1 Improvement of RELAP5 Models for Condensation of Steam and Steam-Gas Mixture in Horizontal and Inclined Tubes Pavel KRÁL NURETH-16, Chicago, 2015

2 Contents 1.Introduction 2.Original wall condensation models of RELAP5 3.Proposed modifications of RELAP5 condensation models 4.Assessment of original and modified RELAP5 against COTINCO data 5.Assessment of original and modified RELAP5 against ITASCO data 6.Summary

3 1. Introduction Objective of the work is to evaluate performance of RELAP5 models for condensation in horizontal and inclined tubes The horizontal tubes are typical not only for VVER steam generators, but also for some passive heat removal system and other heat exchanger Both condensation of pure steam and steam-gas mixture are studied Importance of high fidelity prediction of condensation of pure steam and steam-gas mixture has increases lastly because of the following trends: Modeling of operator actions in course of analyzed accident (system cool-down and depressurization etc.) Passive heat removal systems Consideration of presence of noncondensable gases in RCS Extension of TH codes application to BDBA/DEC analyses The presented paper follows a more general paper “Kral, Hyvarinen, Prosek, Guba: Sources and Effects of Non-Condensable Gases in Reactor Coolant System of LWR” which identifies and quantifies individual sources of NC gas and is also presented at the NURETH-16.

4 2. Original Wall Condensation Model of RELAP5 Schematic illustration of differences in condensation in vertical and horizontal/inclined tubes: More complex character of condensation in horizontal/inclined tubes (2D flow of condensate) Usage of horizontal condensation model in default RELAP5 is limited to strictly horizontal angles and determined by elevation parameter “delgrv”: delgrv < (it corresponds to θ < 0.005° for 1 m long volume) Usage of vertical condensation model (Nusselt) for small inclined angles (delgrv > 0.001) leads to unphysical results and strong underprediction of heat transfer coefficient (HTC)

5 Original Wall Condensation Model of RELAP5 The default condensation model for vertical and inclined surfaces uses Nusselt (laminar) and Shah (turbulent) correlations. For strictly horizontal walls, Chato correlation is used for laminar film flow conditions. Nusselt (1916) laminar film model for condensation in vertical pipe: Shah (1979) turbulent film model is applied both to vertical and horizontal volumes: Final wall condensation heat transfer coefficient for vertical/horizontal walls uses maximum of laminar and turbulent film model: For horizontal volumes (delgrv<0.001), Chato (1962) modification of Nusselt theory is used:

6 Original Wall Condensation Model of RELAP5 Boiling and condensation curve of RELAP5 (for both pure steam and steam-gas mixture) Flow chart of subroutine CONDEN

7 Original Wall Condensation Model of RELAP5 The Colburn-Hougen (1934) iterative diffusion method is used to solve condensation heat transfer in presence of noncondensable gas. Prediction of vapor-gas mixture condensation with the heat and mass transfer analogy approach. The condensate film model employs a heat transfer correlation to calculate mass transfer by replacing Nusselt and Prandtl numbers with Sherwood and Schmidt numbers. The iterative solution method is applied to solve for liquid/gas interface temperature Wall condensation in presence of noncondensable gas(es):

8 3. Modification of RELAP5 Condensation Model Based on UJV works in , we proposed the following set of modifications in RELAP5 condensation model for horizontal and sloped tubes: Replace Chato correlation for horizontal condensation by Jaster and Kosky modification, which instead of constant F (see above) uses a function of local void fraction. Extend usage of horizontal condensation model from strictly horizontal volumes also to inclined volumes. Change parameter for decision between horizontal and vertical condensation models from elevation parameter „delgrv“ to slope parameters „sing“. Optional relaxation for inlet volume of condensation tube (weighting between condensation models with/without Colburn-Hougen to reflect not yet fully developed NCG concentration profile at the tube inlet).  The parameter F in original Chato correlation should reflect decrease of effective wall condensation surface due to forming of bottom liquid pool. In original RELAP5, the value of F is constant  The first term of Jaster-Kosky correlation is based on local void fraction and so it is more flexible and relevant to capabilities of 6-equation computer code

9 Original Condensation Model of RELAP5 and its Modifications Proposed (cont’d) Original source coding: if (abs(delgrv).lt ) then c Horizontal stratified condensation heat transfer. twsub = -dtpps hcond =.296*((rhof(iv)*max((rhof(iv) - rhog(iv)),0.0)*gravcn* & hfg*thconf(iv)**3)/(htdiam*viscf(iv)*(max(1.,twsub))))**.25 else Modified source coding: if (abs(sinb(iv)).lt ) then c Laminar film condensation models c Horizontal stratified condensation heat transfer (Jaster-Kosky) twsub = -dtpps hcond =.725*voidg(iv)**.75*((rhof(iv)*max((rhof(iv) - rhog(iv)),0.0)*gravcn* & hfg*thconf(iv)**3)/(htdiam*viscf(iv)*(max(1.,twsub))))**.25 else Example of changes in RELAP5 source code:

10 4. Assessment of RELAP5 against COTINCO The COTINCO experimental facility has been designed at the University of Rome “La Sapienza” with the following objectives:  to investigate physical phenomena involved in condensation of steam inside tubes;  to study influence of presence non-condensable gases on steam condensation;  to study influence of geometry (namely, the tube axis slope) on the heat transfer rate. Caruso G., Naviglio A.: The Influence of Noncondensable Gases on Condensation inside Tubes: An Experimental Analysis. Universita di Roma „La Sapienza“. 1999

11 Assessment of RELAP5 against COTINCO (cont’d) The main components of the COTINCO experimental facility are:  Atmospheric steam generator, electrically heated, with power ranging from 375 W to a maximum of 6 kW;  Mixing tank, where the air-steam mixture is generated, with a volume of about 1000 litres;  Condensation test section, manufactured with a 22/25 mm internal/external diameter tube, AISI 304, 1.5 m long. The air-steam mixture flows inside the tube. The coolant water flows through an annular zone realised with an external tube of 32 mm internal diameter;  Auxiliary air system to feed the test section with a fixed air flow rate, to perform tests in steady or quasi-steady conditions. A pressurized air cylinder and an electrical heater to preheat air are the main components of this system;  Cooling system, with water reservoir, pumps, heaters and a water softener;  Instrumentation and a data acquisition system.

