US nuclear utility Exelon’s Zion nuclear power station, comprising twin Westinghouse four-loop 1100 MWe PWRs, 40 miles north of Chicago on lake Michigan, was shut down in 1998. The station, similar in design to Byron, Donald Cook and Sequoyah, needed new steam generators, which then-owner Commonwealth Edison concluded was uneconomical, given licence renewal was in 2013. Instead, it decided to cut its losses and decided to shut the plant down. Fuel was unloaded from the core and loaded into the spent fuel pond in 1998. Exelon bought ComEd’s nuclear fleet in 2003.

Instead of decommissioning the station itself, however, Exelon has sold the entire project to EnergySolutions on a firm fixed price contract basis. This US-based radwaste and decommissioning contractor has been involved in most of the US commercial nuclear station decommissioning projects, carrying out jobs such as component removal, transport and disposal, and waste disposal in other US nuclear station decommissioning projects such as La Crosse, Big Rock Point, Rancho Seco and Maine Yankee. But it had never done it all before.

Patrick Daly is general manager of EnergySolutions’ dedicated subsidiary, ZionSolutions. He says: “This is different than the previous approach. The previous approach was that utilities who own the plants decommission them. They tend to maintain a complete operating staff of 500-600 people, and then bring in decommissioning contractors with trades and labour. This leads to overspend and overbudgeting. In our approach, we’re coming in with a smaller group, with a flatter and leaner structure. Right now if we count professional and administrative and office staff, there are 150 people. It will go up to 200-250 in 2011, and including subcontractors as high as 400.” Under Daly are eight vice-presidents, each with a different patch: operations; business systems/CFO; project controls; environmental, safety & health; decontamination & decommissioning and construction; fuel transfer & dry fuel storage; regulatory affairs & quality assurance; engineering and decommissioning plant manager; a senior technical advisor and Exelon asset manager.

Daly says that another common mistake utilities make during decommissioning is underestimating the amount of low-level waste (LLW) of which they will need to dispose, particularly concrete. (It estimates the project will generate more than four million cubic feet of LLW waste; see Table 1). “They can spend thousands on surveying and scabbling, and still at the end realise that they can’t release the material. The way the plants are constructed, you can’t prove joints and cracks are clean with scabbling. Our approach is not surgical removal, but bulk removal. We go in with as large a machine as can get into the space, and take 6-12 inches off until we get down to clean material.”

EnergySolutions can control the risk of overspend with LLW waste because it owns and operates the commercial low-level waste repository in Clive, Utah, which it says is large enough to accept all the LLW from all operating US nuclear power plants. Spokesman Mark Walker said, “We have Clive. There are cost savings there. We’ll put it [LLW radwaste] on our gondolas [railcars] and take it to our site.”

The Zion project is a model for how EnergySolutions would approach decommissioning of other commercial nuclear sites in the US that are shut down or in decommissioning, such as Fermi 1, Indian Point 1, Dresden 1, Humboldt Bay 3, Peach Bottom 1, Millstone 1 and San Onofre 1.

As part of the estimated $900 million project, on 1 September 2010, EnergySolutions took title not only of the waste but the entire nuclear site. In 10 years, it plans to: pack up the spent fuel and move it to a new dry cask storage pad that it will also build; section and remove major components, decontaminate the buildings, demolish everything, and remove the non-rad waste and radwaste. The original plan would have taken twice as long.

Fuel transfer

The most expensive part of the project is also one of the first: removal of the spent fuel. A total of 2226 fuel assemblies lie in storage in the plant’s one fuel cooling pool. Power supplies for the pond have already been transferred from the plant to a new dedicated power supply, dedicated water treatment system and dedicated cooling tower, in preparation for D&D work in the main reactor building. Fuel rods will be transferred to a vented dry cask storage. NAC International has already won a $65 million contract to supply 64 Magnastor 37-element canisters, most of which are for spent fuel, but four will be used to store greater-than-class-C (GTCC) waste. In addition, some class B & C low-level waste will be packaged and stored separately. Spent fuel canisters will be received by the end of 2012; their concrete overpacks will be cast on site. They will sit on a 3ft-thick, reinforced, seismically-qualified concrete pad, the so-called interim spent fuel storage installation, or ISFSI. The 120 ton casks are designed to survive a tip-over, but have a low centre of gravity to reduce the risk of toppling. There they will wait for a permanent US repository for spent fuel. The fuel loading campaign will last two years, and at its peak one can will be loaded on the pad per week.

Fuel assemblies are lifted into canisters contained within a 125 ton transfer cask underwater, using a fuel bridge. (A $5 million crane and building upgrade, which will take until late 2012 to finish, will enable them to carry out this work). The transfer cask is then dewatered, dryed, welded, placed into a concrete overpack, and loaded on to a fuel carrier machine, which drives it to the ISFSI.

Unfortunately the crane upgrade is not all of the remediation work required. Two-thirds of the fuel assemblies, 1454, are pre-1985 Westinghouse fuel assemblies that are known to be at risk of a lifting point failure weakness that may cause a fuel assembly to break off of the top nozzle and fall back into the fuel pool. Such an incident did in fact occur at the North Anna nuclear power plant in 2001. The most conservative fix for affected fuel assemblies involves using electrical discharge machining (EDM) to drill a hole in the top of the assembly, and then inserting a tie bar which can then be used to raise the assembly. Westinghouse has been contracted for this work.

In addition, about 25% of assemblies will be ‘sipped’, assessed for leakages, before packing up, based on power history chemistry. This test involves lowering a canister into the fuel pool, pumping out the water, and monitoring for fission product release. Should any leaking assemblies be found, they will be placed into an additional overpack before insertion in the fuel canister. Westinghouse has won a six-month contract for the fuel assembly inspections and modifications work, which will begin around June 2011.

