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Inerting and venting of mark II containments in BWR nuclear plants
Nuclear Engineering and Design 120 (1990) 57-65 North-Holland
57
I N E R T I N G A N D V E N T I N G O F M A R K II C O N T A I N M E N T S
IN BWR NUCLEAR PLANTS
Fabio F I N E S C H I , Marco C A R C A S S I and Fabrizio C A R N A S C I A L I Dipartimento di Costruzioni Meccaniche e Nucleari, Universitd degli Studi di Pisa, via Diotisalvi 2-56126 Pisa, Italy
Received July 1989
An analysis of hydrogen control systems corroborates containment inerting as the only way of preventing hydrogen explosions which may jeopardize the integrity of BWR Mark II containments during severe accidents. A severe Large Break LOCA and a severe Stuck Open Relief Valve Accident are simulated by the MARCH 2.0 code to compare the advantages and disadvantages of pre-inerting and post-inerting, with or without venting, in BWR Mark II containments.
1. I n t r o d u c t i o n
In a Boiling Water Reactor (BWR) a severe accident generates a sufficient quantity of hydrogen to form flammable mixtures with the air in a Mark II containment [1,21. For this reason the ENEA, the atomic energy authority in Italy, has entrusted the Department of Mechanical and Nuclear Constructions of the University of Pisa with the task of carrying out a preliminary analysis of suitable hydrogen control systems in BWR Mark II containments, be they vented or not. In severe accidents flammable gas mixtures in the containment cannot be prevented with the recombination/dilution systems which are installed for coping with hydrogen dangers in Design Basis Accidents (DBAs) [2,3]. Deliberate ignition must be rejected because a deflagration might compromise the integrity of a BWR Mark II containment even if it occurs as soon as the gas mixture becomes flammable [4,5]. Therefore inerting is the only way of coping with the hydrogen danger in this containment. Attention has thus been focused on a comparison between pre- and post-inerting. In order to compare inerting methods on the basis of how they modify accident sequences and transients, two typical severe accidents, Large Break LOCA, AE, and Stuck Open Relief Valve Accident, TPE, were considered because they give rise to the highest hydrogen release rates in the safety containment, in the drywell and wetwell, respectively. Their transients were calculated by the M A R C H 2.0 code [6] (see ref. [4] for a
detailed description of the code inputs used). For purposes of comparison, this classic code was selected as a standard yardstick of judgement because it is well known and widely used by every expert in safety analysis. For the same purposes, the calculated values were non truncated, even if they indicate a greater degree of accuracy than possible. Containment failure, due to overpressure, is the inevitable consequence of the analyzed accidents, if it is not prevented by containment venting. The reliability and costs of the inerting systems are not the concern of this preliminary analysis, but they would obviously have to be considered in a definitive analysis as well as an optimized system design.
2. Accident
sequences
In both the analyzed accidents it is assumed that: the reactor protection system is available; - the emergency core cooling systems and the spray system in the drywell are unavailable; - the suppression pool cooling system can be utilized with just one pump and the pool water stays subcooled throughout the whole accident; - just one R H R heat exchanger starts, 600 s after the beginning of the accident, and continues operation until the containment fails; - the floor of the cavity underneath the reactor vessel does not have any vent tubes and is dry; when two thirds of its thickness has been attacked by the corium, it fails. -
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © 1990 - Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )
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F. Fineschi et a L / Inerting and venting of Mark H containments
as to reduce and maintain the concentration of oxygen at such levels so as to prevent flames, however, much hydrogen is produced during an accident. Nitrogen is used due to its low cost and its chemical stability. This method was chosen by many countries for BWR Mark I and II plants after the accident at the TMI-2 station (USA). Italy has only recently taken this line, when there were really no doubts [7] about preinerting increasing plant safety. The atmosphere in the containment is diluted so as to have an oxygen molar fraction which is less than 5% (4% in the USA, 3% in Japan, 1% in Sweden). The main disadvantage of this system is the fact that de-inerting or life-support systems are required for personnel entering the containment. In an emergency situation, access to the containment might be delayed from three to ten hours while the containment is being de-inerted. Furthermore, pre-inerting may increase the number of non-programmed reactor shutdowns because the operator might not be immediately sure about the safety aspects of small core coolant losses. Recombination and dilution systems need to be used for coping with air infiltrations from outside, losses from pneumatic systems, and water radiolysis. Apart from eliminating deflagration overpressures, pre-inerting does not modify the accident transients. The most notable accident events are summarized in table 1. In a Large Break LOCA without pre-inerting the drywell would be inerted by steam throughout the whole transient, but as soon as the mixture in the wetwell reaches the downward flammability limit a deflagration might fail the containment, table 2. The lower downward flammability limit is the minimum hydrogen molar fraction (8%) for which the deflagration is very likely and the combustion is adiabatic and complete [5]. A transient of Small Break LOCA (SE) was also studied using the MARCH 2.0 code with regard to pre-inerting; this too is shown in tables 1 and 2.
o 0.8 0.6 0.4
"---\
0.2 •
0.2
0.4
(b)\ 0.6
0.8
Carbon dioxide molar fraction
Fig. l. Air-H 2-CO 2 flammability limits. Studies were also made of the sensitivity of the code in order to appreciate the effects of delayed interventions by the emergency core cooling systems. Comparisons were also made between hydrogen production as calculated using the MARCH 2.0, and values, available in the literature, evaluated by using other codes, such as MAAP, without any essential differences being noted. The results [4], although interesting in themselves, were not particularly relevant when comparing the various inerting methods.
4. Post-inerting with or without partial pre-inerting Post-inerting consists of diluting the air in the containment with an inert gas at the beginning of the accident. Post-inerting could be preceded, during normal plant operation, by pre-inerting the wetwell alone (partial pre-inerting) and also by reducing the oxygen in the drywell to the breathability limit (conditioning).
