The present invention relates to a dust radiation monitor apparatus used in radiation source handling facilities such as nuclear power plants to measure the radioactivity concentration of dust in the air in the facilities, and a dust sampling apparatus used in the dust radiation monitor apparatus.
Conventionally, in radiation source handling facilities such as nuclear power plants, dust radiation monitor apparatuses for measuring the radioactivity concentration of dust in the air in the facilities are widely used.
FIG. 1 is a schematic view showing an example of the arrangement of a dust radiation monitor apparatus widely used in practice in radiation source handling facilities such as nuclear power plants.
In the conventional dust radiation monitor apparatus in FIG. 1, when air is drawn into a chamber C through a sampling pipe 1, radiation dust in the air is collected through a filter (filter paper or the like) in a dust collection section 2. The air after dust collection is exhausted through a sampling pump 3. A .beta. ray detection section 5 detects .beta. rays from the radiation dust collected through the filter, and converts it into an electrical signal. When a .beta. ray measurement value is obtained by a .beta. ray measuring section 7, a data processing section 8 compares the measurement value with a warning set value and determines contamination if any.
Natural radioactive substances exist in nature. Radon is a naturally occurring substance (to be referred to as a natural nuclide hereinafter). Since radon exists in a rare gas form in nature, it also floats in the air in the facilities. A nuclide of this kind emits .alpha. and .beta. rays in the process of decaying into a daughter nucleus and granddaughter nucleus.
The radiation dust collected by the above dust radiation monitor apparatus therefore contains natural nuclides. For this reason, as shown in FIG. 2, a total .beta. ray measurement value Va includes a contribution V2 of natural nuclides as well as a contribution V1 of measurement nuclides due to leakage. Owing to the influences of such natural nuclides, many radioactive nuclides (artificial contamination nuclides) as measurement targets may be determined to exist in amounts larger than actual amounts. For this reason, the influences of natural nuclides must be separately evaluated.
As a technique of solving this problem, a technique of accurately determining the presence/absence of radioactive contamination without separately evaluating the influences of natural nuclides is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-211133. According to this technique, .alpha. and .beta. rays emitted from natural nuclides are separately measured in advance by a measurement system, and the emission ratio of the measured .alpha. and .beta. rays from natural nuclides is obtained in advance. In actual contamination determination, radiation emitted from measurement targets is detected by a radiation detector, and .alpha. and .beta. rays are separately measured from the detection signal. A correction processing means then obtains a .beta. ray value base on natural nuclides which are contained in the .beta. ray measurement value on the basis of the emission ratio of .alpha. and .beta. rays and the .alpha. ray measurement value, and subtracts the .beta. ray value base on natural nuclides from the .beta. ray measurement value. As a result, a .beta. ray value free from the influences of natural nuclides can be obtained.
In the dust radiation monitor apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-211133, however, .alpha. and .beta. rays are measured by using a single radiation detector. More specifically, detection/measurement is performed by a single radiation detector having a detection section made up of two layers (.alpha. and .beta. ray detection layers). For this reason, the following problems are posed.
(1) Low-energy .beta. rays are absorbed by the .alpha. ray detection layer.
More specifically, with the structure formed by stacking the .alpha. and .beta. ray detection layers on each other, .beta. rays output from the dust collection section always reach the .beta. ray detection layer through the a ray detection layer. The detection efficiency of low-energy .beta. rays, which are easily absorbed, decreases, resulting in a measurement error.
(2) Separate measurement of .alpha. and .beta. rays has its own limitation.
More specifically, light components from the .alpha. and .beta. ray detection layers mix with each other. The mixed light components are separated by a subsequent circuit in accordance with differences in rise characteristics and emission amount, but they are not perfectly separated. Since .alpha. and .beta. rays as measurement targets vary in energy, they mix with each other in a certain energy region, resulting in a measurement error. Especially, the low-energy side of a rays tend to mix with the high-energy side of .beta. rays.
Demands therefore have arisen for a dust radiation monitor apparatus which has a structure that prevents light components from .alpha. and .beta. ray detection layers from mixing with each other, and can eliminate any measurement error due to mixing of light components. Demands has also arisen for a dust radiation monitor apparatus which can prevent the other detection layer from absorbing .beta. rays in detecting .beta. rays, and improve the detection efficiency of low-energy .beta. rays, thereby eliminating any measurement error due to .beta. ray absorption.
