atw - International Journal for Nuclear Power | 05.2020

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atw Vol. 65 (2020) | Issue 5 ı May 276 ENVIRONMENT AND SAFETY Physical and Chemical Effects of Containment Debris on the Emergency Coolant Recirculation Jisu Kim and Jong Woon Park Physical and chemical effects of containment debris on the performance of emergency coolant recirculation are investigated to get insight on the cost-effective plant modifications to resolve USNRC’s Generic Safety Issue-191. The effects of debris sources on the sump screen performance are evaluated through the head loss calculation using NUREG/ CR-6224 correlation. The amount of three predominant types of precipitates, i.e., sodium aluminum silicate ( NaAlSi3O8), aluminum oxyhydroxide (AlOOH), calcium phosphate (Ca3(PO4)2) after 30 days of ECCS mission time are evaluated under various environmental conditions using WCAP-16530-NP chemical models. The debris interceptor is considered as a viable design option to reduce particulate debris such as unqualified coatings. The key parameters of each effect are deduced and recommendations for reducing their adverse effects are made through the present analysis: (a) The amount of unqualified coating debris is a major source of particulate debris and has a great adverse effect on the sump screen head loss by reducing porosity in the fibrous insulation, (b) The Cal-Sil insulation reacts with TSP buffer and significantly increases the generation of a gum-like chemical precipitant, (c) Spray time increases the chemical byproducts but the effect is smaller than that of buffer agent type and unqualified coating, (d) The debris interceptor, when verified, may play a vital role reducing head loss generated by coatings and fibrous debris mix. 1 Introduction A primary safety issue regarding long-term recirculation core cooling following a LOCA (Loss of Coolant Accident) is that LOCA-generated debris may be transported to the recirculation sump screen, resulting in adverse blockage on the sump screen and deterioration of available NPSH (Net Positive Suction Head) of ECCS (Emergency Core Cooling System). USNRC identified this as Generic Safety Issue (GSI) 191 [1] and issued the Generic Letter 04-02 [2] to resolve the issue. The GL required that all PWR owners perform an engineering assessment of their containment recirculation sumps to ensure they will not suffer from excessive blockage. The guidance report (GR) [3] for PWR sump performance evaluation has been developed by NEI (Nuclear Energy Institute) and approved by the USNRC [4]. The objective of the assessment is to derive required plant modifications including new insulation, sump screen, etc. of a Korean nuclear power plant for 10-year life extension. To derive the cost-effective modifications, the effects of physical and chemical conditions on the performance of the ECCS recirculation sump with respect to head loss are parametrically investigated. The physical and chemical conditions are debris source, containment environments and debris interceptor as a candidate design option. 2 Analysis methods 2.1 NUREG/CR-6224 head loss correlation The effects of debris sources on the sump screen are evaluated through head loss calculations using NUREG/ CR-6224 correlation [5]. This is applicable for laminar, transient, and turbulent flow regimes through mixed debris beds (i.e., debris beds composed of fibrous and particulate debris). This correlation is approved by USNRC for the determination of head loss [4] and is given by: (1) The fluid velocity (U), is given by simply in terms of the volumetric flow rate (Q) and the effective screen area (A) as: (2) The mixed debris bed solidity a m is given by: (3) For debris deposition on a flat surface of a constant size, the compression rate (c) relates the actual debris bed thickness (DL m ) and the theoretical fibrous debris bed thickness (DL o ) via the relation: (4) Compression of the fibrous bed due to the pressure gradient across the bed is also accounted, which must be satisfied in parallel to the previous head loss equation, Eq.(1), is given by (valid for ratios of DH/DL o > 0.5 ft-water/inch-insulation): (5) where “K” is a constant that depends on the insulation type. The value of K is 1.0 for NUKON fiber. Test data or a similitude analysis are required to determine “K” for fibrous materials that are dissimilar to NUKON insulations. Each constituent of debris has a surface-to-volume ratio associated with it based on the characteristic shape of that debris type. For typical debris type, we have [3]: Cylindrically shaped debris: S v = 5/diam Spherically shaped debris: S v = 6/daim Flakes (flat plates): S v = 2/thick where “diam” is the diameter in feet of the fiber or spherical particle, and “thick” is the thickness in feet of the flake/chip. Environment and Safety Physical and Chemical Effects of Containment Debris on the Emergency Coolant Recirculation ı Jisu Kim and Jong Woon Park
