFR 50.82(a)(3) requires decommissioning to be completed within 60 years of permanent cessation of operations, and provides that completion beyond 60 years will be approved by the Nuclear Regulatory Commission (NRC) only when necessary to protect public health and safety. These 60 years are the entire period from shutdown to completion of decommissioning, and differ from a value that refers only to the period during which a facility is held in safe storage under deferred dismantling. DECON (immediate dismantling), SAFSTOR (long-term safe storage for deferred dismantling) and ENTOMB (entombment) are decommissioning terms defined in the NRC glossary. Projects in other jurisdictions apply the deadlines and procedures set by the relevant regulatory framework.

5.1.2 Conditions for Entombment

Entombment is an approach in which all or part of a facility is encased in a long-lived structural material and left in place. Responsibility for control continues after the structure has been covered or sealed, and the long-term behaviour of the residual radionuclides, the performance of the isolating structure, restrictions on site use and the responsibility for control must all be assessed.

IAEA SSG-47 does not consider entombment an acceptable strategy for planned decommissioning. It explains, however, that entombment may be considered as a last resort for a facility damaged in an accident, where other options are not possible because of worker exposure or technical difficulties. Even then, the extent of the entombed portion and the radioactivity left on site, in particular the amount of long-lived radionuclides, should be reduced as far as possible, and the remaining barriers must be capable of monitoring and maintenance. Entombment leaves the burden of long-term monitoring to future generations.

5.1.3 Strategy Comparison Table and Plan Updates

The following table is an example of a qualitative comparison table that can be used in a project review. Where weights are assigned to the items, the basis for them is explained for each project. Cost advantages are compared only among options that meet the essential safety and licensing requirements.

A decommissioning plan becomes more detailed as knowledge of operating records and facility conditions grows. Re-evaluation of the radionuclide inventory, changes in the dismantling scope, changes in waste acceptance conditions and changes in the availability of storage facilities are all reasons to review the schedule and cost again. IAEA SSG-47 explains that, in accordance with GSR Part 6, the operator updates the decommissioning plan and the regulatory body reviews it periodically (typically every five years or at an interval set by the regulatory body), and that the plan is also updated when special circumstances arise. The final decommissioning plan is a document that is kept up to date to reflect the current state of the facility and the progress of the project. Each revision of the plan records what changed, the basis for the decision, when the change applies and the results of the review.

5.2 Decommissioning Cost Estimation and the ISDC

The International Structure for Decommissioning Costing (ISDC) is a framework for presenting decommissioning costs, published jointly by the OECD/NEA, the IAEA and the European Commission in 2012. It arranges decommissioning costs under common items so that the scope and content of estimates can be compared. A direct cost comparison requires not only the same item names but also the same project scope and estimating conditions. A cost estimate report for an individual project is an application of the ISDC structure and is distinct from the document that defines the ISDC.

The ISDC is a numbered three-level hierarchy. The top level consists of 11 principal activities, and the levels below are shown by adding digits to the parent number, as in 01.0100 (decommissioning planning) and 01.0101 (strategic planning). The following table explains the 11 top-level items. The numbers are ISDC item numbers and are unrelated to WIA data fields.

5.2.1 Comparable Cost Estimates

A cost estimate requires a basis for quantities, work productivity, unit rates, work durations and common maintenance costs. Waste costs must distinguish the volume of structures before dismantling, the volume of waste before processing and the volume after packaging. If a disposal facility charges per waste package, the disposal unit rate is applied to the packaged volume, not to the structural volume.

As a hypothetical example, if a project has 100 m³ of material to be processed and the unit rate for that processing is assumed to be KRW 2 million per m³, the direct processing cost is 100 × KRW 2 million = KRW 200 million. This figure is a hypothetical example intended only to show the method of calculation. If packaging, transport, storage, disposal, project management costs and a contingency for uncertain elements are not included, this must be stated separately.

For comparisons between projects, the price base date, currency, project boundary, inclusion or exclusion of spent nuclear fuel management costs, storage period and extent of site restoration are aligned. Discounted present values and undiscounted sums of expenditure are also distinguished. Even where deferred dismantling yields a lower present value, the actual workload and total expenditure are checked separately as undiscounted values.

