2 - FUMO CODE DESCRIP1 ION

FUMO is a BE thermal hydraulic code for multicompartment systems It is a lumped parameter code and contains the necessary models to realistically describe physical phenomena occurring in a containment system following a DBA or an SA

The containment can be described by a set of up to 100 volumes, 200 junctions, 30 of which can have time dependent areas, and 100 heat transfer structures The thermodynamic conditions in each control volume are calculated according to the following input options

1 a stagnant homogeneous mixture of steam, liquid water and air

2 a two regions model, consisting of a homogeneous gas/steam mixture region and a liquid pool region The pool region may or may not be in thermal equilibrium with the atmosphere one In the gaseous region it is possible to solve the mass balance for seven gases Different deentramment models are available for evaluating the water mass condensed in the atmosphere region and its falling velocity They allow the drop diameter and the atmosphere turbulence and composition to be taken into account

The control volumes are connected together through junctions The flow area can be time dependent It is also possible to model pressure dependent junctions which are opened by a trip on the pressure difference between the two joined volumes Junctions with the inlet or/and the outlet submersed by the water, present in the source or/and m the receiver node, can also be simulated This model can be used to analyse the pressure suppression containment systems or the water management in the LWRs with gravity driven safety systems

The junction flow rate is calculated as follows

1 The Bernouilh equation is solved for the evaluation of water flow rate

2 The momentum balance equation is solved for transients characterized by rapid thermofluid dynamic variations The flow rate is evaluated using one of the following models

Moody critical flow, homogeneous menial flow, orifice polvtropic flow

The method requires small time steps and theretoic should not be used in a long term transient because ol the considuable CPU time needed

^ In order to study the long term phase of an accident a model is used which assumes the pressure differences between control volumes to be negligible, the flow rates are evaluated in such a way that the pressure in the connected compartments is balanced This option turns TUMO into a fast running code and a large number of control volumes even it they are connected in complex ways, can be used

The natural circulation model [3| is able to predict the buoyancy driven flow, both in serial volumes and in complex loops t urthermore a simplified model that simulates the hydrogen vertical distribution m a control node is implemented in the code The model is utilized to modify the hydrogen miss percentage of junction flow rates, depending on the junction heights in the source volumes

Particular care is given in the code to the flexibility in modelling the heat sinks and the heat transfer coefficients The temperature distribution inside a structure is evaluated by solving the Fourier equation by a finite difference method or with a coarse-mesh method, specifically developed by DCMN [7]

Several models, covering a wide spectrum of physical processes that may occur within the containment, were introduced in the code These models include processes like hydrogen combustion heat and mass transfer (convection and/or condensation on cold structures), corium water interaction, as well as BWR specific features (suppression pool and safety relief valve discharge) A detailed model for the description of thermal transient of a metallic structure cooled by a passive external spray was also developed [8] (briefly described in section 4) As the code does not consider in vessel processes, it must rely upon separate analysis performed with a primary system code, to provide sources of mass and energy to the containment To overcome this limit, a simplified model, which can simulate a transient in the primary system, is under development This model will allow the simultaneous analysis of the relevant processes both in the primary and in containment systems

3 - FUMO CODE ASSESSMENT AND QUALIFICATION

The FUMO code was tested to verify its reliability in simulating the abnormal transients that can take place m a m u In compartment contaminent system The code validation was based on the comparison between the values of the most important thermal hydraulic variables measured during expenmental tests and the values of the same variables calculated by the code

3.1 Review of the past assessment work

First assessment activities of FUMO include some numerical benchmark problems, in order to evaluate the degree of approximation of us models A series of separate effect tests were carried out in a facility installed at the DCMN, in order to obtain a better understanding of the influence on the containment behaviour of some important phenomena, such as the deentramment of droplets and the effect of the high turbulence, induced by the blowdown jet expansion, on the heat transfer to the thermal structures

