workshop on research reactor ageing and self...

International Atomic Energy Agency

Workshop on Research Reactor Ageing Workshop on Research Reactor Ageing and Self-Assessment Methodologiesand Self-Assessment Methodologies

Module 3 Ageing Phenomena

S.3.2 Mechanisms and consequences

This presentation is excerpted from material originally presented by Kim, Yong-soo of the Hanyang University Nuclear Engineering Department, Seoul, Korea

CONTENTS

3. Introduction

4. General Service Conditions and Ageing

5. Physical Mechanisms and Effects of Ageing

6. Radiation Damage and Effects

7. Corrosion

6. Non-physical Mechanisms and Effects on Ageing

7. Current Trends and Future Activities in Research on Ageing

1.INTRODUCTIONService Conditions

Service conditions which contribute to ageing act through chemical and physical processes that affect material properties or functional capabilities. These are:

- Stress and/or strain

- Temperature

- Environmental factors such as radiation, high humidity or the presence of chemically active liquids or gases (before or during operation)

- Service wear and corrosion, including changes in the dimensions and/or the relative position of individual parts of assemblies

- Excessive testing

- Inadequate design, improper installation or maintenance.

In addition to these service conditions, there are conditions not related to chemical or physical processes which can lead to obsolescence and affect reactor safety. These are:

- Technology changes- Safety requirements changes- Obsolete documentation- Inadequate design- Improper maintenance

Degradation of Materials

The main effect of ageing is degradation of materials. This degradation may be:

- A change in physical properties (e.g. electrical conductivity)

- Irradiation embrittlement

- Thermal embrittlement

- Creep

- Fatigue

- Corrosion, including corrosion erosion and corrosion assisted cracking

- Wear (e.g. fretting) and wear assisted cracking (e.g. fretting fatigue).

2. GENERAL SERVICE CONDITIONS AND AGEING

Ageing effects are normally discussed in terms of undesirable effects or failures. However, the basic causes of ageing phenomena are frequently service conditions which support the actuation of the particular ageing mechanism leading to these effects. In brief it can be said that:

Service Conditions plus

Ageing Mechanisms lead to

Undesirable Effects or Failures.

Normal Operation Conditions

Normal operation conditions such as radiation, temperature or pressure will affect the physical properties of a material. Radiation affects components in and outside the reactor core. Other components may be affected by radiation from radioactive materials circulating with the coolant. While the effects of temperature and pressure are more noticeable in power reactors, they are also present in research reactors in materials such as gaskets. Cycling temperature or pressure variations may accelerate deterioration. Table 1 provides summary information on specific ageing mechanisms and Table 2 lists the ageing degradation mechanisms and susceptible materials and components.

Anticipated Operational Occurrences Conditions

Following anticipated operational occurrences (e.g. fire, flooding, overheating or power excursions) acceleration of ageing effects may occur. It is advisable to investigate and to take corrective actions to stop accelerated ageing. The effects of ageing under abnormal conditions are summarized in Table 3.

Environmental Conditions

Environmental conditions include climatic conditions such as humidity, frost and winds as well as site conditions such as salinity, sand, dust or chemical agents. The effects of these conditions are, in general, corrosion, erosion or undesirable chemical reactions occurring to the equipment exposed to such conditions. Table 4 summarizes information on these conditions and induced ageing mechanisms.

Table 1. Effect of Ageing Under Several Service Conditions

Conditions Ageing mechanism Consequence/failure

Radiation Change of properties - chemical decomposition- strength change- ductility change- colour change- swelling- resistivity change- burnup

Temperature Change of properties - strength change- resistivity change- ductility change- colour change

Stress (pressure) Creep -changes of geometry (e.g. break collapse)

Cycling of temperature Motion - displacement- change of position or set point- loosing connections

- Wear - deterioration of surface- change of dimensions

Oscillating stress Fatigue - break, collapse- deformation

Flow Erosion - strength change

Fluids chemist Corrosion/galvanic cells - release of radioactive material- strength change- deposition of particles- short circuits- leakage conditions