The spent fuel pool leads to the reactor by the existing equipment hatch. Since the fuel building will itself be busy with its own decommissioning work, the existing hatch will be no good for removing main components from the reactor building. Instead, the EnergySolutions team will bore through the containment wall, in the same spot where the primary circuit components were installed into containment in the first place. Then, a gap was left open as the structure took shape, and was then filled in with concrete and reinforcing steel. Now, however, that spot is no weak point; the post-tensioned concrete containment building is ribbed with a lattice of hundreds of vertical and horizontal tendons. As of March 2011, the tendons in both reactor containments have been removed, so a hole can be cut into the side. A temporary hatch with doors will be built to protect the reactor from the weather.

Work will begin on reactor 2 first, because the presence of high-tension power lines partly block access to unit 1. These will eventually come down, but were in use for the last year or so. Exelon has been running the Zion generators through the neighbouring switchyard to help stabilise the grid against large voltage fluctuations, but has since installed capacitors to perform the same function. Following decommissioning, the switchyard will remain.

Inside containment, the reactor vessel will be sectioned before removal. Although EnergySolutions had success packaging the entire decommissioned Big Rock reactor vessel in a container, and then transporting it by rail, that 75MWe reactor was less than a tenth of Zion’s power output. The Zion RVs are simply too large to repeat that process. They originally arrived on site by barge, which pulled up at a temporary harbour; now, however, there is no navigable waterway to its final destination of the Clive facility in Utah.

As of September 2010, the reactor vessel refueling cavity was dry. After the reactor vessel is isolated from the rest of the primary circuit and the vessel head has been removed, then the reactor cavity will be flooded 24 feet above the reactor head, so the head and vessel internals can be cut underwater. The head will be laid on its side for cutting, sorting and packaging. (The vessel walls and head will be class A waste). A jacking system will raise the reactor vessel partially out of its cavity for cutting with diamond wire. (The beltline region of the vessel will be greater-than-class-C waste). Siempelkamp Nuclear Technology has won the contract for mechanical cutting of both the reactor internals and the reactor vessel walls.

As of March 2011, RI tooling was being fabricated; the contractor is planning to mobilise on site for reactor vessel cutting (again starting with unit 2) in August-September. Also in March, the ZionSolutions team was in the midst of a major asbestos abatement effort involving the removal of all the thermal insulation material with asbestos in the turbine hall building.

Steam generators will be cut above the tube bundle. They and reactor coolant pumps are to be sealed and transported intact. Once other primary components such as pressurisers are out, other pipework and equipment can be removed. Then concrete to the liner will be mined out, and then the liner will be removed. Once gutted, the building will be ready to demolish. A similar process will also apply to the fuel building: fuel and racks will be removed, the water will be pumped out, cleaned and recycled, and then the liner will be cut out using shears, thermal cutting and other demolition techniques. Any contamination underneath will need to be removed, although the process will be simplified because the presence of contamination is unlikely.

An altogether more painstaking D&D process will apply to the auxiliary building, which houses equipment used to support and safeguard the primary circuit. Four of its six levels are below grade. Its small, congested rooms will require hands-on decommissioning. Pipe lagging contains asbestos. To save time, the EnergySolutions strategy is to drain the circuits, search for hot spots in piping and cut them out using small tools, cap them, and pack them up. Non-contaminated areas will be left in place, and eventually be removed when the building is demolished.

Final D&D tasks include removing the turbines and generators and demolishing the conventional plant house and crib house. Feedwater tunnels into the lake will be isolated with a cofferdam at the shore to isolate the plant from the lake. The tunnels themselves will be abandoned, as will the plant’s foundation on land. The project has undertaken to demolish structures to a depth of three feet below grade; therefore it will leave behind the plant’s basemat, which goes down 60 feet. One of the project’s final acts after final surveys and release will be to grade and seed the site to, in its words, “achieve a natural contour that blends in with the local environment.”

Best-laid plans

EnergySolutions submitted its original Zion decommissioning proposal to Exelon in spring 2006; a letter of intent followed in July. Further elaboration and negotiations led to an asset sale agreement signed in December 2007. The transfer of the nuclear licence required submitting an application to the NRC, and documentation of the high-level schedule and scope of work in the post-shutdown decommissioning activities report, PSDAR, submitted annually to the US NRC. In 2008, ZionSolutions submitted a change in the PSDAR that outlined its proposal. The document was published and a public hearing was held.

More onerous still were financial requirements: a $200 million letter of credit, and an irrevocable easement of asset disposal. The latter ensures that even if EnergySolutions goes bankrupt, all the LLW will still go to the Clive repository, even if it is sold. As for the letter of credit, the beneficiary is Exelon; should EnergySolutions go bankrupt, it will have the option to the plant back.

EnergySolutions hoped to close the deal in 2008, but the market crash of August/September knocked $150 million off the value of a trust fund set up to finance the project; it could not continue. Eighty-five people ready to start work had to be demobilised until markets eventually recovered two years later.

According to the September schedule, the company’s next regulatory move will come in 2014, when it will submit a licence termination plan in late 2014, which covers the latter stages of decontamination, decommissioning and land remediation.

Table 1: Zion estimated waste volumes

4.0 MMCF (millions of cubic feet) of class A (LLW) radwaste: equipment, piping, debris, secondary waste
2.7 MMCF fill materials
1.3 MMCF recycle (copper, steel, rebar)
3200 CF (cubic feet) class B/C (LLW) radwaste: RV internals
700 CF greater-than-class-C (LLW) radwaste:
RV internals

FilesFigure 1: Zion decommissioning project schedule (as of September 2010)