Corrosion Resistance of Radwaste Storage
Corrosion Protection Experimental Studies and Tests under Gamma-Radiation and Heating Combined Effect in BN-350 Spent Fuel Dry Long-Term Storage Facility Silo Models
Tech Area / Field
- FIR-DEC/Decommissioning/Fission Reactors
- ENV-WDS/Waste Disposal/Environment
3 Approved without Funding
National Nuclear Center of the Republic of Kazakstan, Kazakstan, Kurchatov
- Nuclear Technology Safety Center, Kazakstan, Almaty\nNational Nuclear Center of the Republic of Kazakstan / Institute of Atomic Energy (2), Kazakstan, Kurchatov
- Argonne National Laboratory (ANL), USA, IL, Argonne
Краткое описание проектаBN-350 reactor is a multi-purpose fast reactor with liquid-metal coolant. The reactor physical start-up was performed in 1973. The reactor facility (RF) was used to produce electric and thermal power for desalination of the sea-water. The steam generated in RF steam-generators was supplied to power station turbines and to a distillate production plant that provided Aktau and Aktau region population and industry with potable and service fresh water. The reactor also produced weapon-grade plutonium. BN-350 20-year design operation life was over in 1993. After this the reactor was operating another 5 years. In March 1998, the reactor was shutdown due to the economical situation that made its further operation inexpedient. On April 22, 1999 the Government of the Republic of Kazakhstan adopted Resolution on BN-350 Nuclear Reactor Decommissioning. The Resolution defined the BN-350 decommissioning strategy: reactor to be transferred to the safe mode and stored in this mode for 50 years. In compliance with this the fuel was removed from the reactor, the work is performed on sodium draining and processing of radioactive/operational waste. BN-350 spent fuel is packed in self-protective, sealed canisters and stored in the cooling pool. The peculiarity of this fuel is that, first, it is highly radioactive waste, and second, it is valuable raw to produce secondary nuclear fuel, third, it is nuclear material that may be captured to produce nuclear weapon.
The spent fuel long-term storage in the cooling pool is difficult due to the following reasons:
– canisters are designed to be stored in water for no more than 5 years;
– the pool design life is over;
– BN-350 reactor site is in the dangerous proximity to the regions with unstable political situation.
The most preferable option to solve the problem is to transport BN-350 spent fuel to Baikal-1 Test Bench Complex of the Institute of Atomic Energy NNC RK (IAE NNC RK) (Kurchatov). This option is preferable due to the following reasons:
– Kurchatov is geographically more isolated than Aktau and more distanced from the regions with unstable political situation;
– the IAE NNC RK has a proper infrastructure necessary to design, construct, and operate a storage facility for nuclear material long-term storage, and meets long-term priorities in the development of nuclear power in the Republic of Kazakhstan;
– nuclear-and radiation hazardous facilities are already located at the Baikal-1 Complex;
– Baikal-1 Complex site is located on the former Semipalatinsk Test Site, which lands are withdrawn from agricultural and other economical use;
– Baikal-1 Complex site is situated at more than 60 km from the nearest populated area.
According to the operation schedule for BN-350 decommissioning the construction of the spent fuel storage facility at Baikal-1 Test Bench Complex is to be accomplished in June 2003. Upon its completion the BN-350 spent fuel will be transported to a newly built storage facility and placed for 50-year storage in dry silos.
The dry storage in shallow silos technology was selected for BN-350 spent fuel long-term storage facility to be built at the site of Baikal-1 Complex. The storage facility silos are similar to those used at the US Radioactive Scrap and Waste Facility (RSWF) in Argonne National Laboratory.
The silo walls are cased with steel casing pipes. From below a bottom is welded to the casing, at the top a lid is welded. A casing with the bottom and lid welded forms a leak tight cavity of a silo and is one of the confinement barriers in the way of radioactive product possible release into the environment. In the silo cavity it is supposed to install one canister including four or six BN-350 spent fuel assemblies.
One of the main problems in designing, constructing and operating a storage facility of this type is the one concerning integrity of confinement barriers, first of all precluding corrosion of casings that directly contact with filler and soil. The work under this project is the study of corrosion processes in casings including material property and condition changing and testing of anticorrosion methods.
