analyses of severe accidents in sfps in kozloduy npp...the sfp is composed of three sections,...

Analyses of Severe Accidents in SFPsin KOZLODUY NPP

KRASEN RASHKOV – SENIOR EXPERT “THERMAL-HYDRAULIC ANALYSES”KOZLODUY NPP PLC, UNITS 5 AND 6,

[email protected]

1

OUTLINEOUTLINE

INTRODUCTION

PLANT CONDITION CATEGORIES

SPENT FUEL POOL DESIGN

DESIGN BASIS ACCIDENTS FOR SFPs

SEVERE ACCIDENT ANALYSIS IN SFP

2222

INTRODUCTIONINTRODUCTION

3

INTRODUCTIONINTRODUCTION

Unit 5• WWER-1000 (B320)• In operation since 1987• Operating license - 06 November 2027 (Long

Term Operation)

Unit 6• WWER-1000 (B320) • In operation since 1991• Operating licence - 02 October 2019 (waiting

license for Long Term Operation)

4

PLANT CONDITION CATEGORIESPLANT CONDITION CATEGORIES

According to the new requirements of the Bulgarian Nuclear RegulatoryAgency, from 2016, published in “Regulation on ensuring the safety ofnuclear power plants”:

The events and conditions of the NPP defined in the project bases aregrouped and analyzed in separate categories with different acceptancecriteria to demonstrate that events with the highest frequency do not haveradiological consequences out of the site and that events with potentialconsequences are very unlikely and fulfill the safety objectives. Dependingon the expected frequency of occurrence and the possible consequences, thefollowing categories are distinguished:

• Normal Operation (NO)• Anticipated operational occurrences (AOO)• Accidents without fuel melting• Accidents with fuel melting

5


Normal Operation (NO)

Anticipated operational

occurrences (AOO)

Accidents without fuel melting

Accidents with fuel melting

Postulated single initiating events

Postulated multiple failure events

6


IAEA

Cla

ssifi

catio

nBG

Cla

ssifi

catio

n

time

- AOO- DBA

- BDBA- SA

- AOO- Accidents without fuel melting (single events;

multiple failures)- Accidents with fuel melting- AOO

- DBA- BDBA

- SA Project for new Chapters (Ch. 15…) in Safety Analysis Report

- AOO- DBA

- DECs (without significant fuel degradation; with core melting)

7

SPENT FUEL POOL DESIGNSPENT FUEL POOL DESIGN

To supply water toSFP, three pumpsand three heatexchangers areused. Each pumpcan supply waterto any SFPcompartment.

For VVER-1000, the SFP is located close to the reactor, in the reactor building (inside of the containment hermetic boundary).

8


The SFP is composed of three sections, separated by walls. When the fuel pool is not used for refueling operations, it is covered by panels.

Refueling of the reactors is carried out after reactor shutdown with open reactor vessel. The refueling pool above the reactor vessel is filled up with water and the replacement of spent FAs with fresh ones is carried out under deep water level by use of a refueling machine.

9


10


The first section (B01) has racks with 225 Fuel Assembly (FA) positions and 28 additional positions for cylindrical hermetic capsules to store problem assemblies.

The second section (B02) – 110 FA positions.

The third (B03) - 228 FA positions, 21 additional positions for cylindrical hermetic capsules and 4 capsules for hermetic tests of the fuel.

Always there are 163 free FA positions intended for emergency moving the fuel from the reactor to the SFP.

11


SFPs cooling system:

12

DESIGN BASIS ACCIDENTS FOR SFPsDESIGN BASIS ACCIDENTS FOR SFPs

There were analyzed three scenarios categorized like Anticipated OperationalOccurrences (AOO):

- Limiting the passage section of racks with FAs – events with falling of objects or instruments (without loss of integrity)- Limiting the cooling of the hermetic capsules - Problems with heat exchangers - Increase of the temperature of the technical water, decrease of the flow rate of the water and decrease of heating surface.

The SFP was modeled and analyzed using RELAP5/mod3.3 patch 02 by analogy with the core analysis.