(Continued)

Table 2. Effect of Ageing Under Abnormal Condition


Power excursion Thermal and mechanical - deterioration of systems damage - accelerating ageing

Flooding Deposition and chemical - Corrosion

contamination

Fire Heat, smoke, - reduction of strength reactive gases - corrosion

Table 3. Effect of Ageing Under Several Environmental Service Conditions


Humidity Corrosion/galvanic cells - leakage- release of radioactive material- strength reduction- deposition of particles- short circuits

Chemical agents Chemical reactions -undesirable chemical products- deterioration of structures

Wind, dust, sand Erosion and deposition - strength change- deterioration of surface- malfunction of components

Table 4. Aging Degradation Mechanisms and Susceptible Materials Components

Degradation mechanisms Susceptible materials and components

General corrosion, pitting, and wastage Crevices and hideout regions, low and no flow (low and high temperature) components, safety injection system,

service water system Stress corrosion cracking on external surface Weld vicinity in components (low and high temperature) (off-normal chemistry conditions) Stress corrosion cracking on external surface Components near leaking valves and (chloride related; low and high temperature) in coastal plants (e.g. insulation) Irradiation assisted stress corrosion Reactor pressure vessels and internals Erosion-corrosion (high temperature) Steam piping and steam seperation,

heat exchanger (i.e. moisture separator reheater),turbine blades

Crevice corrosion (low and high temperature) Stagnant regions, weld vicinity, sleeved regions,weld with backing rings


Microbial influenced corrosion Service water, heat exchangers,(low temperature) equipment where pressure tests are performed,

equipment laid up, anchor bolts, diesel generatorsCorrosion fatigue Thermal mixing regions, (low and high temperature) especially carbon and alloy steelsFatigue (low and high temperature) Rotating equipment supports

and piping attached to large componentsWeld related cracking (lack of fusion, Similar metal welds, wrought materials to castings,hot ductility, ferrite depletion, low ferrite filler joints, seam weldscrevice formation; high or low temperature)Dilution zone cracking Dissimilar metal welds, vessel to clad interface,(high or low temperature) nozzle to safe-ends, valves or pump to pipe

(carbon steel to stainless steel)

(Continued)


(Continued)

Low temperature sensitization Stainless steel components,

(high temperature) cast components

Thermal embrittlement Ferritic stainless steels, cast stainless steels

(high temperature)

Irradiation embrittlement Reactor pressure vessel,

internals and support structure

Hydrogen embrittlement High strength, low alloy components,

(low temperature) vessel cladding (ferrite phase),

interface between vessel cladding and vessel,

anchor blots, vessel and pressurizer supports

Mechanical wear, fretting Rotating equipment

(low and high temperature)

Binding and wear Components within pumps and valves

Creep and swelling (high temperature) Vessel internals (radiation assisted)

Insulation embrittlement and degradation Cables, motor windings, transformers

Thermal runaway (dielectric materials) Capacitors, inductors,

medium and high voltage equipment

Partial discharges Transformers, inductors,

medium and high voltage equipment

Oxidation Relay and breaker contacts, lubricants,

Insulation materials associated

with electrical components


(Continued)

3. PHYSICAL MECHANISMS AND EFFECTS OF AGEING

Following subsections deal with the general effects (physical changes) of several ageing mechanisms on materials, components and systems of a research reactor which may be induced by specific service conditions. It provides a checklist of ageing related problems which would be expected for different service conditions. If several of the following conditions exist simultaneously, the ageing process may be accelerated. The statistics of reported ageing mechanisms in Table 5 reveals that corrosion and radiation are the dominant mechanisms in the research reactor ageing. Therefore, the issues on the corrosion and the radiation are dealt with separately in Sections 4 and 5, following brief review of previously listed mechanisms in this section.

