Transcript
OPERATING EXPERIENCE WITH SHIELDING FUEL ASSEMBLIES AT RINGHALS 3 AND 4 AUDRIUS JASIULEVICIUS Vattenfall Nuclear Fuel AB SE-16956 Stockholm, Sweden
URBAN BERGENSTRÅHLE, HENRIK NYLÉN Ringhals AB SE-432 85 Väröbacka, Sweden
JÖRG PEUCKER AREVA GmbH D-91052 Erlangen, Germany
ABSTRACT Shielding Fuel Assemblies (SFA) of HTP™ design were introduced in Ringhals units 3 and 4 in 2009, in 12 peripheral positions of the core in order to prolong the vessel lifetime by reducing the beltline-weld fluence. The first generation SFA operated in Ringhals Unit 3 until 2015. This year, the first generation design has been replaced with a second generation of SFA, with higher shielding factor, to enable further lifetime extension for Unit 3 reactor-pressure vessel. Ringhals Unit 4 continues operating with first generation SFAs. During the several years of operation in the core peripheries, a considerable amount of data and operational experience was accumulated for the first generation SFA in Ringhals PWR. In addition, the HTP™ fuel is in operation in Ringhals unit 3 and 4 cores in reload quantities since 2003. A number of inspections are being performed during each outage on the fuel in Ringhals 3 and 4, including the SFAs. Typical visual examinations are: fuel assembly length, fuel assembly bow, oxide layer thickness, fuel rod length, and fuel rod diameter measurements. This paper presents an overview of the SFA implementation at Ringhals NPP, operational experience accumulated at Ringhals units 3 and 4 with SFA of HTP™ design, and conclusions regarding the efficiency of the SFA-concept. The design of the second generation of SFA, which replaced the first generation in Ringhals 3 this year in order to further strongly reduce the RPV fluence, is also briefly described.
1.
Introduction
Ringhals Nuclear Power Plant in Sweden operates four nuclear reactors: one boiling water reactor (R1) and three pressurized water reactors (R2, R3 and R4). The plant is situated on the west coast of Sweden, about 60 km south from Gothenburg city. With total power rating of 3717 MW e it is the largest power plant in Scandinavia, generating about 20% of electrical power used in Sweden. In 2014 Ringhals celebrates 40 years as an electricity supplier, with the first kilowatthour generated in 1974. The youngest, unit 4 was put in operation in 1983. The 3-loop PWR reactors are of Westinghouse design, operating on 12-month cycles.
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As the reactors age, measures to prolong the lifetime are being implemented at the Ringhals NPP. The beltline weld on the reactor pressure vessel of Ringhals units 3 and 4 has a lifetime estimate of 40 years under the current operating conditions. For the uprated reactor-power conditions, the irradiation-induced embrittlement of the beltline weld becomes a limiting condition. One of the solutions to extend the lifetime of the reactor pressure vessels is to limit the neutron fluence on the beltline welds. For Ringhals units 3 and 4, this was implemented by replacing some of the fuel assemblies at the periphery of the cores, at positions closest to the beltline weld by modified fuel assemblies, designed to reduce the fluence on the reactor structures. As a solution, SFA of HTP™ design made by AREVA were introduced in 2009, in 12 peripheral positions in the reactor cores, shown in Fig. 1 together with the schematic SFA-design.
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Fig 1. Positions of SFA in the reactor core (left figure) and the overview of SFA Gen I (right figure).
2.
Design of SFA Gen I
The initial goal was to reduce the fluence of fast neutrons (> 1MeV) at the critical positions by at least 50%. A shielding factor was defined as the ratio of the fast neutron fluence of the standard fuel assembly and the SFA-designs, evaluated at the core main axis in the position of the beltline weld, with a target value of > 2.0 in the design stage. The background to the optimization of the SFA, and the final design is described in more detail in [1]. The results from the fluence calculations of the inside of the reactor pressure vessel, performed with MCNPX code for Ringhals 3, cycle 25, show a decrease of fluence by the factor of 2 (Fig. 2). As result of the optimization, the SFA were designed with the three outermost rows of fuel rods in HTP™ fuel assemblies being replaced by stainless steel rods. The remaining rows of the fuel 205/441
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rods were rods with low content of U-235 uranium; five rows with depleted uranium of 0.2 w/o U235 and nine rows with 1.0 w/o U-235. The guide thimbles in the fuel assemblies were replaced by solid stainless steel bars for additional shielding and to further strengthen the assemblies. This provides sufficient dimensional stability to the SFA, in particular resistance to assembly bowing, so that they can be loaded at the same core location without rotation for the envisaged lifetime of 8 cycles. Special attention has been spent on axial dimensioning and hold-down device, since the relaxation of the hold-down springs is not compensated for growth due to the steel bars which replaced the guide thimbles. For all other components such as spacer grids, nozzles and the fuel rods, modifications could be avoided, relying on the well-proven HTP™ design.