In order to obtain an in depth validation of the code the qualification program continued with participation in some international exercises based on tests carried out in apparatuses with various volumetric scales Particular attention was paid to the validation of the models which describe the long term phase of an incidental sequence During this phase the following tests were simulated

the OFCD CSNI Benchmark Problem 2 \2\ proposed in the framework of the PWG 2 Task group on Ex Vessel Thermal hydraulics, in particular to quantify the pressunzation and the combustible gas distribution m a containment system during a core melt down accident in a LWR

the OECD CSNI 1SP 16 |2|, carried out in the HDR Facility at the end of 1982, for the analysis of the first 12(X) s of a large LOCA sequence, and the second phase of the same test, named HDR V44, for the study of the temperature stratification,

the pre and post test calculations [9] of the LACE tests 4 and 6, performed at the CSTF (Hanford W USA)

3 2 - Recent assessment work

FUMO assessment has recently focused on some complex phenomena that may occur in the containment of new simplified plants, such as buoyancy driven natural convection flows, condensation of steam on structures and long term cooling of the plant by passive features This was achieved again by comparing expenmental data (from HDR and BMC facilities) and code calculation results

3 2.1 - The HDR facility

The HDR containment (11300 m"5) consists of a steel shell 60 m in height and 20 m m diameter [10] This steel shell is surrounded by a secondary concrete shell which is separated from the former by an annular gap The containment is subdivided into 69 subcompartmems of different shapes and sizes by concrete partition walls Two spray systems internal and external to the steel liner can be activated during the transient

Two HDR tests 15] 111 ] were analyzed, simulating the containment behaviour after the rupture of a large diameter steam pipe (HDR T31 'S) and a small break LOCA (Test El 12) respectively Their timing characteristics allow a good assessment of the code particularly for the long term analysis

Analysis of the HDR T31.5 test

A seven volume nodalization was used for the analysis with FUMO of the T31 5 test [51 This test is characterized by a relevant stratification over the height of the containment, due to the heat exchange processes and to the evolution of the convective flows A stable temperature distribution can be identified, with a measured temperature differences of roughly 50 °C between the upper and lower parts of the containment Stratification decreases gradually dunng the last phase of the test, with the exception of a localized region around the blowdown node The good evaluation of temperature trends, obtained with FUMO, is a necessary condition for a correct simulation of the hydrogen distribution

<2 o

•—D— Experiment

—^—— Without NC -O—— With NC

0 0 2 0 3 0

Time (s) *104

Fig 1 HDR T31 5 - Hydrogen concentration in the blow-down volume

.1 5

30 5

Experiment -A— Without NC -O— With NC

1 0 2 0 3 0

Time (s) *104

Fia 2 HDR T31 5 - Hvdrooen concentratjon in the central volume

Hydrogen volumetric concentrations are compared with the experimental ones in Figs 1 and 2 The hydrogen injection starts at 35 mm into the transient and lasts for 12 mm Most of the released hydrogen leaves the blowdown volume, along the staircases, and reaches the upper dome region The zones below the injection position are also affected, due to diffusion and convective flows Figure 1 shows the results for the blowdown volume, the agreement between experimental and calculated values is quite good, especially in the long term The natural circulation and the hydrogen distribution models are fundamental for this goal In the same plots also the results from a calculation performed without activating the model for the natural circulation are shown In this case FUMO calculates an unrealistic hydrogen peak, due to the lack of convective flows

For the other control volumes the influence of the natural circulation is not so relevant In a volume located at the same height of the blow-down node (Fig 2) there is a very good agreement in the results, with the exception of the first few minutes

Fig 3 Natural circulation flows inside HDR containment

after the hydrogen injection The differences are probably due to the technique used for hydrogen concentration measurement, which requires some time to initiate the catalytic process in the hydrogen transducers

Analysis of the HDR E11.2 test

The experimental Ell 2 conditions simulate, in the first phase (740 mm), a SB-LOCA (Fig 3) During the following phase a steam and hydrogen/helium mixture is injected at an elevation of +23 5 m, near the staircase This mixture, being lighter than the air present in the containment rises up to the dome The air is thus pushed through the spiral stair into the lower region of the HDR containment, like in a plug flow A very large amount of air is displaced in the first 100 mm, so that 50% of the air has been expelled from the dome at this time The injected steam strongly accumulates in volumes above the break location, but u does not flow down in a substantial quantity The temperature transients make this impressively clear, with a peak temperature of 130 °C in the region above +25 m, whilst only 65 °C was measured at +15 m At the bottom of the containment, below 10 m, the temperatures are almost constant between 22-26 °C

