ONR-GDA-AR-11-008, Revision 0 - Office for Nuclear Regulation

316 Pages·2011·1.35 MB·English

Office for Nuclear Regulation
An agency of HSE

Generic Design Assessment – New Civil Reactor Build

Step 4 Reactor Chemistry Assessment of the Westinghouse AP1000® Reactor

Assessment Report: ONR-GDA-AR-11-008
Revision 0
11 November 2011
Office for Nuclear Regulation Report ONR-GDA-AR-11-008
Revision 0
An agency of HSE

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© Crown copyright 2011

First published December 2011

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PREFACE
The Office for Nuclear Regulation (ONR) was created on 1st April 2011 as an Agency of the Health
and Safety Executive (HSE). It was formed from HSE's Nuclear Directorate (ND) and has the same
role. Any references in this document to the Nuclear Directorate (ND) or the Nuclear Installations
Inspectorate (NII) should be taken as references to ONR.
The assessments supporting this report, undertaken as part of our Generic Design Assessment
(GDA) process and the submissions made by Westinghouse relating to the AP1000® reactor
design, were established prior to the events at Fukushima, Japan. Therefore, this report makes no
reference to Fukushima in any of it’s findings or conclusions. However, ONR has raised a GDA
Issue which requires Westinghouse to demonstrate how they will be taking account of the lessons
learnt from the events at Fukushima, including those lessons and recommendations that are
identified in the ONR Chief Inspector’s interim and final reports. The details of this GDA Issue can
be found on the Joint Regulators’ new build website www.hse.gov.uk/newreactors and in
ONR’s Step 4 Cross-cutting Topics Assessment of the Westinghouse AP1000® reactor.

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EXECUTIVE SUMMARY
This report presents the findings of the Reactor Chemistry assessment of the AP1000 reactor
undertaken as part of Step 4 of the Health and Safety Executive’s (HSE) Generic Design
Assessment (GDA). The assessment has been carried out on the December 2009 Pre-
construction Safety Report (PCSR) and supporting documentation submitted by Westinghouse
during Step 4.
This assessment has followed a step-wise-approach in a claims-argument-evidence hierarchy. In
Step 3 the claims and arguments made by Westinghouse were examined.
The scope of the Step 4 assessment was to review the safety aspects of the AP1000 reactor in
greater detail, by examining the evidence, supporting arguments and claims made in the safety
documentation, building on the assessments already carried out during Step 3, and to make a
judgement on the adequacy of the Reactor Chemistry information contained within the PCSR and
supporting documentation.
It is seldom possible, or necessary, to assess a safety case in its entirety, therefore sampling is
used to limit the areas scrutinised, and to improve the overall efficiency of the assessment process.
Sampling is done in a focused, targeted and structured manner with a view to revealing any topic-
specific, or generic, weaknesses in the safety case. To identify the sampling for the Reactor
Chemistry an assessment plan for Step 4 was set-out in advance.
My assessment has focused on obtaining further evidence from Westinghouse and assessing the
chemistry of:
 The Westinghouse safety case for the justification, implications and control of primary coolant
chemistry during all modes of operation. This included consideration of nuclear reactivity
control using boron, the effects of coolant chemistry on the integrity of pressure boundaries,
protection of fuel and core components and production, transport and deposition of
radioactivity, including its influence on radiological doses to workers and ultimately to wastes.
 Those features of the design, material choices or chemistry controls which reduce radioactivity
so far as is reasonably practicable.
 The main secondary circuit systems which control or are influenced by chemistry. This includes
consideration of the implications of system design on chemistry choices and the interaction of
chemistry with materials and corrosion susceptibility.
 Those engineered systems which allow the operator to control, monitor or change the plant
chemistry.
 The storage of nuclear fuel within ponds, including the effects of pool chemistry.
 Those systems which mitigate the release of radioactivity to the environment in either the liquid
or gaseous form.
 Design basis and beyond design basis accidents, including the production, release and control
of hydrogen and fission product nuclides.
 The arrangements for moving the safety case to an operating regime, including the derivation
of suitable limits and conditions and the arrangements for specifying plant chemistry.
A number of items have been agreed with Westinghouse as being outside the scope of the GDA
process and hence have not been included in my assessment. A full list and description of these
items can be found in the text of the report.
From my assessment, I have concluded that:

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 Westinghouse has been continuously developing the design of AP1000 throughout the GDA
assessment. While this has caused questions to be raised regarding design definition and the
cohesiveness of the safety case, a number of positive design changes have been incorporated
which have allayed many of my concerns in the original design.
 Westinghouse has struggled to meet my expectations in regard to defining and limiting the
chemistry required for safe operation of AP1000. As a plant vendor, Westinghouse do not
specify the operational chemistry regimes for AP1000, referring instead to industry guidelines
which, in some cases, allow the operator a degree of freedom. Westinghouse also does not
propose limits and conditions related to the vast majority of chemical or radiochemical
parameters. This latter case is a significant shortfall and will need to be addressed before
nuclear safety-related construction and is the subject of a cross-cutting GDA Issue.
 I have assessed the major chemistry systems which allow the operator to control, monitor and
change the primary chemistry, including those that are used during accidents. The AP1000 has
a number of systems for these aspects which have been simplified and made passive, in line
with the overall plant design philosophy, but are functionally very similar to existing PWRs in a
number of regards. I have assessed the Chemical and Volume Control System (CVS) in some
detail as part of my assessment. This system is novel in a number of aspects but I am content
that an adequate case has been made from a Reactor Chemistry perspective for this system,
with the exception of the hydrogen dosing control, which I have raised as a GDA Issue which
requires resolution. Similarly, sampling of the primary coolant is an important chemistry
function allowing the operator to maintain control. Despite a positive design change made late
in Step 4 I have still not been convinced that the design of the AP1000 is adequate in this
regard and has been shown to meet relevant good practice. I have raised this as a GDA Issue.
 The Westinghouse case for materials in AP1000 is well reasoned and sound, both from a
radioactivity and corrosion prevention perspective. Westinghouse have engineered many
known ‘problem’ alloys out of the AP1000 design entirely, have reduced many to levels
consistent with ALARP as well as demonstrating an appropriate level of control over aspects
such as surface finishing and fabrication. Westinghouse has proposed to electropolish the
steam-generator channel heads.
 Westinghouse proposes to add zinc to the primary coolant of AP1000 to further reduce the
plant dose rates. Based on the evidence presented to me I consider that zinc addition is
justified for AP1000 and the use of zinc during commissioning appears to be a welcome
addition, provided the depleted form is chosen by the Licensee.
 Estimates of the radioactive materials such as tritium and cobalt isotopes that would be
produced by AP1000, have been provided by Westinghouse based on a standard US method.
These estimates took no account of different management schemes for AP1000, nor some
specifics of the design. I commissioned independent analysis which showed that AP1000 may
produce more of the cobalt isotopes that some current PWRs for a comparable power output.
Westinghouse also predict some tolerable fuel crud generation. I consider that AP1000 may be
more prone to the development of fuel crud and production of cobalt isotopes although this
could be manageable provided robust and strict controls, limits and conditions are put in place
by the Licensee.
 Despite Westinghouse identifying all aspects of the secondary circuit as being in the scope of
the GDA assessment, a number of the important chemistry systems are not yet fully designed.
Despite these gaps I have assessed a number of aspects of the design including principal
material choices, corrosion threats, chemistry control and tolerance of abnormal chemistry. At a
high level it is apparent that Westinghouse has incorporated operating experience and
feedback into the design and main material choices for AP1000. A number of detailed material
choices, and the operating chemistry, are not yet decided and will only be defined by the