5.2.2 Cost Classification and Funding

Estimated costs and the funds actually available are different things. IAEA SSG-47 recommends that cost estimates and funding provisions be reviewed periodically and adjusted for inflation, technological developments, waste management costs and regulatory changes, and explains that such reviews are particularly important for deferred dismantling, where decommissioning may be completed several decades after shutdown. Totals by ISDC item support these reviews, while the adequacy of funding is demonstrated separately through a funding review.

The levels of the ISDC hierarchy and the number of digits in a code are different concepts. When an ISDC code is assigned to a task, the item is checked against the official code list. Because the WIA-ENE-026 v1.0 data model deals with records at the level of the waste package, the cost structure is managed in project management documents and linked to the related waste records through packageId.

5.3 End State and Site Release

5.3.1 Exemption, Clearance and Site Release

Decommissioning requires a distinction between the control status of materials and the use status of the site. Clearance of some demolition materials and release of the entire site are separate steps, and responsibility for waste remaining in other areas continues after part of the site has been released. The following definitions follow the concepts of the IAEA Safety Glossary.

Exemption

A determination by the regulatory body that a source or practice need not be subject to some or all aspects of regulatory control, for reasons such as the exposures and potential exposures due to the source or practice being sufficiently small.

Clearance

The removal of regulatory control by the regulatory body from radioactive material or radioactive objects within notified or authorized facilities and activities. Obligations relating to chemical hazards or to conventional waste remain in place.

Site release

A decision, based on an assessment of the residual radioactivity on the site and the conditions of its future use, to end radiological regulatory control over the site or to adjust that control with restrictive conditions.

Release for unrestricted use and release with restrictions on use are concepts that describe the end state. IAEA SSG-47 regards unrestricted release from regulatory control as the preferred end state and ultimate goal of decommissioning. Even where unrestricted radiological use is permitted, other land-use regulations continue to apply. Where restrictions on use remain, the content of the restrictions, responsibility for implementing them, and how monitoring and records are to be maintained must be defined together.

5.3.2 Demonstrating the End State

A site release decision requires information on the residual radionuclides, the spatial distribution of contamination, the media to be assessed such as soil, buildings and groundwater, and the future use scenarios and exposure pathways. On this basis the survey scope and assessment methods are defined, and the results are checked against the criteria that apply to the project.

Public dose limits, clearance criteria for materials and assessment criteria for site reuse apply in different contexts. The dose limits in §5.1 of the WIA-ENE-026 v1.0 main text serve to control the exposure of workers and the public, and the surface contamination criteria in §5.3 serve to control surface contamination. These values are not substituted directly as site release criteria; site release applies the criteria approved in the relevant jurisdiction.

The final survey record documents the measurement locations and objects, the measurement methods, calibration information, detection limits and uncertainty, and the assumptions used in the assessment. “Not detected” does not mean that no radioactivity is present; it means that none was detected under the given measurement conditions. Only when survey results are linked to the basis for approval can the end state be reviewed again.

Where a spent nuclear fuel storage facility or a waste storage area remains on the site, the boundary between the area to be released and the area that remains under control is clearly defined. Completion of dismantling work, cessation of storage facility operation and release of the entire site are each subject to separate decisions.

5.4 Classification and Management Routes for Decommissioning Waste

5.4.1 Scope of Use of the International Classification

IAEA General Safety Guide GSG-1, Classification of Radioactive Waste (2009), divides radioactive waste into six classes. The definition of each class was given in Section 1.1, so this section sets out the points to confirm when applying them to decommissioning waste. The classification considers not only activity concentration but also radionuclide half-lives, the need for long-term isolation, heat generation and the applicable disposal concepts. GSG-1 leaves the permissible radionuclide-specific activity values to be set in the safety assessment for each individual disposal site. The following table is an example of applying the classification concepts to decommissioning; national licensing criteria and the acceptance criteria of individual facilities are confirmed separately.

Class boundaries depend on the radionuclides and the conditions of application, so the boundaries between EW, VLLW and LLW are also confirmed radionuclide by radionuclide. Total activity, activity concentration, surface contamination level, external dose rate and thermal output are different physical quantities. Suitability for disposal is judged after each of these quantities has been confirmed.

