Corresponding author: Sergey V. Rosnovsky (RosnovskySV@nvnpp1.rosenergoatom.ru)
Academic editor: Georgy Tikhomirov
Expensive permanent storage facilities with massive engineered structures are used traditionally to ensure safe temporary storage of solidified radioactive waste at the NPP sites. Such approach is dictated by the need to comply with the regulatory requirements for limiting the gamma background in the area adjacent to the storage facility.
The costs involved in temporary storage of solidified
The Novovoronezh NPP, a branch of JSC Concern Rosenergoatom, operates successfully light hangar-type facilities for temporary storage of solidified
A methodology has been developed to assess the radiation situation around storage facilities during temporary storage of
Permanent storage facilities with massive engineered structures are used traditionally to ensure safe temporary storage of solidified radioactive waste (
With regard for the fact that the global volume of low- and medium-level waste to be conditioned exceeds 6 000 thsd. m3 (
The optimization of costs during temporary storage of solidified
Since the mid-2000s, a number of casks have been developed in the Russian Federation to ensure safe conditioning of radioactive waste fully in compliance with regulatory requirements (
Reinforced-concrete casks of the NZK-150-1,5P type have been used extensively in the Russian nuclear industry to condition different types of radioactive waste (
Since 2008, light storage facilities of a hangar type have been used successively by the Novovoronezh NPP, a branch of JSC Concern Rosenergoatom, for the interim storage of solidified
The following practical tasks have been solved as part of the undertaken activities:
a methodology and a method for the solidified RW conditioning and temporary storage have been developed to ensure the RW removal for final disposal while avoiding extra process operations and unreasonable costs;
a methodology has been developed to assess the radiological situation around storage facilities during temporary storage of RW;
procedures have been developed to measure the RW activity and radionuclide composition using analytical methods without cask opening and sampling;
a dedicated software package has been developed to calculate the optimal loading of the temporary storage facility for casks so that to ensure the minimization of the radiation fields in the adjoining area.
The results obtained in the process of investigations make it possible to arrange for the interim storage of solidified waste at the NPP site using NZK casks and unheated light storage facilities of a hangar type. The developed models and procedures allow assessing the in-situ radiological situation.
As part of the developed conditioning technology, primary waste in the form of the evaporation-to-the-maximum-salt concentration plant’s saline product is placed into steel drums which are packed into prismatic concrete casks of the NZK-150-1,5P type with the free space inside of these being filled with a substance that attenuates ionizing radiation (sand, concrete, etc.) (
For temporary storage, pending the shipment to the national operator, concrete casks are accommodated on a concreted platform above the ground level. At the top, the storage facility is covered with a light-weight steel structure that protects the contents from atmospheric phenomena, while providing little radiological protection.
The protection against ionizing radiation is achieved through dedicated procedures with waste placed inside a secondary package and through a particular arrangement of such packages within the storage facility.
Fig.
An example of the asymmetrical NZK-150-1,5P cask (secondary package) loading pattern: 1 – concrete cask wall, 2 – filler (extra biological shielding).
Diagram of the external cask orientation in the storage facility’s peripheral rows.
An analysis of the pattern shows that the equivalent dose rate differs greatly at different points of the concrete casks. Therefore, one can control the radiological safety parameters through a particular arrangement of outer casks in the storage facility. Fig.
Directional pattern of gamma radiation from a filled NZK-150-1,5P cask.
To improve the efficiency of this approach, mathematical tools and software have been developed to calculate the optimal loading for the storage facility to ensure the smallest possible radiation impacts on the environment (
Procedures were adopted during the commissioning of hangar-type storage facilities for pilot operation to find out if it is possible to fill these based on determining the allowable equivalent dose rate values.
The following values were used as the reference maximum allowable levels during operation of hangar-type storage facilities based on radiation safety rules and internal regulations:
6.0 μSv/h, in the controlled access area;
1.2 μSv/h, at the buffer area boundary.
It is taken into account in assessing the radiological situation that, due to a low activity of the waste in storage, the casks in the outer row are the only contributor to radiation beyond the storage facility.
No radiation from casks in the inner rows penetrates the outer cask row in practically significant amounts. Therefore, the outer row of casks isolates the ionizing radiation from the casks in the storage facility’s inner rows.