12 Assessment of RELAP5 against COTINCO (cont’d) Test section specifications: max available power: 6 kW design power (without non condensables): 3 ÷ 4 kW design pressure: 1 ate operating pressure: about 0 ate air concentration: 100 ÷ 0 % high accuracy on process variable measurements high accuracy on heat transfer coefficients determination steam-air mixture inside tube external coolant: water coolant temperature increase: 5 ÷ 10 °C steam flow inside tubes (horizontal lay-out): basically, stratified Test matrix

13 Assessment of RELAP5 against COTINCO (cont’d) Example of test results:

14 Assessment of RELAP5 against COTINCO (cont’d) Example of test results:

15 Assessment of RELAP5 against COTINCO (cont’d) Major findings from COTINCO experimental campaign [Caruso 1999]:  Experimental results with vapour-air mixtures show that non-condensable has a strong degradation influence on condensation HTC: o The condensation heat transfer coefficient decreases to 50% of the pure steam values when the air mass fraction changes from 0% to 2% o With 10% of air flowing within the flow, the condensation heat transfer coefficient is reduced to roughly 10%.  Tests with different inclinations of the experimental facility show that condensation HTC increase with inclination of the test section with low air mixture concentration. o This effect does not occur at higher gas concentrations. In fact by increasing noncondensable mass fraction inside the air-vapour mixture, the thermal resistance of the condensate film becomes negligible in comparison with the thermal resistance on the gas side and hence the test section inclination has no appreciable effect on the condensation heat transfer.

16 Assessment of RELAP5 against COTINCO (cont’d) Nodalization of COTINCO input model for RELAP5:

17 Assessment of RELAP5 against COTINCO (cont’d) Pure steam condensation - measured and calculated HTC for various tube slopes: Improvements: 1) Removal of non-physical HTC decrease for small slope angles 2) Reduction of RMS from 3533 W/m 2 K to 1492 W/m 2 K

18 Assessment of RELAP5 against COTINCO (cont’d) Pure steam condensation - measured and calculated condensation length: Original RELAP5: Modified RELAP5: Improvement: 1) Consistent L cond prediction for all slope angles 2) Reduction of RMS from 0,304 m to 0,112 m

19 Assessment of RELAP5 against COTINCO (cont’d) Measurement and calculation of HTC for various noncondensable qualities X n : Improvement: 1a) Reduction of RMS in whole X n range from 501 W/m 2 K na 330 W/m 2 K 1b) Reduction of RMS for low X n from 550 W/m 2 K na 229 W/m 2 K

20 5. Assessment of RELAP5 against ITASCO  The ITASCO experimental facility has been designed at the University of Rome “La Sapienza” as the follower of COTINCO  The major difference of ITASCO is ability to measure local HTC Main parts of ITASCO: A. pure water steam; B. noncondensable gases; C. mixture steam/noncondensable gases; D. condensate; E. steam/noncondensable gases; F. Air Caruso G., Di Maio D. V., Naviglio A.: Condensation heat transfer coefficient with noncondensable gases inside near horizontal tubes. Universita di Roma „La Sapienza“

21 Assessment of RELAP5 against ITASCO (cont’d) Nodalization of ITASCO input model for RELAP5:

22 Assessment of RELAP5 against ITASCO (cont’d) Measured and calculated HTC for various tube slopes and noncondensable qualities X n : Measured HTC: Calculated HTC (modified Relap5) Result: Good agreement or measured data and calculated HTC for various tube slope and noncondensable gas concentrations.

23 Assessment of RELAP5 against ITASCO (cont’d) Further assessment of modified version of RELAP5:  Continuation of assessment against data from SET facilities  Assessment against data from integral test facility PMK (stability proven)  Test calculations with plant SG model (with defined boundary conditions)  Test application to NPP calculations  Also communication and information exchange with other researchers working in the field of condensation of steam and steam-gas mixture and in area of RELAP5 development is going on.  At the NURETH-16 more than 10 relevant papers have been presented!

24 6. Summary Major parts of the presented work:  Review and analysis of condensation models in RELAP5  Explanation of differences between condensation in vertical and horizontal/inclined tubes  Proposed improvement of RELAP5 condensation models for horizontal and inclined tubes  Assessment of original and modified models against data from COTINCO and ITASCO separate effect test facility  Major part of proposed modifications were applied in new US NRC version of RELAP5/Mod3.3kc (as options at Card 001 – see below)

25 Summary (cont’d) Applied set of modifications in RELAP5/MOD3.3kc: Option 46 extends the horizontal condensation criterion to 5 degrees (plus decision based on slope parameter “sinb” instead of elevation parameter “delgrv”) Option 47 extends the horizontal condensation criterion to 45 degrees (plus decision based on slope parameter “sinb” instead of elevation parameter “delgrv”) Option 49 replaces the Chato correlation with the Jaster-Kosky correlation for horizontal condensation: These modifications were applied in standard US NRC version of RELAP5 in beginning of 2015 – starting from version MOD3.3kc.

26 Thank you for your attention