Table 2 Non-inerting - adiabatic isochoric complete combustion in the wetwell. Hydrogen molar fraction = 8%; containment failure pressure = 1.07 MPa [2] Accident Deflagration start Pressure before combustion Temperature before combustion Pressure after combustion
(s) (MPa) (K) (MPa)
AE
TPE
SE
2142 0.290 314 1.055
1920 0.117 327 0.401
1950 0.302 319 1.067
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F. Fineschi et al. / Inerting and venting of Mark I I containments
Since in post-inerting the inert gas is added to the air while in pre-inerting it substitutes the air, the pressure in post-inerting increases more than in pre-inerting, and the greater the quantity of gas injected the greater the increase. The quantity of gas to be injected depends on the inerting characteristics of the gas. Carbon dioxide, CO 2, seems to be the best at the moment, as it has a greater specific heat than nitrogen and a greater chemical stability than Halon [2]. The a i r - H 2 - C O z flammability region (F) is shown in the triangular diagram in fig. 1 [8], where LFL represents the lowest flammability limits, line (s) is the stoichiometric line, line (a) represents the mixtures with the COz/air molar ratio which is necessary for inerting the containment via oxygen deficiency, however, much hydrogen there is, and line (b) represents the mixtures with the C O 2 / H 2 molar ratio which is necessary to prevent, via hydrogen deficiency, the formation of flammable mixtures, however, much air there is. Different amounts of steam and other inert gases are produced during different accidents, but they were neglected in determining the preliminary specifications of the CO 2 injection system in order to provide a reliable post-inerting in any accident scenario. The carbon dioxide injection rate must be able to dilute the hydrogen flow released into the containment, so as to prevent flammable mixtures from being formed, until the oxygen has been diluted enough to make any mixture inert. At that point the CO 2 injection can be stopped. Since the free volume of a BWR Mark II containment is around 8000 m3, 24 500 kg of carbon dioxide is needed to ensure that the atmosphere is inerted. It should be injected (the condition is necessary but not sufficient) before the hydrogen/air ratio in the containment reaches the value which is characteristic of the tangency point of line (a) in fig. 1 with the flammability limit curve and which is virtually the same as the stoichiometric value. But the flammability limits do not depend on the composition of the mixture alone [8,9] and they cannot always be determined exactly. It is therefore wise to inject all the CO 2 before the H z / a i r ratio reaches that of the intersection point of the lines (a) and (b) in fig. 1, i.e. before the released hydrogen goes above 150 kg. In the drywell and the wetwell two types of hydrogen sources can be identified: - a direct source, into the drywell via a pipe break and into the wetwell via a relief/safety valve; an indirect source, from one compartment to another, via the vent tubes and the vacuum breakers. -
A tentative value of the CO 2 injection rate is calculated by assuming that there is a hydrogen release into the drywell which is the same as the direct one which there would be in a AE, and at the same time there is one into the wetwell which is the same as the direct one in a TPE, with the two compartments mutually isolated. The initial quantities of air in the drywell and in the wetwell are 14 800 and 9700 kg respectively. The maximum quantities of hydrogen that could be tolerated in the two compartments, just before the inerting via oxygen deficiency is reached, are therefore 90 and 60 kg, respectively. Figs. 2a (drywell) and 2b (wetwell) show the masses of hydrogen (calculated with the M A R C H 2.0 code) produced in an AE and a TPE respectively, and the quantities of carbon dioxide needed to inert the two compartments via hydrogen deficiency (i.e. such as to maintain the representative point of the a i r - H 2 - C O 2 mixture on the lower part of the flammability curve, LFL in fig. 1), assuming that there are no other inert gases and that the two compartments are mutually isolated. 90 kg of hydrogen is reached after 1260 s in the drywell, and 60 kg after 1560 s in the wetwell. If the injection begins immediately after the event which caused the accident, a constant 11.9 k g / s injection rate of CO 2 into the drywell and 6.1 k g / s into the wetwell are therefore necessary to complete inerting by these times (solid lines in figs. 2a and 2b). These injection rates are also sufficient (if the two compartments are mutually isolated) to maintain an inert atmosphere in the two compartments during the whole injection period, because the hydrogen production rate in the first stage of the accident increases in time: the points which represent the mixture compositions are below the line (b) in fig. 1 until the intersection point (a) (b) is reached. Four per cent hydrogen is the lower upward flammability limit for H2-air mixtures and corresponds to 16 kg of hydrogen in the drywell and 10 kg in the wetwell, which are reached after 420 s and 1260 s, respectively. Such values can be considered to be the upper limits of time available for starting CO 2 injection in the two compartments. If injection is delayed, the flow rates must be increased. Supposing (to provide an example of delayed injection) that, when a 4% H z / ( H 2 + air) molar ratio is reached, enough CO 2 must be in the drywell and wetwell to inert the hydrogen which will be in the compartment a minute later, in the dryweil, the injection should be started after 360 s from the beginning of the accident and the injection rate should be 19.4 k g / s until 900 s (dotted -
61
F. Fineschi et al. / Inerting and venting of Mark H containments
2°°/
20000
15°l
200
. / _ _ _ _ _ . 15000"~
/oo /
lO0 0
ioooo~
.-- I0000
.
15o
7500?
100
* o 5000 o
Z 0
50
~/
5000
50
25oo
o
/o¢. oH
1'0
0
20
10
TIME (min) a)
Large
Break
LOCA
~,
,
0
30
20
30
TIME (min) - Drywell
b)
Stuck
Open
Accident
Relief
Valve
- Wetwell
LEGEND: o
H 2 in the mutually isolated contalnment compartments.
+
Minimum CO 2 to inert compartment atmosphere via 0 9 deficiency
-- • - -
Minimum CO 2 to inert the
compartment via 0 2 deficiency.
Immediate CO 2 in3ection Delayed CO 2 injection.
Fig. 2. Post-inerting:tentativccalculation of CO2i~ectionrates. line in fig. 2a) and, afterwards, 11.9 k g / s until inerting has been completed; in the wetwell, the injection should be started at 1176 s with a constant flow rate of 24.3 k g / s (dotted line in fig. 2b). Post-inerting can be preceded by pre-inerting with nitrogen in the wetwell, which is the compartment where access during normal operation is less important and where in an accident the inerting action of steam cannot be counted on. In this case the post-inerting of the drywell can be done with the above-mentioned procedures, while there is no need to inject C O 2 in the wetwell. The quantity of gas to be injected is reduced (14 800 kg instead of 24 500 kg) as well as containment -
pressure, without over-penalizing the normal plant operation. Using the M A R C H 2.0 code and the tentative C O 2 injection rates, A E and T P E accidents have been compared [4] where there is: no inerting, total pre-inerting, - total post-inerting with immediate injection, - total post-inerting with delayed injection, post-inerting with pre-inerting in the wetwell and immediate injection, post-inerting with pre-inerting in the wetwell and delayed injection. Since the tentative injection rates have been calcu-
-
-
-
Table 3 Post-inerting - containment failure Accident
Large break LOCA
Wetwell inerting Gas injection delay Cont. failure time (s)
no no 4100
no yes 4077
Stuck open relief valve yes no 7962
yes yes 8178
no no 14 297
no yes 13 932
yes no 15 495
yes yes 14 718
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F. Fineschi et aL / Inerting and venting of Mark 11 containments
lated on the assumption of a hypothetical large hydrogen release, but without taking into account the redistribution of incondensable gases between the two compartments, in A E with total post-inerting the wetwell is non-inerted for around 600 s immediately after the end of the CO 2 injection. It is a short period of time, and furthermore it is calculated with a code which simulates a very complex situation in an approximate manner, but it underlines the fact that the gas transfer between the compartments needs to be considered in a more accurate design of the injection system. If the wetwell has been pre-inerted, this problem does not arise and therefore the safety analysis could be significantly simplified and could also offer more certain conclusions. Table 3 outlines the failure times of a post-inerted containment: - In a TPE the failure time is brought forward for 1637 s or 2835 s, depending on whether or not the wetwell has been pre-inerted, compared to the 17132 s in which the containment would break with total pre-inerting (table 1). - In an A E the reduction in containment failure time is more accentuated, 3806 s or 7668 s, depending on whether or not the wetwell has been pre-inerted, compared to the 11 768 s in which the containment would break with total pre-inerting (table 1). The delay with respect to the 2142 s in which the non-inerted containment could break, if there were a deflagration of a hydrogen mixture (table 2), is 5820 s and 1958 s respectively. If the wetwell is not pre-inerted the containment and the vessel failure coincide. In any case, the delay in CO 2 injection has little influence on containment failure times.
