The Zorita project: providing insights for PLEx

When the cargo ship Wilson Gaeta left the port of Santander, Spain, on 19 July 2013, it was carrying a treasure trove for nuclear industry stakeholders: a lead-lined cask holding 70kg of highly irradiated metal samples removed from Spain's Jose Cabrera reactor.

These metal samples contain insights from nearly 40 years of exposure to high-energy neutrons and gamma rays at the Cabrera plant, widely known as Zorita because of its location. They were bound for the Studsvik laboratories in Sweden, where teams are now working to understand the precise nature of the material's degradation.

As plants age, operators need additional information to manage degradation of irradiated materials. The Zorita project aims to fill a knowledge gap regarding the effect of long-term irradiation of materials. Precise characterisation of material properties after prolonged exposure will enable nuclear plant owners to make better-informed decisions about asset operations, maintenance, and continued investment. Such knowledge also can help evaluate nuclear plant life extensions

A rare opportunity

When Zorita closed in 2006, international nuclear stakeholders saw its decommissioning as the perfect opportunity to study the effects of long-term radiation on reactor materials.

"These opportunities are really rare," says Kurt Edsinger, director of materials at US research organisation EPRI. "It's extraordinary to find a reactor that has the material you want under the conditions you want, from a plant being decommissioned by a utility that is willing to take the time to remove the material and give it to you." Utility Union Fenosa was also willing to provide critical background information to support the research -- the operating history, cycle by cycle, including temperature and fluence (neutron exposure) and the makeup of the original material.

Successfully executing the research required coordination between utilities, regulators and vendors from different countries in the context of a nuclear plant decommissioning, when "The general feeling during any decommissioning is that this is a very large project with tight schedules and defined budgets -- there is no time for science," says Edsinger.

From 2007 to 2013, EPRI principal technical leader Rick Reid worked to convene the stakeholders, define their roles in the project and gain agreement about how it would move forward. EPRI members and the US Nuclear Regulatory Commission (NRC) provided expertise and funding. Spanish regulator CSN provided approval and support that were critical in bringing to the project the Spanish nuclear utilities and Enresa, which is responsible for nuclear waste management and decommissioning in Spain.

Enresa and UNESA, the umbrella organisation for the Spanish nuclear utilities, funded part of the material extraction. Also providing funding are European utilities Tractebel and AXPO, Swedish regulatory agency SSM, and five Japanese utilities. Mitsubishi Heavy Industries is conducting additional characterisation of the materials.

As the Wilson Gaeta neared Sweden, hundreds of people and dozens of organisations celebrated the result of seven years of complex planning and negotiations.

Matt Hiser, materials engineer with the NRC, says: "The Zorita materials are unique. Most existing knowledge of the performance of materials irradiated to these levels has been obtained from test reactors. However, the conditions in test reactors are not as representative as commercial reactor conditions."

The US NRC's interaction with EPRI focuses primarily on technical aspects of the project, to help ensure that materials tested and the test conditions are representative of commercial reactor conditions for reactor pressure vessel internals. The collaborative agreement between the organisations allows the industry and the regulator to jointly obtain technical data, which NRC will then use to independently assess regulatory implications.

Extracting samples and collecting operational history

Extracting the test samples became an adjunct to decommissioning. Union Fenosa transferred ownership of Zorita to Enresa, which hired subcontractors to perform various decommissioning services. Westinghouse was contracted to cut the reactor internals into smaller pieces. EPRI arranged separately with Westinghouse to cut the samples from the larger pieces after they had been moved to the fuel pool.

Westinghouse took eight strips from the one-inch thick baffle plates that had been bolted together and arrayed around the fuel assembly. "The longest sample taken was about four feet long," says Jean Smith, EPRI senior technical leader, who took over the project from Reid in 2012. "Once in the fuel pool, our strips were cut out and set aside, and the rest of the plate was transferred to the waste disposal liner."

The plant operations log can shed much light on historical radiation levels and is therefore essential in understanding material performance under high-radiation conditions.

“As part of the project, engineers had to determine with a high degree of accuracy the radiation exposure of the material over time," says Smith. "This is based on the operational history of the plant, the fuel loading for each cycle, and the precise location of the material. Because the parts of the baffle closer to the fuel have more exposure, we took sections with different neutron exposures." Heating of materials from radiation exposure also can have significant effects on material degradation, so researchers had to characterise how the various baffle samples had been heated. "The material with the highest neutron exposure has the highest peak temperature during operations," she said.