3.1. Radiation

The effect of neutron irradiation on metals is mainly to increase the yield and the ultimate strength and to reduce the toughness. Helium or fission gas production within the metal matrix leads to changes in material properties and also to swelling. Swelling is particularly important in reactor control devices made of boron compounds.

Fast neutron irradiation of graphite causes displacement of lattice atoms and leads to graphite growth and distortion. The Wigner effect in graphite is also a problem to some high power reactors. For these reactors, embrittlement of beryllium components must also be considered.

Concrete is traditionally used as a shielding material. However, severe damage from radiation is not expected under most research reactor operating conditions because the concrete is usually not in a high radiation field.

Electrical and electronic equipment is generally located in low radiation level fields. Where that is not possible, (e.g. coaxial and other cables) proper action should be taken to inspect and renew equipment.

All organic materials and glass are radiation sensitive and they should be carefully selected and monitored during use.

3.2. Temperature

Attention should be paid to proper cooling of experimental facilities and reactor structures such as thermal columns and concrete shields, as well as to electrical and instrumentation cables which may be located in unventilated hot areas. A temperature above 60o may cause degradation of concrete by dehydration with a corresponding loss of integrity and neutron shielding effectiveness.

Elevated temperature in polymers results in hardening or a loss in tensile strength and elastic qualities even at the temperatures associated with research reactors.

3.3. Pressure

Research reactors operate at much lower pressure than power reactors. Therefore, pressure alone usually does not impose high stress on components in these reactors. Local high stress areas should be considered separately. Special care should be taken with experimental devices operated at high temperature and/or pressure.

3.4. Cycling

Vibrations, cycling of pressure, and cycling of temperature develop loading stresses which may cause cracking of material and eventually a fatigue fracture. Vibrations may cause degradation of electronics components and instrumentation. Vibration associated with the integrity of bonds and seals may be an important factor in their integrity. Change of position or of a set point is another phenomenon connected to vibration. Repeated relative motion of adjacent parts may result in fretting or wear.

3.5. Corrosion

Corrosion is the reaction of metal with its environment. Corrosion leads to material loss with surface degradation and loss of strength. Some types of corrosion (e.g. intergranular corrosion, stress/strain corrosion, corrosion fatigue) lead to loss of strength through crack enhancing.

Another effect of corrosion is deposition of particles (corrosion products) in vulnerable places (e.g. valve seat) to impair the function of a component. These particles may contain radioisotopes, which complicate maintenance work. As corrosion products occupy a larger volume than that of the metal itself, fill up of crevices and narrowing of passages can also be expected. Corrosion of reinforcing bars in concrete should be taken into account.:

3.6. Chemical reactions (other than corrosion)

Some environmental conditions may cause deterioration of structure or equipment through chemical reactions other than corrosion, e.g. reaction of the structure or equipment with ozone or NO2. Use of chemicals may cause damage to equipment. Special care should be taken when irradiating capsules containing materials such as copper or mercury which may cause strong corrosion in aluminum alloys.

3.7. Erosion

Operational conditions such as high velocity of coolant fluid may cause erosion in equipment such as pipes and heat exchangers. Erosion results in deterioration of surfaces and reduction in strength. Environmental conditions such as high winds and sand storms may cause erosion in outside structures.

Table 5. Statistical Distribution of Reported Ageing Mechanisms

Radiation 1 XXXXXXXXXXTemperature 2Pressure 3Cycling 4 XXXCorrosion 5 XXXXXXXXXXXXXXXXChemical 6 XXXXXXXErosion 7 XXXTechnology changes 8 XXXXXSafety requirements 9Documentation 10Human factors 11Design/operation/maintenance 12 XXXXXXXXX

Keys:X Actual problem Concern, problem not confirmed

4. RADIATION DAMAGE AND EFFECTS

Radiation damage: primary and microscopic events preceding the appearance of gross change in the solid

Radiation effects: macroscopic, observable, and often crucial results of exposure of solids to energetic particles

Characteristic time of radiation damage is less than 10-11 sec while subsequent processes require much longer times: i.e., the diffusion of radiation-produced point defects to sinks takes milliseconds and the nucleation and growth of voids is of the order of months.

