atw 2018-02

inforum

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

calculations. Moreover, the results of

the two severe accident scenarios

(MBL and ND*) for the base cases

with increased containment leakages

are compared regarding their effect

on the accident consequences.

2.1 Base case with containment

design leakage

The hydrogen concentration in the RB

annulus is presented in Figure 2 (left

side). In the base case no formation of

combustible gas mixtures (> 4 vol.-%

hydrogen) in the RB annulus is

observed during the calculated time

period, and some fire protection

doors and flaps between the separated

rooms of the annulus close automatically

when the atmosphere

temperature reaches 70 °C limiting

the hydrogen and radionuclide inflow

into these areas (Figure 2 right side).

Due to the operation of the RB annulus

exhaust air system, the hydrogen

concentration remains below 1 vol.-%

and decreases further in the long term

when the containment filtered venting

starts reducing the hydrogen leakage

from the containment. Gas stratification

with slightly different gas

concentrations at different elevations

is formed in the annulus gap due to

the operation of the annulus exhaust

air system.

| | Fig. 2.

H 2 concentration in the RB annulus for base case scenario (MBL) with operation of RB annulus exhaust air system;

RB annular gap (left) and RB annulus rooms (right).

| | Fig. 3.

H 2 concentration in the RB annulus for base case (left) and variant case (right) with a 10 times larger containment leakage,

both cases with operation of RB annulus exhaust air system.

ENVIRONMENT AND SAFETY 87

2.2 Variant calculation with a

10 times larger containment

leakage

As already mentioned, one of the

goals is to investigate the conditions in

the RB annulus in case of increased

containment leakages. For this purpose,

a COCOSYS variant calculation

was performed assuming a 10 times

larger containment leakage. The RB

annulus exhaust air system was

assumed to be in operation as in the

base case. It sucks steam-air mixture

from one selected location of the RB

annulus at about 12 m level. Figure 3

compares the hydrogen concentration

and Figure 4 the aerosol concentration

in the base case and the variant

calculation. The overall behaviour in

the RB annulus is the same, but the

variant with 10 times larger containment

leakage leads to the formation of

combustible gas mixtures (> 4 vol.-%

hydrogen) in the upper annulus area

and a higher aerosol concentration

especially in the early accident phase

with large releases from the reactor

circuit during core melting. The

results show that the RB annulus

exhaust air system is not efficient

enough to keep the H 2 concentration

below the lower combustible limit of

| | Fig. 4.

Aerosol concentration in the RB annulus for base case (left) and variant case (right) with a 10 times larger containment leakage,

both cases with operation of RB annulus exhaust air system.

| | Fig. 5.

Comparison of pressure in the containment (left) and MCCI gas generation (right) for the cases with and without melt relocation.

4 vol.-% H2 in all RB annulus areas.

The following three gas concentration

zones are established (Figure 3 right):

• RB annulus above 16 m with

hydrogen concentrations up to

~ 5 vol.-%.

• RB annulus at ~12 m (leak location)

with low hydrogen concentrations

up to ~ 2 vol.-%.

• RB annulus at ~ 6 m and below

with very low hydrogen concentrations

< 0.1 vol.-%.

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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