Decommissioning of CANDU Nuclear Power Stations - of CANDU Nuclear Power Stations by G.N. Unsworth Mr. Unsworth is Head, Maintenance and Construction Branch, Whiteshell ... associated

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  • Decommissioning of CANDUNuclear Power Stationsby G.N. Unsworth

  • This publication It a revised edition of a report previouslypublished as Chapter S of AECL-S800.

  • Decommissioning of CANDUNuclear Power Stationsby G.N. Unsworth

    Mr. Unsworth is Head, Maintenance and Construction Branch, WhiteshellNuclear Research Establishment at Pinawa, Manitoba. He is an electrical

    engineer with 20 years' experience in reactor operations.

    April 1979 AECL - 6332

  • This report summarizes the results of a detailed studyof the various procedures and costs associated withdecommissioning a CANDU reactor. The three inter-nationally recognized "stages" of decommissioning(mothballing, encasement, and dismantling) arediscussed. It is concluded that decommissioning ispossible with presently available technology, and thacosts could be financed by only a marginal increasein the cost of electricity during the life of the reactor.The environmental impact would be no greater thanthat of any large construction project.

    Ce rapport resume les resultats d'une etude detailleedes diverses methodes et des couts associes a lamise hors service d'un reacteur CANDU. Les troisetapes de mise hors service (la mise en reserve, lescellement et le demontage) reconnues international-ement sont examinees. Cette etude conclut que lamise hors service peut etre realisee dans le cadre dela technologie actuelle et que les frais encourus peu-vent etre finances facilement par un accroissementmarginal du cout de I'electricite durant la vie utile dureacteur. L'incidence de I'impact sur I'environnementne serait pas plus levee que cede provenant de toutgrand projet de construction.

  • INTRODUCTION

    The term "decommissioning" means shutting downand placing a plant permanently out of service. Theoperation must be capable of being performed safelyand in such a manner as to impose no responsibility onfuture generations. The decommissioning of nuclearreactors will involve the expenditure of material andmanpower and will require careful planning. This sum-mary is based on a study*11 of decommissioning a single600 MW(e) CANDU* reactor station (Figure 1).

    The "design" life of a reactor is normally con-sidered to be 30 years and usually this is chosen as thecapital repayment period. There is little likelihood thata utility will choose to decommission a reactor beforethe end of its design life and, indeed, there will be con-siderable economic incentive to operate it beyond thedesign life. Therefore, barring major incidents involvingdamage to the reactor, it is reasonable to assume that autility will not consider decommissioning a facility untilat least 30 years after start-up and, more likely, will notbe involved in decommissioning until more than 40years after start-up.

    There are many possible ways to decommission areactor. They range from simply shutting down the

    Figure 1 Simplified Vitw of a 600 MW(t) Reactor Building

    italnment building

    Operating

    Calandrla (containingheavy watar moderator

    Figure 2 Stage* of Decommissioning a Nuclear Power Station

    ' CANidt Deutarlum Uranium

  • reactor, to completely dismantling the reactor and re-leasing the site for other uses. It has been agreed inter-nationally12' that, when referring to nuclear power facili-ties, three "stages" for decommissioning may be de-fined. In the order of least complexity to greatest com-plexity, these are: Mothballing, Encasement, and Dis-mantling and Removal. Figure 2 shows a schematic re-presentation of the different stages.

    ACTIVITIES PRIOR TO DECOMMISSIONING

    A number of factors must be considered prior to decom-missioning a reactor and they may affect the optionselected.

    The total inventory of radioactive substances on thesite must be determined. This allows an assessment of:

    the man-rem * exposure expected for any operation,

    the merits of delaying completion of decom-missioning to permit the radioactivity to decay,

    the weight, volume sr>d specific activity of thematerial to be shipped to and buried in a wasterepository,

    the weight, volume and specific activity of equip-ment which will be left on site.

    Removal of the fuel, heavy water and radioactivewaste material will reduce the long-lived radioactive in-ventory by approximately 98.5 per cent. The remaining"fixed" radioactivity will be due to activation ofpressure tube, calandria and shielding materials.Negligible amounts of fission products and actinideswill be present. After two to three years, the primarysource of gamma activity is cobalt-60. After 20 years,the rate of decay of total activity will be governed by thehalf-life of nickel-63 and there will be approximately 106

    curies of activity. After 25 years, most of the materialwill meet IAEA transport regulations for Low SpecificActivity material'3'. The major proportion of the activityis associated with the calandria shell, end shields andassociated equipment.