PHASE-1 uses “half-life less than 100 days” at the VSLW decision branch. This criterion refers to the half-life of the radionuclide; the time needed to reach clearance levels depends on the ratio of the initial activity to the target value. Where long-lived radionuclides are also present, or where a radionuclide continues to be produced from a parent radionuclide, a separate assessment is needed. Actual classification also checks the criteria of the applicable jurisdiction.

5.4.2 Origin Versus Final Class

Surface contamination is the state in which radioactive material adheres to the surface of a material or object, whereas activation is the production of radionuclides within the material itself. Waste volumes and processing methods can only be assessed once the portion removable by surface decontamination has been distinguished from the radioactivity that remains in the material itself.

The class of activated material is determined by its radionuclide composition, activity concentration, content of long-lived radionuclides, heat generation and the applicable classification scheme. On these criteria activated material, too, falls into whichever class applies, from VLLW to HLW. Knowing that a waste item originated as a reactor component is the starting point for characterization; the final class is determined by the results of characterization.

Waste volumes are estimated by location of origin and by material, and secondary waste generated during processing, such as filters, cleaning solutions and consumables, is also taken into account. Distinguishing the quantity of material before dismantling, the quantity of waste after processing and the packaged storage and disposal volume allows cost and facility capacity to be compared on the same basis.

5.4.3 Link to Disposal Facility Acceptance Conditions

The waste class is only part of the information used to select a disposal facility. Actual acceptance is governed by the facility’s waste acceptance criteria, including radionuclide-specific limits, waste form, chemical properties, waste package performance, thermal output and documentation requirements. Spare capacity at storage facilities and whether a particular waste package can be accepted are also confirmed separately.

For intermediate level waste, §2.1.2 of the standard gives intermediate-depth disposal for ILW-1, deep disposal for ILW-2 and “deep geological disposal” (written in this book as geological disposal) for ILW-3 as the disposal methods. EW and VLLW differ in whether clearance applies and in the acceptance conditions of the permitted facilities, so a separate route is set for each. The start of operation and the operator of future disposal facilities are reflected in the plan when confirmed project information is available.

5.5 Recording Classification Codes and Decay Calculations

5.5.1 Applicable Documents and Classification Codes

The descriptions of waste packages and the API in this chapter are based on the main text of the standard, v1.0, and implementation documents state the source and version used. For the differences between the main text and PHASE-1 in the half-life value of Cs-137 and in the detailed classification codes (LLW-1 to LLW-4 versus LLW-A and LLW-B, HLW-1 versus HLW-SF, and so on), see the Standard Reading Notes in Chapter 1. An implementation that uses both code schemes states in its interface contract which document’s codes serve as the primary codes, and preserves the codes of the other scheme alongside as reference values.

When IAEA classification categories and WIA detailed codes are used together, the international class name, the management class of the applicable jurisdiction and the WIA wasteClass are each recorded, and the basis for the judgement about how they relate is preserved. The classification table in §2 of the main text and the decision diagram in PHASE-1 are each cited with their source.

5.5.2 Meaning and Limits of a Simple Decay Calculation

The activity of a single radionuclide, disregarding ingrowth from other radionuclides and external input, is calculated with the following equation. The equation and symbols are the same as the decay equation in Section 1.4.2. A₀ is the activity at the reference date, A(t) is the activity after elapsed time t, and T_½ is the half-life. The same time unit is used for t and T_½.

A(t) = A0 * 2^(-t / T_half)

As a hypothetical example, take Co-60 with an initial activity of 1.0 × 10⁶ Bq and apply the half-life of 5.27 years given in both the v1.0 main text and PHASE-1. Since 10.54 years is twice the half-life, the activity falls to one quarter of its initial value, 2.5 × 10⁵ Bq. This is a hypothetical single-radionuclide example intended only to show the method of calculation.

The result describes only the decrease in the activity of that radionuclide. Whether mixed waste can be cleared, the spatial radiation field, worker dose and the suitability of a waste package for disposal are judged by separate assessments. Where decay chains are significant, ingrowth of progeny is also considered, and actual assessments use the reference date and uncertainty of the measured values. Recording the half-life value used, and its source, alongside results calculated through the API allows those results to be reproduced in later reviews.

