Design Requirements and Engineering Considerations by NKB4nmzL

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									Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                                                    1

                           PROPRIETARY COMMERCIAL CONFIDENTIALITY
 This document contains technical, financial and commercial information of a confidential nature, which is
 proprietary to Milan Gacesa and Mukesh Tayal. It is submitted with the understanding that there will be
    no exploitation and no direct or indirect disclosure to third parties of the information contained in or
                                         derived from this document.



                                                             Chapter 5
                                                                   Fuel
                                                prepared by
                           Milan Gacesa and Mukesh Tayal – Independent Consultants

Summary:
In this section we will summarize the contents of Sections 2, 3, 4, and 5, i.e.:
    1) The design/operating requirements imposed on CANDU Fuel with reference to the
         specific interfacing systems which impose those requirements
    2) The process for selecting the Fuel design in response to those requirements (including
         the alternatives considered for all components of the Fuel bundle)
    3) The verification processes used to demonstrate that the selected design can withstand the
         specified requirements without failing. For this verification step we describe two
         approaches that have been used successfully for CANDU Fuel, the traditional
         “applications based” verification process, and the more recent “generic a-priory”
         verification process, and
    4) The feed-back process for monitoring the behaviour of Fuel in operating CANDU plants
         as well as the influence of that feed-back information (including the role of NRU tests) on
         Fuel design evolution.

                                                     Table of Contents
1    Introduction ............................................................................................................................. 3
  1.1     Overview ......................................................................................................................... 3
  1.2     Learning Outcomes ......................................................................................................... 3
2    CANDU Fuel Design Requirements ....................................................................................... 4
  2.1     System A (for instance HTS) .......................................................................................... 4
  2.2     System B (for instance Fuel Channels) ........................................................................... 5
  2.3     System C (for instance Fuel Handling) ........................................................................... 5
  2.4     System D (for instance Physics) ..................................................................................... 5
  2.5     System E (for instance Defective Fuel Removal) ........................................................... 5
  2.6     Licensing and/or Jurisdictional ....................................................................................... 5
3    CANDU Fuel Design - Concept Decision .............................................................................. 5
  3.1     Sheath Component – Alloying Metal for Attaching the Appendages to the Tubes........ 5
  3.2     Sheath Component – Appendage Material and Configuration ....................................... 5
  3.3     Sheath Component – Tube Material and Configuration ................................................. 5
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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                                                2
  3.4     Element Component – Filling Gas .................................................................................. 6
  3.5     Element Component – CANLUB Layer ......................................................................... 6
  3.6     Element Component – Endcap........................................................................................ 6
  3.7     Element Component – Diametral Clearance................................................................... 6
  3.8     Element Component – UO2 Pellet .................................................................................. 6
  3.9     Bundle Assembly – End Plate......................................................................................... 6
  3.10 Bundle Assembly – Cross Section .................................................................................. 6
4    CANDU Fuel Design Verification.......................................................................................... 6
  4.1     Traditional Verification Methodology ............................................................................ 7
     4.1.1 Verification Topics ..................................................................................................... 7
     4.1.2 Verification Strategy ................................................................................................... 7
     4.1.3 Verification Assessments ............................................................................................ 7
  4.2     Generic Verification Methodology ................................................................................. 7
     4.2.1 Defect Mechanisms and Design Acceptance Criteria ................................................. 9
     4.2.2 Damage Scenarios ....................................................................................................... 9
     4.2.3 Verification Strategy and Assessments ....................................................................... 9
     4.2.4 Operational Constraints – Input to SOE ................................................................... 10
  4.3     Illustrative Examples .................................................................................................... 10
5    Operational Feedback ........................................................................................................... 10
  5.1     Use of NRX and NRU in Establishing CANDU Fuel Principal Parameters ................ 10
  5.2     Power Plant Operational Feedback ............................................................................... 11
     5.2.1 Douglas Point ............................................................................................................ 11
     5.2.2 Pickering ................................................................................................................... 11
     5.2.3 Bruce ......................................................................................................................... 11
     5.2.4 CANDU 6 ................................................................................................................. 12
     5.2.5 Darlington ................................................................................................................. 12
     5.2.6 Fuel Defect Statistics ................................................................................................ 12




