46-04

SCALING FACTORS FOR ¹²⁹I IN PWR'S WASTES

C.Leuthrot
Commissariat à l'energie atomique
CEA/DRN - CE CADARACHE

P. Ridoux, A. Harrer
Electricité de France
EDF/SEPTEN

ABSTRACT

The characterization of long lived fission products, actinides and activation products contained in the wastes or deposited on reactor circuit surfaces of the PWR's plants is necessary for the management of the wastes produced during reactor operations as well as for the subsequent decommissioning of the plants.

The current methods for long lived nuclides activities determination, using simple ratios correlated to ⁶⁰Co or ¹³⁷Cs activities, by statistical methods applied to the feedback of the wastes producers, do not take into account the peculiarities of each reactor and are not accurate enough for the nuclides which present a physico-chemical behaviour completely different from this one of the references nuclides ( for instance ⁹⁰Sr or ¹²⁹I ).

The acquired knowledge at CEA on the short lived fission products, actinides and corrosion products behaviour in the french PWR's primary circuits and the EDF feedback in this field allowed to work out models for prediction of long lived nuclides activities in the primary water as well as deposited on the primary circuit surfaces. These estimates allow to calculate wastes activities such as CVCS filters and resins and deposited activities on the surfaces of the primary circuit; they also allow to calculate standard alpha, beta and gamma spectra for the auxiliary circuits wastes and deposits.

^{129I is one of the most important nuclide in terms of wastes disposal activities and accurate predictions of the ¹²⁹I activities is of importance. We describe in this paper, a method to determine ¹²⁹I activities, related to measurements of gamma emitters in the primary water of PWR's (rare gases, iodines and cesiums).
INTRODUCTION
In several countries, and especially in France, ¹²⁹I, which is a low energy beta emitter with a very long half life ( 1.6 10⁷ years), is the "critical" nuclide for the low activities wastes disposal. Due to its large mobility, this nuclide dominates ground water doses ( # 90% contribution of the dose). Activities of ¹²⁹I, which can be disposed on this disposal sites are then limitated and it is necessary to know with a good accuracy the activities in the wastes.
Due to the difficulties of quantitative activities measurements of ¹²⁹I in the wastes, only evaluation of the ¹²⁹I activities by calculations can be considered in an industrial approach; these evaluations are at the present time generally performed by using generic ratios between the activities of ¹²⁹I and one easily measurable gamma emitters such as ⁶⁰Co or ¹³⁷Cs. These ratios are supposed to be invariable for all wastes and are obtained from statistical regressions on the available experimental data.
Such an approach of the problem for ¹²⁹I, lead to a statistical scattering of several order of magnitude /1/; even if the study is restricted to the main solid wastes of the french PWR's reactors, the uncertainety on the ¹²⁹I /¹³⁷Cs ratio is considerable (figure 1). The need of conservatism by using this method of activities determination lead generally to a large overestimate of the wastes activities calculations and then to virtually saturate the disposal site.
The models of fission products and corrosion products contamination transport in the primary circuit, of a PWR, developed at CEA and already qualified on the short lived fission and corrosion products, allowed a different approach of this problem and to develop correlations between ¹²⁹I activities in the main PWR's wastes and the activities in the primary water of the short lived fission products ( rare gases and iodines) measured by gamma spectrometry during the cycle. By this method it is possible, for one specific reactor and cycle, to derive scaling factors for ¹²⁹I particularely for CVCS filters and resins and also for the other main solid wastes of the reactor.

Fig. 1. 129I /137Cs Ratio for EDF PWR's Wastes
SOURCE TERMS FOR GASES, IODINES AND CESIUMS IN A PWR'S PRIMARY CIRCUIT
The release of volatiles fission products ( rare gases, iodines and cesiums) out of the defected fuel assemblies and the tramp uranium deposited on the fuel cladding, as well as the behaviour of these fission products into the primary circuit of PWR's have been extensively studied at CEA and a computer code (PROFIP /2/) has been developed in order to calculate primary activities for most of the short lived or long lived fission products.
A parametric study which take into account:

defected fuel power (between 150 to 250 W/cm)

burnup (between 0 an 40000 MWJ/t)

size of the defect (between 10 to 100 µm)

tramp uranium

UO2 and MOX fuel rods

allowed to derive scaling factors and uncertainety for ¹²⁹I in the primary circuit related to ¹³⁷Cs, ¹³¹I and ¹³³Xe.
Different kinds of scaling factors were studied:

^{129I / ¹³⁷Cs source term ratio in the primary circuit; this ratio may be used for all types of wastes whatever the running conditions of the reactor are.
^{129I /¹³⁷Cs, ¹²⁹I /¹³¹I and ¹²⁹I / ¹³³Xe primary activities ratios; these ratios may be used for wastes such as primary water, CVCS filters and resins, but the operation conditions of the reactor and the waste has to be precisely known.}}

The main results of this parametric study are given in table I below.
Table I: Theoretical Scaling Factors Deduced from the PROFIP Code Calculations

From the results of the PROFIP computer code parametric study, it is possible to derive the following conclusions related to the best 129I tracer:

137Cs

For:

Similar behaviour as 129I
Similar scaling factors for defected fuel and tramp uranium
Similar scaling factors for steady state operations and transients
Easily measurable in the wastes

Against:

Not easily measurable in primary water in steady state operations
Large dependence of the scaling factors with CVCS resins efficiencies

131I

For:

Same behaviour as 129I
Easily measurable in the primary water

Against:

Differents scaling factors for defected fuel and tramp uranium
Large dependence of the size of defects on scaling factors

133Xe

For:

Low dependence of the operations parameters on scaling factors in steady state operations
Easily measurable in the primary water

Against:

Differents scaling factors for defected fuel and tramp uranium

In short, if we are able to estimate the contributions of defected fuel and tramp uranium to the primary activity of ¹³³Xe in steady state operations, this nuclide seems the best tracer of the primary activity in ¹²⁹I. When both ¹³³Xe or ¹³¹I are not measurable,or during power transients ¹²⁹I /¹³⁷Cs scaling factor will be used.
SCALING FACTORS FOR FILTERS
CVCS Filters
The conclusions of the previous chapter show that we are able to derive with a resonably accuracy ¹²⁹I activities in the primary water from the short lived fission products activities (¹³³Xe, ¹³¹I, ¹³⁴I); It will then be possible to calculate trapped activities in filters such as CVCS filters if we are able to estimate the efficiency of trapping of these filters for iodine species.
From measurements performed by CEA with the Mini-RCVdevice /3/ show ( Table II) that trapping factors can be derived for iodine in CVCS filters below 10^-3 during power operations and below 5 10^-2 during cold shutdowns.
Table II: Iodine Trapping Efficiency for the CVCS Filters

^{129I activities trapped in the CVCS filters (A_F) can be derived from the primary activity (A_pw) by the relation:

(eq 1)

where:

T_u: in operation time of the filter
Q_d: Flow rate in the filter
K: 10^-3for normal operation
5. 10^-2for shutdown operation

Others Filters
For all filters connected to auxiliary circuits such as TEP (Boron Recycle system) or TEU (Liquid Waste Treatment System), activities are not directly linked to the primary water activity, and then it is necessary to derive scaling factors from the ¹³⁷Cs activity of the specific filter. In order to determine these scaling factors, it is necessary to know the trapping efficiency of the filter for iodines and cesiums species; for iodine (cf table II) values have been derived from the experimenta feedback of EDF and CEA. For cesiums, adsorption on paper filters lead to a trapping factor (F) depending of the operation time of the filter; an empirical correlation has been derived from the CEA measurements on specific EDF plants filters ( cf. Figure 2), which may be expressed by:

(eq 2)

where To is the operation time of the filter
Taking into account the previously determined trapping factor for iodines species, it is possible to derive scaling factors for filters (SF_F) from the scaling factor in the primay water (SF_w):

(eq 3)

Fig. 2. Trapping Factor of 137Cs in the Paper Media of French Plants CVCS Filters

SCALING FACTORS FOR RESINS
On the french PWR's power plants, specific gamma measurements are not achievable on individual purification resin tanks, but only on the mixture of several resins used in two contiguous plants; therefore only ¹²⁹I /¹³⁷Cs source terms scaling factors (cf Table I) may be used to characterize this waste stream.
MODELS VALIDATION
Primary Water Activities
The validation of the theoretical scaling factors for primary water activities in power operations has been performed from specific ¹²⁹I measurements in the primary water of several EDF PWR's plants, associated to the gamma measurements of ¹³¹I, ¹³⁴I, ¹³³Xe and ¹³⁷Cs during the corresponding cycles; ¹³¹I, ¹³⁴I and ¹³³Xe are used to derive the contribution of defected fuel rods and tramp uranium to the total activity of ¹³³Xe in the primary water. Comparison between measured and calculated ¹²⁹I activities from ¹³³Xe and ¹³⁷Cs primary water activities is shown in table III.
Predicted to measured activities ratio are better than a factor 3 when using ¹³³Xe as tracer and a factor 10 when using ¹³⁷Cs.
Table III. Comparison Between Calculated and Measured ¹²⁹I Primary Activities

CVCS Filters Activities
As for primary water, the validation of the theoretical scaling factors for CVCS filters activities in power operations has been performed from specific ¹²⁹I measurements of CVCS filters of several EDF PWR's plants, associated to the gamma measurements of ¹³¹I, ¹³⁴I, ¹³³Xe and ¹³⁷Cs in the primary water during the time of operation of the filters. The results of this validation are shown in table IV.For the whole set of data, calculations uncertainety are similar to those of primary water and below a factor of 3.
Table IV. Comparison Between Calculated and Measured ¹²⁹I Activities in CVCS Filters

Others Filters
Comparisons between measured and calculated scaling factors for filters using equation 3 are shown in table V.
Table V. Comparison Between Calculated and Measured ¹²⁹I Scaling Factors for Filters

The accuracy of prediction for the ¹²⁹I activities in filters from the ¹²⁹I / ¹³⁷Cs scaling factor is about an oder of magnitude as for primary water activities.
Resins
The theoretical ¹²⁹I / ¹³⁷Cs source terms scaling factor ( 3. 10^-7) has been compared to specific measurements on resins for several plants ; some results are given in the table VI. The mean value of the experimental scaling factor is close the theoretical one, but it can be noticed a large scattering of experimental values ( one order of magnitude).
Table VI. ¹²⁹I / ¹³⁷Cs experimental scaling factor for resins

CONCLUSION
The present study, performed at CEA Cadarache and based on the theoretical models of fission products behaviour in the primary circuit of PWR's and validated by the experimental feedback on the EDF power plants, showed that current methods for ¹²⁹I activities determination may be improved for the main wastes produced in PWR's plants such as CVCS filters, resins and primary water by using scaling factors specific to the reactor and cycle and calculated from the primary water coolant activities in ¹³³Xe, ¹³¹I and ¹³⁴I. These methods allowed to decrease the uncertainety on ¹²⁹I activities by a factor of 3. When this calculation method cannot be applied, ¹²⁹I /¹³⁷Cs scaling factors will be used; the uncertainety on ¹²⁹I activities calculation is by an oder of magnitude
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