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Testing and qualification of Control & Safety Rod and its drive mechanism of Fast Breeder Reactor
Nuclear Engineering and Design 240 (2010) 1728–1738
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Testing and qualification of Control & Safety Rod and its drive mechanism of Fast Breeder Reactor V. Rajan Babu ∗ , R. Veerasamy, Sudheer Patri, S. Ignatius Sundar Raj, S.C.S.P. Kumar Krovvidi, S.K. Dash, C. Meikandamurthy, K.K. Rajan, P. Puthiyavinayagam, P. Chellapandi, G. Vaidyanathan, S.C. Chetal Indira Gandhi Centre for Atomic Research, Department of Atomic Energy, Kalpakkam 603 102, India
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Article history: Received 6 November 2009 Received in revised form 12 February 2010 Accepted 18 February 2010
a b s t r a c t Prototype Fast Breeder Reactor (PFBR) has two independent fast acting diverse shutdown systems. The absorber rod of the first system is called Control & Safety Rod (CSR). CSR and its Drive Mechanism (CSRDM) are used for reactor control and for safe shutdown of the reactor by scram action. In view of the safety role, the qualification of CSRDM is one of the important requirements. CSR & CSRDM were qualified in two stages by extensive testing. In the first stage, the critical subassemblies of the mechanism, such as scram release electromagnet, hydraulic dashpot & dynamic seals and CSR subassembly, were tested and qualified individually simulating the operating conditions of the reactor. Experiments were also carried out on sodium vapour deposition in the annular gaps between the stationary and mobile parts of the mechanism. In the second stage, full-scale CSRDM and CSR were subjected to all the integrated functional tests in air, hot argon and subsequently in sodium simulating the operating conditions of the reactor and finally subjected to endurance tests. Since the damage occurring in CSRDM & CSR is mainly due to fatigue cycles during scram actions, the number of test cycles was decided based on the guidelines given in ASME, Section III, Div. 1. The results show that the performance of CSRDM & CSR is satisfactory. Subsequent to the testing in sodium, the assemblies having contact with liquid sodium/sodium vapour were cleaned using CO2 process and the total cleaning process has been established, so that the mechanism can be reused in sodium. The various stages of qualification programmes have raised the confidence level on the performance of the system as a whole for the intended and reliable operation in the reactor. © 2010 Elsevier B.V. All rights reserved.
1. Introduction PFBR is a 500 MWe power, (U-Pu)O2 fuelled, sodium cooled, pool type fast reactor. It is under construction at Kalpakkam, India (Chetal et al., 2006). The reactor assembly consists of core, grid plate, core support structure, main vessel, safety vessel, inner vessel, top shield and absorber rod drive mechanisms as shown in Fig. 1. It holds 1150 t of primary sodium, blanketed by argon cover gas. The inner vessel separates the sodium in the hot and cold pools, and is supported on the grid plate. The core is homogeneous with two enrichment zones having radial and axial blankets as shown in Fig. 2. Top shield consists of roof slab, Large Rotatable Plug (LRP), small rotatable plug and control plug. It provides biological and thermal shielding in the upper axial direction of the reactor. The roof slab supports the LRP, Primary Sodium Pump, Intermediate Heat Exchanger and heat exchangers of decay heat removal system. The flow sheet of PFBR is shown in Fig. 3. Heat energy from
∗ Corresponding author. Tel.: +91 44 274 80 221; fax: +91 44 274 80 104. E-mail address: [email protected] (V. Rajan Babu). 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.02.037
the core is transferred from the primary sodium loop to the tertiary steam-water circuit through an intermediate secondary sodium loop. FBR systems are designed with defence-in-depth approach having redundancy, diversity and independence. The safety measures provided are two independent, fast acting & diverse reactor shutdown systems, two decay heat removal systems, a core catcher and Reactor Containment Building. Each shutdown system, consists of sensors, logic circuit, Drive Mechanisms and neutron absorber rods having B4 C pellets (Rajan Babu et al., 1995a,b). The absorber rod of the first system is called as Control & Safety Rod (CSR) and that of the second system as Diverse Safety Rod (DSR). The respective drive mechanisms, Control & Safety Rod Drive Mechanism (CSRDM) and Diverse Safety Rod Drive Mechanism (DSRDM), are housed in the control plug, which is a part of top shield of the reactor. The schematic arrangement of the absorber rods and their mechanisms in the reactor assembly is shown in Fig. 4. Core subassemblies are arranged in a hexagonal array and nine CSR and three DSR subassemblies are kept in two pitch circles as shown in Fig. 2.
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Nomenclature ASME CSR CSRDM DSR DSRDM EM LRP PFBR RCB
American Society of Mechanical Engineers Control & Safety Rod Control & Safety Rod Drive Mechanism Diverse Safety Rod Diverse Safety Rod Drive Mechanism Electromagnet Large Rotatable Plug Prototype Fast Breeder Reactor Reactor Containment Building
The functions of CSRDM are to facilitate • start-up & controlled shutdown of the reactor and control of reactor power by raising and lowering of CSR and • shutdown of the reactor at off-normal conditions by rapid insertion of CSR into the core (i.e., by scram action) under gravitational force.
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nism of Phenix reactor was tested for 2470 translations and 2431 scram operations for 6842 h in sodium, of which 4905 h at 560 ◦ C, whereas Type-2 mechanism was tested for 1880 translations and 1460 scram operations for 5940 h in sodium, of which 4950 h at 560 ◦ C (Delemontey et al., 1972). The mechanism of KNK-II was tested for 1750 scram cycles whereas the expected number of scrams in reactor was 350 (Glinsky et al., 1978). Similarly, an extensive qualification programme was envisaged from the onset of the design of CSR and CSRDM. Critical components of the system were identified, developed and qualified by testing the full-scale models. Later, integrated assembly of full-scale prototype CSRDM and CSR was manufactured indigenously and qualified by elaborate performance testing and endurance testing under simulated reactor-operating conditions. The purpose of this paper is to bring out the details of the systematic developmental activities and the qualification tests carried out on the subsystems and integrated assembly of CSR and CSRDM resulting in a reliable shutdown system for PFBR. 2. Design features 2.1. CSR subassembly
However, the function of DSRDM is only to facilitate shutdown of the reactor by scram action of DSR. Safety of the reactor as a whole depends on the reliable operation of CSR & DSR and their mechanisms. So, this demands a very detailed design, analysis, technology development in manufacturing and testing of the entire system under simulated reactor-operating conditions to check and ensure the intended functions in the reactor. The shutdown systems of various fast reactors were qualified by extensive testing carried out in dedicated sodium test loops simulating the operating conditions in the reactor. In the case of SPX-1, the mechanism aligned with absorber rod was tested at 565 ◦ C for 100 shutdowns, 4000 impulsions & 200 complete translations and with misalignment for 400 shutdowns, 16,000 impulsions & 800 complete translations (Pignatelli et al., 1982). Type-1 mecha-
CSR subassembly consists of mobile CSR and stationary sheath as shown in Fig. 5. Its length is 4.46 m and is supported on grid plate. 19 absorber pins housed in mobile CSR are held by guide rails and freely hanging from the top. They are arranged in triangular pitch as a bundle and covered by a hexagonal sheath. B4 C pellets are stacked in clad tubes of the absorber pins. Sodium enters at the foot of the subassembly, passes through the absorber pin bundle and also parallely through the annular gap between the mobile CSR and stationary sheath and leaves out through the head. The mobile CSR moves inside the stationary sheath and is guided at two levels. The contact surfaces of the mobile rod are hard faced with nickel based alloy and the corresponding internal surfaces of the stationary sheath are hard chromium plated to avoid self-welding between the parts having relative motion in sodium.