                                                       List of Figures
Figure 1 Schematic Diagram Illustrating Minimum Acceptable Margin in Fuel Design............. 14

                                                        List of Tables
Table 1 - The Complete List of Fuel DR. .................................................................................. 13




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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      3


1 Introduction
1.1 Overview

Fuel is a system that confers on a CANDU plant some unique nuclear characteristics and high-
level requirements, from the need to create the environment for generating the nuclear heat, to
the requirements to provide the facilities to remove the heat, to the requirements to provide
radiological protection, to the requirement to provide appropriate containment of the fission
products both during normal operation and during the postulated design basis accidents, and
finally to the requirement for safe storage of used fuel. Having had these high level requirements
imposed by fuel accepted and implemented by the interfacing systems, the fuel in turn must
accept and satisfy the requirements imposed on it by those interfacing systems.

In this Chapter we describe the methodology used to: (1) derive the design/operating
requirements for CANDU fuel, (2) assess the design concept alternatives and select the design of
each fuel bundle component, and (3) verify that the selected design meets the defined
requirements without experiencing defects. For the verification phase we describe two
approaches that have been used successfully for CANDU fuel, the traditional test based
verification process, and the more generic verification process developed recently. The feed-back
process for monitoring fuel behaviour in operating CANDU plants is described as well as the
influence of the feed-back information on fuel design evolution.

The unique features of the CANDU reactor design that enable it to utilize a variety of fuel types
are described in other Chapters of this book. Alternative Fuel Cycles that have been considered
for application in the CANDU rector and Long Term storage and disposal of irradiated CANDU
fuel are also described elsewhere.

1.2 Learning Outcomes

The goal of this chapter is for the student to understand:
    The derivation of the design and operating requirements on CANDU fuel.
    The range of potential alternatives available and the rationale for selecting the specific
      fuel component design features in response to the specified requirements.
    The verification processes used to demonstrate that the selected component and assembly
      (fuel bundle) features can meet the specified requirements without experiencing failures,
      and that there are two distinctly different approaches employed for CANDU fuel.
    The methodology used for defective fuel detection and removal in CANDU reactors and
      the use of this operating reactor information in fuel design evolution.


1.3 Proposal Specifics


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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      4
1.3.1 Schedule

The following milestones are based on a 12 month production of the final draft. The
scheduled milestones are based upon a start date of 1 Jan 2012, and will be adjusted as needed.
The tasks, listed below, indicate the chapter sub-headings.

  Milestone        Month            Possible dates                     Tasks (sub-headings)
      1          1              2012 January                  Sign contract and agree on scope
      2          4              2012 April                    Draft sections 1 to 3
      3          6              2012 June                     Draft section 4.1
      4          7              2012 July                     Submission of draft chapter
      5          9              2012 September                Receive consolidated comments
      6          11             2012 November                 Revise draft and issue final

1.1 Costs (exclusive of HST).

                         Item                                         Hours                       Cost
Milestone 1 – 2012 January                                                             $1000
Milestone 2 – 2012 April                                                               $2500
Milestone 3 – 2012 June                                                                $2500
Milestone 4 – 2012 July                                                                $2500
Milestone 6 – 2012 November                                                            $1500

Travel                                                                                 $1000

Total                                                                                  $11,000

2 CANDU Fuel Design Requirements
Design/Operating Requirements that are imposed on CANDU Fuel are derived from several
sources, including the reactor systems that interface with the Fuel and from Licensing and/or
other jurisdictional entities.

The reactor systems that have the strongest (interfacing) influence on fuel are: A, B, C, D, and E.
Jurisdictional and/or licensing requirements are F and G. The requirements that are derived from
each of these sources are described in sub-sections below. The complete list of fuel Design
Requirements (DR) will be given in Table 1.


