atw 2018-02

inforum

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

operation as AM measure at approx.

50 h after the accident onset (Figure 9

right). The results show a significantly

increased hydrogen concentration in

the RB annulus in case of a failure of

the RB annulus exhaust air system

(Figure 9 left).

Further, in this case the use of

the RB annulus air supply/exhaust

systems is efficient to reduce the

hydrogen concentration and prevent

the formation of combustible gas

mixtures in the annulus rooms. With

the operation of the system the hydrogen

is removed from the annulus

quickly and the hydrogen concentration

remains below 1 vol.-% for the

long term. In that case, the use of the

emergency air filtration system of the

plant is needed in addition to limit

the radionuclide releases into the

environment.

In addition, a possible alternative

method for hydrogen reduction in the

annulus was investigated assuming

the installation of a small number of

medium size PARs in the upper RB

annulus (Figure 10 right). The results

are compared with a variant calculation

with a 10 times larger design

leakage and failure of the RB annulus

exhaust air system (Figure 10 left).

The results show that already the

implementation of PARs of medium

size can significantly reduce the

hydrogen concentration in the RB

annulus and keep it well below

lower combustible limits. The hydrogen

depletion starts at approx. 40 h

(150,000 s) after the accident onset

if the concentration exceeds about

1 to 2 vol.-%. Thus, an AM concept

with the installation of some PARs in

the annulus is considered a very

efficient mitigation measure for preventing

formation of combustible gas

mixtures in the RB annulus not just in

the case presented.

3 Conclusions

The behaviour of hydrogen as well as

aerosol and noble gases released into

the reactor building annulus of a

German PWR KONVOI reference plant

resulting from increased containment

leakages under severe accident conditions

was investigated using the

GRS code COCOSYS. Two representative

and different severe accident

scenarios – the base cases – have been

selected for the analyses.

The calculation results show no

formation of combustible gas mixtures

in the RB annulus during the observation

period for the base case with

containment design leakage and

operation of RB annulus exhaust

| | Fig. 9.

H 2 concentration in the RB annulus for variant cases with 10 times larger leakages and failure of RB annulus exhaust air system (left)

and with AM measure “operation of RB annulus air supply/exhaust systems” (right).

| | Fig. 10.

H 2 concentration in the RB annulus for variant cases with 10 times larger leakages and failure of RB annulus exhaust air system (left)

and with AM measure “PARs in the RB annulus” (right).

air system. It was identified that in

this case separate annulus rooms are

isolated at an early stage by the automatic

closing of fire protection doors,

thus preventing a further increase in

the hydrogen concentration in these

rooms.

In contrast, the variant calculation

with a 10 times larger containment

design leakage leads to formation of

combustible mixtures in the upper RB

annulus area. In this case, the RB

annulus exhaust air system is not

efficient enough to prevent formation

of combustible gas mixtures in the

upper RB annulus area.

Further, the variant calculation

assuming melt relocation into the

containment sump demonstrated that

the corium spreading into the sump

results in a higher steam generation,

which leads to a faster long-term

containment pressurization. After the

melt relocation into the sump, the

corium solidifies within a short time

and the generation of combustible

gases (H 2 and CO) coming from

MCCI is terminated. As a result, the

H 2 concentrations in the containment

as well as in the RB annulus are

significantly lower compared to those

in the case without melt relocation. In

this case, the lower combustible limit

of 4 vol.% in the RB annulus is no

longer reached.

Moreover, the results of the two

analyzed severe accident scenarios

(MBL and ND*) were compared in

order to investigate their effect on

the accident consequences. From the

comparison it was identified that in

the ND* base case, the filtered

containment venting starts about

16 hours earlier than in the MBL base

case. As a result, the maximum hydrogen

concentration in the RB annulus,

calculated for the ND* base case, is

lower than that in the MBL base case.

The comparison showed that in the

ND* base case the hydrogen concentration

does not exceed the lower

combustible limit of 4 vol.% until the

beginning of the containment depressurization.

Within the scope of the project, the

efficiency of different AM measures

for mitigation of accident consequences

in the reactor building annulus

was analyzed. The assessment

results show that the operation of RB

annulus air supply/suction system

significantly reduces the hydrogen

concentration and prevents formation

of combustible gas mixtures in RB

annulus. Therefore, the use of these

ventilation systems is considered as a

very promising accident management

measure for reducing the hydrogen

concentration in the reactor building

annulus. However, in that case the

ENVIRONMENT AND SAFETY 89

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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