A detailed nodahzation was set up for FUMO, based on 15 control volumes, 85 heat structures and 64 junctions The compartments inside the steel shell were nodahzed using 12 control nodes two nodes were used for the gap and one for simulating the external back-up volume of the gas sampling sensors

Figure 4 shows the comparison between the measured and the predicted total pressure in the containment up to 1300 mm The very good prediction of the long

term phase shows UK capability of F-UMO to simulate the overall ttiennal-hydraulic transient In the figuie the data obtained without using the natural circulation model aie also presented In this case, the steam concentiation in the dome is underestimated this leads to too low a condensation and therefoie to too a high pressure trend I his result highlights the effect of natural uiculauon flows on the thermal hydraulic transient and in particular on the sfeam concentration and distribution over the containment volumes Furthermore without the simulation of convective flows, the hydrogen volumetric concentration is overestimated in the nodes located near the staircase (Fig 5), while it is underestimated in other compartments Using the natural circulation model a better prediction of this fundamental variable is obtained

0 30

0 22

If. 0 14

-2 0 4 0 6 0 8 0 Time (s) *104

Fig 4 HDR Ell 2 - Tota± pressure

0 0 2 0 6 0 8 0

Time (s) *104

-il 2 - dydrcgen concentration in oentral volumes

0 0 2 0

Time (s) *104

Fig 6 HDR Ell 2 Hydrogen concentration in upper dome

At the end of the El 1 2 test, the external spray system for cooling the spherical region of the steel liner is activated As a consequence, a sudden increase in the steam condensation rate takes place inside the dome, followed by an increase in the local hydrogen volumetric concentrations in the same volume, due to the suction" effect from the lower nodes of the facility Both FUMO (Fig 6) and other participants- in the international exercise [12] show a general underestimation of this local phenomenon

3.2.2 - The Battelle Frankfurt Model Containment facility (BMC) The BMC, with a total volume of 640 m3, was originally designed tor DBA analysis and later was also used for hydrogen distribution experiments and for the DEMONA aerosols depletion tests 16) The facility is made of reinforced concrete and its internal volume is subdivided into several compartments, sizes and locations of the vent openings between the individual compartments can be adjusted to the specific test conditions The height of the facility is reduced by a factor 4/5 compared with a real containment, which might have some influence on the natural circulation behaviour

Analysis of the FIPLOC F2 test

The FIPLOC-F2 test is focused on the analysis of natural circulation phenomena in a multicompartment containment system Dunng the first phase of the test (48 h) the pressure and the temperature inside the facility are step wise increased by steam injection, until a partial pressure of the vapour of about 2 7 bar is reached In the following phases steam, air and sensible heat are injected into different containment subcompartments, developing convection flows which have a direction and a velocity depending on the injection locations (Fig 7) A detailed nodahzation was set up for FUMO analysis 9 control volumes, 45 heat structures and 16 junctions were used

O\

Steam ——

7 Natural circulation *lows in BMC facility

0 30

-18 0 22 0 26 0

Time (s) *.»04

Fig 8 FIPLOC F2 - Total pressure

130 0

—O—Experiment

—A—Without NC

—O—With NC

110 0

18 0 22 0 26 0

Time (s) *104

Fig 9 FIPLOC F2 - Temperature

In Figs 8 and 9 the measured and calculated global pressure and temperature with and without the use of natural circulation model are compared The underestimation of pressure data (Fig 8), calculated without using the natural circulation model, can be explained through the prediction of a higher steam concentration m the dome of the BMC facility this induces an overestimation of the condensation on thermal structure

More insight into the problem of a correct prediction of the long term containment condition is obtained from the analysis of temperature trends (Fig 9) The difference between measured and calculated values, of about 8 °C during all the transient, is due to uncertainties of initial value The calculation performed without the use of natural circulation model shows an oscillatory behaviour due to an unrealistic evaluation of heat exchange coefficients

