R&D drive on concrete ageing30 March 2016
Concrete ageing is a key factor in plant life design and asset management, and more research is underway. Report by Patrick Reynolds.
Concrete ageing in nuclear power plants is one of the critical issues for asset life management and extended operation of the facilities, and over the last year notable steps forward have been made in this field of research.
In Spain, the Jose Cabrera nuclear power plant (NPP), currently being decommissioned, is a focal point for concrete ageing research alongside another major materials study into the long-term effects of irradiation in operating plants.
Late last year, the International Committee on Irradiated Concrete (ICIC) held its first general meeting (see Box Panel 1).
Most recently, the International Atomic Energy Agency (IAEA) published a new technical report, 'Ageing Management of Concrete Structures in Nuclear Power Plants', as part of its Nuclear Energy Series (see Box Panel 2).
Research location: Jose Cabrera NPP
The Jose Cabrera facility - or 'Zorita', after its location in Guadalajara - was a 160MWe pressurized water reactor, and the first NPP to commence operations in Spain, in the late 1960s. The licensee was held by Union Fenosa. After 38 years of operation, the plant was closed in April 2006.
With 26 years of effective full power (EFP), the NPP operated with a core average temperature of 293°C, with inlet and exit temperatures of 282°C and 305°C, respectively. The reactor coolant system pressure was 140kg/cm2. The plant was converted to up-flow eight years before decommissioning.
Ownership of the NPP was transferred to Enresa, the Spanish organisation responsible for nuclear waste management and decommissioning. The task of decommissioning Jose Cabrera began in 2010, and upon completion the site is to be returned to the utility owner - Gas Natural Fenosa, since the acquisition of Union Fenosa in 2008.
Facilitated by extensive and prolonged preparatory international discussions and planning, the Jose Cabrera NPP holds an unusual position in the nuclear sector: the plant has been made available as a focal point for a number of post-operation research projects. The data will be valuable to future NPP design and asset management during operational lives.
Research projects underway at Jose Cabrera include study of long-term irradiation of metals (Zorita Internal Research project - ZIRP), and also concrete ageing.
For the irradiated study, a 70kg consignment of samples was transported for metallurgical testing almost three years ago to researchers working at the Studsvik laboratories, in Sweden. The research is looking into the manner, rate and influencing factors in the severity of irradiation damage.
In the concrete ageing studies, researchers are investigating the effect of irradiation on the concrete structures of an operational NPP. Previously, most research into concrete ageing in NPPs has been performed mainly in laboratory conditions, and the data extrapolated, according to the project team.
The materials research projects at Jose Cabrera NPP fall under the auspices of the Spanish Strategic Platform on Nuclear R&D (CEIDEN), and through which almost 100 public and private organisations are collaborating in the studies. The initiatives underway at the former power plant are only part of the nuclear sector research programmes underway in Spain, some of which are being performed through international co-operation.
Research plan: concrete ageing
Planning for concrete ageing research at Jose Cabrera NPP began in 2009, when CSN began exploring the feasibility of studies that would use structural material from the plant. Within months, a working group was set up as part of CEIDEN.
Partners in the concrete ageing research project are Enresa, Gas Natural Fenosa, the utilities Endesa and Iberdrola, national nuclear regular Consejo de Seguridad Nuclear (CSN), and the R&D lab Instituto Eduardo Torroja de Ciencias de la Construccion (IETcc).
The concrete ageing research project is being directed through steering and technical committees, respectively, drawn from the current partners - though the group is open others to join, domestically or internationally. For the tasks, Enresa is responsible for management coordination, and IETcc the technical coordination.
GAP analyses looked at key factors affecting the plant's concrete structures, especially the effects of high radiation (neutron, gamma) and temperature, respectively.
The IAEA book says fluence (neutron exposure) levels of 1 x 1018n/cm2 has been cited in relation to altering the mechanical properties of reinforcing steel, though this level is unlikely to be experienced by concrete structures, except possibly in the concrete primary biological shield wall over an extended operating period.
For the Zorita project's irradiation analysis, Enresa's preliminary estimates for fluence on the concrete of the biological shielding is approximately 8 x 1018n/cm2. Based on literature references, the degradation threshold for concrete structures in NPPs was taken to be 1 x 1019n/cm2.
The concrete mix design in the shield wall has Type 1 Ordinary Portland Cement. The density of the concrete is 2300kg/m3.
From the ZIRP studies, it was determined the reactor vessel internals had an accumulated fluence of 52dpa (Displacements Per Atom, or estimated energy absorbed by an atom as a consequence of collision with radiation particles) in some areas. This level is more than other commercial reactors, so far, checked at the decommissioning stage, say the researchers.
Calculations on neutron and gamma sources come from a compilation of 594 fuel elements, and 29 cycles of operation over the EFP years, and consider: load patterns; cycle durations; power and burnup monthly history; moderated high and low leaks configuration; and upflow-downflow configuration of core bypass.
Concrete areas surrounding the reactor vessel are, consequently, expected also to have high levels of fluence and gamma radiation, making the structures 'high value' for researching the effects of irradiation, they add.
On temperature, the researchers note that compared to short events, such as fires, there is little known about the effects of prolonged medium to high temperatures on concrete. But the effects of temperature in the biological shielding around the reactor depends on interaction with another variable - gamma radiation. While literature is scarce, they say it indicates that dose of more than 1010 rads might produce effects.
In addition to radiation and temperature, another stressor selected for analysis is chemical - studying the effects of possible boric acid leaks (from the spent fuel pool or other structures) on concrete.
The research project also involves non- destructive testing (NDT) for corrosion on the liner under the concrete slab. This is to verify the reliability of electrochemical techniques to detect corrosion activity.
Research: concrete sampling & testing
The biological shield wall is the key site for sampling concrete to investigate the effects of radiation and temperature stressors on concrete - and their combined effects.
Researchers say eight or more core samples with the maximum level of irradiation, and symmetrically located around the shield wall, can be obtained - having been identified by the same methodology as used in the ZIRP project.
The samples are being taken by drilling the concrete in-situ. Each sample is to be long enough to provide a number of test sections, or 'coupons', at different locations - taken from closest, farthest and intermediate points with respect to the reactor vessel. Researchers say the approach also enables them to determine the attenuation effects.
Extra - spare - core samples are being collected.
In the focus on predominantly high temperature effects, the core samples are taken near the outcome section of the reactor coolant system. For the chemical stressor, the sampling will is focused on the spent fuel pool/transfer channel. The core samples are to be identified through analysis of the operating experience, and then taken from extracted blocks of concrete (2m x 2m square section, length to be determined). Test coupons are taken from along the length of each sample.
The various test coupons are to be examined in the IETcc laboratory or Enresa's facilities. Types of tests will include mechanical, electrical resistivity, ultrasound velocity, and microstructural (eg composition, microscopic structure, cracks, porosity, carbonation, etc) say researchers.
Concrete from the containment building is taken for reference samples and data.