Fig. 1. Reactor assembly of PFBR.
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Fig. 2. CSR & DSR in reactor core.
2.2. CSR drive mechanism Fig. 6 shows the schematic diagram of CSRDM along with CSR and the vertical section of CSRDM. Overall length of CSRDM is about 12.3 m. Its lower part is partially immersed in hot pool sodium and the upper part is in argon/air atmosphere. The mobile assembly consisting of guide tube, translation tube and gripper subassembly at its bottom end is guided by a stationary tube sheath. Three fingers in the gripper subassembly are actuated to hold/release the head of mobile CSR. They are operated manually from the top of CSRDM. The mobile assembly of CSRDM is held by an electromagnet (EM), which is attached to the nut of a translation screw–nut mechanism. Both the EM and mobile assembly slide over two guide columns fixed to a support tube. Motor drive assembly rotates the translation screw to raise or lower the EM and hence the mobile assembly. When CSR is coupled to the gripper of the mobile assembly, it also translates up or down based on the direction of rotation of the motor.
The mobile assembly is also guided by two bushes (made of nickel based alloy), one at the level of control plug top and the other in sodium at the lower end of the stationary tube sheath and the corresponding surfaces of the translation tube are hard chromium plated to avoid self-welding of stainless parts having relative movement in sodium/sodium vapour environment. The inbuilt load cells facilitate on-line measurement of frictional force encountered by the mobile assembly of CSRDM and CSR. On receiving the scram signal, the EM is de-energised and the mobile assembly of CSRDM along with CSR is released to fall under gravity. At the end of free fall travel (835 mm), the mobile assembly is decelerated by an oil dashpot for the remaining 250 mm travel. The design parameters are given in Table 1 and the operating conditions of CSR & CSRDM are listed in Table 2. Primary leak-tightness is achieved by the seals in the lower part and secondary leak-tightness by the seals in the upper part. They prevent the leakage of radioactive sodium vapour and argon cover gas into Reactor Containment Building (RCB) and deposition
Fig. 3. PFBR-flow sheet.
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Fig. 4. CSR & DSR with their Drive Mechanisms.
Fig. 6. Control & Safety Rod Drive Mechanism.
Table 1 Design parameters of CSR & CSRDM. Total travel of CSR Free fall travel Braking travel Raising/lowering speed Response time of electromagnet Fast drop time in flowing sodium including response time of EM
1085 mm 835 mm 250 mm 2 mm/s <100 ms <1 s
of sodium vapour at low temperature region of the mechanism. The inter-seal argon space between the primary and secondary leak tight barriers is maintained at a positive pressure with respect to reactor cover gas. Temperature at the top of control plug is maintained at 383 K, so that sodium vapour deposition in the annular gap does not hinder the free movement of the mobile assembly. ‘O’ ring seals are used wherever there is no/slow relative movement which is of no safety concern. Dynamic lip type ‘V’ ring seals are provided Table 2 Operating conditions in reactor.
Fig. 5. Control & Safety Rod Subassembly.
Max. temperature experienced by Lower part of CSRDM Upper part of CSRDM
845 K (572 ◦ C) 373 K (100 ◦ C)
CSR Inlet temperature of sodium Outlet temperature of sodium
670 K (397 ◦ C) 783 K (510 ◦ C)
Number of scram operations in 40 years life time of CSRDM 2 years life time of CSR
752 38
Number of gripper operations
∼120
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between the mobile assembly and the stationary tube sheath at the level of control plug top to act as a barrier restricting the sodium vapour entering in to the upper part. The configuration of the Vring seal (as explained in Section 3.4) is such that it does not arrest the downward movement of freely falling mobile assembly. The major material of construction of the lower part of CSRDM, having contact with sodium/sodium vapour, is SS 316LN. The fingers, axle pins, guide pins and screws in the gripper assembly are made of precipitation hardened austenitic stainless steel (equivalent to ASTM A453, Grade 660, Class B) having high strength at elevated temperature. The upper part operating at temperature less than 100 ◦ C, is made of carbon steel and alloy steel. The satisfactory performance of CSRDM and CSR is monitored and ensured by on-line measurement of the following: • Frictional force. • Response time of EM, drop time of mobile assembly of CSRDM along with CSR and braking time during scram action. • Leak-tightness. 3. Qualification tests After the detailed design and analyses of the entire system, the qualification of CSRDM and CSR was carried out in two stages. In the first stage, the full-scale models of the critical subassemblies of the mechanism such as hydraulic dashpot, scram release electromagnet, dynamic seals and CSR subassembly were tested individually in air simulating the actual operating temperature and pressure as in the reactor. The design parameters were fine-tuned based on the test results. Experiments were also carried out on sodium vapour deposition in the annular gaps between the stationary and mobile parts. In the second stage, prototype CSRDM and CSR were manufactured and subjected to various functional tests in air, hot argon and subsequently in sodium simulating the operating conditions of the reactor. The developmental activities are detailed out in the subsequent paragraphs. 3.1. Hydraulic dashpot Fig. 7. Hydraulic dashpot.
During scram action, the mobile assembly of CSRDM along with CSR falls freely under gravity and acquires kinetic energy. An oil dashpot having piston-cylinder arrangement dissipates the kinetic energy before the mobile assembly comes to rest. It provides deceleration to the fast moving mobile assembly and brings down the velocity to near zero at the end of travel of CSR. The dashpot has a central hollow passage surrounded with two annuli. The inner annulus is for the movement of the piston and the outer one is for oil collection. The weight of the freely falling mobile assembly of CSRDM & CSR is about 3600 N and that of the piston is 290 N. The mobile assembly of the mechanism passes through the central hollow passage. The configuration of the dashpot is shown in Fig. 7. Teflon pads are provided to avoid direct metal to metal contact between the mobile assembly and the piston at the time of impact and also to transfer the momentum gradually. A pre-compressed spring provided below the piston pushes it to its top most position when the mobile assembly is lifted up and hence the dashpot is ready for the next drop. The wall between the inner and the outer annuli has several holes (orifices) located at different elevations in a helical manner to facilitate flow of displaced oil. The rapid movement of the piston and mobile assembly is decelerated by the upward thrust due to the oil pressure developed. As the piston moves down, its velocity reduces and also it closes the orifices at different heights one after the other. Therefore, the effect of reduction in velocity of the piston on oil flow velocity is balanced
by the reduction in number of orifices available for oil flow. The size, number and positions of the orifices were optimised using computer softwares developed in-house for the design and analysis of the dashpot. A prototype dashpot was manufactured and tested in a dedicated test facility shown in Fig. 8. The test facility was provided with a short length mobile part loadable with adjustable dummy weights to simulate the weight of the mobile assembly of CSRDM & CSR. Free fall and decelerated travel heights were maintained same as in the actual CSRDM. Dynamic characteristics of the oil pressure, deceleration and displacement during braking travel were recorded. Based on the functional behaviour of the dashpot during the preliminary tests and fine tuning of the computer softwares, the number of orifices at the top end of the cylinder was increased to reduce the first pressure peak, which is mainly due to acceleration of the piston and hence the oil from rest at the time of mobile assembly hitting on the piston. Fig. 9 shows the decrease in first pressure peak of the oil asymptotically as a function of number of orifices. There is no significant change in pressure peak when number is increased beyond 16 for the orifices ϕ 8 mm and for the given configuration of the dashpot. Further reduction in pressure peak was achieved with increase in thickness of pad at the contact surface
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Fig. 10. Typical dynamic pressure of oil during dashpot action.