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eventual Licensee. A novel design choice for the AP1000 secondary circuit is the use of
Electrodeionisation (EDI) for purification of the steam generator blowdown. I have assessed a
number of features of this design in detail and have been satisfied with the responses from
Westinghouse for the most part, with some areas requiring further work. Thus while reasonable
arguments have been made by Westinghouse in these areas, the principal deficiency is that a
holistic assessment for the secondary circuit, considering chemistry and material choices, is
not yet available, although I am content that there should be no fundamental hindrances to safe
operation of the plant.
 The AP1000 Spent Fuel Pool (SFP) safety case related to criticality and loss of cooling events
relates to Reactor Chemistry in a number of areas including boron control and the potential for
radioactive releases. This has been a cross-cutting area during GDA. In response to our
assessment, Westinghouse has proposed a number of design improvements to the SFP and
associated systems and has revised the safety case in a number of areas. Whilst broadly in
line with my expectations, these were received late in GDA and will need further assessment. A
cross-cutting GDA Issue has been raised in this area, which also needs to be satisfactorily
resolved before an adequate Reactor Chemistry safety case can be made.
 The AP1000 has been designed to prevent accidents and make unplanned releases smaller
and less likely. I assessed the chemistry occurring during Steam Generator Tube Rupture
(SGTR) events, during accidents which involve generation and release of combustible gases
and in the unlikely event of an accident severe enough to melt fuel. These have all been areas
of challenge to past reactor designs. In general, while further work will be required by the
Licensee in many of these areas, the overall Westinghouse case for chemistry during accidents
is acceptable for GDA. The main exception to this is for the control of fission products in an
accident. While the containment of AP1000 has been designed to retain radioactive material in
an accident, which simplifies the management of radioiodine, AP1000 does not include a
recirculating, pH buffered spray system for fission product control as in many PWRs, instead
relying on passive deposition mechanism driven by external cooling supplemented by a spray if
necessary. The chemistry aspects of this case were presented to ND late in GDA and I have
not yet completed my assessment in this important area. I have raised this as a GDA Issue.
 As a result of the GDA assessments, the consolidated PCSR for AP1000 has been updated
and now includes a chapter dedicated to Reactor Chemistry. This is a valuable addition to the
safety case and will provide a basis for further developments of the plant chemistry. As the
Step 4 PCSR contained no chemistry chapter, and this chapter was issued during March 2011,
I have not yet fully assessed this document. I have raised this as part of a cross-cutting GDA
Issue.
In some areas there has been a lack of detailed information which has limited the extent of my
assessment. As a result ND will need additional information to underpin my conclusions and these
are identified as Assessment Findings to be carried forward as normal regulatory business. These
are listed in Annex 1.
Some of the observations identified within this report are of particular significance and will require
resolution before HSE would agree to the commencement of nuclear safety-related construction of
an AP1000 reactor in the UK. These are identified in this report as GDA Issues are formally
defined in Annex 2 of this report. In summary these relate to:
GI-AP1000-RC-01 - Westinghouse need to provide further evidence that the source term for
severe accident release has been appropriately applied for the AP1000 design, including
fractions and timing of release in both the short and long term.
GI-AP1000-RC-02 - Further justification, potentially including further design changes, will be
needed for the primary circuit sampling systems to meet UK expectations.

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GI-AP1000-RC-03 - Westinghouse will need to provide further evidence to support the
design of the primary circuit hydrogen injection system.
In addition, there are cross-cutting Issues relating to assessment of the consolidated GDA PCSR
(GI-AP1000-CC-01), operating limits and conditions (GI-AP1000-CC-02) and the spent fuel pool
(GI-AP1000-FS-01), which require a satisfactory resolution before an adequate Reactor Chemistry
safety case can be made.
Overall, based on the sample undertaken in accordance with ND procedures, I am broadly
satisfied that the claims, arguments and evidence laid down within the PCSR and supporting
documentation submitted as part of the GDA process present an adequate safety case for the
generic AP1000 reactor design. The AP1000 reactor is therefore suitable for construction in the
UK, subject to satisfactory progression and resolution of Issues to be addressed during the forward
programme for this reactor and assessment of additional information that becomes available as the
GDA Design Reference is supplemented with additional details on a site-by-site basis.

