nuclear plant radiation inspection

NUCLEAR PLANT RADIATION INSPECTION

V. A. Kremnev and S. G. Tsypin UDC 621o039.538

In the USSR, the first two units of the Beloyarsk, Novovoronezh and Armenia nuclear plants have been taken off line. This is an ongoing process. The preparations for decommissioning include fuel unloading and removal, building and equipment decontamination, re- processing and burial of the accumulated radioactive wastes, the performance of a plant technical inspection (PTI), including a radiation inspection (RI) and a technical-economic investigation (TEl) and study (TES), and drawing up a work organization plan (WOP) for the specific unit [I] (cf. Table i).

The radiation investigation is conducted following the developed program, which in- cludes:

choosing or developing methods, for example, of measuring the depth distribution of nuclide activities in materials, determining the distribution and number of points for measuring radiation dose magnitude in areas and on radioactive equipment, choosing the measuring samples from materials of unrestricted usage, etc.;

choosing and preparing (calibrating) dosimetric and spectrometric sensors and instru- ments and low-background units for measuring the low activity of materials of unrestricted usage, for example, a 137Cs decay rate of 4/hour [2];

marking points and regions in rooms and closed areas and on equipment for measuring 7-, 6-, and a-activity;

TABLE i.

Opera- tion

Removing a Nuclear Plant from service

Removal from service

Preparations for extending service life: PTI (including RI), TEl, TES, WOP

Unloading and removal of fuel, decontamination and removal of radioactive wastes

Preparation for decon~nissioning:

PTI (including RI), TEl, TES, WOP

Extending service life: remodelling, reconstruction, conversion

i0 I00 years

Decommissioning

Conversion/ burial/liquida- tion

~a

Translated from Atomnaya Energiya, Vol. 71, No. 3, pp. 242-244, September, 1991. Orig- inal article submitted June 6, 1990.

0038-531X/91/7103-0755512.50 �9 1992 Plenum Publishing Corporation 755

H, rad

~2

/0 7

Ig fO $0 ~g 50 ~ON, cm

Fig. i. Radiation inspection of Armenia unit i.

drawing up specifications of the materials' element composition, including that of the reactor vessel and in-core structure and in serpentine (primary shielding) and ordinary concrete, both for the basic elements and for trace ones (from 10 -2 to i0 -~ by mass); the con- tent of the latter in materials is determined, for example, by neutron activation analysis;

choosing methods, programs, software for constants, and models for performing neutron activation calculations for elements (including trace elements) and dose magnitude calculations for materials and structures during equipment and structure disassembly (for 70 years) and burial (for 7,000 years);

instructing and training personnel;

drawing up documents (including plans of the areas and equipment inspected, certificates of the radioactive material samples taken, etc.), including those for the computer data base.

Radiation inspection consists of measuring the dose magnitude and volumetric activity of y-radiation, the surface activity of 6- and s-radiation, and the isotopic composition of emitters in the following areas:

the reactor, the in-core structures and reactor vessel, the serpentine and ordinary concrete, and the area around the reactor (for induced neutron activity and radioactive contamination);

the primary loop pipes, the steam generators, main circulation pumps and gate valves, pressurizer, and other equipment, and areas where this equipment was located (for radionuclide contamination);

the primary loop water purification system, the spent fuel holding pool, the liquid waste disposal system, the ventilation and gas purification system, the tanks, etc. (for radionuclide contamination);

the solid and liquid waste storage areas;

other areas.

As a result of the radiation inspection of Armenia unit i in 1990, cartograms of the 7- and R-radiation fields for rooms and equipment were obtained and the isotopic composition of materials and structures due to neutron-induced activity and radioactive contamination of the primary loop and other loops was determined. For depth penetration of concrete by nuclides, the main contaminants were 134,1~7Cs. Detailed calculations of the neutron activation of several materials in this unit (from in-core structures to ordinary concrete) were performed for ii nuclear reactions involving neutron activation~ The largest contribution to, for example, the y-activity of serpentine concrete is made by 6~ for 20-30 years after reactor shutdown.

As an example of radiation inspection, Figure I gives the v-radiation absorption dose measurement data (for 22 hours of irradiation) for the serpentine concrete shielding as a function of its thickness for measurements performed on October 9, 1990. They were

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carried out in an experimental channel which was radially situated at the center level of the VV~R-440 reactor core using thermeluminescent dosimeters [4]. The absorption dose as a function of thickness has a distinct maximum at a point located 8 mm from the edge of the serpentine concrete shielding on the reactor side (according to [5], this maximum occurs at a distance of 8-10 cm); for an additional increase in the shielding thickness, the curve falls off with a relaxation length of 9-10 cm, which is in accord with the data in [6].

A comparison of the y-radiation absorption dose magnitude at the maximum point with that calculated based on the assumption that the primary contributor to the dose magnitude is y-radiation from 6~ shows that the two agree within the limits of measurement and calculational error.

The data from the radiation inspection of Armenia unit i will be used in selecting strategies and tactics for its decommissioning.

LITERATURE CITED

i. V. A. Kremnev, V. A. Elin, and S. D. Gavrilov, "The concept of nuclear plant decommissioning in USSR and the Comecon member-nations," At. Energ., 68, No. 5, 371-373 (1990).

2. I. A. Lototskaya, A. A. Alekseev, S. G. Tsypin, and N. S. Orekhova, "A unit for measuring the activity of sources in a technologically altered radiation background," Tekhni- cheskii protsess v atomnoi promyshlennosti, Ser. Izotopy v SSSR, No. 74, 116-118 (1989).

3. P. A. Lavdanskii, V. M. Nazarov, N. I. Stefanov, and M. V. Frontas'eva, "Induced activity in concrete used for shielding nuclear plants," At. Energ., 64, No. 6, 419-422 (1988).

4. V. Yu. Ifraimov, Yu. I. Kolevatov, V. V. Lysenko, et al., "Calculational and experimental research on radiation fields in areas near reactors," Radiatsionnaya bezopasnost' i zashchita AES, 1982, No. 7, 189-194.

5. S. May, D. Diccot, L. Bergemann, et al., "Activation of biological shields," in: Decom- missioning of NPP, Proc. of a European Conf. Held Luxemburg, 22-24 May 1984, pp. 47-60.

6. V. P. Mashkovich, Ionizing Radiation Shielding: A Handbook [in Russian], Energoizdat, Moscow (1982).

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nuclear plant radiation inspection

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