Three Mile Island owner tackles fuel storage risks as closure looms

Exelon will launch into decommissioning work at Three Mile Island Unit 1 when it closes in September and the operator is focusing on dry storage construction and shared facility challenges to minimize spent fuel costs, Michael Gallagher, Vice President License Renewal at Exelon Generation, told Nuclear Energy Insider.

On May 8, Exelon confirmed it will close its 852 MW Three Mile Island (TMI) Generating Station Unit 1 in Pennsylvania by September 30 after a proposed price support mechanism stalled in the state legislature.

"It is clear a state policy solution will not be enacted before June 1, in time to reverse the premature retirement of the plant," Exelon said in a statement.

Exelon plans to decommission TMI Unit 1 under the deferred SAFSTOR process. The operator filed its Post Shutdown Decommissioning Activities Report (PSDAR) to the U.S. Nuclear Regulatory Commission (NRC) on April 5. The total decommissioning cost is estimated at $1.2 billion, it said.

Exelon plans to transfer spent fuel to dry cask storage by the end of 2022. On-site staffing numbers will fall from 515 full-time employees currently, to 50 by 2022, in order to minimize costs.

Around 46 dry storage canisters will be placed inside a new Independent Spent Fuel Storage Installation (ISFSI) to be completed by late-2021.

The distinctive Three Mile Island site on the Susquehanna River will present Exelon with site-specific challenges as it looks to optimize spent fuel transfer.

Upcoming challenges include the mitigation of flood risk at the planned ISFSI storage facility and optimization of shared fuel handling buildings with First Energy's TMI 2 reactor.

Spent fuel costs

Decommissioning specialists are developing new technologies and practices to speed up the transfer of spent fuel into dry storage facilities.

Rapid transfer of spent fuel to dry storage significantly reduces plant monitoring and labor requirements.

Annual decommissioning costs at TMI Unit 1 are forecast to fall from around $58.5 million per year during wet fuel storage, to $10.2 million/year in dry storage, according to the PSDAR. Spent fuel management costs are forecast to drop from $20.1 million/year to $3.5 million/year.

                        US plants set for closure in 2018-2015

Source: Energy Information Administration (EIA), September 2018.

During a "preparations for dormancy" period from September 2019 until February 2021, Exelon has earmarked $39.9 million of spending on spent fuel management and $93.7 million on radiological decommissioning.

During this period, smaller systems no longer required, such as those containing hydraulic fluid and circulating water systems, will be removed from service and either dismantled or placed in long-term storage, Gallagher told Nuclear Energy Insider.

Exelon will implement a new fuel handler certification program and shutdown emergency and security procedures for the period, he said.

“Most of the costs during this period are related to staffing," Gallagher noted.

"Our focus will be around safely and efficiently moving the fuel into dry storage, which will allow for the staffing reductions that come with reduced operational risk,” he said.

The number of on-site employees is forecast to fall from 515 at present to 300 shortly after the core is offloaded, then 200 in 2021, falling to 50 full-time staff in 2022. The company will work with employees to identify potential placements within the company.

Storage construction

The spent fuel from TMI 1 will be held in the ISFSI until 2034, when it will be transferred to an external consolidated interim storage facility (CISF) or permanent storage site. The dismantling of large structures, including the station's cooling towers, is scheduled to start in 2074.

Exelon plans to build the ISFSI in the "south parking area” of the nuclear plant site, according to the PSDAR.

The siting and construction of the ISFSI facility must take into account flooding risk from the Susquehanna River.

In 2017, the US Nuclear Regulatory Commission (NRC) re-evaluated Exelon's risk mitigation strategies at TMI 1 against "beyond-design-basis" flood events. The NRC found that current measures provide sufficient protection against hazard scenarios such as torrential rainfall, failure of dams and onsite water/control storage structures, and ice-induced flood-causing mechanisms.

The ISFSI will be “built to withstand a 100-year flood,” Gallagher said.

Twin reactor

TMI 1 also shares certain structures with the TMI 2 plant, owned by FirstEnergy. TMI 2 has been shut down since the accident in 1979 and has been in the "post-defueling monitored storage" phase of the SAFSTOR process since 1993.

TMI 1 and TMI 2 use separate spent fuel pool systems, located in the same building.

“This may present some interesting design challenges for Exelon regarding how to isolate the spent fuel pool,” Adam Levin, principal at AHL Consulting and a former Director of Spent Fuel and Decommissioning at Exelon, told Nuclear Energy Insider.

According to Gallagher, the shared facilities will "not impact the decommissioning of TMI Unit 1.”

Exelon and FirstEnergy have previously agreed to "synergize decommissioning efforts" and are working together to optimize schedules, Gallagher noted.

“We are working closely with FirstEnergy on a timeline for TMI Unit 2 decommissioning,” he said.

Spreading costs

Many other operators have chosen "immediate" decommissioning under the DECON process in order to reduce cost and safety risks.

Exelon chose SAFSTOR because it provides a "safer environment for our decommissioning workforce by allowing additional time for normal radioactive decay, which results in less waste and lower radiation exposure,” the company said in a statement.

SAFSTOR also delays spending and allows operators to build up decommissioning trust funds (DTFs) through long-term investment products. The DTF balance for TMI 1 is currently well below the estimated decommissioning cost of $1.2 billion.

The DTF balance for the plant was $625.9 million at the end of 2016 and projected to rise to $885.0 billion by 2034, the previous shutdown date for the plant, according to the NRC's 2017 Decommissioning Funding Status Report.

“The TMI Unit 1 decommissioning trust fund should fully cover the cost of decommissioning. If there were a shortfall in the fund, Exelon would be responsible to meet its obligations,” Gallagher said.

By Neil Ford