Validation of Dose Calculation Codes for Clearance
Authors: Menon Shankar , Wirendal Bo , Bjerler Jan , Teunckens Lucien 
Published: March 2009

All proposals for clearance from regulatory control of very low level radioactive material are based on predicted scenarios for subsequent utilisation of the released materials. The calculation models used in these scenarios tend to utilise conservative data regarding exposure times and dose uptake as well as other assumptions as a safeguard against uncertainties. Another aspects is common to all these calculation models and codes: none of them has ever been validated by comparison with the actual real life practice of recycling. An international project has recently been concluded where two calculation codes used for this purpose (the RESRAD-RECYCLE and CERISE codes) were used to calculate the dose uptake by workers, during the segmentation and melting of a contaminated fuel rack at Studsvik RadWaste, Sweden. These calculated doses were compared with electronic dosimeter measurements on workers participating in the various operations. The measurements showed that segmenting was the work operation that gave the highest dose, almost 65% of the total dose incurred, while melting itself accounted for only about 13%. The project was a co-operation between the Swedish Radiation Protection Institute, Studsvik (Sweden), the US Department of Energy, Argonne National Laboratory (USA), the Institute de Radioprotection et Securite´ Nucle´aire (France) and Belgoprocess (Belgium). The comparison of the calculation results indicated that, even with a carefully controlled reflection of reality with respect to geometry and exposure time and with a “best judgment” choice of densities for each operation, the calculation programmes have tended to overestimate the dose uptake by a factor 4 to 7, i.e. about an order of magnitude. An obvious explanation is the fact that the workers are not static, they move about constantly, changing the geometry, thus not taking the assumed doses. There are also some other practical aspects difficult to reflect exactly in the calculations. It should be noted that the Swedish Radiation Protection Institute were not completely of the same opinion as the project team, pointing out that the codes also underestimated doses for certain operations. We feel, however, that this is irrelevant, as only the maximum estimated doses for any operation in the process are used for the determination of clearance levels. It seems reasonable to state that the use of ‘enveloping’ scenarios, which necessarily cover a wide range of scenarios range of scenarios in connection with the calculation of clearance levels, would tend to accentuate this tendency of overestimation of dose uptake in most individual cases of recycling by melting. Taking into account the sensitivity of the modelling and the practical aspects listed above, the estimated doses can be, say, one or even more orders of magnitude higher than those actually taken. A side aspect of the execution of the Validation Project — specifically the background measurements — was the revelation of radioactivity in unexpected places: the paint used for the painting of moulds at A˚kers (3–5 Bq/g), the slag binding product (twice background radiation), the stamp mass, insulation and new asphalt at the Studsvik furnace (all at three to four times background). This serves to illustrate the undetected omnipresence of radioactivity in the human habitat at dose rate levels considerably higher (up to 400% over background) than the levels (ca 1% over background) at which the currently proposed clearance criteria are based on. Finally, it is important to note that the degree of overestimation (a factor of 4 −7), as recorded in the validation project, is generally regarded as ‘acceptable’ by dose modellers. The results will most probably not lead to any revision or refinement of these codes. For the nuclear decommissioner and the other producers of large volumes of only slightly radioactively contaminated material, the clearance levels resulting from such a degree of conservatism can lead to huge amounts of material unnecessarily being condemned to burial as radioactive waste. Considering that most such producers transfer their costs to the public, it is society at large that will foot the bill for this exercise in conservatism.