Fig. 8. Dashpot test set-up.
mobile assembly and the inner core rests on the nut of screw–nut mechanism. Central hole provided in the inner core facilitates passage of translation screw. As indicated in Table 1, response time of EM is to be less than 100 ms. It is the time interval between the instant at which the power supply to EM is switched off and the instant at which the mobile assembly moves down by 1 mm, ensuring positive detachment. The development and qualification of EM were completed in three campaigns of experiments. Series of tests were carried out to ascertain the load carrying capacity, temperature rise and response time. Fig. 12 shows the set-up for EM testing and Fig. 13 shows the electrical circuit used for the EM with features for measuring response time. The increase in temperature of the coil was less than 5 ◦ C, which has no implication on the performance. Initial configurations met the requirements of load carrying capacity, but the response time was found to be exceeding the limit of 100 ms. Then radial slots were cut in the inner and outer cores of EM to reduce eddy current induced and hence to reduce the response time. The test results are shown in Fig. 14. It is to be noted that when the dif-
Fig. 9. Effect of increasing number of holes on first pressure peak.
between the mobile assembly and the piston. A typical characteristic of variation of oil pressure with respect to time is shown in Fig. 10. Also the length of the air vent pot was increased with the introduction of appropriate baffles to avoid spraying out of oil from the dashpot during fast movement of the piston. The fast drop tests were carried out at room temperature and also at 353 K (80 ◦ C). Endurance testing was carried out for 1000 drops and the test results were satisfactory. 3.2. Scram release electromagnet The configuration of EM is shown in Fig. 11. DC power supply is provided to the EM. The outer core of EM holds the top end of the
Fig. 11. Scram release electromagnet.
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Fig. 14. Test results of electromagnet.
Fig. 12. Electromagnet & its test set-up.
Fig. 13. Circuit for EM response time measurement.
ference between the load carrying capacity and the attached weight to the EM is high, then the response time is larger. The response time is not influenced by the air gap thickness at the contact face beyond the load difference of 2500 N. In actual case, air gap of 0.1 mm is provided at the contact face. The response time was found to be 81 ms for the parameters corresponding to reactor-operating conditions and hence the design requirements were satisfied. 3.3. Elastomeric seals As explained in Section 2, lip type V-ring seals are provided at the top end of the annulus between the stationary tube sheath and the mobile assembly, where the temperature is maintained at about 383 K. In the process of development of lip type ‘V’ ring seals, several
configurations of the seal (as shown in Fig. 15) were formulated and tested. A photograph of the selected V-ring seal (Type-D) having labyrinth at the sliding contact surface is shown in Fig. 16. Leaktightness and frictional force offered by the seals were measured with respect to the differential pressure across the seals. Two V-ring seals were mounted in series to improve leak-tightness. Endurance test was carried out simulating the translation and scram action of the mobile assembly. The performance of the seal related to leak-tightness and frictional force was found to be satisfactory and no damage was noticed. The leak rate is less than 0.5 cm3 /s, which is within the allowable limit of 1 cm3 /s. This corresponds to leakage of fresh argon getting into reactor cover gas and there is no release of radioactive argon. When the seal was subjected to a reverse differential pressure of 100 mbar (g) (situation arises when inter-seal argon space opens up to RCB), there was no appreciable change in frictional force.
3.4. Sodium vapour deposition studies In CSRDM, long annuli with narrow gaps having bottom end exposed to hot pool sodium are present. The large temperature gradient between the top and bottom ends induces natural convection of argon gas filled in the annular space. At high temperature, sodium vapour is formed which quickly condenses to form sodium mist in the argon gas. The argon gas carries sodium mist when it travels up along the vertical annuli due to natural convection. Some of the sodium mist gets deposited on the annulus walls where the temperature is below 373 K (100 ◦ C). Continuous accumulation of sodium deposit between the mobile assembly and the stationary tube sheath may resist the safety scram action during off-normal condition.
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Fig. 15. Cross section of V-ring seals tested.
Fig. 16. Dynamic lip type V-ring seal.
The above phenomena of sodium vapour generation in sodium pool and deposition in narrow annular gaps were studied in an experimental set up to articulate an effective method to control the deposition. Experiments were carried out in two phases. In the first phase, 1.3 m long annulus models were used. A sleeve was introduced in the annulus at a particular height from sodium level to reduce sodium deposition above the sleeve. In this experiment, sleeve length was varied from 20 to 120 mm. Also the radial gap at the sleeve was varied from 1.5 to 3 mm. Multiple sleeves were also used in the experiment. The bottom of the model was dipped in sodium. The sodium temperature was set to 820 K (547 ◦ C) and the sodium vapour was allowed to deposit in the annulus for about 750 h. After the test, the models were dismantled and the deposited sodium was collected by dissolving in water. By chemical analysis, the amount of sodium deposited in the annulus above and below the sleeve was determined. In the second phase, 3.5 m long annulus models were used. Configuration of the test vessel along with a typical model is shown in Fig. 17. Addition of sleeve in the annulus decreases the sodium vapour deposition rate by one order of magnitude. It is found that as the aspect ratio (i.e., the ratio of length to gap) increases, the sodium vapour deposition rate decreases. Based on the experimental results, inside surface of top end of the tube sheath of CSRDM has been configured to reduce sodium vapour deposition at low temperature region of the annulus. 3.5. Testing of prototype CSRDM along with CSR 3.5.1. Test vessel The integrated full-scale assembly of prototype CSRDM and CSR were manufactured and testing of the same was carried out in Test Vessel-1 (TV-1) of Large Component Test Rig (LCTR). TV-1 is shown in Fig. 18. LCTR is a test facility in which many of the full-scale reactor components are being tested in static sodium. The temperature of the sodium and pressure of cover gas in the test vessel are maintained similar to that in the reactor. TV-1 is a vertical cylindrical vessel of size ϕ 1 m × 12.5 m long. It has an inner vessel, which is bolted to the top flange of TV-1. Inner vessel consists of two parts. The upper part of the inner vessel simulates the reactor control plug. It houses 48 baffle plates stacked with differential gap between the adjacent plates to achieve the desired temperature distribution as in the reactor. The lower part of the inner vessel has grid plate sleeve which supports the CSR subassembly. CSRDM is supported on the top flange of the inner vessel. After successful completion of testing in air, the inner vessel along with CSR and CSRDM is lowered in to TV-1. Sodium is filled in TV-1 for a height similar to that in the reactor and above the sodium pool, argon cover gas is filled at a pressure of 100 mbar(g).
Fig. 17. Na vapour deposition in annulus—test vessel.
3.5.2. Tests carried out Table 2 shows the operating conditions of CSRDM & CSR and the number of expected scram operations during life time of the reactor. Since the damage occurring in CSRDM & CSR is mainly due to fatigue cycles during scram actions, the number of test cycles
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Fig. 19. Dynamic pressure of oil in dashpot.