Damage is isolated with electrons and light ions while it is clustered with heavy ions and fast neutrons.

Radiation Effects

Four broad categories of mechanical behavior pertinent to reactor performance: Irradiation hardening

Embrittlement and fracture Swelling Irradiation creep

Evolution of Microstructure During Neutron Irradiation

Black-dot structure Observed when irradiated at ~100 oC

by a fast neutron fluence of ~1021/cm2. Believed to represent the depleted zone or vacancy clusters.

Dislocation loops (Frank sessile loops) Formed by condensation of radiation-produced vacancies or interstitials into

roughly circular disks followed by collapse of the atomic planes adjacent to the platelet.

Voids Embryo collection of vacancies grows in

3-D manner rather than collapse into a dislocation loop

(Carbide) Precipitates In pure metal, only voids and dislocation loops

are produced by intermediate-temperature irradiation.

In a material as complex as stainless steel, however, the irradiation also cause different solid phases to precipitate.

Between 400 and 900 oC, exposure of austenitic stainless steel to fast neutron fluence between 1021 and 1022 n/cm2 produces carbide precipitation, Cr23C6 or Fe23C6.

Helium bubbles n (B10, Li7) n (Ni58, Ni59) n (Ni59, Fe56)

5. CORROSION

5.1. Corrosion in General

Definition

Chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material or of its properties.

Forms of Corrosion 1. Uniform or general corrosion

2. Galvanic or bimetallic corrosion3. Crevice corrosion4. Pitting corrosion5. Intergranular corrosion6. Selective leaching or dealloying7. Impact corrosion (erosion corrosion, impingement corrosion, cavitation corrosion and fretting corrosion)8. Stress-corrosion cracking (including corrosion fatigue)

Uniform corrosion

Uniform corrosion, also known as general corrosion, takes place evenly over the entire surface of the metal. This is because the anodic and cathodic reactions are uniformly divided over the surface. Uniform corrosion is the commonest form of corrosion.

Localized corrosion

In all other cases the attack is more localized as a result of the presence of heterogeneous situations. The essence of localized corrosion is that fixed anodic sites on the surface can be indicated where the oxidation reaction dominates, surrounded by a cathodic zone where the reduction reaction takes place.

Localized corrosion is far more treacherous in nature and far less readily predictable and controllable, and it is moreover capable of leading to unexpected damage with disastrous consequences.

Galvanic corrosion

The coupling of two metals with different potentials in a conductive electrolyte solution results in accelerated attack to the anodic metal and reduced attack to the cathodic metal.

In galvanic corrosion, the entire metal surface becomes anodic due to contact with another more noble metal. At the anode the following reaction takes place to the bivalent metal M:

At the (noble) cathode, in aerated, neutral or alkaline media oxygen reduction will take place according to the following reaction:

In certain cases, especially in acid media or in the absence of oxygen, the cathodic reaction can continue, releasing hydrogen:

or, in a neutral medium:

Galvanic corrosion is a very common phenomenon, often in unexpected places. Another form of galvanic corrosion is the pitting galvanic corrosion caused by the deposition of noble metals or metal oxides on less noble metal, which is also known as deposition corrosion (corrosion due to cathodic contamination of the anode).

eMM 22 + +

++ OHeOHO 222 22

223 222 HOHeOH +++

22 222 HOHeOH +++

Crevice corrosion

Crevice corrosion is a common form of corrosion to which active-passive metals are particularly sensitive. Crevice corrosion is generally observed where small amounts of standing electrolyte solution occur, e.g. between flanges, bolts, nuts etc.

