GB2461868A

GB2461868A - Portable radiation detectors

Info

Publication number: GB2461868A
Application number: GB0812787A
Authority: GB
Inventors: Malcolm John Joyce; Robert Mackin
Original assignee: Lancaster University
Current assignee: Lancaster University
Priority date: 2008-07-12
Filing date: 2008-07-12
Publication date: 2010-01-20
Also published as: GB0812787D0

Abstract

Portable apparatus for the extraction and measurement of radioactive substances from composite materials includes a microwave generator 201 arranged to deliver microwave radiation to a horn 205 to irradiate a workpiece 101. Vapours displaced from the workpiece by the microwave radiation are drawn by a pump 121 through pipework 129 for measurement by an instrument 125 such as a portable mass spectrometer. The apparatus may be used to determine the level of tritium contamination in concrete.

Description

PORTABLE RADIATION DETECTORS

DESCRIPTION

PROBLEM ADDRESSED

During operation and decommissioning of nuclear facilities, quantities of radioactive waste materials are produced. It is normal practice to categorise each item of waste in order to proceed to the most efficient means of processing, storage and/or disposal -with due regard to safety.

In general the higher the level of radioactivity, the more care that must be taken in processing, and the greater the safety measures that must be in place to protect the people involved in the processing; and the greater the cost.

For many isotopes (such as caesium 137) there is a high-energy gamma-ray label that affords easy identification of the extent of radioactive contamination. However, in other cases such as tritium (a radioactive isotope of hydrogen), the situation is more difficult because no characteristic gamma-ray emission is observed.

The present invention is a novel portable system for detection and measurement of contaminants where no gamma label exists, preferably for application in-situ to composite materials.

BACKGROUND

Background -tritium

Contamination by tritium is believed to be widespread at nuclear sites. Tritium has been produced for nuclear weapons programmes, and effluent and spillage has resulted in contamination of infrastructure. In civil nuclear engineering, tritium arises from the neutron activation of water in water-cooled reactors, steam generators, asbestos and spent-fuel storage ponds. * S

Tritium is an isotope of hydrogen. It is radioactively unstable and emits low energy beta-particles with an average energy of 5.7 keV. These fast electrons have insufficient energy to penetrate the outer layer of human skin. Since tritium has no other higher-energy emissions (such as gamma-rays) the external risks are small, but there is a significant internal exposure risk if tritium is ingested and/or inhaled. The radioactive half-life of tritium (12.3 years) means that it cannot be dealt with by delayed storage, as the timescales for decay are too long.

The chemistry of tritium is almost identical to that of hydrogen, and tritium easily replaces hydrogen in water and organic materials. These types of substance are readily taken up by the body. Tritium also displaces hydrogen in the water of hydration in extended molecular complexes, for example in the calcium silicates used in construction materials such as concrete.

The extremely low energy of the beta-particles emitted from tritium results in an extremely low yield of electrical current evolved as a result of ionisation in radiation detectors. This means that accurate measurement is difficult, as detectors are often forced to operate at the limits of their sensitivity, where calibration is difficult and expensive.

Background -concrete

Concrete is a ubiquitous substance used in industrial plant construction, and is composed of a variety of calcium silicates. For example, tn-calcium silicate (3CaO.2S102.4H20) contributes to strength in the early (days) stages of cu ring; whilst di-calcium silicate (2CaO.2SiO2.4H20) provides strength later in the curing timescale.

Water is essential in the bonding of the concrete because the complex of water molecules with silicates delivers its strength. Even though cured concrete is dry, it incorporates significant quantities of water, providing a location for tritium atoms. Tritium migrates readily through concrete structures, both in response to agitation and by diffusion.

Existing techniques to attempt to measure tritium in concrete are believed to be inaccurate because the act of measurement is likely to change the distribution.

Furthermore, as discussed earlier, the measurement of tritium is intrinsically difficult.

ESSENTIAL FEATURES OF THE PRESENT INVENTION

The present invention operates by using microwave radiation to excite small molecules within a porous composite material. The molecules are liberated from the matrix and extracted through the porous material by application of a negative pressure at the surface. The extracted molecules are then routed to a detector.

The use of microwave radiation provides a direct means of heating the target molecules, and drives off not only free and capillary molecules, but in the case of water in concrete also the water molecules complexed with the silicates. This may have a side-effect of impairment of the structural properties of the concrete, but this is not a problem as the material is being decommissioned.

Where the target is tritium, molecules of regular (light) water and tritiated water are extracted and analysed, and the detector preferably determines the proportion of the water molecules which are tritiated.

Microwave delivery The present invention uses a klystron (similar to those used in domestic microwave ovens) to generate microwaves. This requires only a domestic-rated electrical power supply, and operates preferably at a microwave frequency of about 2 GHz and with a power in the range 500 to 2,000 watts. Modern heating klystrons are engineered to be robust and to couple radiation into a wide range of target impedances without suffering problems from reflected power (this is a requirement for coping with the wide range of cooking materials and geometries presented in microwave ovens).

As a non-limiting example at a frequency of 2.40Hz typical relative permittivity lies between 4.5 for dry concrete and 7 for concrete in the environment. For a normal (right angle) transition from air to concrete, applying equation (1) gives a reflection coefficient between 0.36 and 0.45 R-_______ Equation (1) -+ In a first embodiment shown in Figure 1 the work-piece (101) is placed inside a chamber (cavity) (103) and irradiated with microwave radiation from an intrinsic klystron (105) for a defined period (of the order of minutes) based on the mass of the work-piece (101).