4 - APPLICATION OP FUMO CODE TO INNOVATIVE LWRs FUMO is being used at present for analysing the containment systems of SBWR and AP600 reactors The nodahzation has been completed and tested for both reactors (Figs 10 and 11) For AP600 some results are already available Below they are compared with data available m literature [H), obtained by Westlnghouse using an independent methodology

4.1 External spray model

The AP-600 containment design is similar to a dry containment arrangement in terms of size and design pressure The safety functions are devoted to a Passive Containment Cooling System, which is capable of transmitting heat directly from the reactor containment structure to the environment [14]

Containment cooling is performed by a falling liquid film and a counter cuirent natural circulation air flow A monodimensional model is used in FUMO Mass and energy balance equations are solved both for the liquid film and for the air/steam mixture The energy balance takes into account terms due to heat conduction through the film, convective heat transfer between film and air and latent heat carried by the evaporating flow The evaporation rate is evaluated using a correlation derived from the analogy between heat and mass transfer in turbulent forced convection, this is acceptable because of the high Reynolds number which characterizes the mixture flow The solution of the mass balance equation along the height of the containment structure means that the film thickness can be calculated this is necessary for the evaluation of the heat transfer phenomena and also for the verification of its hydro dynamic stability

A minimum flow-rate is required to guarantee the stability of the liquid film, but there is a large spread in the experimental values of this limit, ranging from 0015 Kg/ms to 60 kg/ms for water at room temperature [15] The exact value depends upon the geometry and the nature of the vertical surface the initial distribution of the liquid and the mass transfer between the liquid and the surrounding gas Specific tests in order to investigate this phenomenon will be earned out at the DCMN In FUMO the value of 0015 Kg/ms is used at present When the liquid film flow rate drops

L_

Fig. 12: Sensitivity analysis for Large LOCA in AP600.

Pressure inside containment.

Fig. 13: Thermal flux through liner internal surface.

Fig. 14: Thermal flux through liner external surface.

below this value the lower portion of the containment is assumed to be dry and the convectlve heat transfer coefficient is evaluated on the basis of the air/steam flow rate induced by natural circulation

4.2 AP600 Large LOCA analysis

A preliminary analysis of AP6(X) containment response to a Large LOCA was performed using FUMO The containment was nodalized with 15 control nodes, 11 inside the steel liner and 4 outside

A reference calculation (case A) was performed without modelling the IRWST pool or injecting steam into the Containment during the whole transient with a total enthalpy flow equivalent to the decay power of the reactor

A sensitivity study was also performed to evaluate the effects of the limit for film instability and of the way of providing the blow down table In the second calculation (case B) the instability test described in the previous paragraph was by passed Figure 12 shows that the assumed minimum flow rate value strongly influences the pressure transient inside the containment In fact, the instability condjtion is reached at about 80000 s, and, starting from this time, the pressure rises significantly

In the third calculation (case C) the IRWST was simulated, and decay power, given by the ANS curve + 20%, was directly added to the pool in the reactor cavity This approach needs a more complex calculation, but it matches the actual situation better, in fact it allows a correct simulation of the water balance inside the containment and of the steam generation in the long term phase, when a pool of saturated water is present The importance of a correct evaluation of the blow-down is evident from Fig 12 The higher steam generation present in case C induces a significantly higher pressunzation of the containment

In Figs 13 and 14 the heat flux trends, internal and external to the containment steel wall, are given The results are compared with those obtained by Westmghouse in a similar analysis [B] The good agreement between the results obtained in the two calculations is highlighted, in particular, a good quantitative agreement is present also m the initial phase of the transient, when the heat transfer process is dominated by the external spray action

5 - CONCLUSIONS

The presented results show FUMO s ability to analjze thermal hydraulic transients in the containment system of the current and the new generation of LWRs during a DBA or a SA sequence In particular the results obtained in the assessment process show the validity of the natural circulation model and its importance for a BE analysis of a containment system

Also the new models, related to the ESFs present in the new LWRs, have been tested with satisfactory results A further assessment may be useful to improve the external spray film model especially as far as the stability condition is concerned

Also the new models, related to the ESFs present in the new LWRs, have been tested with satisfactory results A further assessment may be useful to improve the external spray film model especially as far as the stability condition is concerned