Pitting corrosion

Pitting corrosion is a dangerous form of localized corrosion, capable of causing holes to occur in the metal. Pitting corrosion is often difficult to observe because of the small diameters of the pits, and because they are often covered with corrosion products. Two types of pitting corrosion can be distinguished, namely:

pitting caused by halides (generally chloride pitting of stainless steel), and

pitting of carbon steel caused by oxygen attack.

Chloride pitting is also an autocatalytic process, in which conditions arise which have a stimulating effect on the process. Inside the pit, the metal M dissolves allowing chloride ions to migrate to the positive charge in the pit. The resultant metal chloride hydrolyses, forming an insoluble hydroxide and hydrochloric acid. Outside the pit, oxygen reacts to hydroxide. These reactions are the same as those discussed for crevice corrosion. In effect, crevice corrosion is a special form of pitting.

The mechanism of oxygen attack to carbon steel is shown in the figure below. Local small differences in structure at the steel surface due to minor differences in carbon content, heat treatment or mechanical deformation allow small anodic and cathodic sites and hence uniform corrosion to occur.

In a neutral to slightly alkaline oxygen-rich medium, the following reactions occur:

At the anode

oxidation:

At the cathode

hydrogen formation:

depolarization:

oxygen reduction:

hydroxide formation over the entire steel surface:

further oxidation:

eFeFe 22 + +

HeH ++

OHOHH 2221 ++

++ OHeOHO 2221

22

22 )(2 OHFeOHFe + +

++ 3222 )(221)(2 OHFeOHOOHFe

This can further react through to .Ultimately a layer of rust forms on the steel, consisting of a whitish-green/black layer of hydrated ferrous oxide ( ) and a reddish-brown layer of hydrated ferric oxide ( ) at the water side. In between, a transitional area may form consisting of a black magnetic layer of hydrated ferroferrite ( ), magnetite.

The rust layer, formed of poorly soluble compounds, reduces the diffusion of to the surface and hence the corrosion rate. Ultimately regions will form underneath the rust layer where oxygen is no longerpresent. This represents the beginning of a new form of corrosion by the development of oxygen concentration cells. This is accompanied by the formation of rust modules, also known as tubercles, belowwhich there is pitting corrosion to the steel. Besides ions, , and ions also migrate through the tubercle to the positive anode surface, further intensifying the corrosion. Although this in effect represents a new corrosion phenomenon (tuberculation), this form of pitting is generally regarded as oxygen potting.

22 )(. OHFeOnHFeO =

FeOOH 32OFe

3232 )(. OHFeOnHOFe =

OnHOFe 243 .

Fe

2O Fe

OH2

3COCl 24SO

Intergranular corrosion

All metals are built up of small crystals or grains, the surface of one grain being adjacent to that of another grain and thereby forming grain boundaries. Under certain conditions, small areas near the grain boundaries can become much more reactive (because they are more anodic) than the bulk of the grains. The corrosion is able to penetrate metal via the grain boundaries, and it is therefore known as intergranular (also: intercrystalline) corrosion.

Selective leaching (dealloying)

Selective leaching is the selective removal of a particular element from an alloy due to the occurrence of corrosion. In many cases this corrosion is not visible to the naked eye, although perforation or fracture may nevertheless occur due to the reduced strength. In this process, the most active component of an alloy will selectively enter into solution, while the rest remains as a porous and mechanically highly weakened mass. The best-known example is dezincification of brass, which occurs especially under deposits of dirt.

Impact corrosion

Erosion corrosion, Impingement corrosion, Cavitation corrosion and Fretting corrosion

Stress corrosion cracking and corrosion fatigue

This involves an interaction between the metal, its environment and a mechanical load. Stress corrosion cracking is caused by a constant tensile strain acting at the surface. Stress corrosion occurs in both intergranular and transgranular form.

