Spent fuel management is becoming a higher priority for many plant operators now that pools are reaching full capacity. Nuclear Energy Insider takes a closer look at how this dynamic is impacting the profitability and long-term O&M responsibilities of today’s nuclear power plants.
One of the most critical parts within the early stages of any nuclear decommissioning project is preparing for the removal of spent fuel, which contains significant amounts of fissile material as well as highly radioactive fission products, says Adrian Bull, Director of External Relations, National Nuclear Laboratory.
Bull points out that the spent fuel is the material that requires the most care when decommissioning a reactor. Even when it is removed, he says, there are still significant levels of radioactivity remaining associated with the reactor internals, which will have been exposed to tremendously high radiation doses for several decades of reactor operation.
Removal of the spent fuel from the reactor installation needs to be considered as early as possible in decommissioning. The timely removal of the fuel from the spent fuel pool into specialised canisters will simplify surveillance requirements and also minimise radiological risk.
Managing spent fuel
Jayant Bondre, vice president and chief operating officer of AREVA TN, highlights that utilities need to consider a number of factors when it comes to managing used fuel at plant sites. Bondre says these factors are unique to each site depending on the available space inside the spent fuel pools and any degradation issues that have arisen with the neutron absorbers located in the spent fuel racks.
Some of the key overriding factors for a US-based utility in determining when to remove fuel from the spent fuel pools include safety and economics. In more detail, this could include the potential for reimbursement from the U.S. Department of Energy, occupational exposure, and public health and safety.
“In managing used fuel at plant sites, utilities have to handle a number of challenges, including preparing a specification and requirements document for the selection of dry cask storage technology, budget, lead time required from preparation of proposal to final deployment of the dry storage system at a new site, and availability and retention of experienced personal to support this operation,” says Bondre.
Utilities may also need to address facility and/or crane modifications and the lead times associated with these modifications, the location for the independent spent fuel storage installation, and any impacts on plant procedures for operations and security.
Many spent fuel pools at nuclear power plants across the US were originally designed to hold a certain number of spent fuel rods. Two factors have in many cases tripled the number of spent fuel rods in today’s spent fuel pools, far beyond original designs. The reason being as plants extended their operating licenses they were not provided a national spent fuel repository to store their rods. To accommodate this, many plantsredesigned and fitted their pools with racks to accommodate more rods in a compact formation.
One example is the Pilgrim Nuclear Power Station in Massachusetts. Its initial 40-year license expired on June 8, 2012 and was granted a license until 2032. The plant will be receiving inspectors from the Nuclear Regulatory Commission to conduct six evaluations of the systems meant for moving its spent fuel.
According to one news report, the plant’s pools are nearing their capacity for spent fuel rods. The report said that the plant has more than 500 metric tons of spent fuel – or 3,222 fuel assemblies – in pools and is licensed for only 3,859 fuel assemblies.
The NRC estimates that more than 70,000 metric tons of spent nuclear fuel has accumulated in the United States, with about three-fourths of it still stored and cooled in 40-foot-deep pools at nuclear power plants.
The Department of Energy (DOE) earlier this year unveiled, albeit without much publicity, a new strategy for the management and disposal of the spent nuclear fuel in the US.
According to news reports, the strategy is suggesting a phased, consent-based approach to siting and implementing a nuclear waste management and disposal system, and it endorses building a pilot interim storage facility by 2021.
While in recent weeks there has been talk of one sparsely populated county in the state of Texas (Loving County), having interest in hosting a spent fuel storage site for the state’s four reactors and potentially more from other states, the topic is still on the discussion board. Geological considerations always have to be seen through, so this will also be a weighing factor of any spent fuel storage facility.
According to an Associated Press report and the New Mexico media, the Texas waste hosting site could be up for discussion at a 2015 legislative session. House Speaker Joe Straus instructed lawmakers to study the issue, said the AP report.
Until a final national resting place for the spent fuel rods is constructed, there will have to be even more skilled personnel to handle on-site storage and at the least amount of cost, but with the upmost safety mechanisms in place.
Quality personnel
Bondre emphasises that it is important to have qualified personnel throughout the canister process. In addition, good coordination, supervision and training, and being aware of the potential for personnel fatigue are vital to ensure that operations are carried out safely.
“To that end, AREVA TN’s NUHOMS University, located in Aiken, South Carolina, provides intensive on-site classroom training and training with real equipment in real time, including practicing “what if” scenarios with the entire team to ensure a streamlined and safe pool to pad operation at the lowest doses in the industry,” shared Bondre.