Fig 2. The shielding effect of SFA Gen I, from [1].
The reduced number of heated fuel rods in combination with an unchanged reactor thermal power level leads to an increase in the average heat flux in the remaining core. Therefore, the core peaking factor limits had to be reduced by 1.4%. The asymmetric design of the SFA was chosen in order to minimize penalties on the fuel cycle. However, the cycle length is reduced by approximately 100 effective full power hours. The final expected SFA burnup after 8 years operation in the reactor is 30 MWd/kgU (Fig. 3). The initial Monte Carlo simulations demonstrated that the fluence of the fast neutrons (>1MeV) at the beltline weld was reduced by more than 50% in the critical positions upon introducing the SFA [1]. However, due to the reactivity and the power increase in the SFA during burnup, the shielding factor will decrease and approach a constant value above 50% in the critical positions with equilibrium content of fissionable Pu after about 4 cycles.
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3.
The shielding performance of SFA Gen I
To verify that the actual flux reduction is achieved, and to provide for comprehensive monitoring of the RPV neutron exposure, ex-vessel neutron dosimetry (EVND) was introduced into Ringhals Unit 4 in May 2008, and into Ringhals Unit 3 in Aug. 2008.
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Fig 3. Burnup of SFA Gen I. The dosimetry installation consists of small aluminium dosimeter capsules connected to and supported by a stainless steel bead chain located in the annular space between the reactor vessel reflective insulation and the concrete primary biological shield. The EVND consists of measurements outside the RPV that extends over the full core height at four azimuthal angles (0°, 15°, 30° and 45° in a single octant). To confirm the reduction of fluence, two consecutive evaluations of the EVND system have been performed. One evaluation was performed before the implementation of the shielding fuel assemblies; thus characteristic of the previous cycles in 2008-2009, and the other evaluation was performed after the cycle with implemented SFA in 2009-2010, Ref. [2]. Fluence calculation and dosimetry evaluation is presented in [3], and verified that the introduced SFA achieved a shielding factor of 2.3 at the azimuthal peak location of 0° for Ringhals Unit 3, and a shielding factor of 2.4 for Unit 4.
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The in-core performance of SFA Gen I
A number of fuel inspections are routinely performed at Ringhals NPP during each reactor outage. Typically, the fuel assembly length is measured on 100% of fuel assemblies while unloading the reactor. More detailed measurements on chosen assemblies are performed in order to assess specific issues, such as fuel assembly bow, fuel rod oxide thickness, fuel rod growth, etc. This section summarizes the inspection results for SFA in Ringhals 3 and Ringhals 4 since the start of the operation of the assemblies in 2009.
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Fig 4. Oxide Layer measurement results in µm (shown at the respective position).
4.1
Summary of results of the measurements on SFA Gen. I at Ringhals unit 3
The visual inspections of the SFA Gen. I show for their burn-up and operational history typical surface conditions. Fuel rod oxide layer thickness measurements were performed on 8 peripheral fuel rods of SFA “SA301” in RH-3 in 2013 (after 4 operating cycles). The measurement was performed in an axial linear trace along the fuel rod surface. Due to the high radial burnup gradient, up to 20 MWd/kgU in row A and around 2 MWd/kgU in row N, results show the expected clear difference, see Fig 4. All values fit well with the AREVA M5 cladding database. Basically the same can be seen in the results of the fuel rod diameter measurements, which have been performed together with the oxide layer measurement after 4 operating cycles. While in row A the diameter shrinkage amounts to about 0.6 %, the effect is about 0.1% lower in rows M and N where the fuel rods have lower burnup and been exposed to less neutron flux. Also these results lie well within AREVA’s database. Also the fuel assembly length has been precisely measured on 2 of the SFA Gen I after 4 cycles of operation. The results of -0,1 and 0,2 mm show that no growth appeared as to be expected due to the fact that the guide thimbles have been replaced by stainless steel bars in the SFA Gen I design.