5.6 Scope of Use of the WIA Data Model and API

5.6.1 Using Waste Package Records

The waste package model in §4 of the standard can be used to record the identification, characteristics, processing, storage and chain of custody (hand-over and acceptance records) of waste arising from decommissioning. It is a record model at the level of the individual waste package; the decommissioning plan and licensing status of the facility as a whole are handled in project management documents.

WIA-ENE-026 v1.0 does not define dedicated data items or APIs for decommissioning phases, decommissioning costs, plan revisions, strategy recommendations or site release decisions. Projects that need such information manage it in separate project management documents and link it to waste package records through packageId.

When material is divided or several streams are combined during dismantling and processing, the relationship between the location of origin and the final waste package must be preserved. traceability.chainOfCustody records the movement and hand-over history of a waste package as entries of time, event, location and responsible person. Where split and merge relationships are supplemented with project records, those items are marked as project-specific extensions to distinguish them from the mandatory fields of the standard.

5.6.2 APIs Defined in the Standard

The API in §10 of the standard covers waste packages, monitoring, dose and decay calculations, and storage facility information. The full list of APIs is covered in Section 2.8; the following table sets out the endpoints that can be used directly in decommissioning waste management and how they are interpreted in decommissioning.

For the range of values of the list filter wasteClass and its relationship to the detailed codes, see the Standard Reading Notes in Chapter 1; the interpretation adopted in an implementation is written into the interface contract.

The following example expresses the Co-60 calculation conditions of Section 5.5 in the request format of POST /dose/decay. The field names and the notation for time units follow the request example in §10.1.3 of the standard.

{
  "isotope": "Co-60",
  "initialActivity": 1.0e6,
  "decayTime": 10.54,
  "unit": "years"
}

In this example the initial activity is taken to be in Bq. The request in §10.1.3 of the standard consists of four fields, isotope, initialActivity, decayTime and unit, and the response returns the result of the decay calculation. When systems are integrated, the unit of the initial activity, the structure of the response fields and the half-life value used in the calculation are recorded in the interface contract agreed by both parties, and calculation results are labelled so that they are kept distinct from clearance approval.

5.6.3 Authorization, Audit Records and the Boundary of the Standard

Security requirements such as authentication methods, transport security, encryption and audit logs are set out in Section 4.8.1. For decommissioning records, the permissions given in §10.1 of the standard apply to registration, retrieval and location changes, while long-term facility management records and the basis for site release are managed separately from audit logs, each under a retention policy suited to its own retention purpose and conditions of application.

§10.3 of the standard defines the webhook events package.created, package.moved, monitoring.alarm, dose.exceeded, inspection.due and safeguards.report. These events report occurrences at the level of waste packages and monitoring; completion of a decommissioning phase for the facility as a whole is confirmed separately in project management records.

5.7 Linking the Project Schedule with Decommissioning Infrastructure

5.7.1 Define Completion Conditions Before Durations

A decommissioning schedule pairs each task with its start conditions and the evidence of its completion. For example, the state in which fuel has been removed from the reactor differs from the state in which spent nuclear fuel has been shipped off the site. Completion of dismantling for part of a facility and completion of waste disposal are likewise distinguished. Stating these differences explicitly prevents the error of reporting different project states under the same word, “complete”. IAEA SSG-47 likewise recommends that the final decommissioning plan define the start and end points of each phase.

Planned schedules, approved schedules and actual performance are managed separately. When a plan changes, the reference date, the tasks changed, the reason for the change and the effect on subsequent tasks are recorded. The durations of parallel tasks must not simply be added together, and past target years must not be presented as current performance.

Waste management routes can be a precondition of the dismantling schedule. A future storage or disposal facility is counted as available only from the point at which its licensing and readiness have been confirmed. The readiness of the facility, the range of waste it is permitted to accept, the date from which it can actually accept waste and the transport conditions are confirmed, and where uncertain elements remain, alternative storage needs and schedule impacts are examined as separate scenarios.

5.7.2 Review Examples by Facility Type

For example, for a Pressurized Water Reactor (PWR), activation and contamination information is organized by distinguishing the materials and operating histories of the pressure vessel, the internals and the coolant systems. For a Pressurized Heavy Water Reactor (PHWR), the pressure tube system and the management of heavy-water-related materials may be additional subjects of review, and the distribution and migration history of tritium are also important to waste characterization. These are general review examples by facility type.