Fig. 20. Displacement characteristics of CSR during scram.
incorporating the design modifications. Then endurance testing was carried out in sodium for another 1093 cycles of scram operations, maintaining the temperature of sodium at 823 K (550 ◦ C) and 120 cycles of gripper operations at 473 K (200 ◦ C), with the misalignment of 30 mm between CSRDM and CSR (Mark-II). In addition to the above, the tests carried out in air, argon and sodium at different temperatures amounts to 450 cycles. Fig. 18. Vessel for testing CSRDM & CSR in sodium.
is decided as per the guidelines given in ASME, Section III, Div. 1, Appendices, Para II-1500 (2004). As per this guide line, the prototype CSRDM has to be tested for 3460 scram cycles at 845 K (572 ◦ C) or 4351 cycles at 823 K (550 ◦ C) or 5062 at 803 K (530 ◦ C) corresponding to 752 cycles in reactor and 552 gripper operations at 473 K (200 ◦ C) corresponding to 120 operations in reactor. However, CSR is required to be tested for 173 scram cycles, since its life span in the reactor is only 2 years. There is a possibility of misalignment between the axes of CSRDM and CSR during operation in reactor. The causes for the misalignment between the mechanism and the CSR in the reactor are due to the manufacturing tolerances, differential thermal expansion and deflection & slope of support flanges, thermal & irradiation induced bowing of core subassemblies, etc. So it is envisaged to carry out the tests in aligned as well as misaligned condition. The prototype CSRDM and CSR were subjected to extensive performance testing to check and ensure all the intended functions in air, hot argon at 473 K (200 ◦ C) and then in sodium at temperature starting from 473 to 823 K (200–550 ◦ C). Then endurance testing was carried out for 500 cycles of scram operations maintaining the temperature of sodium at 803 K (530 ◦ C) and 60 cycles of gripper operations at 473 K (200 ◦ C), keeping CSRDM and CSR (Mark-I) in aligned condition. Based on the test results, the cross section of the mobile CSR was changed from circular to hexagon to have uniform and more gap within its sheath which is hexagon in shape. The number of guides for the mobile CSR was changed from three to two to reduce the frictional force. The CSR (Mark-II) was manufactured
Fig. 21. Flow sheet of sodium removal facility (water vapour–CO2 process).
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Table 3 Summary of test results of prototype CSRDM & CSR. Measured parameter
Testing In air
Frictional force (N) during translation Min. holding current (A) of EM Drop time (ms) (including EM response time) Braking time (ms) Dashpot oil pressure (MPa) a
Aligned
Misaligned
60 0.65 531 186 3.06
280 0.65 561 194 3.11
In argon (misaligned) at 473 K
In sodium (misaligned) at (K) 473
573
673
773
823
823a
275 0.65 563 189 3.19
260 0.65 600 212 2.84
220 0.65 596 206 2.67
192 0.65 597 195 2.71
290 0.65 596 202 2.65
315 0.65 590 205 2.73
350 0.65 610 220 2.66
Operation after endurance test, i.e., after 1593 scram cycles.
Fig. 22. CSRDM (lower part) after testing and sodium cleaning.
Hence, as per the ASME guidelines, CSRDM has been qualified for 14 years of life in the reactor and CSR has been qualified for its total life in the reactor. Fig. 19 shows the dynamic pressure of oil in the hydraulic dashpot during scram action and Fig. 20 shows the displacement characteristics of CSR during scram action. Table 3 shows the summary of the test results. The variation in frictional force with respect to the operating temperature is only about 90 N and the maximum frictional force is less than 10% of the total weight of the mobile assembly. Hence increase in drop time due to frictional force is only about 25 ms. The maximum drop time of CSR during scram action is 610 ms, which is less than the allowable time, 1 s. There is no change in minimum holding current of the electromagnet. Hence, the results of all the tests show that the performance of CSRDM along with CSR is satisfactory throughout the endurance testing and there is no significant change in the performance. 4. Sodium cleaning After the qualification tests, CSRDM lower part wetted with sodium was cleaned for subsequent handling, inspection and reuse. Out of various methods of cleaning available, water vapour–carbon dioxide (CO2 ) process was selected for the reusable components of PFBR. The flow sheet for sodium removal facility is shown in Fig. 21. The process involves bubbling of CO2 through demineralised (DM) water (kept at temperature of 323–333 K) in a re-circulating stream of nitrogen. Initially sodium reacts with humid water
vapour carried by CO2 gas stream to form caustic sodium hydroxide (NaOH) with the release of hydrogen. Further reaction of NaOH with CO2 produces sodium bicarbonate (NaHCO3 ) and sodium carbonate (Na2 CO3 ). The carbonate and bicarbonate so formed can be easily removed by washing with DM water, thus preventing caustic stress corrosion of austenitic stainless steel by NaOH. The cleaning process involves the following four steps: • • • •
Sodium draining from the components. Vapour phase reaction both with water vapour and CO2 . Rinsing the component with water. Drying with hot nitrogen.
Vapour phase reaction was continued for about 80 h and hydrogen release during the reaction was kept well below 1% and there was no pressure build-up in the system due to reaction. Subsequently the mechanism was cleaned with DM water with several fill, recirculation and drain cycles. Finally, drying the component with hot air/nitrogen completed the cleaning process. The CO2 cleaning process was successfully demonstrated for removal of sodium from CSRDM lower part for reuse. Fig. 22 shows some of the photographs of CSRDM taken while handling it after testing and cleaning the sodium. The reaction products formed during sodium removal from the CSRDM are shown in the photographs.
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5. Conclusion CSR and its drive mechanism, which are the major constituents of the first shutdown system of PFBR, have been designed, critically analysed and thoroughly tested in air, argon and in sodium for their fail-safe operation. The sodium vapour deposition in the annular spaces of CSRDM was studied. Sodium removal from the sodiumwetted parts of CSRDM was done successfully and the cleaning process has been established, so that the mechanism can be reused in sodium. The test results of the individual components and the system as a whole are satisfactory and hence CSR & CSRDM have been qualified for their intended and reliable operation in the reactor. Acknowledgments The authors gratefully acknowledge the immense contributions of their colleagues in the Reactor Engineering Group, Fast Reac-
tor Technology Group, Quality Assurance Division, Electronics & Instrumentation Division, and Central Workshop. References Chetal, S.C., et al., 2006. Design of the Prototype Fast Breeder Reactor. Nuclear Engineering and Design 236 (7–8), 852–860. Delemontey, et al., 1972. Mecanismes de barres de controle pour reacteurs rapides. In: Proceedings of ‘Engineering of Fast Reactors for safe & Reliable operation’, vol. I, BBW, Kernforschungszentrum Karlsruche, pp. 350–357. Glinsky, H.W., et al., 1978. The shutdown systems of KNK and SNR 300. In: Design, Construction and Operating Experience of Demonstration LMFBRs, IAEA, Bologna, STI/PUB/490, pp. 191–203. Pignatelli, et al., 1982. The Superphenix shutdown system. In: Proceedings of LMFBR Safety Topical Meeting, Lyon-Euelly, pp. II 485–495. Rajan Babu, V., et al., 1995a. Design philosophy of PFBR shutdown systems. In: Proceedings of Technical Committee Meeting, IAEA-TECDOC-884, Obninsk, pp. 81–88. Rajan Babu, V., et al., 1995b. Design of shutdown systems for PFBR. In: Proceedings of Technical Committee Meeting, IAEA-TECDOC-884, Obninsk, pp. 89–96.