    Decontamination will be required if the gammafields associated with the out-of-core portions of theprimary heat transport system are high or if the systemis contaminated with fission products and actinides asa result of fuel defects during the life of the reactor. De-contamination costs can be high, and some of the out-of-core portions of the primary heat transport systemmay be seriously contaminated. Therefore, considera-tion should be given to allowing the activity to decay(and paying for surveillance costs during the decayperiod) rather than carrying out decontamination. How-ever, on the assumption that decontamination is to bedone, aggressive reagents should be used to ensure

    2 that a high decontamination factor is obtained. This is* A measure of human radiation exposure; it Is the total dose In rem receiv-

    ed by the Individuals In a given population.

    possible since corrosion of the components by thesereagents need no longer be a major concern.

    In the case of a 600 MW(e) CANDU reactor the costof a full decontamination would be approximately$1 200 000*. It is assumed that decontamination willreduce the radioactive fields to a level that will not re-quire any significant man-rem expenditures.

    Removal of the fuel, heavy water and radwaste willeliminate the capability of operating the reactor andgreatly reduce the probability of release of any radio-active material. Therefore, an application to the regula-tory authorities for a change in operating licence wouldbe justified so that manpower and surveillance require-ments can be reduced.

    The availability of facilities for the storage of radio-active waste material and the cost of transportation tothe storage site will also affect decisions regardingdecommissioning. Decommissioning wastes whichcontain long-lived radioactivity would be placed in adeep underground waste repository when it isavailable14*.

    Some points that must be considered whendeveloping the specifications for a repository for wastefrom the decommissioned station and for transportingthe waste to it are:

    (a) The decontamination procedures associated withdecommissioning will be effective in removinglong-lived fission products or actinides. Therefore,the material to be stored will be solids of relativelylow specific activity. Most of the gamma activitywill have short half-lives and will decay fast enoughto eliminate the necessity of constructing gammashielding of high integrity and resistance to aging.The only major concern is to provide facilities thatensure the stored solid components will not cor-rode or dissolve at a rapid rate.

    (b) The fuel will be stored, as described elsewhere*4),and it is expected that the amount of actinides andfission products in the decommissioning wasteswill be reduced to negligible values by decon-tamination. Therefore, the only material containingsignificant amounts of these radioactive sub-stances will be those that can easily be separatedand will have relatively small volumes, e.g., ion ex-change resins, decontamination products, etc.

    (c) Consideration should be given to retrievable stor-age. It would seem desirable to store some of thedecommissioning wastes in such a fashion thatthey could be retrieved and re-used after the radio-activity has decayed. There are advantages to thisapproach since it means the disposal facility doesnot need to accommodate them and a future scrapvalue can be attached to some of the waste.

    (d) The hazards and risks involved in transporting thedecommissioned material will be low and easy tomanage, since the specific activity levels are low

    ' All cost estimates are In 1975 dollars.

  • and the material will be made up primarily of largesolid pieces. It can be assumed that the regulationspublished by the IAEA will apply.

    Time and cost estimates are required to determinewhat activities are significant cost items and to obtainan appreciation of what effect the cost of decommis-sioning might have on a utility's financing.

    The reactor designer will want to know in whichareas the significant cost items lie so that he can in-vestigate whether changes can be made to the originaldesign to reduce decommissioning costs. The owner ofa nuclear power plant may use the cost estimates tohelp decide whether it is more economical to repair amajor breakdown or to decommission the reactor.

    If sinking fund financing is used to defray the even-tual costs, only minor changes in cost per kilowatt hourof electricity would be required to cope with large dif-ferences in decommissioning costs (Table 1). Actual de-commissioning costs are discussed in the next section.

    Table 1 Increments in power cost required todevelop a sinking fund of $100 000 000after "N" years

    "N"(years)

    Increment In Cott(mills per kilowatt hour)

    2030

    40

    0.820.45

    0.27

    Assumptions: