atw 2018-05v6

inforum

atw Vol. 63 (2018) | Issue 5 ı May

OPERATION AND NEW BUILD 304

to be insufficient as their position

varies after each operation with those

boxes due to replacement of probes.

The conclusion is that in case of the

IVR-Ex strategy implementation, the

replacement of some boxes is needed

with modified ones, which will include

the burst membrane and sufficient

flow channel.

The analytical studies on RPV

coolability showed very low margin to

critical heat flux (CHF) if the cooling

is in a water pool. So the idea of

the intensification of heat removal

appeared and feasibility study on

the application of the deflector was

performed. The conclusion is that the

deflector is technically feasible, but

too many new risks are related to its

implementation that the deflector

cannot be acceptable as the measure

for a safety increase. Just a few

examples of those risks. The deflector

would need to be removed during

every outage so the process of its

disassembling and later re-assembling

using the manipulators would be very

risk for the hiting RPV. Also storing of

the deflector components, which are

irradiated, would be high risk for

additional dose rates to personal. So

the conclusion is that the deflector is

not reasonably applicable.

Verification of IVR-EX efficiency

for VVER-1000. This topic covers

practically all plant analyses for the

IVR-EX and also the analytical support

for the designing of the experimental

facility (called THS-15) on the RPV

coolability. This topic also covers the

designing of the facility itself. As

examples of the activities, it can be

mentioned – a study on the heat flux

distribution from corium to RPV wall

using the FLUENT code, RELAP

analyses of RPV cooling with various

parameters of deflector, pre-test

analyses of the THS-15 facility,

supporting scaling analyses, but also

a preparation of the methodology for

an evaluation of experimental results,

a preparation of the experimental

matrix and so on.

Although it is not part of the project,

it is logical to put into this subchapter

the information on the building of the

THS-15 facility. The facility purpose is

to perform tests on the cooling of RPV

of the VVER-1000 reactor under IVR-EX

conditions. The tests will be focused on

the verification of a coolability of heat

fluxes, an identification of CHF, and to

produce data for the code validation.

The experiments will be performed as

the part of the WP4 of the EC H2020

Project IVMR (grant agreement

number 662157). The recent situation

in the facility building is that the

production of the last big component

is finished and the facility assembling

is ongoing. The scheduled com missioning

of the facility has to be at the

end of November 2017. Then the experimental

program will be launched.

3.4.4 Strategy ExVC

The alternative strategy to the IVR-EX

is the ex-vessel corium cooling one.

The idea of the strategy is to spread

the corium into the cavity neighboring

room (named GA302) and cover the

corium with the water to cool it down.

Several analytical studies as well as

the experimental data from the OECD

MCCI and MCCI2 project showed, that

the MCCI with the siliceous concrete

is not possible to terminate. Based

on this observation the idea of the

refractory material, which would

survive at least several hours till

significant corium temperature reduction

appeared. The project also

defined some topics of a solution for

this strategy, starting with additional

analysis, but also with the feasibility

study on modifications of doors

between cavity and room GA302 to

make spreading much faster and

easier. The second activity was a

feasibility study of an instalation of

the refractory material. It is very

complicated mainly in the cavity due

to need of the concrete cooling (it

must not be influenced) and also from

the perspective of those modification

performace as there is really high

radiation in the cavity, which practically

precludes any work in the cavity.

The updated measurement of the

radiation in the cavity (in 2017) not

only confirmed the level of radiation,

but it was more detailed and identified

sources, which is not only the

RPV itself, but also the thermal and

biological shielding near the cavity

walls. So it is practically impossible

to work there and this shielding was

expected to be replaced with new

one together with refractory layer

installation. So the situation in the

cavity is recently not yet solved and

will need an alternative solution,

which is not yet prepared.

Concerning the refractory material,

the preliminary study on potential

candidates was performed, but before

the final decision the experimental

testing has to be performed.

3.4.5 Stabilization of

containment conditions

The title of this chapter differs in the

wording with the bullet above, but its

meaning is same, because the main

task of the activities is to maintain the

containment pressure and temperature

on an appropriate level for long

time. During this year, the project

opened this task and the proposal

for technical solutions of the heat

removal from the containment

atmosphere are under development.

About nine technical proposals were

prepared as basic scheme and two of

them are be selected for the feasibility

study. Those nine solutions use

various approaches – spraying, steam

condensation and heat removal,

using sophisticated systems with

supercritical CO 2 and so on.

4 Conclusions

A continuous process of safety

enhancement of VVER units in the

Czech Republic has been briefly

described including a presentation of

important milestones and examples of

particular safety measures already

implemented. Despite the high level

of safety reached mainly by the

preventive means in the last decades

at both VVER sites in the context

of Fukushima accident, a new period

of enhancement process has been

initiated following the stress tests

exercise. Use of advanced deterministic

analytical approaches and

implementation of new preventive

safety measures has then taken place

together with special attention to

mitigative part of potential accidents

and relevant strategies and measures.

In the paper a special attention was

given to the evaluation and implementation

of safety measures following

stress tests conclusions and R&D

activities supporting this process. As

examples an implementation of the

„design extension condition without

core melt“ concept and various activities

related to severe accident mitigation

strategies are presented in more

detailed way.

Within the systematic approach to

enhance safety also the PSA has been

recognized as a very useful tool. The

PSA has been mainly used for identification

of weak points in design and

operation and for an evaluation

and prioritization of potential safety

measures. The example of new cooling

tower measure for the Dukovany

site has been used to demonstrate this

approach.

The work on DEC-A (previously

BDBA) safety analyses for Czech NPPs

was described as a consequence of the

Periodical Safety Review (PSR) after

20 year of the operation of the

Dukovany NPP. This effort has been

influenced also by initiatives and

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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