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Testing and qualification of Control & Safety Rod and its drive mechanism of Fast Breeder Reactor V. Rajan Babu ∗ , R. Veerasamy, Sudheer Patri, S. Ignatius Sundar Raj, S.C.S.P. Kumar Krovvidi, S.K. Dash, C. Meikandamurthy, K.K. Rajan, P. Puthiyavinayagam, P. Chellapandi, G. Vaidyanathan, S.C. Chetal Indira Gandhi Centre for Atomic Research, Department of Atomic Energy, Kalpakkam 603 102, India
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Article history: Received 6 November 2009 Received in revised form 12 February 2010 Accepted 18 February 2010
a b s t r a c t Prototype Fast Breeder Reactor (PFBR) has two independent fast acting diverse shutdown systems. The absorber rod of the first system is called Control & Safety Rod (CSR). CSR and its Drive Mechanism (CSRDM) are used for reactor control and for safe shutdown of the reactor by scram action. In view of the safety role, the qualification of CSRDM is one of the important requirements. CSR & CSRDM were qualified in two stages by extensive testing. In the first stage, the critical subassemblies of the mechanism, such as scram release electromagnet, hydraulic dashpot & dynamic seals and CSR subassembly, were tested and qualified individually simulating the operating conditions of the reactor. Experiments were also carried out on sodium vapour deposition in the annular gaps between the stationary and mobile parts of the mechanism. In the second stage, full-scale CSRDM and CSR were subjected to all the integrated functional tests in air, hot argon and subsequently in sodium simulating the operating conditions of the reactor and finally subjected to endurance tests. Since the damage occurring in CSRDM & CSR is mainly due to fatigue cycles during scram actions, the number of test cycles was decided based on the guidelines given in ASME, Section III, Div. 1. The results show that the performance of CSRDM & CSR is satisfactory. Subsequent to the testing in sodium, the assemblies having contact with liquid sodium/sodium vapour were cleaned using CO2 process and the total cleaning process has been established, so that the mechanism can be reused in sodium. The various stages of qualification programmes have raised the confidence level on the performance of the system as a whole for the intended and reliable operation in the reactor. © 2010 Elsevier B.V. All rights reserved.
1. Introduction PFBR is a 500 MWe power, (U-Pu)O2 fuelled, sodium cooled, pool type fast reactor. It is under construction at Kalpakkam, India (Chetal et al., 2006). The reactor assembly consists of core, grid plate, core support structure, main vessel, safety vessel, inner vessel, top shield and absorber rod drive mechanisms as shown in Fig. 1. It holds 1150 t of primary sodium, blanketed by argon cover gas. The inner vessel separates the sodium in the hot and cold pools, and is supported on the grid plate. The core is homogeneous with two enrichment zones having radial and axial blankets as shown in Fig. 2. Top shield consists of roof slab, Large Rotatable Plug (LRP), small rotatable plug and control plug. It provides biological and thermal shielding in the upper axial direction of the reactor. The roof slab supports the LRP, Primary Sodium Pump, Intermediate Heat Exchanger and heat exchangers of decay heat removal system. The flow sheet of PFBR is shown in Fig. 3. Heat energy from
∗ Corresponding author. Tel.: +91 44 274 80 221; fax: +91 44 274 80 104. E-mail address: [email protected] (V. Rajan Babu). 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.02.037
the core is transferred from the primary sodium loop to the tertiary steam-water circuit through an intermediate secondary sodium loop. FBR systems are designed with defence-in-depth approach having redundancy, diversity and independence. The safety measures provided are two independent, fast acting & diverse reactor shutdown systems, two decay heat removal systems, a core catcher and Reactor Containment Building. Each shutdown system, consists of sensors, logic circuit, Drive Mechanisms and neutron absorber rods having B4 C pellets (Rajan Babu et al., 1995a,b). The absorber rod of the first system is called as Control & Safety Rod (CSR) and that of the second system as Diverse Safety Rod (DSR). The respective drive mechanisms, Control & Safety Rod Drive Mechanism (CSRDM) and Diverse Safety Rod Drive Mechanism (DSRDM), are housed in the control plug, which is a part of top shield of the reactor. The schematic arrangement of the absorber rods and their mechanisms in the reactor assembly is shown in Fig. 4. Core subassemblies are arranged in a hexagonal array and nine CSR and three DSR subassemblies are kept in two pitch circles as shown in Fig. 2.
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Nomenclature ASME CSR CSRDM DSR DSRDM EM LRP PFBR RCB
American Society of Mechanical Engineers Control & Safety Rod Control & Safety Rod Drive Mechanism Diverse Safety Rod Diverse Safety Rod Drive Mechanism Electromagnet Large Rotatable Plug Prototype Fast Breeder Reactor Reactor Containment Building
The functions of CSRDM are to facilitate • start-up & controlled shutdown of the reactor and control of reactor power by raising and lowering of CSR and • shutdown of the reactor at off-normal conditions by rapid insertion of CSR into the core (i.e., by scram action) under gravitational force.
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nism of Phenix reactor was tested for 2470 translations and 2431 scram operations for 6842 h in sodium, of which 4905 h at 560 ◦ C, whereas Type-2 mechanism was tested for 1880 translations and 1460 scram operations for 5940 h in sodium, of which 4950 h at 560 ◦ C (Delemontey et al., 1972). The mechanism of KNK-II was tested for 1750 scram cycles whereas the expected number of scrams in reactor was 350 (Glinsky et al., 1978). Similarly, an extensive qualification programme was envisaged from the onset of the design of CSR and CSRDM. Critical components of the system were identified, developed and qualified by testing the full-scale models. Later, integrated assembly of full-scale prototype CSRDM and CSR was manufactured indigenously and qualified by elaborate performance testing and endurance testing under simulated reactor-operating conditions. The purpose of this paper is to bring out the details of the systematic developmental activities and the qualification tests carried out on the subsystems and integrated assembly of CSR and CSRDM resulting in a reliable shutdown system for PFBR. 2. Design features 2.1. CSR subassembly
However, the function of DSRDM is only to facilitate shutdown of the reactor by scram action of DSR. Safety of the reactor as a whole depends on the reliable operation of CSR & DSR and their mechanisms. So, this demands a very detailed design, analysis, technology development in manufacturing and testing of the entire system under simulated reactor-operating conditions to check and ensure the intended functions in the reactor. The shutdown systems of various fast reactors were qualified by extensive testing carried out in dedicated sodium test loops simulating the operating conditions in the reactor. In the case of SPX-1, the mechanism aligned with absorber rod was tested at 565 ◦ C for 100 shutdowns, 4000 impulsions & 200 complete translations and with misalignment for 400 shutdowns, 16,000 impulsions & 800 complete translations (Pignatelli et al., 1982). Type-1 mecha-
CSR subassembly consists of mobile CSR and stationary sheath as shown in Fig. 5. Its length is 4.46 m and is supported on grid plate. 19 absorber pins housed in mobile CSR are held by guide rails and freely hanging from the top. They are arranged in triangular pitch as a bundle and covered by a hexagonal sheath. B4 C pellets are stacked in clad tubes of the absorber pins. Sodium enters at the foot of the subassembly, passes through the absorber pin bundle and also parallely through the annular gap between the mobile CSR and stationary sheath and leaves out through the head. The mobile CSR moves inside the stationary sheath and is guided at two levels. The contact surfaces of the mobile rod are hard faced with nickel based alloy and the corresponding internal surfaces of the stationary sheath are hard chromium plated to avoid self-welding between the parts having relative motion in sodium.