2.1 System A (for instance HTS)
A brief description of the interfacing system is provided with emphasis on aspects that affect fuel
design/operation. The requirements are enumerated and described. Typically, each major
interfacing system gives rise to a dozen or so requirements stated in the following format:
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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      5
      Fuel bundle shall be capable of withstanding the full range of operating pressures
       imposed by the HTS, including the hydrostatic test pressure.
      Etc.

2.2 System B (for instance Fuel Channels)
Similar to Section 2.1, for requirements that derive from System B

2.3 System C (for instance Fuel Handling)
Similar to Section 2.1, for requirements that derive from System C

2.4 System D (for instance Physics)
Similar to Section 2.1, for requirements that derive from System D

2.5 System E (for instance Defective Fuel Removal)
Similar to Section 2.1, for requirements that derive from System D

2.6 Licensing and/or Jurisdictional
Similar to Section 2.1, for requirements that derive from Licensing and/or other jurisdictions.



3 CANDU Fuel Design - Concept Decision
CANDU fuel bundle has two principal components, the end plate and the fuel element. The fuel
element has five (5) sub-components: the fuel pellet, the fuel sheath, the endcap, the CANLUB
layer, and the filling gas. The fuel sheath has three sub-sub-components, the Zircaloy tube, the
Zircaloy appendages, and the alloying metal for attaching the appendages to the tubes. Fuel
elements can be assembled into bundles having a number of configurations (cross sections) and
fuel element diameters. In the sections below we provide the rationale that is applied to selecting
the appropriate material/configuration for each of the components and configurations.

3.1 Sheath Component – Alloying Metal for Attaching the Appendages to the
    Tubes
Why is Beryllium used and not Iron or Silver, for instance?

3.2 Sheath Component – Appendage Material and Configuration
Why Zircaloy-4?
Why coined in a spiral configuration?

3.3 Sheath Component – Tube Material and Configuration
Why Zircaloy-4?
What thickness?

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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      6
3.4 Element Component – Filling Gas
Why helium/air mixture?

3.5 Element Component – CANLUB Layer
What material and thickness?

3.6 Element Component – Endcap
Why Zircaloy-4?

3.7 Element Component – Diametral Clearance
Why “collapsible”?

3.8 Element Component – UO2 Pellet
Why not U-metal or uranium carbide?

3.9 Bundle Assembly – End Plate
Why Zircaloy-4?
Why the selected thickness?

3.10 Bundle Assembly – Cross Section
28-element? 37-element? 43-element? 61-element? Element-to-element clearance?



4 CANDU Fuel Design Verification
Unlike the fuel used for chemical energy generation, nuclear fuel does not undergo a significant
change in its physical appearance during irradiation. The energy is produced by the splitting of
fissile uranium isotopes at the atomic level, which generate significant heat and irradiation
byproducts, at the atomic level, without exhibiting significant physical changes at the macro
level. The bundle is not “consumed” and looks essentially the same when it comes out of the
reactor as when it went in. Nevertheless the changes that do occur (at the atomic level), both in
the uranium dioxide pellets and the bundle structural material do accumulate to produce macro
changes and these are the effects that must be taken into account in assessing compatibility of the
bundle design in relation to the specified requirements. In this section we describe those
verification assessments.

Two different approaches have been used successfully to verify that CANDU fuel can meet the
specified design/operating requirements without experiencing defects. The traditional approach,
which was applied to all fuel types that are currently in use in operating CANDU plants, is
primarily a test based approach. A more generic approach has been developed in the recent past
that applies quantitative limits (design acceptance criteria) to the relevant material conditions,
with “all effects in”, for all defect mechanisms. In the past, this approach was applied to two
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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                        7
defect mechanisms, fuel overheating and power ramps. The approach, with some refinements
particularly related to the application of minimum acceptable margins, has now been extended to
all identified defect mechanisms for CANDU fuel.

In this section we will describe the two verification assessment approaches in detail including a
clarification of the main differences and how they affect the verification process.


