IMPACTS OF OPERATIONAL CHANGES ON LLW SCALING FACTORS

Transcription

1 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ IMPACTS OF OPERATIONAL CHANGES ON LLW SCALING FACTORS D. W. James D. W. James & Associates 855 Village Ceter Drive #330 North Oaks, MN 557 ABSTRACT Sice the implemetatio of 0CFR6 i 984, US uclear power plats have adopted, as a routie, samplig ad radiochemical aalysis for the determiatio of difficult to measure radiouclides limited by 0CFR6 for shallow lad disposal. Samples are collected o, approximately, a aual basis ad the results are used i the determiatio of scalig factors. NRC guidace o the characterizatio ad classificatio idetifies the eed to update the sample basis for the scalig factors more frequetly if there are sigificat chages i plat coditios that might potetially chage the scalig factors by more tha a factor of 0. A umber of examiatios of idustry experiece with samplig ad scalig factor developmet have bee completed. I these studies data is assembled as a sigle database ad ratios are aalyzed to determie overall averages broke dow by streams. All of the coditios that might give rise to chages i the scalig factors for a idividual plat are averaged over i the composite groupig. This gives little guidace o how to hadle local chages i plat coditios. Sice 984, several sigificat chages i plat water chemistry maagemet have bee itroduced. These iclude for BWRs, zic ijectio to limit the icorporatig of Co-60 i the corrosio layer, hydroge water chemistry to displace oxyge to limit stress corrosio. There have also bee chages i PWR water chemistry such as varyig lithium cocetratios to cotrol formatio of corrosio layers. I additio, there have bee both BWR ad PWRs that have experieced fuel failure episodes, exteded outages, ad have udergoe major decotamiatios. It is geerally believed that such chages ad trasiet coditios cause temporary or permaet chages i scalig factors but to date o attempt has bee made to show which scalig factors were affected or the extet of the effect. This work was udertake for the Electric Power Research Istitute. The paper will focus o a explaatio of the project, aalytical methods, ad provide a overview of some of the fidigs. INTRODUCTION With the 0CFR6 rulemakig i 983, the NRC issued two Brach Techical Positios (BTPs) to provide guidace to uclear power plat operators o the implemetatio of the ew rules. These BTPs addressed waste form ad waste characterizatio. Radiological characterizatio icluded the determiatio ad reportig of activities of a umber of difficult to measure radiouclides specifically idetified i 0CFR6. This was t a ew idea. Prior to 0CFR6, low level waste geerators were required to report cocetratios of trasuraic radiouclides i LLW to assure that disposal restrictios previously established for AEC/DOE LLW disposal