Fig. 1. Reactor assembly of PFBR.
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Fig. 2. CSR & DSR in reactor core.
2.2. CSR drive mechanism Fig. 6 shows the schematic diagram of CSRDM along with CSR and the vertical section of CSRDM. Overall length of CSRDM is about 12.3 m. Its lower part is partially immersed in hot pool sodium and the upper part is in argon/air atmosphere. The mobile assembly consisting of guide tube, translation tube and gripper subassembly at its bottom end is guided by a stationary tube sheath. Three fingers in the gripper subassembly are actuated to hold/release the head of mobile CSR. They are operated manually from the top of CSRDM. The mobile assembly of CSRDM is held by an electromagnet (EM), which is attached to the nut of a translation screw–nut mechanism. Both the EM and mobile assembly slide over two guide columns fixed to a support tube. Motor drive assembly rotates the translation screw to raise or lower the EM and hence the mobile assembly. When CSR is coupled to the gripper of the mobile assembly, it also translates up or down based on the direction of rotation of the motor.
The mobile assembly is also guided by two bushes (made of nickel based alloy), one at the level of control plug top and the other in sodium at the lower end of the stationary tube sheath and the corresponding surfaces of the translation tube are hard chromium plated to avoid self-welding of stainless parts having relative movement in sodium/sodium vapour environment. The inbuilt load cells facilitate on-line measurement of frictional force encountered by the mobile assembly of CSRDM and CSR. On receiving the scram signal, the EM is de-energised and the mobile assembly of CSRDM along with CSR is released to fall under gravity. At the end of free fall travel (835 mm), the mobile assembly is decelerated by an oil dashpot for the remaining 250 mm travel. The design parameters are given in Table 1 and the operating conditions of CSR & CSRDM are listed in Table 2. Primary leak-tightness is achieved by the seals in the lower part and secondary leak-tightness by the seals in the upper part. They prevent the leakage of radioactive sodium vapour and argon cover gas into Reactor Containment Building (RCB) and deposition
Fig. 3. PFBR-flow sheet.
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Fig. 4. CSR & DSR with their Drive Mechanisms.
Fig. 6. Control & Safety Rod Drive Mechanism.
Table 1 Design parameters of CSR & CSRDM. Total travel of CSR Free fall travel Braking travel Raising/lowering speed Response time of electromagnet Fast drop time in flowing sodium including response time of EM
1085 mm 835 mm 250 mm 2 mm/s <100 ms <1 s
of sodium vapour at low temperature region of the mechanism. The inter-seal argon space between the primary and secondary leak tight barriers is maintained at a positive pressure with respect to reactor cover gas. Temperature at the top of control plug is maintained at 383 K, so that sodium vapour deposition in the annular gap does not hinder the free movement of the mobile assembly. ‘O’ ring seals are used wherever there is no/slow relative movement which is of no safety concern. Dynamic lip type ‘V’ ring seals are provided Table 2 Operating conditions in reactor.
Fig. 5. Control & Safety Rod Subassembly.
Max. temperature experienced by Lower part of CSRDM Upper part of CSRDM
845 K (572 ◦ C) 373 K (100 ◦ C)
CSR Inlet temperature of sodium Outlet temperature of sodium
670 K (397 ◦ C) 783 K (510 ◦ C)
Number of scram operations in 40 years life time of CSRDM 2 years life time of CSR
752 38
Number of gripper operations
∼120
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between the mobile assembly and the stationary tube sheath at the level of control plug top to act as a barrier restricting the sodium vapour entering in to the upper part. The configuration of the Vring seal (as explained in Section 3.4) is such that it does not arrest the downward movement of freely falling mobile assembly. The major material of construction of the lower part of CSRDM, having contact with sodium/sodium vapour, is SS 316LN. The fingers, axle pins, guide pins and screws in the gripper assembly are made of precipitation hardened austenitic stainless steel (equivalent to ASTM A453, Grade 660, Class B) having high strength at elevated temperature. The upper part operating at temperature less than 100 ◦ C, is made of carbon steel and alloy steel. The satisfactory performance of CSRDM and CSR is monitored and ensured by on-line measurement of the following: • Frictional force. • Response time of EM, drop time of mobile assembly of CSRDM along with CSR and braking time during scram action. • Leak-tightness. 3. Qualification tests After the detailed design and analyses of the entire system, the qualification of CSRDM and CSR was carried out in two stages. In the first stage, the full-scale models of the critical subassemblies of the mechanism such as hydraulic dashpot, scram release electromagnet, dynamic seals and CSR subassembly were tested individually in air simulating the actual operating temperature and pressure as in the reactor. The design parameters were fine-tuned based on the test results. Experiments were also carried out on sodium vapour deposition in the annular gaps between the stationary and mobile parts. In the second stage, prototype CSRDM and CSR were manufactured and subjected to various functional tests in air, hot argon and subsequently in sodium simulating the operating conditions of the reactor. The developmental activities are detailed out in the subsequent paragraphs. 3.1. Hydraulic dashpot Fig. 7. Hydraulic dashpot.
During scram action, the mobile assembly of CSRDM along with CSR falls freely under gravity and acquires kinetic energy. An oil dashpot having piston-cylinder arrangement dissipates the kinetic energy before the mobile assembly comes to rest. It provides deceleration to the fast moving mobile assembly and brings down the velocity to near zero at the end of travel of CSR. The dashpot has a central hollow passage surrounded with two annuli. The inner annulus is for the movement of the piston and the outer one is for oil collection. The weight of the freely falling mobile assembly of CSRDM & CSR is about 3600 N and that of the piston is 290 N. The mobile assembly of the mechanism passes through the central hollow passage. The configuration of the dashpot is shown in Fig. 7. Teflon pads are provided to avoid direct metal to metal contact between the mobile assembly and the piston at the time of impact and also to transfer the momentum gradually. A pre-compressed spring provided below the piston pushes it to its top most position when the mobile assembly is lifted up and hence the dashpot is ready for the next drop. The wall between the inner and the outer annuli has several holes (orifices) located at different elevations in a helical manner to facilitate flow of displaced oil. The rapid movement of the piston and mobile assembly is decelerated by the upward thrust due to the oil pressure developed. As the piston moves down, its velocity reduces and also it closes the orifices at different heights one after the other. Therefore, the effect of reduction in velocity of the piston on oil flow velocity is balanced
by the reduction in number of orifices available for oil flow. The size, number and positions of the orifices were optimised using computer softwares developed in-house for the design and analysis of the dashpot. A prototype dashpot was manufactured and tested in a dedicated test facility shown in Fig. 8. The test facility was provided with a short length mobile part loadable with adjustable dummy weights to simulate the weight of the mobile assembly of CSRDM & CSR. Free fall and decelerated travel heights were maintained same as in the actual CSRDM. Dynamic characteristics of the oil pressure, deceleration and displacement during braking travel were recorded. Based on the functional behaviour of the dashpot during the preliminary tests and fine tuning of the computer softwares, the number of orifices at the top end of the cylinder was increased to reduce the first pressure peak, which is mainly due to acceleration of the piston and hence the oil from rest at the time of mobile assembly hitting on the piston. Fig. 9 shows the decrease in first pressure peak of the oil asymptotically as a function of number of orifices. There is no significant change in pressure peak when number is increased beyond 16 for the orifices ϕ 8 mm and for the given configuration of the dashpot. Further reduction in pressure peak was achieved with increase in thickness of pad at the contact surface
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Fig. 10. Typical dynamic pressure of oil during dashpot action.
