Fast Reactor Materials Properties
Application of Structural Materials Data from the BN-350 Fast Reactor to Life Extension of Light Water Reactors
Tech Area / Field
- FIR-MAT/Materials and Materials Conversion/Fission Reactors
- FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors
8 Project completed
Senior Project Manager
Pradas-Poveda J I
Nuclear Technology Safety Center, Kazakstan, Almaty
- National Nuclear Center of the Republic of Kazakstan / Institute of Nuclear Physics, Kazakstan, Almaty\nKazakh National University / Scientific Research Institute of Experimental and Theoretical Physics, Kazakstan, Almaty\nMAEC-Kazatomprom, Kazakstan, Aktau
- EDF, France, Paris\nArgonne National Laboratory (ANL), USA, IL, Argonne
There are plans to build a light water reactor (LWR) on Lake Balkash in central Kazakhstan to lessen dependency on foreign energy supplies. There is presently little direct experience with materials problems with this type of reactor, but many materials experts in Institutes in Kazakhstan and a wealth of possible data on irradiated structural materials to be obtained from the BN-350 fast reactor at Aktau on the Caspian Sea being in decommissioning process. This proposal seeks to capture BN-350 materials data for future peaceful application of nuclear energy in Kazakhstan. It would employ scientists and engineers formerly associated with nuclear weapons and thus directly supports ISTC objectives.
In brief, neutron damage to the permanent internal structures of light-water reactors (LWRs) is a major limiting factor in the usable life of these reactors. Neutron damage causes embrittlement and may lead to swelling of the various grades of stainless steel from which components like the core barrel are manufactured. Although neutron damage to LWR components can be simulated by various ion bombardment techniques, there is always some modeling extrapolation necessary, so that there is no substitute for direct measurement on irradiated specimens. Unfortunately, there are very few reactor facilities world wide for specimen irradiation and, when available, such irradiations take about the same time to achieve end-of-life neutron doses as in actual LWRs. Similarly, the alternative method of obtaining samples from the permanent internal structures of decommissioned plants is extremely difficult and expensive. In consequence, the case for life extension of pressurized water reactors (PWRs) and the Russian equivalents (VVERs) is presently made on the basis of extrapolating trends observed in lower dose materials.
It is planned to use stainless steel components from the BN-350 fast reactor to project to beyond end-of-life conditions for LWR internal components. A variety of stainless steels have been used for the hexagonal ducts of fuel assemblies in the active core and blanket regions of BN-350. The steels include the Russian equivalents of AISI Types 304 and 347, which are also predominantly used for LWR components. During 25 years of BN-350 operation these steels have been bombarded with neutrons over a wide range of flux, fluence and temperature. Expressing neutron damage in terms of displacements per atom (dpa), conditions in irradiated BN-350 steel components have ranged over 5-75 dpa, accumulated at damage rates of 10-8 to 10-6 dpa/sec and over temperatures of 280-550 °C. By comparison, LWR components which are not in the immediate vicinity of fuellike the core barrelsustain damage of 2.5-25 dpa over a 40-year life accumulated at damage rates of 10-9 to 2ґ10-8 dpa/sec at temperatures of 300-350 °C. The low inlet temperature of BN-350 (280 °C) means that the conditions of dose rate, dose and temperature in-reactor bound the values occurring in LWRs. Because BN-350 has been operated as a power reactor, not a materials research facility, component examinations in the past have been very limited; this project will be an opportunity to redress that situation.
The proposed investigations of samples of steel ducts will run the gamut of techniques used for materials characterization and will include but not be limited to: mechanical property testing, microstructural evaluation by optical and electron microscopy, density measurements to determine swelling, determination of impurities content by means of mass-spectrometry and determination of gas content from (n,p) and (n,a) reactions in-reactor by means of thermal desorption spectroscopy (TDS). The intent is to build a thoroughly researched materials database for potential application to LWRs. The project will naturally pide into seven distinct tasks: six tasks to be performed with MAEC involvement (one with input from ANL), NNC will be involved in implementation of four tasks and IETP specialists in performing two tasks. Management of these tasks will be performed by the NTSC, which will be the organization responsible for monitoring progress of the tasks, ensuring adequate documentation and acting as task integrator. NTSC will ensure that the needs of the reactor designer and regulatory agency are factored into the work. Besides, NTSC will be involved in implementing two tasks.
In the first task irradiation conditions will be calculated for candidate blanket assemblies to establish the axial distribution of operating temperature of the ducts, and the axial distribution of dpa/sec and fluence. MAEC and NTSC will perform this task, with guidance from ANL. The second task will involve machining the parts of the ducts from blanket assemblies and cutting out specimens from them at appropriate locations in the MAEC hot cells. The third task will involve the packaging and transportation of specimens between Aktau and the NNC facilities in Alatau; this small task will be shared between MAEC and NNC to ensure specimen activities (sizes) are compatible with the NNC facilities. The forth task will involve determination of swelling in the samples and will be performed in MAEC and NNC laboratories. This task includes profilometry of the ducts, sample preparation for density measurements and electron microscopy, and also implementation of corresponding investigations. Additionally, phase-structure investigations and mass-spectrometric determination of the impurities doped to radiation resistant steels can be performed. The fifth task will be devoted to investigation of the mechanical property changes as a result of reactor irradiation.
MAEC will perform “preliminary” mechanical tensile tests, while NNC will perform “precision” mechanical tensile and bending tests. Besides, optical metallography and electron microscopy will be applied. The sixth task will determine the gas content of samples using thermal desorption spectroscopy; this task will be performed by IETP specialists. The seven task will be devoted to the analysis of the results of complex investigations to predict the steels behavior under LWR irradiation conditions over a long period of time. This final task will be performed by all the participants of the project including ANL and EdF.
Similar work is being performed by ANL scientists on equivalent steel samples removed from the blanket region of the EBR-II fast reactor in Idaho. The inlet temperature of EBR-II was, however, 370 °C, requiring some extrapolation to LWR operating conditions. The proposed work in Kazakhstan is likely to greatly benefit from a comparison of results obtained at ANL. Collaboration with EdF is seen as bringing the needs and experience of the commercial nuclear sector to bear on the required work. It could be that details of the final workscope depend on the input of such a commercial enterprise.
Both ANL and EdF will participate in the analyses of the comprehensive research results for duct materials of BN-350 reactor fuel assemblies to predict materials behavior for a long-term irradiation in light water reactors.
The International Science and Technology Center (ISTC) is an intergovernmental organization connecting scientists from Kazakhstan, Armenia, Tajikistan, Kyrgyzstan, and Georgia with their peers and research organizations in the EU, Japan, Republic of Korea, Norway and the United States.
ISTC facilitates international science projects and assists the global scientific and business community to source and engage with CIS and Georgian institutes that develop or possess an excellence of scientific know-how.