In a second embodiment shown in Figure 2 there is no cavity, and all the apparatus is external to the work-piece (101). The apparatus is lightweight and modular to make it human-portable. A klystron and its power supply in a suitable shielded enclosure (201) deliver microwave radiation via a short waveguide incorporating a 3 stub or other suitable matching section (203). The radiation is coupled into the work-piece (101) using a horn (205) designed according to well-known practice to maximise power delivery.

The horn (205) has an aperture to suit the required area of the work-piece (101)to be evaluated. The face (207) of the horn placed against the work-piece (101) is composed of a low-loss microwave-permeable closure with channels to allow displaced vapours to flow to the sampler directly without entering the horn (205). Preferably radiation screening material (preferably metal sheet, optionally perforated; not shown in the figures) is placed around the horn (205) and the other sides of the work-piece (101) in order to prevent environmental leakage of microwave radiation.

In both of these embodiments, a pumping system (121) draws air and other fluids (such as steam, water vapour) at a known rate through pipework (129) for measurement by a suitable instrument (125), preferably a portable mass spectrometer. The radiation may scour dust from the work-piece (101), and a dust-trap (123) is preferably placed upstream from the analysis componentry. The system may optionally incorporate a suitable absorber (127) for water vapour and associated tritium in order for its regeneration or disposal. This may be a condenser or a molecular sieve or composed of a substance such as silica gel, as well known to those skilled in the art.

Standard interlocking precautions are preferably incorporated in all embodiments to ensure that the microwave activation fails safe in the event of detachment from the concrete face.

Detector Preferably the invention incorporates a mass spectrometer (for example a Kore MS 200 portable instrument resolving from 1 to 1,000 AMU with a resolution on 1 AMU, giving much greater sensitivity than an ionisation detector. It is thus able to establish the relative concentration of isotopes of one or more elements of interest (for example hydrogen/deuterium/tritium) by mass and is not dependent on the weaker ionising properties of tritium, for example. The instrument sensitivity may be varied by controlling the microwave source power and the period of irradiation.

Alternative detectors to the mass spectrometer may be used, such as tritium detector instrumentation (such as the Overhoff 1400), in correspondence with the related standard (IEC 60761). However, it is widely understood that the sensitivity of even the best of such ionisation detectors is low.

While the present invention has been described in terms of several embodiments, those skilled in the art will recognize that the present invention is not limited to the embodiments described, but can be practised with modification and alteration within the spirit and scope of the appended claims. The Description is thus to be regarded as illustrative instead of limiting.

STATE OF THE ART

Heating composite materials is well known to those skilled in the art as a means to release light molecules.

Patent JP1273000 ("Method and device for treating concrete contaminated with tritium") discloses a method of heating a piece of concrete in an electric furnace to drive off water (including tritiated water) and uses a device to detect moisture content (or lack of it) in the discharged gas stream to detect an endpoint. JP3051788 is similar ("Method and apparatus for measuring radioactivity') Patent J P3041 386 ("Non-destructive measurement of quantity of tritium in concrete and device therefo?') discloses a method of applying a heating probe to the surface of a piece of concrete and using a pump to suck any resultant steam to a condenser for subsequent analysis.

None of the above use microwave radiation as a means of extraction.

Patent GB2242060 ("Tritium removaf') discloses the use of microwaves to heat concrete. However it is different to and inferior to the present invention in a number of ways: * GB2242060 includes a variable power 25kW, 896MHz microwave generator and about 3.5 metres of waveguide (248 x 124 mm), making it very far from portable and requiring a special dedicated electrical power source.

* The radio-frequency design of GB2242060 is inefficient. It uses a cuboid cavity adjacent to the sample and requires a water-cooled circulator to protect the generator from reflected microwave energy -implying poor coupling into the concrete sample.

In some cases concrete may be contaminated only in a surface layer, while the core is intact. It is desirable to separate these two regions for different processing. There are well known techniques of using high power microwaves to heat water rapidly in the surface concrete to cause chunks of the surface to break off, for example patent US61 57013 ("Microwave applicator and method for the surface scarification of contaminated concrete"). In some circumstances the present invention may be used as a precursor to this in order to indicate the depth of surface contamination.

Example

Figure 3 shows the loss of mass experienced as a number of concrete samples were exposed to microwave radiation. The vertical axis shows the normalised mass, and the horizontal axis shows the time in minutes.

The source of microwave radiation was rated as 900 watts (lEG 60705) and was run at 50% power. The experimental cycle was: 1 minute irradiation, followed by 1 minute standing (and cooling), and one minute of weighing and standing. This three minute cycle was repeated multiple times. Figure 3 shows the total time elapsed.

Samplel (symbol o): High density concrete from a commercial edging strip with mixed aggregate taken from 10mm down, substantially cuboid 125 x 50m x 10 millimetres Sample2 (symbol x): High density concrete from a commercial edging strip with mixed aggregate taken from edge of block, substantially cuboid 125 x 50m x 10 millimetres Sample3 (symbol +): High density concrete from building brick with aggregate from 3 millimetres down to dust limestone. Dimensions 100 x 65 x 10 millimetres Figure 4 shows the same data over a longer timescale, with a logarithmic time axis extending past the period of irradiation. After irradiation, the mass slowly rises again indicating the take up of atmospheric water. This is believed to be chemical rehyd ration, indicating that microwave irradiation removes water of hydration and not just pore water.