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The detailed fuel assembly bow measurements are performed on 20-27 fuel assemblies during each reactor outage. During the first five cycles in the reactor no bow above 2 mm was noticed for SFA Gen I assemblies in Ringhals 3 and Ringhals 4. 2
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Fig 5. Fuel rod growth results for SA301 in mm (maximum per fuel rod, shown at the respective position). Finally the fuel rod length has been measured on all peripheral rods, see Fig. 5. As before for oxide layer thickness and fuel rod diameter, the dimensional changes of the fuel rods in row A are with a length increase of 9 – 12 mm clearly larger than for the fuel rods in rows N (around 3 mm), which again can be explained by the remarkable radial burnup and flux gradient. Also the fuel rod growth results match with the AREVA database and sufficient axial space for further growth in the remaining lifetime of the SFA Gen I is available. The visual inspections and the holddown spring force measurements performed during the outage at Ringhals 3 in 2014 revealed an unusual loss of spring height (setting) and deformation of holddown springs of some shielding assemblies. Evaluation of the measured holddown forces and spring rate showed that the minimum holddown margins were still covered by the mechanical design calculations and the spring rate was not affected. Similar unusual settings were seen during the outage 2015 in Ringhals 3. In this case also indications of cracks in some single leaf springs of holddown spring package were observed. No complete broken springs were found. The holddown spring forces were re-assessed. For power operation the holddown margins were still assured, even when assuming complete failure of one spring package. With realistic boundary conditions, a positive holddown margin can be shown for all operating 209/441
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conditions. However, in order to assure safe future operation of SFA Gen I in Ringhals 4 all top nozzles were replaced in 2015 with new top nozzles to prevent future degradation of this component. A new generation of shielding assemblies were introduced in Ringhals 3 in 2015 (see next section) and therefore the top nozzle replacement was not necessary for this reactor. A cause analysis concerning the cracks and unusual deformations was initiated and performed by AREVA. It suggests that high stresses in the spring leafs during operating conditions due to the specifics of the steel structure caused the initiation of inter-granular stress corrosion cracking in these spring leafs after an incubation time of a few years. Respective design modifications for future SFA are in discussion between Vattenfall and AREVA.
5.
Development of shielding fuel assemblies: SFA Gen II
In order to further increase the protection of the RPV beltline weld against fast neutron flux the second generation shielding fuel assemblies (SFA Gen II) are designed and installed in in Ringhals 3 in 2015. These assemblies are designed with two axial zones of about the same length. The top zone contains short fuel rods with low-enriched 235U while the bottom zone (shielding part) contains only steel bars. The skeleton of the SFA Gen II is of the same design as for SFA Gen I. This design of SFA Gen II results in an estimated shielding factor of ≥ 7.5 throughout the planned cycles [4] and has a small (moderate, low) impact on cycle length and core design. The reduced length of the fuel pins in SFA Gen II will result in a core peaking factor penalty of 3.8 %.
6.
Summary and discussion
The SFA in Ringhals Unit 3 and Unit 4 reactors had proven to provide expected shielding factor on the beltline weld of the reactor pressure vessels. Dosimetry evaluations have demonstrated a corresponding shielding factor of 2.3 at the azimuthal peak location of 0° for Ringhals Unit 3, and a factor of 2.4 for Unit 4. The mechanical performance of the SFA of the HTP™ type had shown that after 5 years in the reactor the fuel assemblies show practically no growth, well below the typical growth values for the fuel types with Zircaloy type material in guide thimbles. The fuel assembly bow values are extremely low for the SFA Gen I type fuel assemblies, on the average around 1.2-1.4 mm compared to 5-6.5 mm maximum bow values for the fuel in the core of Ringhals 3 and Ringhals 4. The oxide thickness measurements for the M5® material in fuel rod cladding are well within the experience database for AREVA fuel. An unusual setting of the holddown springs was first noted after 5 years of operation, indications of cracks in the material of single leaf springs have been observed after six operating cycles. The analysis performed by AREVA suggest that high stresses in the material during operating conditions caused inter-granular stress corrosion cracking after an incubation time of a few years. Top nozzle replacement was done on the twelve SFA Gen II assemblies in Ringhals 4 in 2015. A new generation of SFA is introduced at Ringhals 3 in order to further reduce the fluence on the reactor pressure vessel beltline weld. The SFA Gen II design will also be based on the HTP™ fuel design, taking advantage of the proven skeleton design of the first generation of SFA, but with modified fuel rods equipped with steel bars in the lower half that enhance the shielding factor in the axial area of the beltline weld to ≥ 7.5. 210/441
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M5 and HTP are registered trademarks or trademarks of AREVA NP in the USA or other countries.
7.
References 1. U. Sandberg, H. Nylén, J. Roudén, P. Efsing, and J. Marten, “Shielding Fuel Assemblies used to protect the Beltline Weld of the Reactor Pressure Vessel from Fast Neutron Radiation in Ringhals unit 3 and 4 “, PHYSOR 2010 – Advances in Reactor Physics to Power the Nuclear Renaissance Pittsburgh, Pennsylvania, USA, May 9-14, 2010, on CDROM, American Nuclear Society, LaGrange Park, IL (2010) 2. Green, E.-L., Roudén, J. and Efsing, P., “Ringhals Unit 3 and 4 – Fluence Determination in a Historic and Future perspective”, J. ASTM Intl., Vol. 9, No. 4. Doi:10.1520/JAI104012 (2012)
3. Kulesza, J. A., Fero, A. H., Roudén, J. and Green, E.-L., “Dosimetry Analyses of the Ringhals 3 and 4 Reactor Pressure Vessels”, J. ASTM Intl., Vol. 9, No. 4, doi:10.1520/JAI104033 (2012) 4. L. Ackermann, C. Möllmer, J. Peucker, “The use of neutron fluence analyses as verification of reactor pressure vessel shielding design”, 46th Annual Meeting on Nuclear Technology, Berlin, Germany, May 05-07, 2015
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