Fig. 8. Dashpot test set-up.
mobile assembly and the inner core rests on the nut of screw–nut mechanism. Central hole provided in the inner core facilitates passage of translation screw. As indicated in Table 1, response time of EM is to be less than 100 ms. It is the time interval between the instant at which the power supply to EM is switched off and the instant at which the mobile assembly moves down by 1 mm, ensuring positive detachment. The development and qualification of EM were completed in three campaigns of experiments. Series of tests were carried out to ascertain the load carrying capacity, temperature rise and response time. Fig. 12 shows the set-up for EM testing and Fig. 13 shows the electrical circuit used for the EM with features for measuring response time. The increase in temperature of the coil was less than 5 ◦ C, which has no implication on the performance. Initial configurations met the requirements of load carrying capacity, but the response time was found to be exceeding the limit of 100 ms. Then radial slots were cut in the inner and outer cores of EM to reduce eddy current induced and hence to reduce the response time. The test results are shown in Fig. 14. It is to be noted that when the dif-
Fig. 9. Effect of increasing number of holes on first pressure peak.
between the mobile assembly and the piston. A typical characteristic of variation of oil pressure with respect to time is shown in Fig. 10. Also the length of the air vent pot was increased with the introduction of appropriate baffles to avoid spraying out of oil from the dashpot during fast movement of the piston. The fast drop tests were carried out at room temperature and also at 353 K (80 ◦ C). Endurance testing was carried out for 1000 drops and the test results were satisfactory. 3.2. Scram release electromagnet The configuration of EM is shown in Fig. 11. DC power supply is provided to the EM. The outer core of EM holds the top end of the
Fig. 11. Scram release electromagnet.
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Fig. 14. Test results of electromagnet.
Fig. 12. Electromagnet & its test set-up.
Fig. 13. Circuit for EM response time measurement.
ference between the load carrying capacity and the attached weight to the EM is high, then the response time is larger. The response time is not influenced by the air gap thickness at the contact face beyond the load difference of 2500 N. In actual case, air gap of 0.1 mm is provided at the contact face. The response time was found to be 81 ms for the parameters corresponding to reactor-operating conditions and hence the design requirements were satisfied. 3.3. Elastomeric seals As explained in Section 2, lip type V-ring seals are provided at the top end of the annulus between the stationary tube sheath and the mobile assembly, where the temperature is maintained at about 383 K. In the process of development of lip type ‘V’ ring seals, several
configurations of the seal (as shown in Fig. 15) were formulated and tested. A photograph of the selected V-ring seal (Type-D) having labyrinth at the sliding contact surface is shown in Fig. 16. Leaktightness and frictional force offered by the seals were measured with respect to the differential pressure across the seals. Two V-ring seals were mounted in series to improve leak-tightness. Endurance test was carried out simulating the translation and scram action of the mobile assembly. The performance of the seal related to leak-tightness and frictional force was found to be satisfactory and no damage was noticed. The leak rate is less than 0.5 cm3 /s, which is within the allowable limit of 1 cm3 /s. This corresponds to leakage of fresh argon getting into reactor cover gas and there is no release of radioactive argon. When the seal was subjected to a reverse differential pressure of 100 mbar (g) (situation arises when inter-seal argon space opens up to RCB), there was no appreciable change in frictional force.
3.4. Sodium vapour deposition studies In CSRDM, long annuli with narrow gaps having bottom end exposed to hot pool sodium are present. The large temperature gradient between the top and bottom ends induces natural convection of argon gas filled in the annular space. At high temperature, sodium vapour is formed which quickly condenses to form sodium mist in the argon gas. The argon gas carries sodium mist when it travels up along the vertical annuli due to natural convection. Some of the sodium mist gets deposited on the annulus walls where the temperature is below 373 K (100 ◦ C). Continuous accumulation of sodium deposit between the mobile assembly and the stationary tube sheath may resist the safety scram action during off-normal condition.
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Fig. 15. Cross section of V-ring seals tested.
Fig. 16. Dynamic lip type V-ring seal.
The above phenomena of sodium vapour generation in sodium pool and deposition in narrow annular gaps were studied in an experimental set up to articulate an effective method to control the deposition. Experiments were carried out in two phases. In the first phase, 1.3 m long annulus models were used. A sleeve was introduced in the annulus at a particular height from sodium level to reduce sodium deposition above the sleeve. In this experiment, sleeve length was varied from 20 to 120 mm. Also the radial gap at the sleeve was varied from 1.5 to 3 mm. Multiple sleeves were also used in the experiment. The bottom of the model was dipped in sodium. The sodium temperature was set to 820 K (547 ◦ C) and the sodium vapour was allowed to deposit in the annulus for about 750 h. After the test, the models were dismantled and the deposited sodium was collected by dissolving in water. By chemical analysis, the amount of sodium deposited in the annulus above and below the sleeve was determined. In the second phase, 3.5 m long annulus models were used. Configuration of the test vessel along with a typical model is shown in Fig. 17. Addition of sleeve in the annulus decreases the sodium vapour deposition rate by one order of magnitude. It is found that as the aspect ratio (i.e., the ratio of length to gap) increases, the sodium vapour deposition rate decreases. Based on the experimental results, inside surface of top end of the tube sheath of CSRDM has been configured to reduce sodium vapour deposition at low temperature region of the annulus. 3.5. Testing of prototype CSRDM along with CSR 3.5.1. Test vessel The integrated full-scale assembly of prototype CSRDM and CSR were manufactured and testing of the same was carried out in Test Vessel-1 (TV-1) of Large Component Test Rig (LCTR). TV-1 is shown in Fig. 18. LCTR is a test facility in which many of the full-scale reactor components are being tested in static sodium. The temperature of the sodium and pressure of cover gas in the test vessel are maintained similar to that in the reactor. TV-1 is a vertical cylindrical vessel of size ϕ 1 m × 12.5 m long. It has an inner vessel, which is bolted to the top flange of TV-1. Inner vessel consists of two parts. The upper part of the inner vessel simulates the reactor control plug. It houses 48 baffle plates stacked with differential gap between the adjacent plates to achieve the desired temperature distribution as in the reactor. The lower part of the inner vessel has grid plate sleeve which supports the CSR subassembly. CSRDM is supported on the top flange of the inner vessel. After successful completion of testing in air, the inner vessel along with CSR and CSRDM is lowered in to TV-1. Sodium is filled in TV-1 for a height similar to that in the reactor and above the sodium pool, argon cover gas is filled at a pressure of 100 mbar(g).
Fig. 17. Na vapour deposition in annulus—test vessel.
3.5.2. Tests carried out Table 2 shows the operating conditions of CSRDM & CSR and the number of expected scram operations during life time of the reactor. Since the damage occurring in CSRDM & CSR is mainly due to fatigue cycles during scram actions, the number of test cycles
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Fig. 19. Dynamic pressure of oil in dashpot.