13

DESIGN BASIS ACCIDENTS FOR SFPsDESIGN BASIS ACCIDENTS FOR SFPs

The proper Aacceptance Criteria for the SFP are:

The temperature of the water in the SFP must not be more than 50°С in normal operation and not more than 70°С when all the fuel assemblies are moved from core to SFP.

The temperature of the water in the SFP can be more than 70°С assuming some failure of the components of the system, but not more than the criteria for damage of the fuel (T cladding >1200 °С).

The analysis were calculated with conservative assumptions – high starting temperature of the water in the SFP, all the FA are in the SFP, failure of one of the channels of the cooling system and no operator actions. There was no need the safety systems to start. The acceptance criterion are fulfilled.

14

SEVERE ACCIDENTS ANALYSIS IN SFPSEVERE ACCIDENTS ANALYSIS IN SFP

The objectives of the analysis were:

Description of the applicable phenomena;

Analysis with MELCOR 2.1 and determination of the chronology ofthe events;

Determination of the possible ways of radioactive releases to theenvironment and their mechanisms. It also includes the time of failureof the respective barriers to the spread of radioactive products to theenvironment;

15


Efficiency and feasibility analysis of the possible technical and (or)administrative measures (basic and alternative) for the management ofSA. It includes an assessment of the critical timeframe within whichapplicable operator actions can be implemented to prevent thedegradation of each of the barriers and the radioactive releases to theenvironment be limited within the normative limits;

Determination of the doses on the site resulting from the loss ofintegrity of the last barrier to the spread of radioactive products to theenvironment. It includes an assessment of the relevant acceptancecriteria.

16


The severe accident in the SFP can be divided into three main phases:

• Phase 1 – covers the processes where the fuel (or melt) is located in the SFP structure without the melt interacting with the concrete;

• Phase 2 - covers the processes where the melt is in the construction of the SFP with interaction of the melt with the concrete (before failure of the concrete structure of the SFP);

• Phase 3 - covers the processes when the melt is located in the rooms of the reactor building (after failure of the floor or walls of SFP ).

17


The main differences of the processes of a severe accident in the SFP compared to severe accident in the reactor vessel are:

• Processes are characterized by low pressure only (the pressure is determined by the pressure in the containment);

• The construction of the SFP (walls and floor) is made of heavy concrete with double-sided stainless steel liner on the inside. In practice, after the failure of the supporting structure in the SFP , the cladding melts in a very short period of times, so there is no metal wall of significant thickness, as in the reactor vessel. This also marks the beginning of the interaction of the melt with the concrete.

18


Main acceptance criteria (AC1,2):AC1 - Cesium-137 (Cs-137) 30 TBq MELCORAC2 - The annual individual effective dose of internal and external radiation of the population

50 mSvduring the first yearafter DBA

mSv EPA DOSE

In addition to these criteria, another criteria were used to determine the effectiveness of the strategies.

19


Additional acceptance criteria (AAC1-7) for determining the effectiveness of strategies:

AAC1 - Temperature of the cooling water in SFP < temperature of saturation

C

AAC2 - Maximum cladding temperature 1200 CAAC3 - Temperature of the debris or the melt < 1227

(1500)* C (К)

AAC4 - Maximum pressure in the containment 0.50 (abs.) MPaAAC5 - Maximum temperature in the containment 150 CAAC6 - Concentrations of non-condensing in the containment

Hydrogen: 4Oxygen: 5Steam: 55

%

AAC7 - Integrity of the containment ** - -

20


Remarks:

* 1500 К is the boundary temperature above which the thermal ablation of the concrete is realized. This temperature is measured in the boundary layer between the concrete and the melt.

** This criteria aims to demonstrate that the performance of otherscriteria is implemented without breaking the last barrier.

.