Canister processing teams require qualified personnel to perform multiple activities during the canister processing, for example, spent fuel assemblies handling, welding, ancillary equipment and crane operations, dose control, etc.
Lessons learned
The decommissioning of a nuclear power plant is a complicated process, and each project has its share of challenges.
In the case of the A1 NPP, termed as the first nuclear power plant in the former CSSR, the decommissioning process has been divided into five stages, and it is scheduled to conclude in 2033 [2]. The second stage is in progress, and slated to end in 2016.
Providing an insight into waste management strategy and the removal of the spent fuel from the reactor installation, Zuzana Hosťovecká, spokesperson of Slovakia’s Nuclear Supervisory Office (ÚJD), says experiences during the operation and post-operation period of A1 NPP show that the storing method for the spent fuel assemblies from the A1 operation resulted in the corrosion of fuel elements associated with the release of fission products into the coolant in long-term spent fuel storage and deformation of their geometry.
Hosťovecká says out of 182 fuel assemblies stored in this way, 128 assemblies became non-manipulated.
“Following the accident in 1977 the problem became more complicated by the fact that other fuel with damaged cladding was stored in the spent fuel pool,” says Hosťovecká.
These conditions became an extremely complex matter requiring a gradual searching, verification and application of new technology procedures. It also called for new technical tools that could decontaminate materials, surfaces and components that could then be ready for transport and storage.
“Out of the total amount of 572 fuel assemblies, 440 fuel assemblies were withdrawn and transported into the Russian Federation in the period 1983 – 1990,” shared Hosťovecká.
For the treatment and removal of the rest of spent fuel, it was necessary to develop and produce special technology equipment by means of which the fuel was withdrawn from the long-term storage facility and transported in special containers into the Russian Federation. This phase proceeded in the period between 1996 and 1999.
Hosťovecká adds, “By removing fuel from the plant, conditions were established for the implementation of the design phase of the A1 decommissioning up to the end of first decommissioning phase.”
Storage
Following the decontamination process, there are several aspects that need to be handled diligently for storage. One of them is sealing the spent fuel in airtight concrete-and-steel canisters. Another is constructing for the correct structural strength and radiation shielding.
As for sealing the spent fuel, Areva’s Bondre says the majority of the canistered dry storage systems use a sealed steel canister filled with inert gas, such as helium, to store spent fuel. These sealed canisters are then stored in vertical or horizontal concrete overpacks, which have inlet and outlet openings to circulate air over the canister surface to remove the decay heat from the used fuel assemblies stored in the sealed airtight canisters.
“These dry storage systems are robust, and meet all the rules and regulation requirements for storage. They are also licensed by U.S. Nuclear Regulatory Commission (NRC) to the requirements of 10 CFR Part 72 regulations for storage onsite,” says Bondre.
He adds, “Radiation shielding, which is used to reduce the neutron and gamma dose, is provided by the storage overpack. In almost all cases, radiation shielding is assumed to be damaged during storage and transportation accident conditions depending on the design and shielding material type used.”
The safety analysis reports for storage and transportation conditions document the evaluation of the system under these accident conditions to demonstrate that they meet all the regulatory criteria and requirements.
By design, dry storage systems are passive systems designed to operate safely under all postulated normal, off-normal and accident conditions.
Bondre says the Nuclear Regulatory Commission specifies monitoring requirements in the Certificate of Compliance and their associated technical specifications. These requirements are based on the review of the container design and contents which the license applicant applies for under 10 CFR Part 72 from the NRC. The details of the monitoring are included in the Safety Analysis Reports.
He adds that the monitoring requirements are again reviewed as part of aging management programmes when an application is submitted to the NRC for a licence extension. The parameters that need to be monitored are optimised or balanced with their safety significance. Care is also taken to avoid breaching the confinement boundary of the storage system, which can introduce an additional failure path, to satisfy the need for monitoring.
Therefore, almost all the monitoring requirements are based on accessible parameters that do not compromise the confinement boundary of the storage system.
Overall, as specialists recommend, there always needs to be a contingency plan in the decommissioning process. But one should not discount the absolute need for diligent planning from the outset, timely removal and storage of the spent fuel into specialised canisters and qualified personnel to perform the requisite tasks.
Source: Nuclear Energy Insider
