atw 2018-02

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

ENVIRONMENT AND SAFETY 86

second and third area comprises the

operating containment compartments

and the containment dome. The

fourth area includes all the compartments

outside the missile protection

cylinder, the periphery of the containment.

The volume of the RB annulus is

subdivided into four areas with a total

volume of 50,000 m 3 . The first area is

the annular gap, located above elevation

21.5 m, which has a total volume

of 14,900 m 3 . This area, in turn, is

divided into six axial levels along the

height of the gap. It is connected to

the lower part of the annular gap

( second area) below elevation 21.5 m

and has a free volume of 4,300 m 3 . In

this area vertical fire protection walls

with metal sheets are located, which

do not allow atmospheric flow in

azimuthal direction. The third area

comprises several separate annulus

rooms located on building floors at

elevation 6 m to 21.5 m. The annulus

rooms at elevation 6 m to 9 m are

separated from the annular gap by

ventilation systems. The connections

between these separate rooms are

provided with fire protection doors

and fire protection flaps, which automatically

close, if the room temperature

exceed ~70° C. The fourth area

represents all annulus rooms below

elevation 6 m with a total volume of

23,100 m 3 . Those rooms have only a

negligible atmosphere exchange with

the annular gap above.

Moreover, the model consists of

all relevant plant systems used during

accidents (e.g. the RB annulus exhaust

air system) or operational systems

foreseen as SAM measures in the

SAMG handbook (e.g. the annulus

air supply/suction system and the

annulus air recirculation systems).

The filtered containment venting

system and the hydrogen recombination

system with about 65 PARs

installed inside the containment are

introduced in the input deck as well,

using the modelling capabilities of the

engineered safety features, integrated

in the COCOSYS code.

The COCOSYS model also includes

the containment design leakage of

0.25 vol.-%/d into the RB annulus.

For the base case analyses the design

leakage is assumed to be at the most

unfavorable place in the area of the

cable penetrations at elevation 12 m

(Figure 1 right side), e.g. the leakage

is located opposite to the single

suction point of the RB annulus

exhaust air system, operated in case

of an accident. In addition, leakages

are defined from the environment

through the auxiliary building main

gate into the lower annulus rooms

(leakages represented by red arrows

in Figure 1).

1.2 Selected representative

Severe Accident Scenarios

Two representative and different

severe accident scenarios – the base

cases – have been selected for the

analyses. Some characteristics of the

scenarios are summarized here, the

timing of main events is provided in

Table 1:

• MBL – a medium break LOCA with

a failure of the emergency core

cooling system after the emergency

water supply tank inventory is

empty; core degradation starts

delayed; sequence results in a

maximum water inventory in the

containment sump and a late

filtered containment venting.

• ND* – a transient with a failure of

steam generator feedwater supply;

failure of injection of active

emergency core cooling systems;

primary circuit depressurization

procedure to avoid reactor pressure

vessel failure at high-pressure;

core degradation starts early;

sequence results in a minimum

water inventory in the containment

sump and an earlier containment

venting.

The two representative base cases

were already used in earlier analyses

[2], [3] with respect to the reassessment

of other mitigative SAM measures.

In both cases, no melt relocation

from the reactor cavity into the containment

sump after melt penetration

of the biological shield was assumed,

just water ingress into the cavity and

therefore extended steam production.

As melt relocation into the sump with

cooling of the relocated melt amount

seems to be a realistic scenario leading

to reduced production of combustible

gases, two additional variant calculations

were done with melt relocation

into the containment sump. Furthermore,

a series of COCOSYS variant

calculations were carried out in order

to investigate the influence of the

following specific aspects:

• Operation/failure of the RB annulus

exhaust air system installed for

accident conditions.

• Variation of the size of containment

leakages into the reactor

building annulus: design leakage

(base case) and a 10 times larger

leakage.

• Variation of the containment

leakage location in the area of

containment cable penetrations.

Moreover, the efficiency of different

SAM measures for mitigation of the

consequences in the RB annulus,

documented in the SAMG handbook

of the reference plant, was analysed.

These measures are as follows:

• Use of RB annulus air supply/

suction system – provision of a

controlled ventilation to reduce

the hydrogen concentration in the

annulus.

• Use of RB annulus air recirculation

system – mixing of the annulus

atmosphere and elimination of gas

stratification.

• Use of emergency air filtration

system – extraction of air from the

RB annulus through a filtration

system to reduce the release of

radionuclides into the environment.

The following SAM measure was

additionally investigated as a possible

alternative method for hydrogen

reduction in the annulus. It is related

to a optional recommendation of the

RSK [1].

• Implementation of a small number

of PARs in the RB annulus upper

part to prevent combustible gas

mixtures.

2 Results – Quantification

of the effectiveness of

selected AM measures

Selected results are presented in the

following only for one base case

scenario (MBL) with the operation of

RB annulus exhaust air system used in

case of accidents and for some variant

Scenario

Start of steam/water

leak flow into

containment

Start of

core melting

RPV failure and

melt release

into cavity

Water ingression into

cavity and possible

melt release into sump

Start of filtered

containmentventing

ND* 1.4 hr 3.5 hr 6.5 hr 17.1 hr 66.5 hr

MBL 0.0 hr 5.8 hr 8.9 hr 13.5 hr 82.2 hr

| | Tab. 1.

Timing of characteristic events of severe accident progression of base case scenarios.

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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