Design-Specific Common Position
CP-HPR1000WG-03 FOR PUBLIC USE
Date: August 2022
Validity: until next update or archiving
Version: 1

Multinational Design Evaluation Programme (MDEP)
Design-Specific Common Position
CP-HPR1000WG-03
HPR1000 Working Group

Common Position addressing In-Vessel
Retention strategy for HPR1000

Participation
Regulators involved in the MDEP working
group discussions:

ARN (Argentina), NNSA (China),
NNR (South Africa), ONR (UK)

Regulators which support the present
common position:

ARN (Argentina), NNSA (China),
NNR (South Africa), ONR (UK)

Regulators with no objection:

none

Regulators which disagree:noneDesign-Specific Common Position
CP-HPR1000WG-03 FOR PUBLIC USE
Date: August 2022
Validity: until next update or archiving
Version: 1

Multinational Design Evaluation Programme
HPR1000 Working Group
Common Position Addressing In-Vessel Retention Strategy for HPR1000
1.

Introduction
Pressurised Water Reactors (PWRs) nuclear power plant designs include safetymeasures with the aim of returning the plant to a safe state following an accident
and preventing the escalation to a severe accident and core melt scenario. In
the low probability occurrence of a core melt scenario with loss of the heatremoval function, there is potential for the molten core to relocate to lower headand melt through the Reactor Pressure Vessel (RPV). When the RPV fails, themolten corium can then fall into the reactor cavity, and the corium can erode
the containment basemat. This may continue until melt through is reached, which
may lead to a radioactive release to the environment. In addition, if there is water
present in the reactor cavity at the time of failure of the RPV, there is potential forthe molten core to interact with the water and result in large steam explosions
that may challenge the containment structures.
Basically, in order to ensure the heat removal as well as confinement function insevere accidents, there are two types of melt retention strategies based on theplanned location of the stabilisation of the corium: In-Vessel melt Retention (IVR)
and Ex-Vessel melt Retention (EVR). Both the two strategies are used in different
types of advanced PWRs, respectively. The IVR strategy is adopted by thedesigner in HPR1000, which uses water to cool the RPV outer surface and retainthe molten core within the RPV. This strategy is designed to prevent RPV failure
during severe accidents and the consequential severe accident phenomena sothat the integrity of the containment is maintained.
Regarding HPR1000 IVR, which is an important severe accident management
strategy, this Common Position (CP) is developed by the MDEP HPR1000 Working
Group (HPR1000 WG) members, consisting of regulators from Argentina, People's
Republic of China, South Africa and the United Kingdom. The national regulatory
requirements of member countries regarding the IVR are of general nature, anddetailed regulatory requirements are not given. The purpose of this CP is to present
an acceptable approach to IVR design and safety assessment strategy for theHPR1000. A Technical Report (TR) on the regulatory requirements and practices ofsevere accidents, as well as a questionnaire and clarification of HPR1000 IVR
design assessment, has been completed as part of other MDEP activities. Theseefforts have provided the basis for the production of this CP.
Firstly, this CP introduces the main design features and assessment strategy of
HPR1000 IVR, and then, an acceptable approach to HPR1000 IVR design andassessment is presented in Section 3.

2. Main design features and assessment of HPR1000 IVR
In order to achieve IVR after a severe accident, relevant design features havebeen adopted in HPR1000 design, and common practices have been adopted
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Design-Specific Common Position
CP-HPR1000WG-03 FOR PUBLIC USE
Date: August 2022
Validity: until next update or archiving
Version: 1

in the assessment of the effectiveness of the IVR strategy. In order to understandthe details of IVR relevant design and assessment, regulators from the WG
members put forward a questionnaire with 34 questions to the HPR1000 designer.
The questionnaire covered the system design, analysis and safety assessment,
Critical Heat Flux (CHF) testing, etc. Based on the detailed response of theHPR1000 designer an overview of the design and the analysis which supports thesafety justification of the IVR strategy are summarised below.
2.1 Design feature
According to the HPR1000 designer, the following design features are ofimportance for achieving the IVR strategy:The combination of active and passive injection measures, including
gravity injection from the elevated passive water tank in the containment,and active injection driven by pump;A special flow channel and its inlet and outlet are designed between theRPV and the insulation layer, which are optimised to ensure efficient heattransfer;

The pressuriser is equipped with two trains of the Severe Accident
Dedicated Valve (SADV) for severe accidents to achieve fulldepressurisation of the RPV;

Related equipment is equipped with diverse power sources, including
offsite power source, emergency diesel generator, Station Black Out (SBO)
diesel generator, mobile diesel generator, etc. The instruments and valves
can also be powered by the severe accident battery;

The control initiation of passive measures can be carried out on Severe
Accident I&C System to ensure that the IVR strategy can be started incase of total loss of AC power;

Startup of the IVR strategy needs to be manually operated by theoperator after a severe accident occurs, and the control of injection flowis realised by the self-adaptive pumps, valves and pipeline, without theneed of active adjustment;In order to avoid the impact on the normal operation, measures toprevent spurious injection are considered in the design;

Operating parameters, such as water level, flow rate and temperature,
are monitored to inform operators of the status of IVR function.