21


Analyses of severe accidents were made with Melcore 2.1Model of the SFP:

22


Model of the containment:

23


The main aspects that have influence to the choice of scenarios are:

24


The initiating events were focused on earthquakes, flooding or othernatural hazards. It is necessary to consider the consistent loss of levels ofDefence-in-Depth, which is determined deterministically, regardless of thenumerical value for the probability of this loss. The loss of safety functionsand severe accidents can occur only when several design protection fails toperform its functions.

25


Possible initiating events to assess the consequences of loss of safetyfunctions are:- loss of off-site power;- loss of ultimate heat sink;- combination of both.

The PSA level 1 results show that it is appropriate to consider thefollowing initiating events: LOCA for SFP, Loss of cooling, Loss of off-sitepower and Loss of ultimate heat sink.

The most conservative operating state is a condition in which the wholecore is in the SFP on the third day after the reactor is stopped.

All initial and boundary conditions are chosen so as the results to showmaximum radioactive releases for the above-mentioned initiating events.

26


There were analyzed 9 scenarios:

1.Total blackout - basic scenario, no operator actions;

2.Total blackout - basic scenario, no operator actions, with some specific assumptions connected with equipment;

3.Total blackout and initial leakage from SFP 5m3/h - basic scenario, no operator actions;

4.,5.,6.,7.,8.,9. Total blackout and initial leakage from SFP 5m3/h –injection of cooling water 45m3/h (scenarios with different initial conditions);

27


Time (h:min)

Phase 1

0:00 Total blackout and initial leakage from SFP 5m3/h (TG21B01)0:12 Temperature in the water in TG21B02, 70оC0:33 Temperature in the water in TG21B02, 98оC3:03 Temperature of saturation in TG21B01

5:02Starting generation of hydrogen. Increase of the temperature of the cladding of the fuel above 900 К (627оС) in TG21B02

8:35 Phase 220:11 Phase 383:20 End of calculation

Results for scenario 3 - Total blackout and initial leakage from SFP 5m3/h -basic scenario, no operator actions

28


Value Limit valueAC1 35 30 TBqAAC1 2113 1200 CAAC 2 TG21B03 – 100 100 CAAC 3 1181 (1454) < 1227 (1500)* C (К)AAC 4 0.226 (2.306) 0.490 (5) (абс.) MPa (kgf/cm2)AAC 5 105 150 CAAC 6 Hydrogen: 3.22

Oxygen: 13Steam: 29

Hydrogen: 4Oxygen: 5Steam: 55

%

AAC 7 Failure of the containment

- -

29


Results for scenario 6 - Total blackout and initial leakage from SFP 5m3/h –injection of cooling water 45m3/h, 4 hours after the water in SFP reaches 98 °C

Time,h:min

Phase 1

4:33Starting injection of water to TG21B01 with от 45 m3/h (12.5 kg/s)

4:46Overflow of water from TG21B01 to TG21B02 – elevation 28.75 m (Start filling TG21B02)

4:56Temperature of the cladding of the fuel 812 K (539 °C) inTG21B02

5:18 Stabilization of the cladding temperature at 108 °C8:56 Water in TG21B01-B04 reaches elevation 28.958 m

13:23End of the calculation. There were injected 400 m3 water to SFP

30


Value Limit valueAC1 1.60E-20 30 TBqAAC1 539 1200 CAAC 2 TG21B02 – 114 100 CAAC 3 - < 1227 (1500)* C (К)AAC 4 0.149 (1.519) 0.490 (5) (абс.) MPa (kgf/cm2)AAC 5 85 150 CAAC 6 - Hydrogen: 4

Oxygen: 5Steam: 55

%

AAC 7 Containmentretains its integrity

- -

31


In these scenarios there were investigate different strategies for severe accident management. Important for SAMG for SFP are:

-The minimum water needed for injection to SFP;

-The time for starting injection water to the SFP - the results clearly showed that the water supply should be realized within the phase 1 of the accident (before the start of the interaction between the melt and the concrete);

- The maximum available time for preparation and implementation of water supply to SFP (time from the entrance criterion for SAMG - temperature of water in SFP 98 C).

analyses of severe accidents in sfps in kozloduy npp...the sfp is composed of three sections,...

Documents