Decommissioning

Marcel Thibodeau's last assignment before retiring is to help plan the decommissioning of the plant where he started his career. He takes Hana to the turbine building, silent now, its big machines already drained. "People think decommissioning is demolition," he says. "It is the opposite. It is the most carefully planned work we ever do, because we are taking apart a system that was built never to be taken apart. And it is regulated just like operation, with its own plans, limits and inspections."

What decommissioning involves

Decommissioning covers all the administrative and technical actions taken to remove some or all of the regulatory controls from a nuclear facility at the end of its life. For a power reactor it includes removing the fuel, draining and decontaminating systems, dismantling contaminated and activated equipment and structures, packaging and shipping waste, remediating the site and surveying it to show that it meets the release criteria. It can take a decade or more, cost hundreds of millions to more than a billion dollars per reactor, and produce large volumes of mostly low-level waste. Planning for it begins at the design stage and continues throughout operation.

Most of the radioactivity in a shut-down reactor, apart from the fuel, is in two forms. Activation products are created when neutrons strike the steel and concrete near the core, making isotopes such as cobalt-60, nickel-63 and, in CANDU reactors, carbon-14 and tritium in the moderator and structures. Contamination is radioactive material deposited on the inside surfaces of systems and occasionally on floors and walls. Activation is bulk and cannot be washed off; contamination can often be removed by chemical or mechanical decontamination. The inventory falls with time as short-lived isotopes decay, which is the main argument for waiting before dismantling.

Strategies

The IAEA describes three broad decommissioning strategies. Immediate dismantling begins soon after shutdown, using the existing workforce and their knowledge of the plant, and returns the site for other use within one or two decades. Deferred dismantling places the facility in a safe storage state, with the fuel removed and systems drained, and waits several decades for radioactivity to decay before dismantling. Entombment encases the facility in a durable structure so that it becomes a disposal site, an approach generally not considered acceptable for power reactors except in special circumstances. The IAEA's safety standards express a preference for immediate dismantling where practicable, but the choice depends on the availability of waste disposal, funds, workforce and the plans for other reactors on the same site.

US rules allow three strategies, called DECON for prompt dismantling, SAFSTOR for safe storage with deferred dismantling and ENTOMB for entombment. Decommissioning must be completed within 60 years of permanent shutdown unless the NRC approves a longer period. Many plants shut down in the 1990s used SAFSTOR, but since the late 2010s a new model has emerged: the licensee transfers the license and the decommissioning fund to a specialist company, such as EnergySolutions, NorthStar or Holtec, which commits to complete DECON quickly. Zion in Illinois, Vermont Yankee and others have been dismantled this way, and Indian Point, San Onofre, Pilgrim and Oyster Creek are in progress.

Planning and licensing

NRC licensees must notify the agency within 30 days of deciding to cease operations permanently and must certify when fuel has been permanently removed from the reactor vessel; the license then no longer authorizes operation. Within two years of shutdown, the licensee submits a post-shutdown decommissioning activities report describing its planned activities, schedule and cost estimate, and the NRC holds a public meeting near the site. Major dismantling can begin 90 days after the report is submitted. Before the license is terminated, the licensee submits a license termination plan with a final survey design, which requires an NRC approval and a public meeting.

Paying for it

Under 10 CFR 50.75, every power reactor licensee must provide reasonable assurance that funds will be available for decommissioning, using a prepaid trust, an external sinking fund to which it contributes over the plant's life, a surety or insurance, or a parent company guarantee in some cases. The rule sets a minimum amount by formula, based on reactor type and power and escalated for labor, energy and waste burial costs, and licensees report the status of their funds every two years. Most licensees use external trust funds. Spent fuel management costs are tracked separately.

The essential calculation is a present-value one. A decommissioning cost incurred decades in the future can be covered by a smaller sum invested today, because the fund earns a return. But costs also escalate, often faster than general inflation, and estimates tend to rise as plans become more detailed. If the fund's real return falls short or costs grow, the fund will fall short. Regulators therefore require regular updates of both the cost estimate and the fund balance.

# Present value of decommissioning strategies and the annual contribution needed (illustrative, real terms).
r = 0.025                     # real discount rate
def pv(amount, year): return amount / (1 + r) ** year
# immediate: 1200 M spread evenly over years 1-12 after shutdown
immediate = sum(pv(1200 / 12, y) for y in range(1, 13))
# deferred: 8 M per year safe storage for 30 years, then 1000 M over years 31-42
deferred = sum(pv(8, y) for y in range(1, 31)) + sum(pv(1000 / 12, y) for y in range(31, 43))
print(f"present value at shutdown: immediate {immediate:7.0f} M  deferred {deferred:7.0f} M")
# annual contribution over 40 operating years to reach the immediate-strategy PV at shutdown
for ret in (0.025, 0.0125):
    n = 40; factor = ((1 + ret) ** n - 1) / ret
    print(f"real return {ret:.2%}: annual contribution {immediate / factor:5.1f} M per year for {n} years")

present value at shutdown: immediate    1026 M  deferred     575 M
real return 2.50%: annual contribution  15.2 M per year for 40 years
real return 1.25%: annual contribution  19.9 M per year for 40 years

The output shows two things. First, deferring the work lowers its present value substantially, because a real discount rate applied over decades shrinks the cost, even though the deferred strategy adds annual safe storage costs. Second, a modest annual contribution over a long operating life can build a large fund. The result is sensitive to the assumed real return: halving it raises the needed contribution considerably. Marcel points out that the present value advantage of deferral is only part of the decision; the loss of experienced staff and the burden on future generations count against it.