5. Venting
of inerted
containments
In order to avoid containment failure in severe accidents, containment venting can be carried out when the pressure goes above the design value, so as to reduce the release of radioactive substances into the ambient which would be verified in a catastrophic containment failure. Venting carried out after the containment design pressure has been reached cannot prevent flammable mixtures and deflagrations during severe accidents, table 4. If venting were carried out before hydrogen and fission products are released from the reactor vessel and the pressure reaches the design pressure (early venting), the resulting decrease in air might cause, or at least contribute to, containment inerting. However, early
Table 4 Non inerting-gas mixture compositions at the containment design pressure (0.45 MPa) [11] Accident
AE
TPE
Design pressure reaching time (s) Temperature drywell (K) wetwell (K) Hydrogen molar fraction drywell (%) wetwell (%) Air molar fraction drywell (%) wetwell (%) Steam molar fraction drywell (%) wetwell (%)
2340
10500
570 370
490 420
20.0 28.0
26.5 64.0
79.0 69.5
54.5 29.0
19.0 2.5
1.0 7.0
venting currently seems too problematic and therefore inadvisable, seeing as its effectiveness depends on the accident development, which might still be unpredictable when it is being initiated. It might cause an unnecessary release of radioactivity if the accident turns out not to be severe or if venting were prolonged when radioactive substances in the containment are no longer negligible. Venting can hardly cope with an explosion and thus replace containment inerting, because the energy release might be so great and rapid in a deflagration that it might not be sufficiently balanced by a limited opening of the containment [8]. Furthermore, as venting causes turbulent flows in the atmosphere of the containment, it might aid flame instability phenomena [10] which could cause detonations and dynamic loads on the structures and safety-related equipment in the containment. Its function should thus be limited to controlling the relatively slow pressurizing of an inerted containment and may be particularly important in post-inerting. A mixture, which was inert in the containment due to oxygen deficiency, when released from the containment may come into contact with the air and become flammable. In order to avoid explosions in the venting system it is necessary: - either to avoid any contact with the air, - or to avoid the mixture being ignited, - or to dilute the mixture coming from the containment with C O 2, for example, so as to obtain a C O 2 / H 2 ratio in the venting flow which is at least equal to that of line (b) in fig. 1. The analysis of venting is carried out [4] by comparing the A E and TPE transients in a pre-inerted contain-
63
F. Fineschi et al. / lnertmg and venting of Mark H containments
ment with those in a post-inerted containment where oxygen is minimized by reducing the initial oxygen molar fraction to 1% in the pre-inerted wetwell and to 15% in the conditioned drywell. Partial pre-inerting and conditioning allow a safe normal operation of the plant and a reduced post-inerting pressurization. In fact, in this case there are 197 kmol of incondensables in the containment during the normal plant operation, of which 141 kmol are of air and 56 kmol of nitrogen. Without taking the 56 kmol of nitrogen into account, conservatively 230 kmol of CO 2 (10 100 kg) is needed to inert the atmosphere, however, much hydrogen there is. The injection rate into the drywell, if activated right at the beginning of the accident and terminated after 1260 s, is reduced to 8 kg/s. The venting start-up pressure has been chosen in such a way so as not to cause an opening if a DBA occurs, and to maintain an adequate safety margin with respect to the containment failure pressure (1.07 MPa [2]). If there is a DBA-LOCA in a pre-inerted containment, the maximum pressure and temperature in the drywell are 0.41 MPa and 413 K, respectively [11]. The design pressure is 0.45 MPa [11]. To estimate the pressure value which might be reached in a DBA-LOCA in a post-inerted containment, the CO 2 partial pressure at 413 K (0.096 MPa) has been added to the pressure of 0.41 MPa. The start-up pressure should therefore range between 0.5 and 1.07 MPa, so as to prevent radioactive releases into the atmosphere in accidents which are slightly more serious than a DBA. Venting can be. carried out either via a "blowout disk" which leaves the containment open, or via an o n / o f f valve which allows the integrity of the containment to be re-established if the engineering safeguards are recovered in time to reduce the containment pressure.
