Promotion work for Scientific Degree of Doctor of Physics
Janis Alksnis
The results of nuclear spectroscopy and beta radioactivity
measurements in the environmental monitoring around the Nuclear
reactor at Salaspils
Latvian Academic of Sciences
Nuclear Research center, 1998
1. Introduction.
The research nuclear reactor IRT at Salaspils has been operating since
September 25, 1961. According to the radiation safety regulations [1,2] and the world
practice, systematic measurements of the environment radioactivity (radiation
monitoring) in the surrounding of the nuclear reactor should be carried out at Salaspils.
The radiation monitoring in order to watch if the operation of the nuclear reactor does
not increase the contents of the radionuclides in the environment. The monitoring since
1966 in the neighbourhood of the Salaspils nuclear reactor was carried out by a group
of scientists of the Institute of Physics. The author of this promotional work had to guide
this group all the time.
After some time as the result of various measurement, it was possible to come
to the conclusion - the reactor has not influenced the environment - this influence was
hardly felt. In fact the monitoring of the reactor became the radiation monitoring of great
significance, that enabled to control different elements in the environment atmosphere, precipitations, water in ponds, rivers etc, subsoil water, soil, plants - their
radioactivity level in rather a large neighbourhood (from Riga to Ikskile) for many years.
Till 1991 the Latvian hydrometeorological service only gathered the environmental
samples. After that they had been sent to Obninsk (Russia) to measure their
radioactivity [39].
In this respect the environmental monitoring group at Salaspils nuclear reactor had
been the only one in Latvia for many years, who carried out rather wide, although
limited in territory , control of the radioactivity in the environment.
The aim of the dissertation is to prove, that Salaspils nuclear reactor has not
done harm to the environment and to show the causes and interconnections of the
radioactivity annual level changes in the environmental elements.
If the pollution is relatively high (in comparison with the natural level), then there are no
problems to ascertain it. If the environmental pollution is unimportant, then it is
necessary to carry out many measurements with the help of which it will be possible to
get statistically safe results of the measurements and to come to well-grounded
conclusions. In this promotional work 3836 measurement (samples) results have been
summarized and analysed that are gathered at Salaspils and its neighbourhood from
1966 to 1995. The number of samples: precipitation-2550, soil-304, water 415, plants329, aerosols 238, besides that, the data of the aerosol radioactivity have been
compared with the data obtained in the air control stations in Germany and Norway
from 1973 to 1985. The measurement and data amount is large enough to come to
definite conclusions.
If the pollution level is relatively high (in comparison with the natural level), then there
are no problems to state it. If the pollution of the environment is unimportant, then many
measurements are necessary in order to obtain effective results of the measurements
and to come to well -grounded conclusions.
The analyses and the conclusions of the monitoring results are problems of the day
now, too, when it is known, that the nuclear reactor at Salaspils like many other
reactors in Europe and America, has been stopped and everything has been done to
carry out decommissioning.
The dissertation consists of the introduction, four chapters and the conclusion.
Chapter 2 gives general description of the radiation monitoring carried out at
Salaspils, short characteristics of its contents, the measurements of samples and the
methods of the processing results; short information about used measuring apparatus
(y-spectrometer, p-radiometer and detectors) and its calibration.
Chapter 3 gives summarized and analysed monitoring results in connection with
Salaspils nuclear reactor monthly and annual energy production data, as well as the
radiation levels of natural objects observed in the monitoring in connection with its
place in realation to the location of the reacor.
In chapter 4 everyone can see analysed atmospheric (precipitations, aerosols,
dust) monthly relative and absolute radioactivity average data, that indicate to its
seasonal character at Salaspils and they are compared with aerosols y - spectroscopy
data, that are obtained in
German air control stations in Germany and in the North of Norway as well as with
similar data in Tashkent.
Chapter 5. The annual radioactivity correlation among the elements of the
environment have been analysed (water, soil, plants, precipitations, aerosols) in yspectroscopy and in radiochemical analyses obtained data about the structure of
radionuclides in the environment and their connection with the explosions of nuclear
weapon tests in the atmosphere. Here have been analysed the consequences of
Chernobil nuclear catastrophe in the neighbourhood of Salaspils, based on the
environmental samples y-spectroscopy data.