4.1 Traditional Verification Methodology
4.1.1 Verification Topics
In this section, we will list the verification topics that have been traditionally covered in a typical
Fuel Design Manual, for example fuel temperature assessment, power ramp assessment, bundle
strength, etc.

4.1.2 Verification Strategy
In this section, we will describe the verification strategy used for each of the verification topics
listed in the section above, for example bundle “endurance” test at operating pressure,
temperature, and flow, etc.

4.1.3 Verification Assessments
In this section, we will summarize the verification assessment for each of the verification topics
listed in the section above, for example the assessment of the “endurance” test results to verify
that the bundle can withstand the relevant in-reactor design requirements without failing.

4.2 Generic Verification Methodology
The above test based methodology (Section 4.1) has served the CANDU fuel industry well for
many decades. The fuel has performed extremely well, as evidenced by the very low defect rate
in the CANDU commercial reactors. Despite a "near-death" experience involving power ramp
defects in early Pickering and Douglas Point fuels, and despite endplate failures in early
Darlington fuel that shut the reactor down for many months, subsequent fuel defect excursions
have been rare. When they did occur, the expertise and facilities available within the industry
were able to identify, isolate, and solve the problems effectively.

But sometimes even good things can be improved upon without taking away from the inherent
excellence of the original. The above test based methodology (Section 4.1) for design
verification does make some a-priori assumptions, out of necessity, that several important
parameters that cannot be easily included in the tests have low impacts on fuel performance.
While that is certainly true in some applications, it may not necessarily be true in other
applications, as illustrated below.

As one illustrative example, consider the out-reactor tests for bundle strength. For reasons of
cost, schedule, and availability of facilities, they are necessarily done on unirradiated fuel
bundles. Unirradiated Zircaloy has a reasonably high ductility -- of the order of some 20% or so
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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      8
minimum. In the C6 and Bruce reactors, the greatest demand for endplate strength occurs when
the fuel is being discharged. At that time, the fuel bundle has undergone significant irradiation
and in addition may also have potentially picked up significant amounts of hydrides depending
on irradiation conditions. Hence, depending on the discharge burnup, the ductility of irradiated
Zircaloy can be significantly lower -- say around 2% or so, a big drop from the unirradiated
value. For this reason, it is not intuitively clear whether or not a strength test on unirradiated
bundles can always give a sufficiently reliable confidence in the ability of irradiated fuel to
withstand the expected in-reactor loads.

As a second illustrative example, consider in-reactor power-ramp tests. Since they are expensive
and require a long time, only a small number can be practical for any new application. But it is
usually not practical to cover in those limited number of tests, statistically sufficient numbers of
credible combinations of the many operational, design, and fabrication parameters (including
tolerances) that affect power-ramp performance of CANDU fuel. This can severely limit the
confidence that the tests alone can provide in the ability of fuel to consistently survive the
expected power ramps. It is noted that, for instance, power ramp defect thresholds and the
relevant acceptance criteria were developed only after statistically significant defect data were
obtained in power reactors (Douglas Point and Pickering).

As a third illustrative example, consider the effects of gravity on fuel performance in a few
damage mechanisms through contributions such as pellet/sheath contact, sag of fuel elements,
etc. Tests done in our test reactors, NRX and NRU, (NRX is no longer in use), which are
vertical do not capture the effect of gravity experienced by fuel in the horizontal commercial
reactors.

For the above reasons, a more generic verification strategy has recently been developed with the
major aim of being more comprehensive and systematic, while ultimately also becoming more
timely and less expensive.

The new generic approach is structured around identifying four basic “building blocks” –
damage mechanisms; material failure limits, minimum acceptable margins, and design
acceptance criteria. These concepts are explained in further detail in Section 4.2.1. For a given
damage mechanism, the interrelationship amongst the latter three parameters is illustrated
schematically in Figure 1.

Once the above are defined, the new generic approach then systematically identifies all
parameters that have major influences on all credible damage mechanisms. This is followed by
crafting a judicious Verification Strategy that that combines appropriate tests and analyses to
optimize a balance of key objectives based on their relative priorities.