Even for the same reactor type, differences in materials, operating history, contamination status and dismantling scope lead to different waste volumes and costs. When applying this to a project in the Republic of Korea, confirmed project documents, the applicable regulations and facility acceptance criteria are linked. The costs, progress rates, completion years and disposal facility operators of named projects are used only after they have been confirmed in official material from the operator and the regulatory body.

5.8 Implementation Worksheet

The following worksheet is an example procedure for applying the content of this chapter to project records and software review. The steps and outputs are illustrative and are adjusted to the conditions of the project.

1. Fix the applicable documents and scope. Record the version of the standard that applies to waste packages and the API, and set out the boundary between the area to be dismantled and the fuel and storage facilities that remain under control. Record the choice of half-life values and classification codes in accordance with the Standard Reading Notes in Chapter 1.
2. Link waste sources to the basis for their characteristics. Compile the location of origin, material, survey results, measurement reference date and uncertainty, and link them to the packageId of the related waste packages. Distinguish quantities before processing from quantities after packaging.
3. Review classification and management routes. Distinguish the classification basis of the applicable jurisdiction from the source of the WIA codes. Confirm separately whether clearance applies, whether processing is needed and the acceptance conditions of storage and disposal facilities.
4. Compare strategies. Prepare the qualitative comparison table of Section 5.1 to suit the project conditions. For decay calculations, state the radionuclide, initial value, reference date, half-life source and time unit, and record calculation results separately from dose and release decisions.
5. Align quantities, costs and schedule to the same scope. Use the cost items of Section 5.2 to explain what is included and excluded. Compare the results of the facility capacity and inventory API queries with the expected times of arising, while distinguishing capacity information from approval of receipt.
6. Check the interfaces. Review waste package registration, retrieval and location updates, monitoring queries and decay calculations against the permissions and request formats in §10 of the standard. Record units, permitted code ranges and response structures in the interface contract.
7. Verify the links between records. Check that locations before and after movement agree with the hand-over and acceptance records, that survey results are linked to waste package characteristics, and that the quantities in the cost estimate are on the same basis as the inventory data. Mark project management items so that they are distinguished from the fields of the standard.
8. Assemble the basis for the end state. Link the final survey, the residual contamination assessment, the applicable criteria and the approval records. Where storage areas or use restrictions remain on the site, clearly record the responsibility for control and the area boundaries.

The outputs of the worksheet are the basis for the chosen strategy, the consistency of quantities, costs and schedule, the tracking record for each waste package and the evidence demonstrating the end state. Each output records, by source, the data obtained from the WIA-ENE-026 v1.0 waste package records and API and the data taken from project management documents.

Chapter Summary

• IAEA SSG-47 (2018) is the current safety guide on decommissioning, superseding WS-G-2.1; it presents immediate and deferred dismantling as decommissioning strategies and does not consider entombment an acceptable strategy for planned decommissioning.
• The choice between immediate and deferred dismantling compares not only radioactive decay but also safety functions, waste management routes, funding and the continuity of records.
• The ISDC provides a common structure for comparison through its 11 top-level items and three-level hierarchy, but costs can be interpreted only when project scope and price basis are the same.
• Exemption, clearance of materials and site release differ in what they apply to and in their procedures, and each is decided under its own criteria and procedures.
• The six classes of GSG-1 take radionuclide half-life, the need for isolation and heat generation into account together; whether material is activated, or a single activity concentration, is only one factor in deciding the class. Classification and disposal facility acceptance conditions are confirmed separately.
• The differences in the Cs-137 half-life and in the WIA detailed classification codes are handled according to the Standard Reading Notes in Chapter 1; the value adopted and its source are recorded, and codes are preserved separately for each classification scheme.
• The waste package, monitoring and calculation APIs of WIA-ENE-026 v1.0 support records and calculations at the level of the waste package, while the decommissioning plan, costs and end state decisions are handled in project management documents and regulatory procedures.
• The decommissioning schedule is managed by completion conditions and evidence, and responsibility for spent nuclear fuel management and for remaining areas is kept distinct from completion of dismantling work.

References

WIA Standards — WIA-ENE-026 — Chapter 5. Hongik Ingan (弘益人間) — Benefit All Humanity.