2. The general exposition of the monitoring.
The contents of the monitoring.
While carrying out the monitoring [3,4,5] at Salaspils since 1966 the following
gross p-radioactivity measurements of the environment have been made:
precipitations (rain. snow, dust and others) - [Bq/m2month].aerosols - (μBq/m3!,
5 cm deep so//- [kBq/m2], plants - their roots and stems are obtained separating them
from the 5 cm deep soil samples - [Bq/kg], water in the river Daugava, in ponds,
draining- ditches [Bq/l], subsoil water 4 -10 m deep just close by the reactor (5-100 m
from the reactor) - [Bq/l], drinking wafer from artesian wells (100-1000 m from the
reactor)- [Bq/l], reactor waste water- [Bq/l]. In order to determine the structure of
radionuclides in the samples more precisely, the following sample measurements have
been made:
y- sectroscopy- to identify radionuclides and to obtain their concentration (these
methods have been worked out and introduced in practice while the author himself was
taking part in the investigation of various nuclear spectrums) [46-53], pmeasurementeto obtain the half-life and Eβmax of radionuclides,
- radiochemical analyses to obtain 90Sr concentration.
Methodic.
Radionuclides, that have produced during the operation of the nuclear reactor
IRT or some other type of nuclear reactor are mainly β - and γ- radiator. In the global
radioactive pollution, too, caused by testing of nuclear weapons, radionuclides radiate β and - γ rays. Because of this, the radiation monitoring can base on gross β radioactivity measurement of the environmental samples, in addition to γ - spectrometry
and radiochemical analysis for identification of radionuclides.
The environmental object samples were gathered in different directions and
distances from the nuclear reactor [3,4,5] and in 1 km protective zone, as well as in
different control points in different distances from the reactor, e.g. in Salaspils Botanical
Garden (1.5 km), in Riga (20 km),at lkskile(10km).
Precipitation samples were gathered without interruption every month in
stainless steel square vessels (40x40x10 cm) that were put up 1.5 m high. Eight such
vessels had been put up in the reactor protective zone and one in Riga, Ikskile and
Salaspils Botanical Garden.
Soil and plant samples were taken twice a year near the places where the
precipitation gathering devices had been placed
The taking of aerosol samples have been carried out at 0.5 km distance from
the reactor by means of the ventilator with sucking power about 1900 m3/h air through
1380 cm2 filter FPP-15 [44] material with exposure 1 - 4 daily. It was done once a
month.
The sample preparation have been carried out in laboratory [3,4,5]. Solid
samples have been dried and reduced to ashes in furnace at temperature less
than+400°C and liquid samples -evaporated and (or) reduced to ashes.
P- and y- radiation of samples had been measured by gas discharge counters
(STS-6, STS-5, MST-17 un SBT-13) and counting assembly with low background
level UMF-1500.
The γ -spectroscopy of the environmental samples had been carried out using the
worked out methods in nuclear spectroscopy by multichannel pulse-height analyzer
with scintilation detectors used in the 60th and Ge(Li) detector during the last years [4653].
Calibration of measuring equipment.
The calibration of UMF-1500 had been carried out by natural KCI standard " and
γ -spectrometer by OSGI un "AMERSHAM" standard. The applied methodic and
measuring equipment corespond to measuring all the characterictic radionuclides [6,7],
which may penetrate into environment from nuclear plants and nuclear weapon tests.
3. The reactor operation and results of the monitoring.
In the situation that there is no significant local radioactive pollution in the
environment, two main general principles can be used in order to compare the results
of the radioactivity measurements in the environment. This should be done in order to
come to the conclusion about the low level radioactivity pollution in the neighourhood
around the nuclear reactor:
- the nuclear reactor as a potential polluter of the envitonment the nearest natural
objects pollute more than distant objects and in some definite direction more than in
others (spatial correlation);
- the intensity of the pollution of the environment as well as the result of the pollution
(the gross radioactivity) is in the direct time correlation with the produced energy
(power), rhythm and with the quantity of the nuclear energy that has been produced
before that.