5.2. Corrosion Behaviour of Aluminum Alloys

Corrosion Behaviour in Aqueous Environments

Aluminium is a very reactive metal with a high affinity for oxygen. The metal is nevertheless highly resistant to most atmospheres and to a great variety of chemical agents. This resistance is due to inert and protective character of the aluminium oxide film which forms on the metal surface. In most environments, therefore, the rate of corrosion of aluminium decreases rapidly with time. In only a few cases, e.g. in caustic soda, does the corrosion rate approximate to the linear. A corrosion rate increasing with time is rarely encountered with aluminium, except in aqueous solutions at high temperatures and pressures.

The corrosion resistance of aluminium and its alloys is largely due to the protective oxide film which within seconds attains a thickness of about 10 on freshly exposed metal; continuation of growth is markedly influenced by the environment, being accelerated by increasing temperature and humidity.

Characteristic Features of Corrosion Behaviour

Pitting is the most commonly encountered form of aluminium corrosion. In curtain near-neutral aqueous solutions a pot once initiated will continue to propagate owing to the fact that the solution within the pit becomes acid, and the alumina is no longer able to form a protective film close to the metal. When the aluminium ions migrate away from the areas of low pH, alumina precipitates as a membrane, further isolating and intensifying local acidity, and pitting of the metal results. Solutions containing chlorides are very harmful, particularly when they are associated with local galvanic cells, which can be formed for example by the deposition of copper from solution or by particles such as iron unintentionally embedded in the metal surface. In alkaline media pitting may occur at mechanical defects in the oxide. Pits usually have no crystallographic shape although structurally indicative etch pits can be produced on aluminium.

Where perforation is the criterion of failure, statistical analysis may be judiciously applied to the distribution and depth of pits.Copper is usually harmful causing increased susceptibility to intercrystalline or general attack, so that alloys containing copper should be regarded as less corrosion resistant than copper-free materials.

With increasing purity of aluminium, greater resistance to corrosion is developed. On high-purity materials, however, any pots which develop are likely to be deeper though fewer in number than those formed in more impure metal.

Aluminium is anodic to many other metals and when it is joined to them in a suitable electrolyte the potential difference causes a current to flow and considerable corrosion can result. In practice, copper, brasses, and bronzes in marine conditions cause the most trouble. Contact with steel, though less harmful, may accelerate attack on aluminium.

5.3. Case Study

[Corrosion Problem in CRENK TRIGA MARK-II Research Reactor, M. W. Kalenga, Kinshasa, Zaire, IAEA-SM-310/4]

5.3.1. Introduction

Twelve pits were identified using an above water telescope. The corrosion problem seemed serious. The tank housing the core was in fact, manufactured in 1965 in Austria using 6061-T67 aluminum. The inspection of the aluminum tank carried out after the removal of the core structure showed a less serious corrosion problem than was anticipated. Only 8 pots were identified as resulting from the corrosion of the aluminum plate. The most advanced corroded spot has a depth of 2.5 mm. The range of the depths of the 8pits was between 1 mm and 2.5 mm. The pit shape vary somewhat but the mouth of a pit tended to be circular, while the cross section was roughly hemispherical.

5.3.2. Evaluation of The Causes of The Corrosion Problem

Many factors can be at the origin of pitting corrosion of aluminum alloy. As far as the initiation of pitting corrosion of aluminum is concerned the factors to be considered fall into three categories:

a) Chemical factors: Chloride, Calcium Bicarbonate, Copper, Mercury, Chromium, Lead, Oxygen.

b) Metallurgical factors: wrong thermal treatment; intermetallics compounds such as (Fe A13), (Al Cu2), (Al Fe Si) which create cathodic areas with respect to pure aluminum, while the compound of aluminum with Zinc or Magnesium produce anodic area with respect to pure aluminum; difference in reaction rate between crystals of different orientation.

Microbiological factors: bacteria, such as desulfovibric desulfuricans acting in the presence of cations or anions as cathodic depolarizing agents to reduce sulphate to sulphide.

As far as the corrosion of the CRENK aluminum tank is concerned one has to take into account the fact that the aluminum plate used to fabricate the bottom of the reactor tank is different from those used to manufacture the reactor tank wall.