In such a dust radiation monitor apparatus, the following problems arise in intermittent measurement and continuous measurement, in addition to the above problems of a decrease in detection efficiency and measurement errors.
FIG. 3 is a schematic view showing an example of the arrangement of a conventional intermittent dust radiation monitor apparatus.
Referring to FIG. 3, a pipe switching unit 1401 sequentially switches connections between the radiation monitor side and a plurality of (n) sampling pipes 1400 which are installed in different sampling places in radiation source handling facilities (not shown) to introduce air from the respective sampling places. As shown in FIG. 3, the pipe switching unit 1401 is constituted by a plurality of solenoid valves 1402.
The air introduced through the sampling pipes 1400 sequentially switched by the pipe switching unit 1401 is drawn by a pump 1406 through a pipe system and continuously sent to a dust collection section 1411. Dust in the air sent to the dust collection section 1411 is collected on filter paper 1403 driven by a filter paper driving section 1407.
In addition, the amount of air drawn is adjusted to a predetermined amount by a flow rate indicator 1404 and a flow rate control valve 1405.
Radiation from the dust collected on the filter paper 1403 is detected by a radiation detector 1410, and the radioactivity concentration of the dust on the filter paper 1403 is then measured by a data converter 1408 using the radiation reading from the radiation detector 1410. The measurement result is output to a display/recording section 1409.
Such an intermittent dust radiation monitor apparatus can perform only intermittent measurement. For this reason, the flow rate of air must be increased to a value equal to or more than a detection limit value, and the flow rate of air must be fixed to maintain a high detection precision.
Under the circumstances, a continuous dust radiation monitor apparatus is proposed, in which dust collection/measurement units are arranged for the respective sampling pipes to perform continuous measurement.
In such a continuous dust radiation monitor apparatus, however, since dust collection/measurement units equal in number to the sampling pipes must be installed, the apparatus arrangement becomes large in size.
Demands have therefore arisen for a dust radiation monitor apparatus which can switch intermittent measurement to continuous measurement to perform continuous monitoring, as needed, while intermittent measurement is performed in normal operation as in the prior art, thereby improving the measurement precision. In addition, demands have arisen for a dust radiation monitor apparatus which can collectively perform dust connection control and measurement processing for each of intermittent and continuous dust radiation monitors by intensively using one data processing section, and can change the schemes of the respective monitors according to the circumstances.
In dust sampling apparatus used in the above dust radiation monitor apparatus, the following problems arise in collecting dust with filter paper.
FIG. 4 shows the arrangement of a conventional dust sampling apparatus.
A chamber 901 is used to draw air from a predetermined place in a radiation management area and exhaust the air after dust collection. A filter paper holder 905 for holding filter paper 903 and a paper filter receiving wire net 904 is mounted in the chamber 901. The filter paper holder 905 is mounted on the chamber 901 through a paper filter holder O-ring 918. A radiation detector 902 for detecting radiation emitted from the dust collected by the filter paper 903 is mounted in the chamber 901. The radiation detector 902 is held by a detector holder 906, which is mounted on the chamber 901, through a detector O-ring 917.
In the dust sampling apparatus having this arrangement, when air is drawn through the inlet of the chamber 901, dust in the air is collected on the filter paper 903 in the filter paper holder 905, and the air after dust collection is exhausted from the outlet of the chamber 901.
In this conventional dust sampling apparatus, to increase the detection sensitivity, the distance between the filter paper 903 and the radiation detector 902 is reduced to several millimeters. Owing to this structure, the flow path of air passing through the filter paper 903 is not uniform, and dust is nonuniformly collected on the filter paper 903. As a result, the detection efficiency decreases due to self-absorption of dust, or a sensitivity calibration deviation occurs with respect to a calibration ray source.
Under the circumstances, demands have arisen for a dust sampling apparatus which can make the flow path of air passing through filter paper uniform, and can uniformly collect dust, thereby preventing a decrease in detection efficiency due to self-absorption of dust and a sensitivity calibration deviation with respect to a calibration ray source.