An o n / o f f valve is analyzed, which opens a 0.017 m2 orifice at 0.76 MPa and closes at 0.66 MPa. The containment pressure and the release rate will obviously oscillate during the intermittent venting. Venting should be done in the wetwell for the following reasons: - the suppression pool can decontaminate gaseous effluents; as long as the integrity of the separation slab between the drywell and the wetwell is maintained, venting in the wetwell increases the steam condensation in the pool and thus the efficiency of the pressure suppression system; - after the initial LOCA blowdown, most of the incondensables are in the wetwell; - in the atmosphere of the wetwell there is a lower quantity of steam and therefore filtering the released gases is easier. Before the reactor cavity floor breaks, i.e. before the pressure suppression pool loses its functionality, the released mixture consists substantially of incondensable gases. When the melted core comes into contact with the water in the pool, a great deal of steam is produced, which keeps the venting open for the longest time during the transient. From then on most of the released gas is steam, both because most of the incondensables have already been released, and because the production rate of carbon monoxide and dioxide by the coriumconcrete reaction is decreasing. Post-inerting brings forward the time of the first venting for 1440 s in a TPE and for 4200 s in a AE. In the latter case venting start precedes vessel failure. Table 5 shows the values which characterise the venting function, the maximum release rate of incondensables and the total mass of incondensables released until the cavity floor is broken. Table 6 shows the
Table 5 Venting characteristics and incondensable release Accident
Pre-inerting AE
Venting start (s) Venting time fraction in the on position Maximum incondensable release rate (kg/s) Total incondensable release until cavity floor failure (kg) at
(s)
Post-inerting TPE
7500 0.30 13.1 14 500 14280
AE
14 340 0.19 14.5 13 200 1 8 840
TPE
3300 0.28 14.5 20 500 14280
12 900 0.20 14.7 20 800 18 840
64
F. Fineschi et al. / Inerting and venting of Mark 11 containments
Table 6 Venting - average release rates Release rate (kg/s)
Time (s)
Incondensables
Steam
Total
Large-break LOCA - pre-inerting 0 7500 7500 15 300 15 300 18000 18000 36000
0 1.73 10.50 0.96
0 0.04 2.73 1.65
0 1.77 13.23 2.61
Large break LOCA - post-inerting 0 3300 3300 7650 7650 15 660 15 660 18 360 18 360 36 000
0 1.03 2.37 10.18 1.02
0 0 0.04 3.15 1.77
0 1.03 2.41 13.33 2.79
Stuck open relief valve accident - pre-inerting 0 14340 14 340 21060 21060 21 960 21 960 36000
0 2.44 12.83 1.94
0 0.10 0.32 0.09
0 2.54 13.15 2.03
Stuck open relief valve accident - post-inerting 0 12900 12900 20700 20 700 21600 21 600 28 800 28 800 36 000
0 3.13 13.33 2.22 0.38
0 0.11 0.42 0.09 0.02
0 3.24 13.75 2.31 0.40
From
To
average release rates for consecutive time intervals. In any case, the first opening of the venting h a p p e n s before the separation slab breakage a n d the consequent loss of the d e c o n t a m i n a t i o n effect of the suppression pool, which occur at the same time b o t h for pre-inerting a n d for post-inerting, table 5. As long as pool effectiveness is maintained, the q u a n t i t y of i n c o n d e n s a b l e s released in post-inerting is greater t h a n in pre-inerting, but the release of radioactive substances to the a m b i e n t is limited by the pool d e c o n t a m i n a t i o n factors (from 600 to 10 000, d e p e n d i n g o n the physical a n d chemical characteristics of the radioactive substances [12]). F u r t h e r m o r e , during the time in which the valve has already been opened in post-inerting, while it is still closed in pre-inerting, venting has reduced the radioactivity of the containm e n t atmosphere. After cavity floor slab failure, the masses of released incondensables, b o t h in pre-inerting a n d post-inerting, are almost the same, table 6. Therefore the total radioactivity released into the a m b i e n t in post-inerting might n o t be m u c h greater t h a n in pre-inerting, a l t h o u g h gases are released for a
longer time: the difference might not b e significant a n d would anyway decrease in time. The release rate values are n o t particularly different in pre-inerting a n d post-inerting, table 6, so the same gas filtering system could b e utilized.
6. Conclusions In order to prevent the integrity of a B W R M a r k II c o n t a i n m e n t b e i n g lost due to h y d r o g e n explosions in severe accidents with partial or total core melting, cont a i n m e n t inerting must inevitably b e utilized. The most drastic solution is to replace the air with nitrogen during n o r m a l p l a n t operation. T h e concentration of oxygen needs to b e controlled d u r i n g the accident so that it stays u n d e r its lower flammability limit. There will be n o gas-explosions, flames or fires in the c o n t a i n m e n t . This m e t h o d offers the safest protection against severe accidents, w i t h o u t aggravating radioactive releases into the a m b i e n t d u r i n g less serious accidents. It does, however, complicate n o r m a l p l a n t oper-
F. Fineschi et al. / Inerting and venting of Mark 11 containments
ation, with an inevitable, though small, reduction in safety, because only after long de-inerting procedures does the containment become accessible. This solution has been adopted by most countries, and experience has proved that this choice is consistent with the plant being run correctly. For particular plants which require more frequent access into the containment, normal operation may be excessively penalized by pre-inerting, even in terms of global safety if the very small likelihood of a severe accident is considered. It might, therefore, be worthwhile inerting the containment at the beginning of the accident. In order to cope with a fast hydrogen production rate, carbon dioxide should be injected, for example, immediately after the emergency core cooling systems have been seen to be working inefficiently. This, however, aggravates the pressure transients even in those accidents where the engineering safeguards are recovered after their initial failure. Unlike pre-inerting, post-inerting must have the gas injection system available right from the beginning of the accident. So far no experience has been had of post-inerting design and operation in situations which can be compared to those of a nuclear plant where a serious accident is developing. Containment venting is nevertheless necessary to prevent containment failure in very severe accidents. The population might receive a dose which is not particularly different, be the containment pre-inerted or post-inerted. Containment inerting does not guarantee in itself the elimination of explosions in a filtered venting system. However, after an accident, there is always the problem of discharging the hydrogen accumulated in an inerted containment into the ambient.
Acknowledgments At the University of Pisa, Antonio Mione and Enrico Penno helped, while doing their theses, to obtain data with the M A R C H 2.0 code, and Professor Salvatore Lanza offered his advice on containment venting.
65
References [1] M. Berman and J.C. Cummings, Hydrogen Behavior in Light Water Reactors, Nucl. Safety 25 1 (1984) 53. [2] A.L. Camp et al., Light Water Reactor Hydrogen Manual, NUREG/CR-2726, Sandia Natl. Lab., Albuquerque (1983). [3] M. Carcassi, F. Carnasciali and F. Fineschi, Sistemi di controllo dell'idrogeno negli impianti nucleari LWR, Tecnica Italiana LII 1 (1987) (in Italian). [4] M. Carcassi and F. Fineschi, I1 controllo dell'idrogeno negli impianti nucleari BWR Mark II, Dipartimento di Costruziani Meccaniche e Nucleari, Universith di Pisa, RL-303/87, (1987) (in Italian). [5] M. Carcassi, F. Carnasciali and F. Fineschi, Flammable gas mixtures and containment integrity during severe accidents in nuclear plants, Proc. I A E A / N E A Int. Symp. on Severe Accidents in Nuclear Power Plants, Sorrento, 21-25 March, 1988, Vol. 2 (IAEA-SM-296/42P, Vienna, 1988) pp. 659-667. [6] R.O. Wooton, P. Cybulskis and S.F. Quayle, MARCH 2.0 (Meltdown Accident Response Characteristics) Code Description and User's Manual, NUREG/CR-3988, Battelle Columbus Lab. (1984). [7] C.D. Heising-Goodman and J. Lepervanche-Valencia, A probabilistic assessment of containment inerting in BWRs, Int. Conf. on Current Nuclear Power Plant Safety Issues, Stockholm, 20-24 October, 1980, IAEA-CN-39/212. [8] B. Lewis and G. Von Elbe, Combustion Flames and Explosions of Gases, 3rd Ed. (Academic Press, New York, 1987). [9] M. Hertzberg, Flammability limits and pressure development in hydrogen-air mixtures, Proc. First Int. Workshop on Impact of Hydrogen on Water Reactor Safety, Albuquerque, 1981, Vol. 3, ed. M. Berman, N U R E G / C R 2017 (Sandia Natl. Lab., Albuquerque, 1981). [10] D.M. Solberg et al., Observations of instabilities in large scale vented gas explosions, 18th Int. Syrup. on Combustion (1981). [11] Rapporto finale di sicurezza della centrale di Caorso, ENEL-CTN (1986) (in Italian). [12] Nuclear power plant response to severe accidents, IDCOR, Technology for Energy Corp., Knoxville Tenessee (1984).
