atw 2018-05v6

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atw Vol. 63 (2018) | Issue 5 ı May

coolability. As the activities have to

produce a reasonable contribution to

the safety enhancement, the following

conditions were defined

• Effectiveness, it means a quantified

benefit to safety (prevention of

early FP releases and minimize FP

releases) plus confirmed physical

fruitfulness with sufficient margin

• Reasonable technical feasibility

• No negative impact to reactor operation

(including all procedures/

activities during outages)

• Simplicity – applicability under

SA condition (limited personal

capacity, limited accessibility …)

• At least partial independence of

functionality assurance in comparison

with existing emergency

systems

• Consistency of approaches with

other utilities operating VVER-1000

(or reactors of similar power) and

VVER-1000 designers

Taking into account above defined

conditions the activities were defined

for following six main topics which

have very well defined structure and

relations each to other

• Primary circuit depressurization

under SA conditions

• Corium cooling with water injection

into RPV (new acronym IVR-IN

is used)

• In-vessel retention with external

reactor vessel cooling strategy

(originally used IVR, but newly

IVR-EX acronym is used)

• Ex-vessel corium cooling strategy

(ExVC)

• Containment response to severe

accident and long term issues

• Severe accident initiated in the

spent fuel pool

Short description of the effort done up

to know is included in following

sub-chapters excluding the last topic,

of which activities are not yet initiated,

but some analyses were already

performed in past.

3.4.1 Primary circuit depressurization

under SA conditions

The activities related to this topic were

mostly focused on the conditions for

the IVR-EX strategy as the primary

pressure is strogly determining

success of this strategy. The analytical

investigation using the MELCOR 1.8.6

code was performed to identify if the

depressurization initiated after the

entry to SAMGs is sufficiently fast to

meet predefined criterion – to reach

the pressure reduction below 2 MPa-g

till the time of starting relocation of

corium into the lower plenum of RPV.

The analytical studies, for variant

opening of one or two safety valves or

additionally the system for a removal

of gases from the pressurizer, confirmed

that the safety valves if can be

kept open are sufficient, but the contribution

of the gas removal system is

negligible. The important condition is

that the valves must be kept open, the

modification of a control of safety

valves is in the final phase of its

preparation with an expectation to

initiate an appropriate technical

negotiation with the SONS.

3.4.2 Corium cooling with

injection into RPV

(IVR-IN strategy)

The extensive analytical investigation

was performed with the MELCOR

1.8.6 code to analyze possibility of a

termination of the core degradation

due to water injection into the RPV.

The analyses varied with the assumed

mass rate of the water injected (to

alternate among the repaired LPI and

HPI, and the new alternative system

called TB50, which has the lowest

mass rate of water injection and also

relatively lower maximum pressure

head). The second variations were in

timing of the water injection activation,

it means different configurations

of core degradation at the time of

water injection initiation. The last

variation was in the initiating event as

the most of analyses were performed

for the large break LOCA initiated

envent (plus loss of all active systems)

and as an alternative the scenario

initiated with postulated SBO was

chosen with the early depressurization

after the entry to SAMGs and also

loss of all active systems.

The conclusion from the analytical

work is that the results are of course

uncertain, but they show that the

initiation of water supply before starting

of the corium relocation into the

lower plenum should prevent the loss

of RPV integrity. More detailed, the

analysis predicted un-failed RPV wall

if the lower head is not dried out, but

such answer seems to be too optimistic

due to some modeling assumptions in

the MELCOR code which favoured

cooldown with even very small

remaining mass of water in the lower

head.

3.4.3 Strategy IVR-EX

The greatest effort was done for this

topic during last two years and the

activity will continue with the experimental

program on the RPV coolability.

The first issue related to the IVR-EX

strategy is the flooding of reactor

cavity and long term water supply

to the cavity. The design of the

VVER-1000/320 containment is absolutely

un-favorable for this IVR-Ex

strategy, because the recirculation

sump is located one floor below the

cavity and the water from the containment

is drained to this sump. So it

is absolutely impossible to close the

water circulation inside of the containment,

like it is in case of the

AP1000 or VVER-440/213 reactors.

The water for flooding of the cavity

must be injected from sources outside

of the containment. The feasibility

study was performed to investigate all

necessary modifications to be done

for a possibility to cool the RPV

from outside. The preliminary project

solution was prepared for new systems

on fast cavity flooding (based on

pressurized tanks located on roof of

the auxiliary building) with the

second part for feeding water from the

second pair of pressurized tanks. This

solution enabled to cover the first

6 hours by this passive system and

thus to have a way sufficient time for

an activation of the new active system

which is capable to feed water from

own tanks till 72 hours of the accident.

The injection of the water is

not the only condition, but the second

one is the protection of water releases

from the cavity. The injection of

water is assumed via. the venting

system channels and those must be

equipped with new valves to prevent

water overflow to the containment.

Generally it means to install 9 new

valves which must all of them be

surely closed before starting of injecting

water to the cavity. This is the

most critical part of the solution,

because of possible failure, which

would cause the loss of the IVR-EX

strategy. The second critical point is

the control of water level for the

second set of pressurized tanks. The

control is proposed to prevent loss of

this water due to overflow, but this

control can be tested and tuned up

only at the experimental facility, so its

credibility is not too high.

The second issue is the release of

steam produced during the cooling

of RPV by boiling. The design drawings

of the Temelín NPP showed that the gap

between the RPV itself and its

supporting ring should be sufficient

(including thermal expansion of RPV),

but the data from measurement after

the vessel installation were not available.

So at least part of this gap was

measured during the outage at the unit

2 in 2016 and it confirmed sufficient

gap. But the gaps among the boxes of

thermal and biological shielding seem

OPERATION AND NEW BUILD 303

Operation and New Build

Continuous Process of Safety Enhancement in Operation of Czech VVER Units ı J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik

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