To preclude corrosion of carbon steel casings that are placed vertically in the ground or to mitigate it the multi-step, multi-layer corrosion protection system is proposed. Presumably, the corrosion protection system will comprise a passive cathodic protection system and polymeric (epoxy) protection coating system. The coating layers will serve as barriers precluding contact between carbon steel and soil oxygen/damp. The filler (cement or sludge mortar) pumped into the gap between a silo wall and casing will be another confinement barrier. The corrosion protection materials selected to include epoxy are effective at temperatures possible when the casing and adjoining material layers get heated. This effort's tasks will be to study the behavior of confinement barrier materials and kinetics of electrochemical processes in the soil and on the margins of heterogeneous materials influenced both by heat and radiation, to analyze results of experimental measurements and to develop recommendations on selection of anticorrosive protection system.
The proposed corrosion protection system differs from the one utilized at RSWF in Argonne where the active (with impressed current) cathodic protection system is used. The corrosion protection system proposed for the spent fuel storage facility at the Baikal-1 Complex applies the proven technologies. The analogous systems have been and are widely used in underground piping. However, there is no experience in operating such systems when the high temperature and strong radiation fields influence jointly and simultaneously. Therefore, additional data are required to ensure that the whole system construction will be efficiently operating during the entire expected service life.
All items under study are the confinement barriers designed to preclude radioactive materials release to the environment. The confinement barriers' integrity during the entire storage period is an obligatory condition for the storage facility accident-free operation. The objective of the proposed studies and tests is to study anticorrosive protection to ensure system protection properties in storing canisters with spent nuclear fuel for 50 years. These studies will provide data necessary to design the system. Moreover, the results of tests will be used for timely detection and repair of possible failures of the confinement barrier, determination of confinement barrier condition at any time, prediction of all confinement barrier materials condition during the entire period of SFA storage, also as a result of tests there will be obtained data necessary to develop proposals and technology for SFA handling when the storage term is over.
The proposals on study and testing are developed at RSE NNC RK and NTSC. The following is suggested: on BN-350 spent fuel storage facility site to construct four control-experimental silos. The experimental canisters containing model assemblies (gamma-radiation sources) and samples of the studied materials were loaded into these silos. The casing material samples shall be placed in such a way that to provide a direct contact with soil or filler on the one side and a good thermal and mechanical contact with the casing outer surface on the other side. The silos, model canisters and assemblies to include material samples shall be provided with proper parameter measuring instruments. The model assemblies placed into canisters loaded into control-experimental silos shall not contain nuclear fuel. Each experimental- control silo shall be equipped with electric heaters maintaining the required temperature of the casing and surrounding soil.
The project objective is to study the behavior of confinement barrier materials and corrosion protection efficiency under heat, radiation and medium (soil) combined effect on kinetics of corrosion processes and on the material of corrosion protection and casings.
3. Experimental Plan.
The project work includes:
Construction of experimental-control silos at the storage facility sites and study behavior of corrosion protection materials and its efficiency, to include behavior of structural materials under conditions maximally similar to those in operated storage silos.
3.1. General Provisions.
The work plan for the first part to study corrosion protection operability and efficiency and carbon steel casing efficiency implies for construction of several control-experimental silos. A design and technology for these experimental silos should be the same as those to be used in construction of regular silos of the storage facility. The protection system of the silos will consist of different combinations of sprayed metallic, e.g. Zn/Al, coatings and polymeric material coating applied over the metmetals, e.g. three-coat epoxy coating. Additionally, all silos will be equipped with passive cathodic corrosion protection system.
As was stated above to fill the gap between a casing and silo wall the cement or sludge mortar is supposed to be used in constructing the storage facility at Baikal-1 site. At present the cement mortar seems to be preferable, as it provides for alkaline medium, which is more favorable for carbon steel casing operational integrity and for radiolysis products neutralization.
3.2. Location of Control-Experimental Silos.
The control-experimental silos shall be located directly at the storage facility site under the same geological, soil, hydrogeological and man-caused conditions as the silos containing canisters with SFA. Soil electric resistivity shall be measured by 4-point Venner's method at different levels down to 20 feet spacing two meters. In constructing control-experimental silos the soil samples shall also be obtained and analyzed.
3.3. Silo Structure.
Vertical silos will be auger drilled. The silo diameter shall be 200-…400 mm larger than the casing outer diameter. The gap between a casing and silo outer wall will be filled with cement or sludge mortar. The silos are drilled every 5 meters. The silo bottom is laid over with cement or sludge mortar. The casing vertical deviation along its whole length shall not exceed 20 mm.
The steel pipes of 450 mm diameter and 10 mm wall thickness, assortment GOST 9567-75 are proposed to be used as casings. Material for pipes manufacturing and engineering requirements are specified in GOST 8733-66 for cold-rolled pipes and in GOST 8731-66 for hot-rolled ones.
3.5. Heat and Radiation Load.
We assume that the interaction between a casing and surrounding soil damp/oxygen to include the interaction of heterogeneous materials influenced both by heat and ionizing radiation cause corrosion processes in the casing material and change conditions of other confinement barrier materials and coatings. To simulate the radiation impact the ionizing radiation sources are proposed to place into model canisters. The thermal impact is suggested to simulate using heat generated by ionizing radiation source and placing additional heaters into the canister when necessary. Ionizing radiation sources are proposed to manufacture in the shape of pipes filled with mixture of radioactive fission products diluted with inert non-radioactive material, if necessary. The samples of irradiated fuel claddings are proposed to place into the model canisters. In the fuel claddings the mechanical load is also to be simulated, which is generated in fuel pins by the swollen fuel pellets and fission gas.
3.6. Data Acquisition and Analyses.
It is proposed to maintain observation and survey in control-experimental silos and surrounding soil during the entire 1– 2 year test period to be followed by dismantling and destructive analysis of the four test casings. At that the levels of radiation and thermal impact on materials in silo shall always comply with the levels existing in silos and canisters loaded with SFA. During a survey the following quantitative and qualitative data are acquitted, analyzed and systemized:
– on migration of Chlorine and other ions causing corrosion;
– on temperature regimes and heat load of model silo and canister structural components;
– on soil moisture and concentration of ions in the soil and ground water;
– on surrounding soil resistivity at the storage facility site;
– on coating controlled defects;
– on radiation intensity and dose loads on the materials;
– on controlled defects and condition of the filler layer in the gap between a silo wall and casing pipe;
– on filler chemistry;
– on interaction between heterogeneous materials at the contact areas;
– radiation damage;
– condition of cathodic protection system elements.
Regular soil sampling and estimation of its physical and chemical characteristics is also required.
When 1-2 year storing test period for the spent fuel storage is over materials of the control-experimental silos’ confinement barriers shall be studied and analyzed in the scope that provides for acquisition of information necessary for selection of anticorrosion protection technology and predicprediction of confinement barriers condition during 50-year period. Based on results of these final surveys preliminary recommendations and proposals on future handling of BN-350 spent fuel assemblies after storage period finishes are also to be developed.
4. Project Criteria.
The design and technology of control-experimental storage facility silo shall meet the following requirements:
– reasonable financial, material and labor costs of construction and operation;
– construction at the site using local labor and materials, if possible;
– minimal or insignificant maintenance;
– possibility to create in a control-experimental silo the radiation and heat loads on protection coating materials and structural materials similar to those occurred in the silo containing canisters with SFA;
– capability to retrieve a radiation source from the silo and canister when sampling for materials structure study and measurements are performed and when the silo and canister conditions are examined;
– possibility to place material structure samples and to retrieve them from control-experimental silos at any time;
– possibility to take continuous measurements and records of condition parameters of the control-experimental silo, protective coating materials, soil and material testing samples;
– control-experimental silo design shall meet requirements of all standard-technical documentation regulating construction of the silo with spent fuel and the storage facility in general.
Project Technical Objectives.
The technical objectives of the joint US-Kazakhstan work are as follows:
– assess the efficiency of different corrosion protection methods for carbon steel construction elements, which have direct contact with the soil under combined influence of gamma-radiation and heat.
– select the most effective corrosion protection method;
– develop recommendations to arrange corrosion protection for casings of BN-350 spent fuel dry storage silos.
Project Management Objectives.
In the project work the representatives of a number of Kazakhstan organizations, RSE NNC RK, NTSC, and IAE NNC RK, and the US Argonne National Laboratory-West take part. In compliance with that, RSE NNC RK as a leading organization shall provide the attaining of the following management objectives:
– distribute optimally the work and functions between the participants;
– separate clearly rights and areas of responsibility for each participant;
– coordinate participants' activities;
– immediate exchange of information and results obtained;
– manage operatively the funding and optimize costs;
– making operatively management decisions and inform the project participants of them;
– unconditional and complete fulfilling of management, engineering and operative decisions made by each project participant;
Project technical strategy.
The implementation of project technical strategy implies stepwise carrying out of a number of tasks providing attainment the aforementioned objectives. The tasks are as follows:
Task 1 – Develop program-methodic documentation.
Task 2 – Develop draft-design and project documentation for control-experimental silos.
Task 3 – Deliver componentry, manufacture and assemble control-experimental silos.
Task 4 – Study corrosion processes in control-experimental silos.
Task 5 – Perform material structure testing studies.
Task 6 – Process and analyze results of the studies of silo corrosion processes and material structure testing studies.
Task 7 – Manage the contract.