57
I N E R T I N G A N D V E N T I N G O F M A R K II C O N T A I N M E N T S
IN BWR NUCLEAR PLANTS
Fabio F I N E S C H I , Marco C A R C A S S I and Fabrizio C A R N A S C I A L I Dipartimento di Costruzioni Meccaniche e Nucleari, Universitd degli Studi di Pisa, via Diotisalvi 2-56126 Pisa, Italy
Received July 1989
An analysis of hydrogen control systems corroborates containment inerting as the only way of preventing hydrogen explosions which may jeopardize the integrity of BWR Mark II containments during severe accidents. A severe Large Break LOCA and a severe Stuck Open Relief Valve Accident are simulated by the MARCH 2.0 code to compare the advantages and disadvantages of pre-inerting and post-inerting, with or without venting, in BWR Mark II containments.
1. I n t r o d u c t i o n
In a Boiling Water Reactor (BWR) a severe accident generates a sufficient quantity of hydrogen to form flammable mixtures with the air in a Mark II containment [1,21. For this reason the ENEA, the atomic energy authority in Italy, has entrusted the Department of Mechanical and Nuclear Constructions of the University of Pisa with the task of carrying out a preliminary analysis of suitable hydrogen control systems in BWR Mark II containments, be they vented or not. In severe accidents flammable gas mixtures in the containment cannot be prevented with the recombination/dilution systems which are installed for coping with hydrogen dangers in Design Basis Accidents (DBAs) [2,3]. Deliberate ignition must be rejected because a deflagration might compromise the integrity of a BWR Mark II containment even if it occurs as soon as the gas mixture becomes flammable [4,5]. Therefore inerting is the only way of coping with the hydrogen danger in this containment. Attention has thus been focused on a comparison between pre- and post-inerting. In order to compare inerting methods on the basis of how they modify accident sequences and transients, two typical severe accidents, Large Break LOCA, AE, and Stuck Open Relief Valve Accident, TPE, were considered because they give rise to the highest hydrogen release rates in the safety containment, in the drywell and wetwell, respectively. Their transients were calculated by the M A R C H 2.0 code [6] (see ref. [4] for a
detailed description of the code inputs used). For purposes of comparison, this classic code was selected as a standard yardstick of judgement because it is well known and widely used by every expert in safety analysis. For the same purposes, the calculated values were non truncated, even if they indicate a greater degree of accuracy than possible. Containment failure, due to overpressure, is the inevitable consequence of the analyzed accidents, if it is not prevented by containment venting. The reliability and costs of the inerting systems are not the concern of this preliminary analysis, but they would obviously have to be considered in a definitive analysis as well as an optimized system design.
2. Accident
sequences
In both the analyzed accidents it is assumed that: the reactor protection system is available; - the emergency core cooling systems and the spray system in the drywell are unavailable; - the suppression pool cooling system can be utilized with just one pump and the pool water stays subcooled throughout the whole accident; - just one R H R heat exchanger starts, 600 s after the beginning of the accident, and continues operation until the containment fails; - the floor of the cavity underneath the reactor vessel does not have any vent tubes and is dry; when two thirds of its thickness has been attacked by the corium, it fails. -
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © 1990 - Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )
59
F. Fineschi et a L / Inerting and venting of Mark H containments
as to reduce and maintain the concentration of oxygen at such levels so as to prevent flames, however, much hydrogen is produced during an accident. Nitrogen is used due to its low cost and its chemical stability. This method was chosen by many countries for BWR Mark I and II plants after the accident at the TMI-2 station (USA). Italy has only recently taken this line, when there were really no doubts [7] about preinerting increasing plant safety. The atmosphere in the containment is diluted so as to have an oxygen molar fraction which is less than 5% (4% in the USA, 3% in Japan, 1% in Sweden). The main disadvantage of this system is the fact that de-inerting or life-support systems are required for personnel entering the containment. In an emergency situation, access to the containment might be delayed from three to ten hours while the containment is being de-inerted. Furthermore, pre-inerting may increase the number of non-programmed reactor shutdowns because the operator might not be immediately sure about the safety aspects of small core coolant losses. Recombination and dilution systems need to be used for coping with air infiltrations from outside, losses from pneumatic systems, and water radiolysis. Apart from eliminating deflagration overpressures, pre-inerting does not modify the accident transients. The most notable accident events are summarized in table 1. In a Large Break LOCA without pre-inerting the drywell would be inerted by steam throughout the whole transient, but as soon as the mixture in the wetwell reaches the downward flammability limit a deflagration might fail the containment, table 2. The lower downward flammability limit is the minimum hydrogen molar fraction (8%) for which the deflagration is very likely and the combustion is adiabatic and complete [5]. A transient of Small Break LOCA (SE) was also studied using the MARCH 2.0 code with regard to pre-inerting; this too is shown in tables 1 and 2.
o 0.8 0.6 0.4
"---\
0.2 •
0.2
0.4
(b)\ 0.6
0.8
Carbon dioxide molar fraction
Fig. l. Air-H 2-CO 2 flammability limits. Studies were also made of the sensitivity of the code in order to appreciate the effects of delayed interventions by the emergency core cooling systems. Comparisons were also made between hydrogen production as calculated using the MARCH 2.0, and values, available in the literature, evaluated by using other codes, such as MAAP, without any essential differences being noted. The results [4], although interesting in themselves, were not particularly relevant when comparing the various inerting methods.
4. Post-inerting with or without partial pre-inerting Post-inerting consists of diluting the air in the containment with an inert gas at the beginning of the accident. Post-inerting could be preceded, during normal plant operation, by pre-inerting the wetwell alone (partial pre-inerting) and also by reducing the oxygen in the drywell to the breathability limit (conditioning).