Even before the materials were extracted from the plant, there were extensive efforts to quantify the changes to the reactor internal elements caused by radiation and thermal effects.

The first step was to summarise the material composition of the various components and the information from the plant operating history during all of the irradiation cycles. The next step was to develop a three-dimensional geometric model of the fuel assemblies, the core barrel and its components, the thermal shield, and the upper and lower core plates to determine the radiation impacts. These calculations were based on neutron and gamma flux, neutron fluence, and the resultant displacements per atom (DPA), which is an estimation of the energy absorbed by an atom as a consequence of its collision with radiation particles.

After modelling the radiation degradation, the results were used to understand the thermal history of the components. Thermal degradation is the result of: heat transfer by convection or conduction with the reactor coolant; conduction between solid components of the structure; gamma heating; and neutron radiation. Again, a detailed 3D model of the core internals was used to calculate coolant flow and heat transfer in the reactor core metallic elements and to map temperature history profiles. The radiation and temperature history allowed researchers to establish a relationship between the historical plant operating conditions and the material degradation.

Testing in Sweden

The materials arrived at Studsvik on 26 July 2013.

The objective of the metallurgical testing is to understand how and at what rate damage occurs and what parameters influence its severity. "We want to know if material damage might plausibly get to the point where there is potential risk to the safety of the plant," says Reid. "The Zorita plant operated for its design life, with no failure in any of those materials. These reactor structural materials are not highly susceptible to failure, but damage can and does occur."

At the macro level, direct mechanical tests will characterise the materials' fracture toughness (ability of a material containing a crack to resist fracture), ductility, and crack growth rates from stress corrosion cracking. Micro-level tests will examine their crystalline structure and chemistry changes and pinpoint where and how degradation begins.

Macro tests completed in 2014 include tensile strength, which tests how much stress a sample can take without breaking or deforming.

The mechanical testing objectives focus on three radiation dose levels: 10, 30, and 50 DPA, as determined by earlier radiation analysis. Tensile testing of all three dose levels at two different temperatures has been successfully completed and complements the existing tensile testing database. Testing is under way to study the effect of the three dose levels at three temperatures and two stress intensity levels on IASCC crack growth rates (CGRs). Preliminary results using compact tension (CT) specimens have been promising with respect to rates being measured.

Also at the macro level, IASCC crack initiation rate studies and fracture toughness testing will begin in 2015 and continue during 2016. The crack initiation testing will be conducted at various load levels on O-ring and uniaxial constant load specimens made of the highest dose material. The results will fill a significant gap in the IASCC initiation database.

As with the tensile and IASCC CGR testing, fracture toughness testing will be performed on 10, 30 and 50 DPA materials. The fracture toughness testing at the lowest dose level will be performed both in air and in a PWR coolant environment. The environment for testing the mid- and high-dose materials will depend on these first results.

Work is also proceeding at a microstructural level. Here the objective is to see how structural changes are correlated with mechanical performance. This involves electron microscopy of the baffle plate material to examine the potential dose or temperature dependency of radiation-induced segregation, void swelling and gas formation.

Degradation management

Performing a complete test programme on a common material of known pedigree from an operating light water reactor greatly increases the overall quality of the resulting body of data by reducing the uncertainty related to differences in operational and irradiation histories.

Insights from the Zorita project offer great potential to inform operations and maintenance activities at ageing nuclear plants and the benefits are likely to reach many countries and accumulate over decades.

“It's hard to quantify the value of Zorita, but it's incredibly important," says Edsinger. "The smarter you get in terms of the real performance of the reactor materials, the more you can optimise the way you do inspections, manage material degradation and even operate the plant."

The programme will inform regulations on managing ageing of nuclear power reactors worldwide.

The results will advance understanding of materials' susceptibility to various degradation mechanisms. The data from the programme are being included in a materials testing database to improve the accuracy and technical robustness of the materials models used in the nuclear industry. The improved models will allow more accurate predictions of long-term irradiation effects and improvement of aging management strategies.

This will enable engineers to assess which components need to be inspected, determine inspection frequency, calculate how quickly cracks may grow, and evaluate mitigation techniques. The ability to optimise plant inspection scopes and frequencies will have a positive effect on plant operations and outages.

In the US, EPRI will develop guidance for appropriate inspection techniques and intervals to identify problems before they affect plant safety or operation.
“Utilities and regulators around the world are asking all the same questions about how to better manage life extension of nuclear plants," says Edsinger. "The Zorita project will provide many important answers."