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LIST OF ABBREVIATIONS
AC Alternating Current
ADS Automatic Depressurisation System
AICC Adiabatic Isochoric Complete Combustion
ALARP As Low As Reasonably Practicable (see also SFAIRP)
ANSI American National Standards Institute
ASME American Society of Mechanical Engineers
ASN L’Autorité de sûreté nucléaire (Nuclear Safety Authority, France)
ASTM American Society for Testing and Materials
AVT All Volatile Treatment
BAST Boric Acid Storage Tank
BDBA Beyond Design Basis Analysis
BDS Steam Generator Blowdown System
BMS (ND) Business Management System
BOA Boron Offset Anomaly
BWR Boiling Water Reactor
CANDU CANada Deuterium-Uranium reactor
CCS Component Cooling Water System
CDS Condensate System
CFR (US) Code of Federal Regulations
CFS (Turbine Island) Chemical Feed System
CHF Critical Heat Flux
CILC Crud-Induced Localised Corrosion
CIPS Crud-Induced Power Shift
CL Cold Leg (of RCS)
CMT Core Make-up Tank
CORS Catalytic Oxygen Reduction System
CoSHH Control of Substances Hazardous to Health (Regulations)
CP Corrosion Product
CPP Condensate Polishing Plant (see also CPS)
CPS Condensate Polishing System
CPVC Chlorinated PolyVinyl Chloride
CRDM Control Rod Drive Mechanism
CSS Containment Spray System
CST Condensate Storage Tank
CVS Chemical and Volume Control System

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LIST OF ABBREVIATIONS
CWS Circulating Water System
DBA Design Basis Analysis
DCD Design Control Document
DCH Direct Containment Heating
DCP Design Change Proposal
DDT Deflagration to Detonation Transition
DE Dose Equivalent
DSEAR Dangerous Substances and Explosive Atmosphere Regulations
DTS Demineralised Water Treatment System
DWS Demineralised Water Storage and Transfer System
DWST Demineralised Water Storage Tank
EDCD European Design Control Document
EDI Electrodeionisation
EMIT Examination, Maintenance, Inspection and Testing
EPRI Electric Power Research Institute (US)
EU European Union
FAC Flow Accelerated Corrosion
FHA Fuel Handling Area
FP Fission Product
FPS Fire Protection System
FRI Fuel Reliability Index
FW FeedWater
FWS Feed Water System
GDA Generic Design Assessment
GSP Grab sample Panel
HDS Heater Drain System
HEPA High Efficiency Particulate Air
HFT Hot Functional Testing
HL Hol Leg (of RCS)
HOR Hide Out Return
HP High Pressure
HPME High Pressure Melt Ejection
HSE (The) Health and Safety Executive
HVAC Heating, Ventilation and Air Conditioning
HX Heat Exchanger
IAEA The International Atomic Energy Agency
IASCC Irradiation Assisted Stress Corrosion Cracking

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LIST OF ABBREVIATIONS
IGA Inter-granular Attack
IGSCC Inter-granular Stress Corrosion Cracking
IHST Integrated Head Storage Tank
IRWST In-containment Refuelling Water Storage Tank, part of the PXS
IVR In-Vessel Retention
LB Large Break
LOCA Loss of Coolant Accident
LP Low Pressure
LTCP Low-Temperature Crack Propagation
LWR Light Water Reactor
MAAP Modular Accident Analysis Programme
MATPRO MATerial PROperties (database)
MCCI Molten Core-Concrete Interaction
MCR Main Control Room
MFCV Main Feedwater Control Valve
MFIV Main Feedwater Isolation Valve
MSL Master Submission List
MSLB Main Steam Line Break
MSR Moisture Separator Reheater
MSS Main Steam System
MTS Main Turbine System
ND (HSE) Nuclear Directorate
NEI Nuclear Energy Institute
NRC Nuclear Regulatory Commission (US)
OECD Organisation for Economic Co-ordination and Development
OEF Operational Experience Feedback
ORE Operator Radiation Exposure
PAR Passive Autocatalytic Recombiner
PASS Post-Accident Sampling System
PCCWST Passive Containment Cooling Water Storage Tank
PCS Passive (containment) Cooling System
PCSR Pre-construction Safety Report
PLS Plant Control System
PORV Power Operated Relief Valve
PRA Probabilistic Risk Assessment (see also PSA)
PRHR Passive Residual Heat Removal system, part of the PXS
PSA Probabilistic Safety Analysis (see also PRA)

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