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High-performance Dust-radiation Monitoring System by Simultaneous Discrimination of Alpha and Beta Rays
Hitachi Review Vol. 49 (2000), No. 2
71
High-performance Dust-radiation Monitoring System by
Simultaneous Discrimination of Alpha and Beta Rays
OVERVIEW: A dust-radiation monitoring system is installed in every nuclear
power plant to collect dust suspended in the air and to monitor radiation
levels in all areas inside the plant for radiation exposure control and work
management purposes. The existing types of radiation monitoring systems
are as sensitive to natural radioactivity as to artificial radioactivity, so it is
necessary to develop a monitoring system that can discriminate between
the two types of radioactivity whenever a change is detected in the measured
radiation level. Since the natural radioactivity depends primarily on radon
that radiates alpha and beta rays, and its decay-product nuclides, we have
developed a system to significantly improve the monitoring performance of
artificial radioactivity. We achieved this by concurrently measuring the
alpha and beta rays with a single detector in order to discriminate natural
radiation according to the ratio of radioactivity between the two types of
rays. The dust sampler is built as an autonomous distributed system capable
of operating independently. For information monitoring, we have adopted
a cutting-edge data-control system consisting of a high-performance
information control server and a general-purpose LAN. This system is open
and highly scalable and can provide trend data monitoring and central-
control of field equipment and devices.
Akihisa Kaihara
Makoto Namai
Hiroshi Arima
Hitoshi Kuwabara
INTRODUCTION
NUCLEAR power plants use various types of dust-
radiation monitoring systems to collect and measure
the radioactivity (beta rays) of dust in the air inside
the plant. These monitoring systems are also sensitive
to natural radioactivity that fluctuates depending on
the state of air ventilation (air-conditioning) inside the
building and on weather conditions (rain, day/night,
etc.). It has been the common demand, therefore, that
the existing dust-radiation monitors should determine,
upon detecting a change in the measured radioactivity
of beta rays, whether the change is attributable to
natural or artificial radiation.
Against this background, we have developed a dust-
radiation monitoring system to monitor artificial
radioactivity (beta rays) at high precision and to
determine natural radioactivity (primarily attributable
to radon and its decay-product nuclide) in real time.
And this system has been installed in Fukushima Daini
Nuclear Power Station of Tokyo Electric Power Co.,
Inc.
CENTRALLY CONTROLLED DUST-
RADIATION MONITORING SYSTEM
The centrally controlled dust-radiation monitoring
system for Fukushima Daini Nuclear Power Station,
the common waste-treatment facilities for Nos. 1 and
2 plants, and the same for Nos. 3 and 4 plants of the
Tokyo Electric Power Company consists of six sets of
dust samplers. These samplers automatically collect
data at 15 measuring points by switching with valves,
and the data are monitored in a security control room
and an administrative building. The primary functions
of the system, including those related to the
discrimination between natural and artificial
radioactivity, improved maintainability and
performance, are described below.
Discrimination Between Natural and Artificial
Radioactivity
Recent dust-radiation monitoring systems measure
the total level of beta rays consisting of both natural
and artificial radioactivity and must efficiently
discriminate artificial radioactivity from the total
radiation level. Moreover, the large percentage of High-performance Dust-radiation Monitoring System by Simultaneous Discrimination of Alpha and Beta Rays
72
natural radioactivity is attributable to radon and its
decay-product nuclides, and the ratio between alpha
and beta rays radiated from natural radioactivity is
nearly constant so long as the environmental conditions
do not change significantly.
Taking this fact into account, we have developed a
dust-radiation monitoring system as a means to
improve the monitor of the artificial radioactivity. We
did this first by evaluating the influence of natural
radioactivity by measuring and monitoring the portion
of alpha rays in natural radioactivity and, then, by
discriminating the portion of artificial radioactivity
from beta rays measured at the same time.
Improvement of System Maintainability
The dust sampler is designed to be detachable from
the rest of the system under the local mode and
operated on its own. The overall maintainability of the
system has been greatly improved by such measures
as the front-panel operation that enables the dust
sampler installed in the front and facilitates the regular
replacement of the paper filters. In previous models,
it was necessary to pull out the movable unit and open
the sealed box in order to replace the filters.
Integrated Central Control
The server computer collects the measurement data
from the multiple dust-samplers installed in each
building through a general-purpose LAN for central
control purposes and provides the security control
room, central operation room, and administrative
building with necessary information. At each terminal,
operators can monitor current radioactivity density
measured by the respective dust samplers. On the
terminal in the security control room, in particular, it
is possible by remote control to switch the dust
collection points of each dust sampler and/or set the
collection timing so as to raise the level of monitoring
and operation.
DUST SAMPLER
Alpha and Beta Rays Discrimination
The detector for simultaneously detecting alpha and
beta rays is a proven composite detector made of ZnS
(Ag) (for detecting alpha rays) and a plastic scintillator
(for detecting beta rays) overlapping one another. Two
types of signal discriminator circuits are available:
crest discrimination and waveform discrimination.
We selected the waveform discrimination because it
is superior in the discrimination characteristics to the
other type for our purpose.
The structure of the radiation measurement section
of the dust sampler is shown in Fig. 2. It concurrently
detects both alpha- and beta-ray signals and transmits
the measured signals to the subsequent stage, rise-time-
to-height converter (RHC). The RHC efficiently
segregates alpha rays and beta rays into two groups of
different crests according to the difference in the rising
time of the respective pulse waveforms. It also outputs
signals from each of the two groups to the following
stage of each counter circuit as an alpha-ray signal or
a beta-ray signal. Alpha-ray-to-beta-ray signal-mixing
ratio is less than 1.0% and beta-ray-to-alpha-ray signal-
mixing ratio is less than 0.1% so as to substantially
Fig. 1 Central Monitoring System
of Fukushima Daini Nuclear Power
Station, Tokyo Electric Power Co.
The nuclear power plant employs a
system developed by Hitachi, for the
first time in Japan. This system can
monitor artificial radioactivity at a
high degree of accuracy through the
determination of natural
radioactivity (e.g., radon), which
fluctuates according to
meteorological and other conditions,
in real time by concurrently
monitoring radiation from alpha and
beta rays measured from dust
suspended in the air. It is the newest
type of a data control system using a
general purpose LAN for central
control through terminal computers,
including trend monitoring (over a
three-year period) and remote
control of field equipment.
Administrative
building
Central control
room,
plant no. 2
Security control room
Waste control
building
control room,
plant nos. 1 & 2
Waste control
building
control room,
plant nos. 3 & 4
Waste control
building site,
plant nos. 1 & 2
Plant no. 2 site
Waste control
building site,
plant nos. 3 & 4
Optical cable
Optical
cable
Optical cable
Dust sampler
(Information-type LAN)
(Data-type LAN)
Server computer
Terminal computer Hitachi Review Vol. 49 (2000), No. 2
73
improve the accuracy of monitoring the ratio of alpha
rays to beta rays. Thus, the influence of natural
radioactivity on the measurement of total radioactivity
could be determined.
Real-time Measurement
Dust radiation is so small that dust must be collected
by sucking dust-containing air into a pump through a
paper filter. It is, therefore, structured to measure
radiation as th