2 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ sites were observed. Samplig ad radiochemical aalysis of waste samples was already practiced prior to 0CFR6 primarily to verify TRU cocetratios were less tha 0 Ci/gm. What 0CFR6 did was expad the list to iclude several pure beta ad low eergy photo emittig radiouclides. The BTP o waste characterizatio oted the impracticality of samplig ad measurig all LLW ad allowed the use of scalig factors based o represetative samplig of separable waste streams. The scalig factors are effectively correlatios betwee a difficult to measure (DTM) radiouclide, such as Sr-90, ad a key radiouclide that could be readily measured usig equipmet ad methods available to the geerator. The Brach Techical Positio said that a scalig factor approach to determiig the cotet of the DTM radiouclides was permissible as log as there was programmatic review ad updatig of the correlatios o a aual basis or more ofte if there was a sigificat chage i plat coditios that might effect the correlatios. I accordace with the BTP, scalig factors must be derived o a plat specific basis. It is ot sufficiet to use idustry data. As a direct result of 0CFR6, operatig plats udertook more aggressive samplig programs. Idepedet streams of LLW were idetified, samplig procedures were writte, cotracts were writte with radiochemistry laboratories, ad a umber of samples bega to be set each year to the laboratories. RELATED EPRI STUDIES Several studies ad iitiatives have bee udertake by EPRI sice the itroductio of 0CFR6. Two milestoe studies were completed i 985 ad 987. These were: EPRI NP-4037, Radiouclides Correlatios i Low-Level Radwaste ad EPRI NP-5077, Updated Scalig Factors i Low Level Radwaste. I both of these studies, sample data were collected from power plats ad correlatios were calculated for major waste streams icludig high activity ad low activity resis, evaporator cocetrates, filters, ad dry active waste. Both studies provide perspective o the rage of data collected, which scalig factors were most promisig i terms of best correlatio, ad provided baselie data for plats i udergoig startup ad uable to develop correlatios from their ow streams. A additioal data collectio exercise was completed i 99 i support of the developmet of the RADSOURCE computer program. At this poit the database was expaded to more tha 3000 samples. Correlatios were agai calculated but his time for a fier breakdow of waste streams. I this examiatio of scalig factor data, a effort was made to examie the impacts o scalig factors from fuel defects. It was observed that most of the ratios would ot be effected by defects sice idividual radiouclide release rates to reactor coolat were similarly effected. This would be the case with iodie ad cesium where both species would proportioately pass through the defect. Alteratively if the release rates were idepedet of the fuel performace, such as i the case of cobalt ad ickel, scalig factors would ot be expected to chage. While this study was ever idepedetly published, it formed a large part of the basis for the 998, EPRI Low Level Waste Characterizatio Guidelies report. (EPRI TR-070). Oe observatio that evolved from these studies was that ratios of certai radiouclides were particularly well behaved. I particular, ickel ad iro to cobalt ratios were geerally cosistet betwee waste streams ad did t vary much over log time spas. There were differeces

3 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ betwee plats but these differeces were geerally cosistet with the domiat materials used i the primary system. Ratios betwee trasuraic radiouclides (TRU) were also early costat betwee waste streams. These also were observed to ot chage much over time. How trasuraic scalig factors are derived depeds o plat practice. Some plats scale all of the trasuraics of iterest idividually to a key gamma radiouclides, either Co-60, Cs-37, or Ce-44. Others, develop oe of the trasuraics (usually Pu-39) as key to the others, ad the evaluate oly the key radiouclide correlatio with oe of the key gamma radiouclides. TRU/TRU ratios show log term costacy as well as costacy betwee waste streams. Aother observatio that evolved from these studies was that certai other radiouclides, i particular I-9 ad Tc-99, were particularly difficult to measure (PDTM). While positive results were reported, there was little correlatio with available key gamma. Most of the reported values stated cocetratios at the limits of detectio ad were ot true cocetratios at all. It is probably true for all of the 3000 attempts to measure I-9 i LLW, it was ever successfully measured by radiochemical methods. Judgmet is reserved o the failure rate of the Tc-99 measuremets. All that was sure was that activities of these two radiouclides were systematically over reported by several orders of magitude. I cojuctio with the developmet of the RADOURCE program, cotrolled experimets were ru i a umber of power plats usig resi colums to collect samples from primary coolat. These samples were the aalyzed by Battelle usig mass spectrometry to determie isotopic compositio. The measuremets cofirmed what was already kow. Radiochemical methods were ot adequate for determiig I-9 ad Tc-99. DRIVE TOWARD CONSTANT SCALING FACTORS Collectio of 0CFR6 samples has bee ogoig for the last 0 years. Plats with effective trackig ad tredig programs are observig a dimiishig retur o the value of these samples. For activatio products ad trasuraic ratios, ew samples ted to reiforce past observatios ad do ot result i sigificat chages to the scalig factors i use. For I-9 ad Tc-99 measuremets, o ew iformatio is provided sice the reported values reflect either the equipmet limits of the laboratory or a detectio limit specified i the laboratory agreemet. For H-3 ad C-4, o previous evaluatio has ever developed a satisfactory correlatio with Co-60 or Cs-37, the tred, i this case is to use the cocetratio from the laboratory as a costat cocetratio ad avoid scalig to a key gamma. This leaves two radiouclides for which ew sample data will cotribute meaigful ew iformatio. These are Sr-90 ad Pu-39 3 that ca be effectively scaled to oe of the key gammas. Sice 0CFR6 sample collectio started, there has bee a relatively dramatic improvemet i fuel performace i the uclear power plats. Few plats have had recet fuel failures. As a result, it has become icreasigly impractical to use Cs-37 as a key radiouclide sice it is ofte reported at the detectio limit ad ot viable as a key ad, i fact, ot viable i a secod order scalig factor relatio. This is because the reciprocal of < is >. A scalig factor with a LLD i the deomiator will uder predict the scaled radiouclide.

4 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ Ce-44 has always bee problematic for use as a key radiouclide. As a fissio product, it is the best predictor for TRU. However, it is most ofte ot reported i waste samples ad whe it is reported, it is ofte ureliable. Agai, if it is reported as limit of detectio, it has o use i scalig factors. So, give that so little ca be derived from the radiochemical samples, there is some motivatio to focus the process so that it will provide ew ad valuable iformatio for LLW characterizatio. Much of the aalysis is useless because the measuremet ca ot be achieved or uecessary because the correlatios are already well kow. It should be acceptable to omit or limit the repetitio of these procedures. The NRC BTP, however, requires a aual update of the scalig factor correlatios. The iterpretatio here is that a ew set of samples is collected ad full set of radiochemical aalyses are performed. Perhaps it is sufficiet to meet the itet of this guidace, that is, that the program for developig correlatios is maitaied curret ad cosistet with expected cocetratios i the LLW beig geerated ad disposed. This objective ca be achieved with cofidece i the scalig factors used. Cofidece ca be achieved through projectios from a history of measuremets ad uderstadig of the effects of plat chages o the correlatios PLANT CHANGES POTENTIALLY EFFECTING SCALING FACTORS There are a umber of differet types of chages i plat coditios that ca potetially affect scalig factors. At this poit early all of the chages have bee experieced at oe plat or aother. Some of the chages cosidered are listed below: BWR Zic Additio Zic is added to reactor coolat to reduce Co-60 activity i scale deposits. The zic displaces cobalt i the scale layer thereby reducig occupatioal exposures aroud system pipig. The effect, at least iitially, is that the cobalt is mobilized i the coolat ad removed via the reactor water cleaup system. Assumig that there is a relatively costat productio rate of Co-60, ad a correspodig exchage rate with the scale layer, it would be expected that there would be o log term impact i the cocetratios of Co-60 i the Radwaste systems. BWR Hydroge Water Chemistry I this case, hydroge is ijected ito the primary coolat. The purpose of the ijectio is to reduce free oxyge i the reactor coolat (ad other aios). Free oxyge cotributes to stress corrosio crackig. The chages to reactor coolat chemistry are subtle from the perspective of radwaste processig ad agai are expected to have little impact o scalig factors. BWR Iro Ijectio Iro is ijected to maitai a miimum iro cocetratio i reactor coolat. It was foud that whe iro cocetratios dropped below certai levels buildup of activity i hotspots was icreased. The iro displaces other elemets that might settle ito the hot spots. The problem was more proouce i plats with deep bed demieralizer systems that were more effective i