Since all the corrosion pits that were identified are located on the tank bottom, it is likely that the initiation of the corrosion process is due essentially to metallurgical factors; either:

a) to impurities in the original aluminum plate used to make the bottom of the tank; or

b) to impurities incrustrated in the plate during the machining process; or

c) to impurities dropped into the tank bottom during the eighteen years of operation of the reactor.

Taking into account the fact that all the pits are located outside the area covered by the core, that is outside the reflector, it is likely that the corrosion process was initiated by galvanic couples, with impurities dropped into the aluminum tank during the operation of the reactor playing the cathodic role. If the pit created survives the initiation phase, it propagates by galvanic reaction with the aluminum being anodic and the impurities being cathodes.

5.3.3. Rate of Penetration of Pits in Aluminum

It is important to evaluate the rate of penetration of pits in aluminum. It allows one to determine if the repair of the aluminum tank bottom is necessary. One can use the following formula to evaluate the rate of penetration:

Where = maximum pit depth; = time; = constant that depends on the alloy and the environment.

Since 17 years separate the time of the manufacture of the tank bottom and the time that the maximum pit depth of 2.5 mm were measured, it follows from relation 1 that in the case of the CRENK aluminum tank the constant is in the range:

If the same rate of penetration holds for the future the deepest pit will go through the aluminum tank bottom at a time, , such that:

With = 2.5 mm; that is:

Taking into account the lower value of, , one can wait for 10 years before carrying out the repair of the aluminum tank bottom. We thought it advisable, however, to repair the tank bottom right away.

3/1)(tKd =

5.2981.0 21 =

5.3.4. The Repair of The Corrosion Damage

To remedy the corrosion problem one has to fill the cavities created by the pits. Three solutions can be considered. The first solution is to fill the pits with aluminum by welding procedure. The second solution is to use concresive epoxy resins. The third solution is to use silicone rubber. The repair was carried out using silicone covered by small pieces of 1 mm aluminum to act as a protective barrier.

5.3.5. Safety Upgrading of The CRENK Nuclear Reactor

To avoid the recurrence of the corrosion problem, the following measures were taken:

a)The top of the aluminum tank was sealed more tightly than before to avoid the drop of foreign objects into the reactor;

b)To improve the water chemistry, and to reduce bacteriological growth in the water, frequent and intensive stirring of the water in the reactor tank is carried out using the primary circuit water pumps;

c) More precise monitors of the pH and resistivity of the deminarelized water was installed;

d) A careful control of Chlorine in the water in the tank is now carried out on routine basis;

e) Since corrosion of aluminium may be noticed by peak values of some elements, such as Fe, Cu, SO2, in the water, chemical analysis of the water is now carried out every week;

f)An underwater telescope using a flexible endoscope is being manufactured to monitor constantly the bottom of the aluminum tank, particularly the area under the core which is not visible from the top of the reactor.

5.3.6. Conclusions

It is our conclusion that the corrosion process of the CRENK reactor aluminum tank bottom was due to galvanic couples initiated by the drop into the reactor tank of materials being cathodic to the aluminum, such as Iron, Titanium, Vanadium, Nickel or Copper. It was not possible to determine the exact galvanic couple but the most likely candidate is Iron-Aluminum.

Although care was taken to insure that the pH of the water in the reactor tank was always close to neutral as possible, it should be mentioned that the tap water in Kinshasa is strongly acidic in nature, which make it a good electrolyte for the galvanic couple. At all events other factors than metallurgical one, such as bacteria, can intervene to make a pit propagates once it has survived the initiation stage.

With the most advanced corroded pits having a depth of only 2.5 mm out a possible maximum of 8 mm the safety of the reactor was certainly not a short term issue. However we thought it worthwhile to meet the challenge and dismantle the reactor in order to gain a better insight of the causes of the corrosion problem.