2.2 Safety Assessment
According to the HPR1000 designer, the effectiveness of IVR shall be guaranteed
only when both thermal as well as structural criterion is satisfied. The IVR
effectiveness assessment work carried out by the HPR1000 designer includes:
Thermal analysis: A 1:1 two-dimensional IVR CHF test facility was established, andthe CHF correlation along the outer wall of RPV was obtained. Based on
representative severe accident scenarios, deterministic analysis was carried out3

Design-Specific Common Position
CP-HPR1000WG-03 FOR PUBLIC USE
Date: August 2022
Validity: until next update or archiving
Version: 1to determine whether IVR is effective in those representative scenarios. Inaddition, based on the Risk Oriented Accident Analysis Method (ROAAM), asampling analysis on heat flux from molten pool to RPV wall was carried out taking
consideration of the uncertainties of parameters of molten pool, including steelmass, zirconium oxidation fraction and decay heat of molten pool, informed by
the deterministic analysis. The assumption of steady state of the two-layer molten
pool was adopted in the sampling analysis. The HPR1000 designer claims that it isproved that the heat flux from molten pool to RPV wall was less than the CHF
obtained from the test, and there is sufficient margin.
Mechanical analysis: The designer claims that the Finite Element Analysis (FEA) showsthat the pressurised thermal shock of spurious injection under normal operation or thepressurised thermal shock at the moment of water injection under accident
conditions will not threaten the integrity of RPV; the static analysis proves that the RPV
residual wall thickness and its maximum tensile stress can maintain the integrity of theRPV; and the finite element analysis shows that the RPV can still maintain its integrity
under the influence of long-term creep and other factors.
3. Common Position
High level requirements/expectations are set by the regulators related to ensuring
heat removal and confinement safety functions during accident conditions. For
severe accident scenarios, this means it is an expectation that adequate heatremoval from the molten core is provided such that the confinement of radioactivity
is maintained. In HPR1000, IVR is adopted to perform the heat removal function
during severe accidents. Since there are no specific requirements on this strategy,
the WG members consider the following common position can be an acceptableapproach to IVR design and assessment of HPR1000.
Scenario Identification
The melting process of different severe accidents should be analysed to identify therepresentative severe accident scenario(s) for IVR verification. Whilst it is generallyrealised that the LB LOCA severe accident presents the most challenging scenario
for IVR based on the engineering judgement and accident sequence analysis, thechoice of representative scenarios should be justified by the designer due to theuncertainties of severe accident progression and phenomena.
Corium Configuration
The stratification behaviour of the molten pool after the melt is relocated to thelower head has a direct impact on the effectiveness of IVR. The behaviour of molten
pool stratification is complicated. Large uncertainties in the corium degradation andrelocation process mean that the geometry and composition of the corium poolcannot be known. Simplifications are therefore applied in deterministic modelling of
the corium pool. There is currently no international consensus on how the pool should
be modelled; however, one internationally established method is to model the poolas two layers consisting of a light metal layer and an oxidised layer. Special attention
should be paid to the thermal focusing effect caused by the stronger convective
heat transfer from the light metal layer to the side wall. In addition, the core melt4Design-Specific Common Position
CP-HPR1000WG-03 FOR PUBLIC USE
Date: August 2022
Validity: until next update or archiving
Version: 1

transient process and interaction should also be paid attention to due to uncertainty
in the corium behaviour.
Effectiveness Assessment
In order to ensure that the decay heat in the RPV can be continuously
removed in the IVR configuration during severe accidents, it should beadequately proved that the heat flux between the outer wall of the RPV
and the water should be lower than the local Critical Heat Flux of the outerwall of the RPV. The CHF along the outer wall of the RPV should reflect thereal nuclear power plant design and typical severe accident conditions.

Influence of pressurised thermal shock on RPV structure integrity should beanalysed under the condition of spurious injection in normal operation.



Influence of pressurised thermal shock on RPV structure integrity at themoment of water injection under typical severe accident scenarios
should be analysed.

The inner wall of the RPV will be heated and ablated after corium
relocation. It should be proved by mechanical analysis that the residualwall thickness after ablation has sufficient mechanical strength to bear
the weight of the melt pool and the lower head and a certain internalpressure, so as to maintain the integrity of RPV.

System DesignIVR system design should be able to bring the system to a stable state andto maintain residual heat removal continuously after a severe accident.

The safety features related to the water inlet, the flow path between RPV
and insulation layer, and the outlet should be designed to an appropriate
level of reliability.

The design of the IVR flow channel should take into the consideration thepotential deformation of RPV lower head caused by heating and ablation,and potential debris in the containment that enter IVR cooling channel
after accident, such that sufficient flow is maintained to provide
adequate cooling to the RPV.

The nuclear power plant design should be equipped with appropriate
supporting systems that enable IVR to perform its safety functions andallow for monitoring of related parameters. These supporting systems
include instrumentation and control, water sources and power supplies.
These should be designed to an appropriate reliability and should bedesigned such that IVR can be implemented and monitored during apotential complete loss of AC power event.

The reliability of the startup of the IVR system should be ensured. At thesame time, the spurious startup of the IVR may affect the normal operation
and RPV performance. Therefore, measures to prevent the spurious
startup of the IVR should be considered in the design.

5Design-Specific Common Position
CP-HPR1000WG-03 FOR PUBLIC USE
Date: August 2022
Validity: until next update or archiving
Version: 1

ABBREVIATIONS
ACAlternating Current

CHF

Critical Heat FluxCPCommon Position

EVR

Ex-Vessel melt Retention

FEA

Finite Element Analysis

HPR1000 WG

MDEP HPR1000 Working GroupIVR

In-Vessel melt Retention

I&C

Instrument and Control

LB LOCA

Large Break Loss of Coolant Accident

MDEP

Multinational Design Evaluation Programme

NPPs

Nuclear Power Plants

PWR

Pressurised Water Reactor

RPVReactor Pressure Vessel

ROAAM

Risk Oriented Accident Analysis Method

SADV

Severe Accident Dedicated Valve

SBO

Station Black OutTR

Technical Report

References[1] HPR1000 MDEP – Severe Accidents TESG In-Vessel Retention questionnaire.
[2] HPR1000 MDEP – Technical Report on Regulatory Requirements and Practices forSevere Accidents.

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