Table 2 Non-inerting - adiabatic isochoric complete combustion in the wetwell. Hydrogen molar fraction = 8%; containment failure pressure = 1.07 MPa [2] Accident Deflagration start Pressure before combustion Temperature before combustion Pressure after combustion
(s) (MPa) (K) (MPa)
AE
TPE
SE
2142 0.290 314 1.055
1920 0.117 327 0.401
1950 0.302 319 1.067
60
F. Fineschi et al. / Inerting and venting of Mark I I containments
Since in post-inerting the inert gas is added to the air while in pre-inerting it substitutes the air, the pressure in post-inerting increases more than in pre-inerting, and the greater the quantity of gas injected the greater the increase. The quantity of gas to be injected depends on the inerting characteristics of the gas. Carbon dioxide, CO 2, seems to be the best at the moment, as it has a greater specific heat than nitrogen and a greater chemical stability than Halon [2]. The a i r - H 2 - C O z flammability region (F) is shown in the triangular diagram in fig. 1 [8], where LFL represents the lowest flammability limits, line (s) is the stoichiometric line, line (a) represents the mixtures with the COz/air molar ratio which is necessary for inerting the containment via oxygen deficiency, however, much hydrogen there is, and line (b) represents the mixtures with the C O 2 / H 2 molar ratio which is necessary to prevent, via hydrogen deficiency, the formation of flammable mixtures, however, much air there is. Different amounts of steam and other inert gases are produced during different accidents, but they were neglected in determining the preliminary specifications of the CO 2 injection system in order to provide a reliable post-inerting in any accident scenario. The carbon dioxide injection rate must be able to dilute the hydrogen flow released into the containment, so as to prevent flammable mixtures from being formed, until the oxygen has been diluted enough to make any mixture inert. At that point the CO 2 injection can be stopped. Since the free volume of a BWR Mark II containment is around 8000 m3, 24 500 kg of carbon dioxide is needed to ensure that the atmosphere is inerted. It should be injected (the condition is necessary but not sufficient) before the hydrogen/air ratio in the containment reaches the value which is characteristic of the tangency point of line (a) in fig. 1 with the flammability limit curve and which is virtually the same as the stoichiometric value. But the flammability limits do not depend on the composition of the mixture alone [8,9] and they cannot always be determined exactly. It is therefore wise to inject all the CO 2 before the H z / a i r ratio reaches that of the intersection point of the lines (a) and (b) in fig. 1, i.e. before the released hydrogen goes above 150 kg. In the drywell and the wetwell two types of hydrogen sources can be identified: - a direct source, into the drywell via a pipe break and into the wetwell via a relief/safety valve; an indirect source, from one compartment to another, via the vent tubes and the vacuum breakers. -
A tentative value of the CO 2 injection rate is calculated by assuming that there is a hydrogen release into the drywell which is the same as the direct one which there would be in a AE, and at the same time there is one into the wetwell which is the same as the direct one in a TPE, with the two compartments mutually isolated. The initial quantities of air in the drywell and in the wetwell are 14 800 and 9700 kg respectively. The maximum quantities of hydrogen that could be tolerated in the two compartments, just before the inerting via oxygen deficiency is reached, are therefore 90 and 60 kg, respectively. Figs. 2a (drywell) and 2b (wetwell) show the masses of hydrogen (calculated with the M A R C H 2.0 code) produced in an AE and a TPE respectively, and the quantities of carbon dioxide needed to inert the two compartments via hydrogen deficiency (i.e. such as to maintain the representative point of the a i r - H 2 - C O 2 mixture on the lower part of the flammability curve, LFL in fig. 1), assuming that there are no other inert gases and that the two compartments are mutually isolated. 90 kg of hydrogen is reached after 1260 s in the drywell, and 60 kg after 1560 s in the wetwell. If the injection begins immediately after the event which caused the accident, a constant 11.9 k g / s injection rate of CO 2 into the drywell and 6.1 k g / s into the wetwell are therefore necessary to complete inerting by these times (solid lines in figs. 2a and 2b). These injection rates are also sufficient (if the two compartments are mutually isolated) to maintain an inert atmosphere in the two compartments during the whole injection period, because the hydrogen production rate in the first stage of the accident increases in time: the points which represent the mixture compositions are below the line (b) in fig. 1 until the intersection point (a) (b) is reached. Four per cent hydrogen is the lower upward flammability limit for H2-air mixtures and corresponds to 16 kg of hydrogen in the drywell and 10 kg in the wetwell, which are reached after 420 s and 1260 s, respectively. Such values can be considered to be the upper limits of time available for starting CO 2 injection in the two compartments. If injection is delayed, the flow rates must be increased. Supposing (to provide an example of delayed injection) that, when a 4% H z / ( H 2 + air) molar ratio is reached, enough CO 2 must be in the drywell and wetwell to inert the hydrogen which will be in the compartment a minute later, in the dryweil, the injection should be started after 360 s from the beginning of the accident and the injection rate should be 19.4 k g / s until 900 s (dotted -
61
F. Fineschi et al. / Inerting and venting of Mark H containments
2°°/
20000
15°l
200
. / _ _ _ _ _ . 15000"~
/oo /
lO0 0
ioooo~
.-- I0000
.
15o
7500?
100
* o 5000 o
Z 0
50
~/
5000
50
25oo
o
/o¢. oH
1'0
0
20
10
TIME (min) a)
Large
Break
LOCA
~,
,
0
30
20
30
TIME (min) - Drywell
b)
Stuck
Open
Accident
Relief
Valve
- Wetwell
LEGEND: o
H 2 in the mutually isolated contalnment compartments.
+
Minimum CO 2 to inert compartment atmosphere via 0 9 deficiency
-- • - -
Minimum CO 2 to inert the
compartment via 0 2 deficiency.
Immediate CO 2 in3ection Delayed CO 2 injection.
Fig. 2. Post-inerting:tentativccalculation of CO2i~ectionrates. line in fig. 2a) and, afterwards, 11.9 k g / s until inerting has been completed; in the wetwell, the injection should be started at 1176 s with a constant flow rate of 24.3 k g / s (dotted line in fig. 2b). Post-inerting can be preceded by pre-inerting with nitrogen in the wetwell, which is the compartment where access during normal operation is less important and where in an accident the inerting action of steam cannot be counted on. In this case the post-inerting of the drywell can be done with the above-mentioned procedures, while there is no need to inject C O 2 in the wetwell. The quantity of gas to be injected is reduced (14 800 kg instead of 24 500 kg) as well as containment -
pressure, without over-penalizing the normal plant operation. Using the M A R C H 2.0 code and the tentative C O 2 injection rates, A E and T P E accidents have been compared [4] where there is: no inerting, total pre-inerting, - total post-inerting with immediate injection, - total post-inerting with delayed injection, post-inerting with pre-inerting in the wetwell and immediate injection, post-inerting with pre-inerting in the wetwell and delayed injection. Since the tentative injection rates have been calcu-
-
-
-
Table 3 Post-inerting - containment failure Accident
Large break LOCA
Wetwell inerting Gas injection delay Cont. failure time (s)
no no 4100
no yes 4077
Stuck open relief valve yes no 7962
yes yes 8178
no no 14 297
no yes 13 932
yes no 15 495
yes yes 14 718
62
F. Fineschi et aL / Inerting and venting of Mark 11 containments
lated on the assumption of a hypothetical large hydrogen release, but without taking into account the redistribution of incondensable gases between the two compartments, in A E with total post-inerting the wetwell is non-inerted for around 600 s immediately after the end of the CO 2 injection. It is a short period of time, and furthermore it is calculated with a code which simulates a very complex situation in an approximate manner, but it underlines the fact that the gas transfer between the compartments needs to be considered in a more accurate design of the injection system. If the wetwell has been pre-inerted, this problem does not arise and therefore the safety analysis could be significantly simplified and could also offer more certain conclusions. Table 3 outlines the failure times of a post-inerted containment: - In a TPE the failure time is brought forward for 1637 s or 2835 s, depending on whether or not the wetwell has been pre-inerted, compared to the 17132 s in which the containment would break with total pre-inerting (table 1). - In an A E the reduction in containment failure time is more accentuated, 3806 s or 7668 s, depending on whether or not the wetwell has been pre-inerted, compared to the 11 768 s in which the containment would break with total pre-inerting (table 1). The delay with respect to the 2142 s in which the non-inerted containment could break, if there were a deflagration of a hydrogen mixture (table 2), is 5820 s and 1958 s respectively. If the wetwell is not pre-inerted the containment and the vessel failure coincide. In any case, the delay in CO 2 injection has little influence on containment failure times.