This approach is very similar to the current practice in CANDU safety analyses and in LWR fuel
verifications, and indeed is similar to design verifications of a large number of everyday
engineered structures such as CANDU pressure tubes, bridges, aircrafts, etc.


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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      9
4.2.1 Defect Mechanisms and Design Acceptance Criteria
In this section we will start by explaining the above four key building blocks of the new generic
approach, namely damage mechanisms, material failure limits, minimum acceptable margins,
and acceptance criteria.

From "first principles", 18 damage mechanisms have been identified that can potentially fail a
CANDU fuel element or bundle; for example: pellet melting, fuel sheath melting, excessive
strain, etc. A review of world-wide experience of actual failures of Zircaloy-clad fuels has
inspired confidence in the completeness of this list.

For each damage mechanism, an acceptance criterion has been identified that keeps operating
values (including uncertainties) below the appropriate failure limit by at least a pre-defined
margin. This minimum acceptable margin ensures that the damage limit is avoided consistently,
after considering “unknown unknowns". Variabilities through tolerances in design, fabrication,
and operational parameters as well as in material properties are included in an uncertainty
analysis.

In this section we will list the 18 damage mechanisms and their acceptance criteria. The numeric
values of the margins, however, are proprietary; they will not be discussed in this report.

4.2.2 Damage Scenarios
In this section, we will discuss the key damage scenarios that can result while the fuel meets its
requirements. For example, one consequence of refuelling in a C6 reactor is that the fuel can
temporarily experience high degree of local end flux peaking, especially when the endplate of a
fuel bundle is exposed to a column of water rather than to a neighbouring fuel bundle. This
results in locally elevated temperatures in the pellet, called end temperature peaking. This in
turn results in an elevated threat from central melting. In this section we will discuss the key
damage scenarios such as the above and the specific damage mechanism(s) that are activated.

4.2.3 Verification Strategy and Assessments
In the illustrative example above, how do we find a reasonably effective yet prudent and efficient
path to ensure that central melting will not occur as a result of end temperature peaking? Should
we, for instance, perform a series of in-reactor experiments in which we first irradiate many fuel
elements to a variety of burnups (to cover the effect of drop in melting temperature with burnup,
and also to cover statistical significance), then expose them to high local powers representative
of local end flux peaking, plus high channel powers, plus margins to cover variabilities? How do
we know that the few such experiments that can be practical, did not marginally escape a
looming precipice and that an unacceptable percentage of other mass-produced fuel bundles,
with slightly different combinations of fabrication variabilities within the permitted ranges of
tolerances, would not fall off that precipice in commercial reactors? Would the cost and timeline
of such a series of experiments be the most effective use of our limited resources, or can we
devise a more effective path? For example, can computer codes such as FEAT and ELESTRES
help provide sufficiently reliable and comprehensive yet faster and less expensive verification for
this damage scenario?
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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                      10

In this section, we will discuss the technique of selecting judicious verification strategies for the
above damage scenarios and the resulting verification assessments, along with some illustrative
examples.

4.2.4 Operational Constraints – Input to SOE
 Intrinsic to the fuel design verification process described above is the establishment of reactor
operating constraints which are driven by fuel; operating conditions that combine to exceed the
fuel design acceptance criteria are unacceptable to fuel and thus constitute operational limits. In
this section we will describe the processes involved in defining the plant operational constraints
based on fuel limitations and how this information is combined with similar information
provided by the designers of other systems and components to construct the overall plant Safe
Operating Envelope (SOE).


4.3 Illustrative Examples
In this section we will provide three illustrative "worked-out" numeric examples of some
activities towards verifying that a given fuel design will perform as intended:
       Determine the collapsibility of the sheath: CANDU fuel sheath is designed to collapse
          under the operating pressure and temperature of the reactor, to facilitate heat removal
          and improve the bundle’s reactivity. A numeric example will guide the student in this
          exercise.
       Determine pellet temperature. This is needed to ensure that the pellet will not overheat
          during operation.
       Determine internal gas pressure in the fuel element. This is needed to prevent
          overpressure failures in the fuel element.