To calculate the values of the equivalent dose rate from the outer casks within and beyond the specialized facility area, we shall present the outer cask row as a portion of the spherical radiator surface with a certain radius,
A model to assess the radiological situation around a hangar-type storage facility.
The dose rate at the given distance from the wall center is expressed by the dependence
where
Calculated constants
where
For the equivalent dose rate on the outer surface of the specialized facility enclosure not to exceed a value of 1.0 μSv/h, in accordance with
As a result of the activities in 2014, a patent of invention has been registered for JSC Concern Rosenergoatom (
In practice, to decide on if it is possible to accommodate a cask in the hangar’s peripheral row, a more suitable criterion is often that based on the value of the saline product activity rather than on the gamma dose rate value. To reduce the earlier developed criteria to the permissible value of the waste activity in the peripheral row, we shall use a model of a spherical source with the activity distributed over the sphere surface (Fig.
A model to calculate the permissible activity of radioactive waste in NZK-150-1,5P casks in the hangar’s peripheral rows.
For the above source, the surface activity value relates to the dose rate as
whence
For the cask layer facing the specialized facility enclosure,
For the cask layers facing the specialized facility,
Then, from relation (4), we have the following specific surface activity values per the cask surface area unit for the row facing
the enclosure: Аs = 2.22 × 10 6 μm 2;
the potential contamination area: Аs = 3.16 × 10 6 Bq/m 2.
With the cask face area being equal to 2.27 m2, we have the following average permissible activity of one unit for the row facing
the specialized facility enclosure: Аnzk = 5.04 × 10 6 Bq;
the potential contamination area: Аnzk = 7.17 × 10 6 Bq.
The calculations were conducted without taking into account the attenuation of the gamma radiation in the outer cask wall, and with no extra attenuating sand layer between the outer and inner cask walls. The total thickness of the protective layer is about 40 mm while the average density is 2 g/cm3. The coefficient of the above screen attenuating the gamma radiation with the energy
Therefore, the final value of the average permissible activity for one outer cask is as follows for the row facing
the enclosure: Аnzk = 1.512 × 10 8 Bq;
he potential contamination area: Аnzk = 2.151 × 10 8 Bq.
To accommodate a new batch of classified casks, a decision is made based on the following rules as to the cask positioning.
Low-level casks are accommodated in the storage facility’s peripheral rows since the radiation dose from this class of casks is as small as possible.
Casks with the maximum activity are accommodated in the central part of the storage facility. It is important that a rule is observed concerning the orientation of the wall with the radiation hazard sign applied to it – the respective cask shall be positioned with its wall with the minimum dose rate facing the peripheral row since positioning the cask otherwise will lead to increased radiation effects on the storage facility walls.
Medium-level casks are placed between the peripheral rows and the central zone with extra-level casks.
A more accurate calculation of the radiation fields around the storage facility requires a procedure based on presenting each cylindrical cask with the EMSC plant’s saline product as a standalone radiator using the field superposition principle and Monte Carlo method.
Software has been developed based on the obtained results in the form of specialized modules and in the form of a separate application for the hangar-type facility control.
Fig.
Software-predicted distribution of the gamma dose rate from a filled NZK-150-1,5P cask.
It is permitted to perform calculations by hand, as well as to develop specialized software based on the developed model. Fig.
The result of using the model: a graphic representation of the region with the given gamma dose rate.
The simulation results were compared with the experimental data. The resultant difference was not more than 20%. The major reason for the discrepancy is expressed by the low accuracy of the input data that describes the contents of the storage facility.
Testing has shown the software to be satisfactorily serviceable and fit for being put into pilot operation at the Novovoronezh NPP. It was demonstrated in the course of testing that the research works considered in the paper had been practically confirmed [
The software for assessing the radiation situation during temporary storage of NZK casks in hangar-type storage facilities has been successfully integrated with the Novovoronezh NPP’s Unified Automated Information and Analytical System for Accounting and Control of Radioactive Substances and Radioactive Waste (
* Russian text published: Izvestiya vuzov. Yadernaya Energetika (ISSN 0204-3327), 2021, n. 2, pp. 96–105.