5. Venting
of inerted
containments
In order to avoid containment failure in severe accidents, containment venting can be carried out when the pressure goes above the design value, so as to reduce the release of radioactive substances into the ambient which would be verified in a catastrophic containment failure. Venting carried out after the containment design pressure has been reached cannot prevent flammable mixtures and deflagrations during severe accidents, table 4. If venting were carried out before hydrogen and fission products are released from the reactor vessel and the pressure reaches the design pressure (early venting), the resulting decrease in air might cause, or at least contribute to, containment inerting. However, early
Table 4 Non inerting-gas mixture compositions at the containment design pressure (0.45 MPa) [11] Accident
AE
TPE
Design pressure reaching time (s) Temperature drywell (K) wetwell (K) Hydrogen molar fraction drywell (%) wetwell (%) Air molar fraction drywell (%) wetwell (%) Steam molar fraction drywell (%) wetwell (%)
2340
10500
570 370
490 420
20.0 28.0
26.5 64.0
79.0 69.5
54.5 29.0
19.0 2.5
1.0 7.0
venting currently seems too problematic and therefore inadvisable, seeing as its effectiveness depends on the accident development, which might still be unpredictable when it is being initiated. It might cause an unnecessary release of radioactivity if the accident turns out not to be severe or if venting were prolonged when radioactive substances in the containment are no longer negligible. Venting can hardly cope with an explosion and thus replace containment inerting, because the energy release might be so great and rapid in a deflagration that it might not be sufficiently balanced by a limited opening of the containment [8]. Furthermore, as venting causes turbulent flows in the atmosphere of the containment, it might aid flame instability phenomena [10] which could cause detonations and dynamic loads on the structures and safety-related equipment in the containment. Its function should thus be limited to controlling the relatively slow pressurizing of an inerted containment and may be particularly important in post-inerting. A mixture, which was inert in the containment due to oxygen deficiency, when released from the containment may come into contact with the air and become flammable. In order to avoid explosions in the venting system it is necessary: - either to avoid any contact with the air, - or to avoid the mixture being ignited, - or to dilute the mixture coming from the containment with C O 2, for example, so as to obtain a C O 2 / H 2 ratio in the venting flow which is at least equal to that of line (b) in fig. 1. The analysis of venting is carried out [4] by comparing the A E and TPE transients in a pre-inerted contain-
63
F. Fineschi et al. / lnertmg and venting of Mark H containments
ment with those in a post-inerted containment where oxygen is minimized by reducing the initial oxygen molar fraction to 1% in the pre-inerted wetwell and to 15% in the conditioned drywell. Partial pre-inerting and conditioning allow a safe normal operation of the plant and a reduced post-inerting pressurization. In fact, in this case there are 197 kmol of incondensables in the containment during the normal plant operation, of which 141 kmol are of air and 56 kmol of nitrogen. Without taking the 56 kmol of nitrogen into account, conservatively 230 kmol of CO 2 (10 100 kg) is needed to inert the atmosphere, however, much hydrogen there is. The injection rate into the drywell, if activated right at the beginning of the accident and terminated after 1260 s, is reduced to 8 kg/s. The venting start-up pressure has been chosen in such a way so as not to cause an opening if a DBA occurs, and to maintain an adequate safety margin with respect to the containment failure pressure (1.07 MPa [2]). If there is a DBA-LOCA in a pre-inerted containment, the maximum pressure and temperature in the drywell are 0.41 MPa and 413 K, respectively [11]. The design pressure is 0.45 MPa [11]. To estimate the pressure value which might be reached in a DBA-LOCA in a post-inerted containment, the CO 2 partial pressure at 413 K (0.096 MPa) has been added to the pressure of 0.41 MPa. The start-up pressure should therefore range between 0.5 and 1.07 MPa, so as to prevent radioactive releases into the atmosphere in accidents which are slightly more serious than a DBA. Venting can be. carried out either via a "blowout disk" which leaves the containment open, or via an o n / o f f valve which allows the integrity of the containment to be re-established if the engineering safeguards are recovered in time to reduce the containment pressure.
An o n / o f f valve is analyzed, which opens a 0.017 m2 orifice at 0.76 MPa and closes at 0.66 MPa. The containment pressure and the release rate will obviously oscillate during the intermittent venting. Venting should be done in the wetwell for the following reasons: - the suppression pool can decontaminate gaseous effluents; as long as the integrity of the separation slab between the drywell and the wetwell is maintained, venting in the wetwell increases the steam condensation in the pool and thus the efficiency of the pressure suppression system; - after the initial LOCA blowdown, most of the incondensables are in the wetwell; - in the atmosphere of the wetwell there is a lower quantity of steam and therefore filtering the released gases is easier. Before the reactor cavity floor breaks, i.e. before the pressure suppression pool loses its functionality, the released mixture consists substantially of incondensable gases. When the melted core comes into contact with the water in the pool, a great deal of steam is produced, which keeps the venting open for the longest time during the transient. From then on most of the released gas is steam, both because most of the incondensables have already been released, and because the production rate of carbon monoxide and dioxide by the coriumconcrete reaction is decreasing. Post-inerting brings forward the time of the first venting for 1440 s in a TPE and for 4200 s in a AE. In the latter case venting start precedes vessel failure. Table 5 shows the values which characterise the venting function, the maximum release rate of incondensables and the total mass of incondensables released until the cavity floor is broken. Table 6 shows the
Table 5 Venting characteristics and incondensable release Accident
Pre-inerting AE
Venting start (s) Venting time fraction in the on position Maximum incondensable release rate (kg/s) Total incondensable release until cavity floor failure (kg) at
(s)
Post-inerting TPE
7500 0.30 13.1 14 500 14280
AE
14 340 0.19 14.5 13 200 1 8 840
TPE
3300 0.28 14.5 20 500 14280
12 900 0.20 14.7 20 800 18 840
64
F. Fineschi et al. / Inerting and venting of Mark 11 containments
Table 6 Venting - average release rates Release rate (kg/s)
Time (s)
Incondensables
Steam
Total