6. NON-PHYSICAL MECHANISMS AND EFFECTS OF AGEING

As mentioned, there are conditions not related to chemical or physical processes which can lead to obsolescence and affect reactor safety.

Technology Changes

Research reactors were built according to the standards and with the equipment available at the time of construction. Since then progress in technology, especially in electronics has been made. Even if the original instrumentation and control (I&C) systems of the reactor still function well, getting spare parts becomes difficult. This may make it necessary to replace the entire I&C system in order to facilitate a proper maintenance program.

Requirements Changes

For many research reactors, the elapsed time has brought many new safety requirements since their construction. The hardware and documentation of the reactor should be changed accordingly. These type of modifications are called "backfitting" activities.

Obsolete Documentation

The utilization of the reactor demands modifications and changes of experiments which tend to make the reactor documentation obsolete. A good ageing management program should include the updating of operational manuals, drawings, specifications and other documentation.

Inadequate Design

Inadequate design includes selection of wrong materials and inassessability for inspection and repair. The consequences may be the accelerated physical ageing. To overcome the effects of inadequate design it may require a decrease of reactor power to lower the rate of ageing or the need for more frequent inspections and tests.

Improper Maintenance

Improper maintenance may increase the physical effects of ageing in various ways. For example, increased pressure on bearings as a result of excessive tensioning of retaining bolts will accelerate wear. The use of a trained staff is important. Records should be appropriately generated and retained. Following table summarizes the effects of ageing due to non-technical mechanisms.

Table 6. Effect of Ageing for Several Technology and Safety Conditions


Technology progress Shortage of spare parts, - maintenance difficulties

- disappearance of suppliers

Change of safety Obsolescence of existing - interference with operation

concepts safety components & - modification of safety related

systems components and systems

Lack of administrative Incomplete updating - incomplete information

procedures, obsolescence

of documentation

Inadequate design Various - accelerated ageing

- may cause or support other

undesirable operating conditions

Improper maintenanceVarious - deterioration of systems

7. CURRENT TRENDS AND FUTURE ACTIVITIES IN RESEARCH ON AGEING

Much information is available on ageing problems of NPPs. Extensive programs are being conducted in the IAEA and in its Member States. The objectives of many of the studies are metal ageing (mostly steels) and degradation of concrete structures, electric equipment, electronic components, elastomers and lubricants. Most of this research has been carried out in research reactors, some of them almost exclusively dedicated to material testing.

Some of these research activities may also apply to nuclear research reactors. Operators of research reactors may benefit from the experience gained in preparing management program for ageing in NPPs. Methodological approaches can be adapted as well as specific results of research on topics such as corrosion and ageing behaviour of electric components and electronics.

Areas have been identified where further development is needed in the methods to detect and mitigate ageing effects. Some of these areas, applicable to research reactors, are as follows:

- Better understanding of ageing mechanisms

- Engineering studies on individual components which may be identified as having ageing related impact on the reliability of safety related systems

- Refining techniques for reliability predictions

- Improving methods for predicting remaining service life

- Improving guidelines for service condition monitoring.

In fact, nuclear research reactors have unique problems which should be specifically dealt with. Operators of research reactors have been dealing with such problems for many years initially as a response to actual problems in specific reactors and more recently as systematic research programs.

The main items of research reactor programs related to ageing are:

- The aluminum reactor tank and other components

- The effects in graphite and beryllium (reflector and moderator materials)

- Obsolescence of electronic equipment

- Corrosion of cooling and other systems components

- Deterioration of heat-exchanger tubes

- Degradation of cooling towers

- Concrete structures

- New safety requirements.

The first item, ageing effects in aluminum components, is unique to research reactors and little information may be found in the NPP literature. The rate of degradation for the other issues may be not as large in research reactors as in NPPs, but the relative age of existing research reactors increases their importance.

In addition, guidelines for research reactors components life extension have been suggested and implemented in some Member States. Also, a decision making process to shutdown, refurbish, modify or decommission research reactor is currently taking place in several Member States.

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