atw 2018-02

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atw Vol. 63 (2018) | Issue 2 ı February

ENVIRONMENT AND SAFETY 88

2.3 Variant calculation with a

10 times larger containment

leakage and consideration

of a potential melt relocation

into containment sump

As already noted, all previous analyses

conducted by GRS have been performed

assuming no melt relocation

from the reactor cavity into the containment

sump after melt penetration

of the biological shield. Since the melt

is very likely to melt-through the biological

shield, a variant calculation

with a 10 times larger containment

leakage and a failure of the RB annulus

exhaust air system was performed

assuming melt relocation into the

containment sump.

After penetration of the biological

shield, the corium spreads into the

containment sump and comes into

contact with the sump water. This

results in a higher steam generation,

which in turn leads to a faster longterm

containment pressurization

compared to the case without melt

relocation (Figure 5). Because of the

higher steam production, the filtered

containment venting starts significantly

earlier than in the case without

melt relocation.

Shortly after the melt relocation

into the sump, the corium solidifies

within a very short time period and

the generation of combustible gases

(H 2 and CO) is terminated. Due to the

overall lower gas production, the H 2

concentrations in the containment,

and thus also in the RB annulus, are

significantly lower compared to those

| | Fig. 6.

Comparison of H 2 concentration in the containment (left) and H 2 concentration in the RB annulus (right) for the cases with and

without melt relocation.

| | Fig. 7.

Comparison of containment pressure (left) and H2 mass generated during MCCI (right) for the MBL and ND* base cases.

in the calculations without melt

relocation (Figure 6) and the lower

combustible limit is no longer reached

in the RB annulus.

2.4 Effect of the selected severe

accident scenarios on the

accident consequences

In order to investigate the effect on

the accident consequences, the results

of the two analyzed severe accident

scenarios (MBL and ND*) have been

compared for the base cases with

increased containment leakages.

A comparison of the containment

pressure response calculated for

the two base cases with increased

containment leakages is shown in

Figure 7 (left). The comparison

demonstrate that in the ND* base

case, the filtered containment venting

starts about 16 hours earlier than in

the MBL base case. Figure 7 (right)

depicts a comparison of the hydrogen

mass generated during the MCCI for

the two accident scenarios. Because of

the earlier venting in the ND* base

case, less hydrogen is generated until

the start of containment depressurization.

This is due to the fact that in the

ND* base case the MCCI duration is

shorter than that in the MBL base

case. Hence, for the ND* case, a total

amount of hydrogen of about 3,700 kg

is generated, while for the MBL case,

the total hydrogen mass, generated

until the start time of filtered containment

venting, is about 4,000 kg. The

hydrogen concentrations in the RB

annulus calculated for the two base

case scenarios are compared in

Figure 8. Due to the earlier start of

containment venting in the ND* base

case the maximum hydrogen concentration

in the RB annulus is lower than

that in the MBL base case. From the

comparison it is evident that the

hydrogen lower combustible limit of

4 vol.% is not exceeded until the

beginning of the containment depressurization.

| | Fig. 8.

Comparison of H 2 concentration in the RB annulus ring (left) and H 2 concentration in the RB annulus rooms (right) for the MBL and

ND* base cases.

2.5 Variant calculations with a

10 times larger containment

leakage and AM measures

As part of the assessment of potential

mitigative AM measures the efficiency

of the RB annulus air supply/suction

system to reduce the hydrogen concentration

in the RB annulus was

investigated. For this purpose, a

variant calculation with a 10 times

larger design leakage and a failure of

the RB annulus exhaust air system

was carried out (Figure 9 left) and

another one assuming that the RB air

supply/exhaust systems are put into

Environment and Safety

Investigation of Conditions Inside the Reactor Building Annulus of a PWR Plant of KONVOI Type in Case of Severe Accidents with Increased Containment Leakages ı Ivan Bakalov and Martin Sonnenkalb

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