Large-break LOCA - pre-inerting 0 7500 7500 15 300 15 300 18000 18000 36000
0 1.73 10.50 0.96
0 0.04 2.73 1.65
0 1.77 13.23 2.61
Large break LOCA - post-inerting 0 3300 3300 7650 7650 15 660 15 660 18 360 18 360 36 000
0 1.03 2.37 10.18 1.02
0 0 0.04 3.15 1.77
0 1.03 2.41 13.33 2.79
Stuck open relief valve accident - pre-inerting 0 14340 14 340 21060 21060 21 960 21 960 36000
0 2.44 12.83 1.94
0 0.10 0.32 0.09
0 2.54 13.15 2.03
Stuck open relief valve accident - post-inerting 0 12900 12900 20700 20 700 21600 21 600 28 800 28 800 36 000
0 3.13 13.33 2.22 0.38
0 0.11 0.42 0.09 0.02
0 3.24 13.75 2.31 0.40
From
To
average release rates for consecutive time intervals. In any case, the first opening of the venting h a p p e n s before the separation slab breakage a n d the consequent loss of the d e c o n t a m i n a t i o n effect of the suppression pool, which occur at the same time b o t h for pre-inerting a n d for post-inerting, table 5. As long as pool effectiveness is maintained, the q u a n t i t y of i n c o n d e n s a b l e s released in post-inerting is greater t h a n in pre-inerting, but the release of radioactive substances to the a m b i e n t is limited by the pool d e c o n t a m i n a t i o n factors (from 600 to 10 000, d e p e n d i n g o n the physical a n d chemical characteristics of the radioactive substances [12]). F u r t h e r m o r e , during the time in which the valve has already been opened in post-inerting, while it is still closed in pre-inerting, venting has reduced the radioactivity of the containm e n t atmosphere. After cavity floor slab failure, the masses of released incondensables, b o t h in pre-inerting a n d post-inerting, are almost the same, table 6. Therefore the total radioactivity released into the a m b i e n t in post-inerting might n o t be m u c h greater t h a n in pre-inerting, a l t h o u g h gases are released for a
longer time: the difference might not b e significant a n d would anyway decrease in time. The release rate values are n o t particularly different in pre-inerting a n d post-inerting, table 6, so the same gas filtering system could b e utilized.
6. Conclusions In order to prevent the integrity of a B W R M a r k II c o n t a i n m e n t b e i n g lost due to h y d r o g e n explosions in severe accidents with partial or total core melting, cont a i n m e n t inerting must inevitably b e utilized. The most drastic solution is to replace the air with nitrogen during n o r m a l p l a n t operation. T h e concentration of oxygen needs to b e controlled d u r i n g the accident so that it stays u n d e r its lower flammability limit. There will be n o gas-explosions, flames or fires in the c o n t a i n m e n t . This m e t h o d offers the safest protection against severe accidents, w i t h o u t aggravating radioactive releases into the a m b i e n t d u r i n g less serious accidents. It does, however, complicate n o r m a l p l a n t oper-
F. Fineschi et al. / Inerting and venting of Mark 11 containments
ation, with an inevitable, though small, reduction in safety, because only after long de-inerting procedures does the containment become accessible. This solution has been adopted by most countries, and experience has proved that this choice is consistent with the plant being run correctly. For particular plants which require more frequent access into the containment, normal operation may be excessively penalized by pre-inerting, even in terms of global safety if the very small likelihood of a severe accident is considered. It might, therefore, be worthwhile inerting the containment at the beginning of the accident. In order to cope with a fast hydrogen production rate, carbon dioxide should be injected, for example, immediately after the emergency core cooling systems have been seen to be working inefficiently. This, however, aggravates the pressure transients even in those accidents where the engineering safeguards are recovered after their initial failure. Unlike pre-inerting, post-inerting must have the gas injection system available right from the beginning of the accident. So far no experience has been had of post-inerting design and operation in situations which can be compared to those of a nuclear plant where a serious accident is developing. Containment venting is nevertheless necessary to prevent containment failure in very severe accidents. The population might receive a dose which is not particularly different, be the containment pre-inerted or post-inerted. Containment inerting does not guarantee in itself the elimination of explosions in a filtered venting system. However, after an accident, there is always the problem of discharging the hydrogen accumulated in an inerted containment into the ambient.
Acknowledgments At the University of Pisa, Antonio Mione and Enrico Penno helped, while doing their theses, to obtain data with the M A R C H 2.0 code, and Professor Salvatore Lanza offered his advice on containment venting.
65
References [1] M. Berman and J.C. Cummings, Hydrogen Behavior in Light Water Reactors, Nucl. Safety 25 1 (1984) 53. [2] A.L. Camp et al., Light Water Reactor Hydrogen Manual, NUREG/CR-2726, Sandia Natl. Lab., Albuquerque (1983). [3] M. Carcassi, F. Carnasciali and F. Fineschi, Sistemi di controllo dell'idrogeno negli impianti nucleari LWR, Tecnica Italiana LII 1 (1987) (in Italian). [4] M. Carcassi and F. Fineschi, I1 controllo dell'idrogeno negli impianti nucleari BWR Mark II, Dipartimento di Costruziani Meccaniche e Nucleari, Universith di Pisa, RL-303/87, (1987) (in Italian). [5] M. Carcassi, F. Carnasciali and F. Fineschi, Flammable gas mixtures and containment integrity during severe accidents in nuclear plants, Proc. I A E A / N E A Int. Symp. on Severe Accidents in Nuclear Power Plants, Sorrento, 21-25 March, 1988, Vol. 2 (IAEA-SM-296/42P, Vienna, 1988) pp. 659-667. [6] R.O. Wooton, P. Cybulskis and S.F. Quayle, MARCH 2.0 (Meltdown Accident Response Characteristics) Code Description and User's Manual, NUREG/CR-3988, Battelle Columbus Lab. (1984). [7] C.D. Heising-Goodman and J. Lepervanche-Valencia, A probabilistic assessment of containment inerting in BWRs, Int. Conf. on Current Nuclear Power Plant Safety Issues, Stockholm, 20-24 October, 1980, IAEA-CN-39/212. [8] B. Lewis and G. Von Elbe, Combustion Flames and Explosions of Gases, 3rd Ed. (Academic Press, New York, 1987). [9] M. Hertzberg, Flammability limits and pressure development in hydrogen-air mixtures, Proc. First Int. Workshop on Impact of Hydrogen on Water Reactor Safety, Albuquerque, 1981, Vol. 3, ed. M. Berman, N U R E G / C R 2017 (Sandia Natl. Lab., Albuquerque, 1981). [10] D.M. Solberg et al., Observations of instabilities in large scale vented gas explosions, 18th Int. Syrup. on Combustion (1981). [11] Rapporto finale di sicurezza della centrale di Caorso, ENEL-CTN (1986) (in Italian). [12] Nuclear power plant response to severe accidents, IDCOR, Technology for Energy Corp., Knoxville Tenessee (1984).