atw Vol. 65 (2020) | Issue 5 ı May The average surface-to-volume ratio of various types of debris constituents is calculated as: (6) where v is the microscopic volume of the constituent and the subscript “n” refers to the n-th constituent. 2.2 WCAP-16530 post-accident chemical effects evaluation method The materials inside containment may dissolve or corrode when exposed to the reactor coolant and spray solutions. This would produce oxide particulate corrosion products and a potential for formation of precipitates due to chemical reactions with other dissolved materials. These chemical products may become another source of debris loading to be considered in sump screen performance and downstream effects. Recently Westinghouse Owners Group (WOG) proposed a four step process for evaluating the postaccident chemical effects in containment sump fluids to support GSI-191 [6]. As shown in Figure 1, using ICET test results and plant data, the chemistry bench tests are performed and a chemical model is developed to identify the type and amount of chemical products that are produced. This chemical product information generated from the bench testing and the chemical model is used as an input to performance testing to be conducted by licenses and vendors of replacement sump screens. Through bench test, three types of predominant chemical precipitates are identified for the plant using NaOH or TSP (Tri-Sodium Phosphate) as a buffer agent: p Sodium aluminum silicate (NaAlSi 3 O 8 ) p Aluminum oxyhydroxide (AlOOH) p Calcium phosphate (Ca 3 (PO 4 ) 2 ) (if TSP is used) Each quantity of precipitate generated can be calculated as followings: where for each chemical species, concentration data generated during the single-effect bench testing at specific chemistry conditions is used in a regression analysis to develop release equations as a function of temperature, pH, and the concentration of that species. Equations are developed for each predominant source material for each chemical species (Ca, Al, and Si). The detailed information about the equations for the material release rate is described in the reference 6. 3 Results and discussion 3.1 Effects of debris sources The sump screen head loss calculations with various debris loadings on the sump screen are performed using USNRC’s NUREG/CR-6224 correlation [5]. The screen area is assumed as 1,000 ft 2 with maximum ECCS flow rate of 7,000 gpm. The sump pool temperature and pressure are assumed as 212 °F and 14.7 psi, respectively. The debris source and their characteristics are summarized in Table 1. It is assumed that the particulate debris mixture consists of 85 % of coatings, 10 % CalSil and 5 % of latent dust/dirt debris by mass. The head loss by RMI (Reflective Metal Insulation) debris bed is not Debris Type As-fabricated Density [lbm/ft 3 ] | Tab. 1. Debris source and characteristics [3,4]. | Fig. 1. WCAP-16350 Post-Accident Chemical Effects Evaluation Methodology. con sidered because its effect on the head loss is negligible The head loss with various loadings of fiber and particulate mixture debris is shown in Figure 2. The head losses by fiber only debris beds are significantly lower than mixed debris beds of fiber and particulate. The head loss appreciably increases with the amount of particulate debris. Head loss drastically increases with the decrease of the amount of fiber debris because the mass ratio of particulate to fiber increases. The debris packing Particle Density [lbm/ft 3 ] Sv [ft -1 ] Fiber 2.4 175 171,700 Coatings - 94 183,000 CalSil - 115 600,000 Dirt/Dust - 169 106,000 ENVIRONMENT AND SAFETY 277 [NaAlSi 3 O 8 ] = 3.11 [Si], if [Si] < 3.12 [Al] = 9.72 [Al], if [Si] > 3.12 [Al] (7) [AlOOH] = 2.22 {[Al] - 0.32 [Si]} (8) [Ca 3 (PO 4 ) 2 ] = 2.58 [Ca] (if TSP is used) (9) | Fig. 2. Head loss with various mixed debris loading on the 1,000 ft 2 sump screen (7000 gpm ECCS flow). Environment and Safety Physical and Chemical Effects of Containment Debris on the Emergency Coolant Recirculation ı Jisu Kim and Jong Woon Park
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