Fig. 20. Displacement characteristics of CSR during scram.
incorporating the design modifications. Then endurance testing was carried out in sodium for another 1093 cycles of scram operations, maintaining the temperature of sodium at 823 K (550 ◦ C) and 120 cycles of gripper operations at 473 K (200 ◦ C), with the misalignment of 30 mm between CSRDM and CSR (Mark-II). In addition to the above, the tests carried out in air, argon and sodium at different temperatures amounts to 450 cycles. Fig. 18. Vessel for testing CSRDM & CSR in sodium.
is decided as per the guidelines given in ASME, Section III, Div. 1, Appendices, Para II-1500 (2004). As per this guide line, the prototype CSRDM has to be tested for 3460 scram cycles at 845 K (572 ◦ C) or 4351 cycles at 823 K (550 ◦ C) or 5062 at 803 K (530 ◦ C) corresponding to 752 cycles in reactor and 552 gripper operations at 473 K (200 ◦ C) corresponding to 120 operations in reactor. However, CSR is required to be tested for 173 scram cycles, since its life span in the reactor is only 2 years. There is a possibility of misalignment between the axes of CSRDM and CSR during operation in reactor. The causes for the misalignment between the mechanism and the CSR in the reactor are due to the manufacturing tolerances, differential thermal expansion and deflection & slope of support flanges, thermal & irradiation induced bowing of core subassemblies, etc. So it is envisaged to carry out the tests in aligned as well as misaligned condition. The prototype CSRDM and CSR were subjected to extensive performance testing to check and ensure all the intended functions in air, hot argon at 473 K (200 ◦ C) and then in sodium at temperature starting from 473 to 823 K (200–550 ◦ C). Then endurance testing was carried out for 500 cycles of scram operations maintaining the temperature of sodium at 803 K (530 ◦ C) and 60 cycles of gripper operations at 473 K (200 ◦ C), keeping CSRDM and CSR (Mark-I) in aligned condition. Based on the test results, the cross section of the mobile CSR was changed from circular to hexagon to have uniform and more gap within its sheath which is hexagon in shape. The number of guides for the mobile CSR was changed from three to two to reduce the frictional force. The CSR (Mark-II) was manufactured
Fig. 21. Flow sheet of sodium removal facility (water vapour–CO2 process).
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Table 3 Summary of test results of prototype CSRDM & CSR. Measured parameter
Testing In air
Frictional force (N) during translation Min. holding current (A) of EM Drop time (ms) (including EM response time) Braking time (ms) Dashpot oil pressure (MPa) a
Aligned
Misaligned
60 0.65 531 186 3.06
280 0.65 561 194 3.11
In argon (misaligned) at 473 K
In sodium (misaligned) at (K) 473
573
673
773
823
823a
275 0.65 563 189 3.19
260 0.65 600 212 2.84
220 0.65 596 206 2.67
192 0.65 597 195 2.71
290 0.65 596 202 2.65
315 0.65 590 205 2.73
350 0.65 610 220 2.66
Operation after endurance test, i.e., after 1593 scram cycles.
Fig. 22. CSRDM (lower part) after testing and sodium cleaning.
Hence, as per the ASME guidelines, CSRDM has been qualified for 14 years of life in the reactor and CSR has been qualified for its total life in the reactor. Fig. 19 shows the dynamic pressure of oil in the hydraulic dashpot during scram action and Fig. 20 shows the displacement characteristics of CSR during scram action. Table 3 shows the summary of the test results. The variation in frictional force with respect to the operating temperature is only about 90 N and the maximum frictional force is less than 10% of the total weight of the mobile assembly. Hence increase in drop time due to frictional force is only about 25 ms. The maximum drop time of CSR during scram action is 610 ms, which is less than the allowable time, 1 s. There is no change in minimum holding current of the electromagnet. Hence, the results of all the tests show that the performance of CSRDM along with CSR is satisfactory throughout the endurance testing and there is no significant change in the performance. 4. Sodium cleaning After the qualification tests, CSRDM lower part wetted with sodium was cleaned for subsequent handling, inspection and reuse. Out of various methods of cleaning available, water vapour–carbon dioxide (CO2 ) process was selected for the reusable components of PFBR. The flow sheet for sodium removal facility is shown in Fig. 21. The process involves bubbling of CO2 through demineralised (DM) water (kept at temperature of 323–333 K) in a re-circulating stream of nitrogen. Initially sodium reacts with humid water
vapour carried by CO2 gas stream to form caustic sodium hydroxide (NaOH) with the release of hydrogen. Further reaction of NaOH with CO2 produces sodium bicarbonate (NaHCO3 ) and sodium carbonate (Na2 CO3 ). The carbonate and bicarbonate so formed can be easily removed by washing with DM water, thus preventing caustic stress corrosion of austenitic stainless steel by NaOH. The cleaning process involves the following four steps: • • • •
Sodium draining from the components. Vapour phase reaction both with water vapour and CO2 . Rinsing the component with water. Drying with hot nitrogen.
Vapour phase reaction was continued for about 80 h and hydrogen release during the reaction was kept well below 1% and there was no pressure build-up in the system due to reaction. Subsequently the mechanism was cleaned with DM water with several fill, recirculation and drain cycles. Finally, drying the component with hot air/nitrogen completed the cleaning process. The CO2 cleaning process was successfully demonstrated for removal of sodium from CSRDM lower part for reuse. Fig. 22 shows some of the photographs of CSRDM taken while handling it after testing and cleaning the sodium. The reaction products formed during sodium removal from the CSRDM are shown in the photographs.
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5. Conclusion CSR and its drive mechanism, which are the major constituents of the first shutdown system of PFBR, have been designed, critically analysed and thoroughly tested in air, argon and in sodium for their fail-safe operation. The sodium vapour deposition in the annular spaces of CSRDM was studied. Sodium removal from the sodiumwetted parts of CSRDM was done successfully and the cleaning process has been established, so that the mechanism can be reused in sodium. The test results of the individual components and the system as a whole are satisfactory and hence CSR & CSRDM have been qualified for their intended and reliable operation in the reactor. Acknowledgments The authors gratefully acknowledge the immense contributions of their colleagues in the Reactor Engineering Group, Fast Reac-
tor Technology Group, Quality Assurance Division, Electronics & Instrumentation Division, and Central Workshop. References Chetal, S.C., et al., 2006. Design of the Prototype Fast Breeder Reactor. Nuclear Engineering and Design 236 (7–8), 852–860. Delemontey, et al., 1972. Mecanismes de barres de controle pour reacteurs rapides. In: Proceedings of ‘Engineering of Fast Reactors for safe & Reliable operation’, vol. I, BBW, Kernforschungszentrum Karlsruche, pp. 350–357. Glinsky, H.W., et al., 1978. The shutdown systems of KNK and SNR 300. In: Design, Construction and Operating Experience of Demonstration LMFBRs, IAEA, Bologna, STI/PUB/490, pp. 191–203. Pignatelli, et al., 1982. The Superphenix shutdown system. In: Proceedings of LMFBR Safety Topical Meeting, Lyon-Euelly, pp. II 485–495. Rajan Babu, V., et al., 1995a. Design philosophy of PFBR shutdown systems. In: Proceedings of Technical Committee Meeting, IAEA-TECDOC-884, Obninsk, pp. 81–88. Rajan Babu, V., et al., 1995b. Design of shutdown systems for PFBR. In: Proceedings of Technical Committee Meeting, IAEA-TECDOC-884, Obninsk, pp. 89–96.