5 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ strippig iro. Ijectio of iro will assure a miimum cocetratio of elemetal iro that could result i a icrease i Fe-55. The extet of the impact would deped o how low the iro was prior to implemetatio. PWR Lithium Ijectio Lithium is used i PWRs i much the same maer as zic is used i BWRs. Sice PWRs used a substatial amout of Icoel, there is a major cocer with amout of ickel that precipitates o core surfaces. Additio of LiOH, reduces the release rates ad the depositio Co-58 which is a major activatio product of Ni-58. Co-58 cotributes sigificatly to occupatioal exposures. Over the loger term, cotrolled lithium cocetratios should reduce the corrosio rate of Icoel ad thus the release of Co-58 ad Co-60. It does ot impact release rates from previously deposited scale so the effect o scalig factors should be gradual. Decotamiatio Efforts Decotamiatio is aother process used to reduce occupatioal exposures. Chemical decotamiatios usually focus o primary pipig ad steam geerators i PWRS ad o recirculatio pipig i BWRs. Some full system decotamiatios have bee performed, The decotamiatio removes deposited scale ad results i a burst of activity collected i processes used to cleaup the decotamiatio solutios. The decotamiatio waste is expected to be batch sampled ad would ot be evaluated usig scalig factor methods. The decotamiatio itself would be expected to impact the amout of cotamiatio available for trasport withi the system ad it should be assumed that there will be a trasitory effect o scalig factors relatig to radiouclides foud i the scale icludig activatio metals ad trasuraics. Fuel Defects Fuel defects are the most geerally recogized evets impactig scalig factors. This subject has bee discussed i previous EPRI reports. The opeig of a defect is pretty readily observable through reactor coolat chemistry measuremets but may ot be immediately see i scalig factors. Small defects will impact the Cs-37 cocetratio i reactor coolat. If Cs-37 is used as a key for Sr-90 or TRU, reductios i the scalig factors are expected. If Co-60 is used as the key, o impact is likely. Larger defects will effect the release rates of all fissio products ad TRU ad will impact Co-60 scalig factors, perhaps more strogly tha those related to Cs-37. Chages i Fuel Desig Most of the chages related to fuels curretly used have take place i the early stages of scalig factor practice. These chages, icludig the tred to smaller pi sizes ad varyig claddig materials have resulted i improved fuel performace ad less observable Cesium 37 i LLW streams. Sice a least the early 990s, there has bee a steady tred withi the power plats to rely more strogly o Co-60 based scalig factors. Whe a ew fuel desig or type is itroduced, it may take several cycles to fully chage over the core. With the premium o fuel storage space, there is eough motivatio to edure the presece of some defects or a recostitutio process i lieu of a full core chage out. As a result there is o clear demarcatio of the chage. Cesium just gradually disappears.

6 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ Exteded Shutdows A exteded shutdow ca occur as a result of major maiteace, fire, flood, or imposed as a result of regulatory issues. Durig a exteded shutdow, the fuel remais withi the plat but may be off-loaded from the reactor. The burde of processig primary water shifts to the fuel pool coolig system. Because the reactor is t beig operated, there is little additioal activatio occurrig ad little productio of fresh fissio products. The expected impact o scalig factors is a relative loss of Co-60 ad other shorter lived radiouclides due to decay durig the shutdow period. Gradual purgig of volatile or otherwise mobile radiouclides is also expected. ANALYTICAL METHODS The purpose of this examiatio is to assess if chages i plat coditios are of sufficiet impact to effect chages i scalig factors ad ecessitate additioal samplig. The primary tools used i the evaluatio of scalig factors ad chages are highlighted below. Geometric Mea ad Dispersio Geometric mea (GM) ad dispersio (GD) have bee the pricipal methods for evaluatio of scalig factor data. The geometric mea (GM) is defied as the th root of the product of values. It is usually calculated by the atilog of the average logarithmic ratio. A fuctio to calculate this value is available i Excel (geomea()). If you wat to perform the calculatio maually, the formula is provided i Equatio (). GM = log - y log i = i x i (Eq. ) Where: y = Scaled Nuclide x = Key Nuclide The GM is effectively a estimate of the media value. It is more robust tha a simple average i that outliers have less impact o the outcome. The dispersio, reported extesively i EPRI studies, ca be iterpreted as a rage of differece i the scalig factor ratios used i the GM calculatio. A dispersio of would idicatio average variatio by a factor of above ad below the GM. To assure predictio withi a factor of 0 a -σ dispersio admits variatio oly withi a factor of 5. The dispersio is the atilog the variace calculated from the log ratios. The variace is the square root of the stadard deviatio. A formula for calculatig the dispersio is provided i equatio ():