Release criteria and the end state

Under the license termination rule in Subpart E of 10 CFR Part 20, a site can be released for unrestricted use if the residual radioactivity would give an average member of the critical group a dose of no more than 0.25 millisievert, or 25 millirem, per year, and the residual radioactivity has been reduced to levels as low as reasonably achievable. Restricted release with institutional controls is possible under stricter conditions. The final status survey usually follows the Multi-Agency Radiation Survey and Site Investigation Manual, MARSSIM, which sets out statistical methods for showing that the criteria are met.

Waste from decommissioning

Decommissioning a large reactor produces tens of thousands of cubic meters of waste, most of it concrete, steel and soil with little or no radioactivity. Managing this waste well is one of the keys to cost. Material that meets clearance levels, set by the regulator as concentrations below which material can be released from regulatory control, can be recycled or sent to ordinary landfill. Low-level waste goes to a licensed disposal facility where one exists, or to storage where it does not. A smaller volume of intermediate-level waste, such as reactor internals and, for CANDU reactors, pressure tubes and calandria tubes, needs shielding and more robust disposal. The used fuel, removed early in the process, goes to the storage and disposal path described in the fuel cycle course.

The availability of disposal sites for low-level waste, such as the facility in Clive, Utah, and others in Texas, South Carolina and Washington, has helped make prompt dismantling practical in the United States. Most decommissioning waste is class A, the least hazardous category under 10 CFR Part 61. Greater-than-class-C waste, including some reactor internals, has no disposal facility yet and is stored on site with the used fuel.

Safety during decommissioning

A shut-down reactor without fuel cannot have a criticality or core damage accident, so many of the hazards of operation disappear. But others remain or become more prominent: radiation exposure to workers cutting activated components, airborne contamination during demolition, industrial hazards from heavy lifting, cutting and working at height, fire, and the management of large volumes of waste. While fuel remains in the pool after shutdown, the safety case must also cover loss of pool cooling and inventory. The decommissioning safety case addresses these hazards and is reviewed by the regulator like any other.

Marcel tells Hana about the techniques used. Remote tools, such as underwater cutting with abrasive water jets or plasma torches, let workers stay at a distance from the most activated components. Chemical decontamination of a reactor's primary circuit before dismantling can remove most of the contamination and greatly reduce doses. Detailed three-dimensional models of radiation fields help plan each cut. "The dose budget for a decommissioning project is set the same way as for an outage," he says. "We plan each job, estimate the dose, and track it. If a job runs over, we stop and ask why."

License transfers and community concerns

The transfer of licenses to decommissioning specialists has raised questions. States and communities have asked whether the companies' funds and financial assurance are adequate, whether they will cut corners to finish within the fund, and how decisions about matters such as releasing treated water from a used fuel pool will be made. Several states have intervened in license transfer proceedings or negotiated settlement agreements with conditions on funding, site restoration and reporting. The NRC reviews each transfer for the new licensee's technical and financial qualifications, and some states have created advisory panels to give local residents a voice.

How decommissioning has gone in practice

Experience from dozens of completed power reactor decommissioning projects around the world shows that the work can be done safely, with worker doses and environmental releases well within limits, and that sites can be returned to other use. In the United States, sites such as Maine Yankee, Trojan, Rancho Seco and Zion have been decommissioned, with only the dry storage installation for used fuel remaining under license. In Germany and Spain, several large reactors have been or are being dismantled under immediate strategies. Projects have faced cost overruns, often because the extent of contamination was greater than expected, waste disposal routes changed or the regulatory end state was renegotiated. The lesson drawn by the industry is that good records from operation, early characterization of the plant and a stable end state are the best ways to control cost.

At the end of their visit Marcel shows Hana the plant's operating logs from its first decade, kept in a vault and now being scanned. "Every spill, every leak, every repair," he says. "The people who wrote these are mostly gone. But their notes tell us where to look for contamination under the floor. That is why record keeping during operation is a decommissioning requirement too."

Decommissioning from the design stage

Modern design requirements in both countries ask designers to consider decommissioning from the start. Choosing materials with low levels of impurities such as cobalt reduces activation; designing systems so that large components can be removed in one piece reduces cutting; avoiding embedded piping that is hard to survey reduces the risk of hidden contamination; and keeping good records of the as-built plant makes characterization easier decades later. Many SMR designs go further, proposing to remove the whole reactor module at the end of life and return it to a central facility for dismantling, which would leave the site with little more than conventional demolition. Whether that model works will depend on transport licensing and on the availability of a facility to receive the modules, questions regulators are only beginning to address.