5 Operational Feedback
5.1 Use of NRX and NRU in Establishing CANDU Fuel Principal Parameters
From the earliest days of CANDU power reactor Fuel development, the researchers and the
designers had the great advantage of being able to do irradiation tests using full size Fuel
elements (in NRX), and full size Fuel bundles (in NRU) under power reactor operating
conditions. In parallel with the development of Fuel design features, these experimental reactors
also provided the ideal test beds for developing the associated defective Fuel detection
instrumentation. In this section we will summarize the types of tests that were done and the
fundamental Fuel features that were established. For instance, one of the principal distinguishing
features of CANDU Fuel is the use of “collapsible sheathing”; another is the use of high density
(stable) ceramic pellets, and so on. The designer’s confidence in pursuing these design options
was driven principally by the empirical data obtained through the use of these facilities. Also, the
experience gained with the use of on-line defective Fuel detection was directly applicable to
power reactor applications.


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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                     11
5.2 Power Plant Operational Feedback
All CANDU plants are equipped with on-line Gaseous Fission Product (GFP) sensors. This
facility provides the operator with timely warning of a fuel defect having occurred. In addition to
the GFP, some plants are also equipped with individual channel Delayed Neutron (DN)
monitoring system that provides the information that enables the operator to identify the fuel
channel in which the defect has occurred. Using the on-power refueling capability, the operator
can detect and remove fuel defects on-power.

In this section we will describe the operation of the defective fuel detection instrumentation
(ideally with reference to a more complete description which may be provided in Chapter 7 –
Instrumentation), and how it aids the operator in detecting and removal of defective fuel. We will
also describe the pool side and hot cell examinations that are routinely done to identify the
defective fuel element(s) and categorize the defect mechanism (see Section 4.2.1).


5.2.1 Douglas Point
Douglas Point prototype reactor was equipped with a GFP and a DN system. We will describe
how these systems were used to detect, discharge, and categorize the large number of fuel
defects that occurred in the late 1960s and early 1970s and how this data was used in the
construction of the earliest versions of the FUELOGRAM (power ramp defect threshold).

5.2.2 Pickering
Pickering is equipped with a GFP system (no DN system). We will describe how the lack of a
DN system exacerbated the fuel defect excursion that occurred in the summer of 1971, how the
defect data was used (in combination with Douglas Point data) to construct the early versions of
FUELOGRAM and how the defect excursion pulled the entire CANDU fuel industry together to
develop CANLUB (elapsed time between the first tests and full implementation in Pickering fuel
– 24 months!).

5.2.3 Bruce
Bruce reactors are equipped with both a GFP and a DN system. We will describe the fuel bundle
design progression from the 28-element Pickering fuel bundles to the 37-element Bruce bundles.
We will also describe the effect of power cycling on fuel performance – Bruce was the first
CANDU reactor in Canada to be used in power cycling mode. Prior to Bruce the CANDU
reactor located in Karachi Pakistan was run in a load following mode for an extended period of
time and generated the most applicable load following data. The effect of power cycling will be
addressed in this section with reference to this data.

Having introduced 37-element bundles (reduced element rating) with CANLUB into Bruce,
helped to essentially eliminate power ramp defects in Bruce fuel except for one case of
“overstuffing” which caused sheath failures near the endcaps. Fuel defects in Bruce that occur
from time to time are of two types: defects related to manufacturing deficiencies, and defects
related to the style of Fuel Handling system used in Bruce. We will describe the “manufacturing
defects” with reference to CANDU Fuel Bundle Manufacturing (see Chapter 6). We are not
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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                     12
proposing to include a discussion on the effect of the Bruce Fuel Handing system on fuel defects,
but are prepared to do so if the client so desires.