7 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ GeometricDispersio = atilog( s ) (Eq. ) where the stadard deviatio (S ) is defied usig the logarithm ratios. Tred Aalysis Tred aalysis is performed usig liear regressio methods. The scalig factor ratios are plotted o a time scale ad the regressio is performed o the ratio versus time. The slope of the regressio ca be equated to the rate of chage. Usig real data, it is ulikely that a slope of 0, idicatig o tred, will be calculated. A statistical test ca be performed to determie withi cofidece limits if the slope is zero. The pertiet equatios are preseted below: The test statistic is defied by: t = b β s s 0 xx (Eq. 3) where b is the slope estimated by liear regressio ad β 0 is the hypothetical real slope. Whe used to test for treds, the hypothetical real slope is equal to 0. Other parameters are defied by: s yy bs s = Ad s xy ( x = i x ), s yy i= i= = ( ), ad = ( xx i xy x Pooled Variace y y s i= xi y i ) (Eq.4) (Eq. 5) Pooled variace is a techique for testig whether two data sets are equivalet (i.e. from the same populatio). Coceivably, if there is o discerable differece i the data, you could combie ad treat it as oe data set. A warig is i order i that more iteral variatio there is i the two data sets, the more likely they are to appear statistically equivalet. You have low cofidece i the true value of either data groupig. The data are the same i that you ca t draw ay coclusio from either set. Pooled variace is calculated to justify averagig betwee varyig streams. It is applied to idividual ratios sice ot all ratios behave similarly across varyig streams. The pooled variace calculatio compares data sets o the basis of their variace ad degrees of freedom to determie if the data sets are statistically the same.

8 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ sb = s + + ( ) s (Eq. 6) The test statistic used for the compariso is give by: t = s b µ µ + (Eq. 7) where: µ = Average of Waste Stream µ = Average of Waste Stream = Sample Cout for Waste Stream = Sample Cout for Waste Stream S b = Pooled Variace The pooled variace as employed i this aalysis will be the pricipal test to determie if a chage is sigificat over loger time frames. That is, it will verify if the data prior to the itroductio of the chage i plat coditios is sigificatly differet from the data collected after the evet. Graphical Aalysis To support the aalysis a graphical depictio of the data was developed to show the scalig factor ratio treded over the collectio period. Superimposed o the graph are importat milestoe evets that could impact the scalig factors as well as the plat operatig history. A sample graph is show i Fig..

9 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ Fig.. Scalig Factor Tred Plot with Milestoes ad Operatig History. I Fig., the milestoes marked are: SR Stellite Reductio FII - Feedwater Iro Ijectio HWC Hydroge Water Chemistry CONSTANT SCALING FACTORS A sigificat umber of operatig power plats have experieced relatively uiform scalig factor values determied through their samplig programs. This, i cocert with a cotiuig desire to reduce the costs of maitaiig, the program has led to more attetio beig give to defiig costat values for some ratios aimed at reducig the umber ad types of radiochemistry aalyses coducted o each sample. Effectively, each radiouclide requested to be examied by the laboratory ivokes a separate aalysis ad a separate cost factor. By reducig the umber of aalyses, the overall cost per sample ca be reduced. No criteria have bee established for defiig costat scalig factors. I the cases of activatio products ad trasuraic ratios, existig radiochemistry data provide a adequate basis for defiig these values withi the geeral characterizatio objective to esure quatificatio withi a factor of 0 i accordace with the Brach Techical Positio o Classificatio. This coditio ca be met if the data cover a sufficiet operatig period ad the variace or dispersio of the data is sufficietly small. The dispersio is a measure of how well the value is kow. A dispersio of 5 idicates that the true value ca be a factor of 5 times higher or 5 times lower tha what we regard as the scalig factor. I either case, applicatio of the cetral value will achieve the geeral objective of predictio withi a factor of 0. For the cadidate scalig

