The RPV of a PWR has to contain an immense amount of energy and radioactivity and the fracture toughness of the pressure vessel materials degrades with irradiation of fast neutrons. This has the potential for a catastrophe as Saleem A. Ansari, S. Nayyar Ali, Inayat Ali, Mansoor Akhtar of the Pakistan Atomic Energy Commission explain.
The sudden failure of a reactor pressure vessel (RPV) could be catastrophic because of the potential radioactive release of the core fission product inventory and the release of a large amount of depressurising energy.
During its service life, the integrity of the RPV depends on multiple factors. These include the initial incidence of flaws during fabrication; embrittlement caused by fast neutron flux, irradiation and thermal effects; the pressure vessel’s operating history; and the frequency and magnitude of pressure and temperature transients during operation.
The fracture toughness of the vessel materials depends upon the reference temperature for nil-ductility transition, which is denoted as RTNDT, or the adjusted reference temperature (ART). The fast neutron flux, energy spectrum and neutron fluence (total number of neutrons per unit area incident on the vessel surface over the total irradiation period) are among the most important parameters which affect the fracture toughness properties, and hence the ART.
The ‘margin to failure’ of the pressure vessel, under pressurized thermal shock, becomes smaller with increasing fluence, eventually limiting the lifetime of the vessel for safety reasons.
All the PWRs at Pakistan’s Chashma site are equipped with the materials necessary to maintain an RPV surveillance programme. In each reactor, eight surveillance capsule assemblies (SCAs) containing vessel material specimens are fitted on the outer surface of the core barrel. After pre-defined irradiation periods the assemblies are withdrawn from the core barrel for structural testing. To facilitate handling and testing of highly radioactive materials, the Pakistan Atomic Energy Commission has constructed a state-of- the-art SCA testing facility (SCATF) at the Chasma site. The facility comprises several hot cells and advanced laboratories that can carry out measurement of in-situ neutron flux, energy and irradiation temperature incident on the specimens, as well as all mandatory mechanical testing.
Pressure vessel surveillance programme at Chasma 1 & 2
Nuclear regulatory authority regulations demand measures to assess the structural integrity of the reactor pressure vessel for light-water-cooled power reactors. In the US, the specific fracture toughness requirements for normal operation and for anticipated operational occurrences for power reactors are set forth in Appendix G, of 10 CFR Part 50. The relevant codes and standards are set out in the box.
International safety standards require that as-built specimens of the materials used in a reactor pressure vessel are irradiated in the reactor and examined at regular intervals during the plant life to assess the irradiation embrittlement trend of the steels used in the pressure vessel and, where necessary, to revise the pressure-temperature (P-T) limit curve used in reactor operation.
There are four PWR plants (two operational and two nearing operation) at Pakistan’s Chashma site. All these reactors are subject to the mandatory RPV Materials Surveillance Programme (MSP) to monitor the material properties of the base metal and weld metal in the beltline of the pressure vessel. That includes the vessel’s heat affected zone.
In the programme, reference specimens of the pressure vessel materials are fixed at the periphery of reactor core and irradiated by exposing them to nearly the same flux and temperature as that experienced by the RPV itself. These specimens are withdrawn after different irradiation periods and a number of mandatory tests are made to determine the fracture toughness properties of the vessel materials. Tests of neutron dosimetry, irradiation temperature measurement, tensile testing, fracture toughness and impact testing are made to determine the conditions under which the vessel can be operated with adequate margins of safety throughout its service life.
Surveillance capsule assemblies
In each reactor at Chasma, eight surveillance capsule assemblies (SCAs) containing the vessel material specimens are fixed on the outer surface of the core barrel (diagram above). The geometrical specifications of the SCAs are:
Each SCA contains 140 material specimens. There are: 54 Charpy V-notch test specimens, 12 fracture toughness test specimens, six tensile test specimens, eight flux monitoring boxes containing 68 radiometric flux sensors and four boxes of irradiation temperature monitors. The upper section of the assembly contains specimens of the materials in the weld seam and the heat affected zone of RPV, whereas the lower section contains base material specimens. After pre-defined irradiation periods the SCAs are withdrawn from the core barrel for measurement of material structural strength and other mandatory tests.
Adjacent test facility
To avoid the need to transport highly radioactive specimens in the SCAs to far-off locations for testing, and then transfer back the resulting radioactive waste to the Chasma site, the Pakistan Atomic Energy Commission has constructed a fully equipped SCA Testing Facility (SCATF) at the Chasma site, comprising hot cells and several test laboratories. The shielded facility is designed to handle radioactive sources with strengths of up to 1000 curies, and is capable of performing all the mandatory surveillance tests by remote means on the irradiated materials specimens.
The SCATF building is 42m long, 21m wide and 14m high and it has three floors.
As regards seismic protection, the building is designed for the peak ground acceleration of 0.25g. The first floor is a big hall reserved for lifting the SCA transportation cask from SCATF garage and lowering the SCA into hot cell 1 from the roof. The ground floor (±0.0m) is the main operating area. This floor contains four hot cells, two shielded glove boxes and a neutron flux measuring room. Each hot cell is capable of different types of remote operations and mandatory testing on irradiated specimens with the help of robust master-slave manipulators and cranes. Inter-cell transfer hatches between hot cells and glove boxes provide maximum maneuverability and allow for remote transfer of specimens.
Hot cell 1 cell is equipped with a shielded hatch for lowering the irradiated SCAs from the rooftop. Its functions include remote visual inspection of SCAs, precise remote cutting and dismantling of the assembly, withdrawal of specimens, and distribution of specimens to other hot cells. Hot cell 2 is used for identification of the surveillance specimens and dimension measurement. Hot cell 3 is the laboratory for Charpy impact testing, and includes an impact-testing machine. Hot cell 4 is the laboratory used for metal tensile and fracture toughness testing to measure strength and elongation of the pressure vessel specimens.
Glove boxes and the flux measurement room are used in taking measurements of the flux, fluence and energy spectrum of neutrons incident on the pressure vessel, with the help of radiometric flux sensors irradiated in SCAs.
Liquid and gaseous waste management and disposal
The SCATF compartments have been divided into three radiation zones – green, amber and red. The containment of radioactivity in the different zones is provided by maintaining negative pressures of varying degrees, using the radioactive gases HVAC system.
The red zone, which houses all the hot cells, has the maximum negative pressure. The airflows from the green area through amber and then to the red areas, with no recirculation. The radioactive air passes through PRE-filters and HEPA filters and is released to the atmosphere through exhaust fans via a 20.5m high stack. The stack height was designed by taking into consideration the plume dispersion pattern.
The radioactive liquid waste collection and disposal system at SCATF comprises two storage tanks, each of 20m3 capacity. They are located in the basement area 6.25m below ground level. The process wastewater from SCA hot cells and maintenance area is transferred to storage tanks with the help of a flow-diverting valve.
SCA removal and testing
The shielded cask containing the assembly is placed by the main crane on top of a shielded lowering chamber provided on the roof of hot cell 1. The bottom end-plate of the SCA cask is pulled out and the SCA is lowered, through the hatch on the roof, into the hot cell. The shielded chamber provides adequate radiation protection for the operator during the SCA lowering process.
The SCATF has already been successfully employed for the testing of two irradiated SCAs withdrawn from Chasma 1 & 2 containing structural materials from the units’ two pressure vessels. These SCAs were removed from the two reactors after 9.71 and 2.37 effective full power years, respectively. These SCAs were stored for some time in the spent fuel bay for cooling, and then transported to the SCATF garage, within the plant’s premises, in shielded casks. The dismantling and testing tasks are described below.
Neutron fluence and energy measurement
The neutron activation and dosimetry measurements for the reactor vessel for Chasma 2 were made following the guidelines given in RG-1.190 and ASTM counting procedures.
For reliable dosimetry over a wide range of neutron energy, all 68 neutron radiometric sensors were used in the analysis of five materials (cobalt, iron, nickel, copper and titanium) irradiated in the base and weld metal sections of the Chasma 2 vessel. Each type of sensor responds to neutrons of different energies. The dosimetry measurements were made with a high- resolution gamma spectrometric system (GSS), with energy resolution of 1.7 KeV at 1332 KeV. Europium-152 disc sources were used to carry out efficiency and energy calibrations of the GSS.
Neutronic calculations were also made, using realistic models of the reactor core, internals and power history of the plant
on a fuel cycle specific basis, for the first two operating cycles of the Chasma 2 reactor. The measured (corrected) reaction rates, calculated energy spectrum and neutron-cross-section data, along with their uncertainty information, were used in the qualified spectrum unfolding methodology to determine the best estimated fluxes at the base and weld metal sections of the SCA for key exposure parameters.
A comparison of the measured (best-estimated) and calculated fast and epithermal neutron fluxes, neutron fluence as incident on SCA/RPV sections adjacent to core mid plane and upper weld section are given in Table one.
The difference between the calculated and the best-estimated flux in the mid-plane is within ±20%, which is acceptable under international standards. In the weld section, the error is slightly higher, which is primarily due to the very small neutron flux in this region. For the weld section the calculated fluence values were used, since these were on higher (conservative) side.
The adjusted and calculated neutron energy spectra at mid-plane and upper weld sections of C-2 SCA are shown in the diagrams one and two.
Measurement of Irradiation Temperature on RPV Surface Irradiation temperature measurements were made in SCATF on the four temperature boxes. It was concluded that the irradiation temperature of the base and weld metal specimens remained between 290oC and 297oC. No exceptional temperature transients were detected during the entire period of operation.
Mechanical impact testing
Impact testing of the specimens taken from the base, weld and heat affected zones of the Chasma assemblies of C-2 was carried out at SCATF. In these tests the absorbed energy, lateral expansion and percent ductile fracture area were measured for all irradiated and unirradiated specimens. A maximum absorbed energy of 245J and lateral expansion of 2.56mm was found at a test temperature of 60 °C.
These parameters were plotted against temperature by using tangent hyperbolic curve fitting. The reference temperature for non-ductility testing (RT) was calculated from these curves at energies of 41J, 68J, lateral expansion of 0.89mm and 50% ductile-to- brittle transition temperature shift. These criteria were used for base metal, weld and HAZ samples. These values are shown in Table two below.
Revision of pressure-temperature limits for Chasma 2
During reactor startup, the temperature of the coolant in contact with the internal surface of the pressure vessel increases with time, such that the stresses near the vessel external surface are tensile and tend to open existing cracks on or near the outer surface. The allowable stress loadings can be conveniently presented as a plot of measured coolant pressure versus temperature.
The pressure-temperature operating envelope is progressively restricted due to developing irradiation embrittlement of the vessel. For the Chasma 2 reactor, the P-T limit curves were determined in accordance with USNRC Standard Review Plan 5.3.2. These curves were obtained from the neutron fluence, the measured RTNDT and the adjusted reference temperature (ART) obtained from the mechanical and neutron fluence testing of SCA-2. The pressure-temperature limit curves for Chasma 2 after over two years of full power operation are shown in Diagram three.