5.2.4 CANDU 6
CANDU 6 reactors are equipped with both a GFP and a DN system. These systems have been
well maintained and have helped to make CANDU 6’s among the cleanest running reactors in
the world. We will describe the attention to detail that was applied to the calibration and
maintenance of the failed fuel detection systems (in particular the DN system) before the first
CANDU 6’s were put into service and how this attention to detail has been an asset to the
operator.

Similarly to Bruce, power ramp defects have been essentially eliminated in CANDU 6 reactors.
There have been several “defect excursions” in CANDU 6 reactors related primarily to debris
and to manufacturing flaws including one case of “under-baked CANLUB” which caused
hydriding failures in the sheath. These defect excursions will be discussed in relation to their
effect on fuel design and manufacturing standards that have been implemented since their
occurrence.


5.2.5 Darlington
Darlington reactors are equipped with a GFP system and an experimental feeder scanner system
(no DN system). Similarly to Bruce and CANDU 6 reactors, power ramp defects have been
essentially eliminated in Darlington reactors.

The most notable fuel defect excursion that occurred in Darlington happened in 1992 in unit #2.
It is noteworthy that the elimination of this defect type was not accomplished by a design change
in fuel. Feedback from this defect excursion will be discussed in the context of fuel Design
Requirements, and the imperative need to understand the effect of all interfacing systems on fuel
integrity.

5.2.6 Fuel Defect Statistics
A recent IAEA survey for 1994-2006 has found that CANDU fuel has by far the lowest defect
rate among all reactor types in the world. In this section we will present the published defect
statistics for all reactors in the world and relate the superior position of CANDU to the other fuel
types.




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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                     13




                           Table 1 - The Complete List of Fuel DR.

                    Interfacing System               Fuel DR
                    System A                         Requirement A1
                    System A                         Requirement A2
                    Etc.                             Etc.




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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                     14




  Figure 1 Schematic Diagram Illustrating Minimum Acceptable Margin in Fuel Design




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Chapter 5 - Fuel, by M. Gacesa and M. Tayal                                                                        15




About this document:
Authors and affiliation:

Milan Gacesa
      Independent Consultant; gacesam@sympatico.ca;                  905-689-9681
Mukesh Tayal
      Independent Consultant; tayal_m@hotmail.com;                   905-827-2846

The authors have between them, over 70 years of professional experience in CANDU fuel
encompassing design, verification, fabrication, performance feedback, research, development,
testing, analyses, model development, storage of spent fuel, and fuel cycles. They have executed
responsibilities in technical, supervisory and managerial roles at Atomic Energy of Canada
Limited and at Westinghouse Canada Limited (subsequently acquired by Zircatac Precision
Industries, now part of CAMECO Corporation).

Revision history:
Revision 2.0, 2011 November 14, Original (rev. 1.0) updated to reflect a request from the
Working Group to separate out Fuel from Fuel Cycles into different chapters.


Source document archive location: See page footer.

Notes:

This Chapter assumes that corroborative information will be provided by other Chapters as follows:
     To fully develop Section 4.2.4 on Operational Constraints, the contents of the book would have to
       be expanded to include a Chapter on Safe Operating Envelope, and commitments would have to
       be obtained from authors producing material for other disciplines to generate the relevant input to
       be included in such a Chapter.
     The DN system is mentioned several times in Section 5.1 and several sub-section of Section 5.2.
       Input from the author of Chapter 8 will be required on the design, installation and calibration of
       the DN system. Likewise some input from the same author will be required for the proper
       coverage of the GFP system.
     Agreement among all authors will be required on the subject of power cycling. We will have to
       agree whether or not the CANDU is capable of power cycling, and if yes, what type. Bereznai’s
       notes claim, rightly, that the plant can operate in the “normal mode”, which entails power changes
       of 2% or 3%. What are we going to say about load following?
     There will have to be a thorough exchange and review of requirements (at least as that aspect is
       envisioned to be handled in Chapter5). Do all the authors of major systems have a section on
       requirements and how they arise?



                                            C:\Docstoc\Working\pdf\55e27e50-2143-45e3-9804-b049562e12dc.doc 2012-11-18

								
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