10 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ factors the dispersios are geerally observed to be much lower depedig o the particular ratio ad the extet of segregatio of streams. Figure, below, provides a good example of a cadidate scalig factor for applyig a costat value. The figure represets the same ratio for five differet waste streams collected from a uit plat over a twelve year period. The overall dispersio is a factor of. Statistical aalysis results show i Table 6- idicate a maximum factor of differece betwee idividual streams. Variatio from the overall geometric mea is oly about 40 %. RESULTS AND CONCLUSIONS Fig. Costat scalig factor example (Ni-63/Co-60) Examiatio of scalig factor impacts requires fairly detailed kowledge of what is occurrig i the plat while the scalig factor data is beig collected. From the example i Fig. 3. A fuel failure evet is idicated followed by more tha a factor 0 of icrease i the Sr-90/Co-60 scalig factor. I the years followig the evet, the scalig factor remais high but appears to be tredig dowward. It is relatively uaffected by subsequet plat operatig chages.

11 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ Fig. 3 Impact of fuel failure evet The secod example preseted here (Fig. 4) demostrates the shift i the Sr-90/Co-60 ratio i RWCU followig the start of zic oxide ijectio ad feedwater iro ijectio. Sice the two chages are so close together i time it is t possible to distiguish which is drivig the shift. However, sice the two process work i cocert to the same ed, they could be viewed as a sigle milestoe. There is a sustaied drop i the scalig factor that is statistically sigificat at a factor 5. The objective of these processes is to mobilize Co-60 to reduce depositios i crud layers. Sice the Co-60 is more mobile i the coolat it is likely that it will be relatively icreased i the RWCU filter/demieralizers thereby lowerig the scalig factor.

12 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ Fig. 4 Scalig factor reductio evets A importat overall cosideratio i this study is whether or ot a particular database ca support examiatio. To uderstad the impact of a chage i plat coditios, it s ecessary to kow whe the chage occurred ad the plat operatig coditios surroudig the chage. I the example show, the plat experieced stable operatio durig the etire period. The data base is well developed with a strog sample record for all of the major waste streams icludig sufficiet data for statistical tests ad tred aalysis. Owig to the maer i which characterizatio is performed at some plats less emphasis is placed o the overall database ad it ca be problematic to attempt to recostruct the sample record. Hopefully, the results of this effort will provide valuable iformatio to all plats cofrotig chages potetially effectig characterizatio efforts. CONCLUSIONS Chages i plat coditios do have the capacity to shift critical scalig factor values. The overall precisio of the process for derivig scalig factors evelopes the impacts observed. Scalig factor shifts caused by these chages are relatively small i cotrast to the overall dispersio of the data ad would ot i themselves ecessitate a reassessmet of the scalig factors. Operators should be cogizat of these chages but they eed ot alter samplig schedules aroud them. Cadidate ratios for costat scalig factors are geerally uaffected by operatioal chages.

13 WM 04 Coferece, February 9- March 4, 004, Tucso, AZ Costat scalig factor evaluatios should iclude a rigorous evaluatio of treds ad should establish actio evelopes for values. REFERENCES Developed by Jee Vace, the RADSOURCE program predicted scalig factors for fissio product radiouclides from reactor coolat cocetratios of radioiodie isotopes. RADSOURCE iwas developed for EPRI based o the ISOSCALE program developed by Jee Vace. D.E. Robertso, et. al., Cocetratios ad Behavior of 9 I, 99 Tc, ad 4 C i Low Level Radioactive Wastes from Commercial Nuclear Power Statios, WM 9, Vol. pp Pu-4 is a viable alterative as the key TRU sice it is preset at the highest cocetratio ad is a beta-photo emitter rather tha a alpha emitter.