THE MEASUREMENT OF RADIOACTIVITY

Abstract: A process for the measurement of the level of radioactivity of a solid by contacting the solid with a chemically stable reactant fluid. The reactant fluid is converted by nuclear radiation to a chemically stable product fluid to an extent dependent upon the radioactive content of the solid. The amount of the reactant fluid converted into the product fluid is detected in order to determine the level of radioactivity of the solid.

Description: THE MEASUREMENT OF RADIOACTIVITY The present invention relates to the measurement of radioactivity. In the nuclear industry there is a need to assess and measure accurately the radioactive content of bodies of solid material, eg bulk waste materials produced after nuclear fuel reprocessing, nuclear plant decommissioning, surface decontamination and land remediation activities. Nuclear solid wastes are classified into low level wastes (LLW) and intermediate level wastes (JO W) according to their activity levels. Storage of ILW typically costs 100 times that of LLW; it is therefore commercially attractive to be able to distinguish accurately between ILW and LLW by measurement of the radioactivity of a given sample. Furthermore it is desirable to distinguish between LLW and so-called 'de-minimus' wastes which are wastes whose activity is essentially negligible. It is the purpose of the present invention to provide a process to measure accurately the radioactivity of a given sample of solid material, eg in order to classify solid waste material as ILW or LLW or as LLW or de-minimus waste. According to the present invention a process for the measurement of the level of radioactivity of a solid includes the steps of contacting the said solid with a chemically stable reactant fluid which is converted by nuclear radiation to a chemically stable product fluid to an extent dependent upon the radioactive content of the solid and detecting the amount of the reactant fluid converted into the product fluid. In a first aspect of the present invention the solid may comprise a material which is porous or has re-entrant surfaces and the process of the present invention includes permeating through a sample of the said material a chemically stable volatile reactant gas which is converted by nuclear radiation to a chemically stable volatile product gas. The said material may comprise bulk solid waste material which is being monitored to determine whether, it is ILW, LLW or de-minimus waste as described above. Thus, the present invention provides a method for classifying or assaying solid waste material. Desirably, the reactant gas is permeated through the sample using a varying pressure, eg using a pump to ensure that the reactant gas breathes into the re-entrant surfaces and contacts any sites of obscured contamination." The sample interrogated by the reactant gas may be contained in a totally or partially sealed container. Where the container is only partially sealed the outleakage is desirably measured. A pressure pulse from the pump is desirably applied at intervals after the initial application of the reactant gas to permit re-circulation through the re¬ entrant surfaces. The reactant gas may comprise as simple hydrocarbon or a halogen- substituted hydrocarbon or a mixture of such compounds. Examples of gaseous compounds suitable for use as or as a component of the reactant gas include methane, ethane, ethylene, propane, propene, butane, butane- 1, butane-2, methyl bromide, ethyl bromide, hexafluoroethane, trifluoromethane and bromotrifluoromethane. Bromotrifluoromethane is preferred as the reactant gas. This produces tetrafluoromethane as a product gas when exposed to gamma radiation. The product gas may comprise a plurality of products produced by irradiation of the reactant gas. This does not affect operation of the invention provided that the relationship between irradiation of the reactant gas and the concentration of the components of the product gas produced thereby is known. This relationship may be known from literature results or may be determined by experimental analysis of calibrant gases or gas. The relative concentration of the components of the product gas may vary depending on whether the incident radiation comprises alpha, beta or gamma radiation. However, the effect is to produce the same components of the product gas in differing concentrations. For assay purposes, the maximum acceptable concentration level of each of the components is set and in the detection step it is found whether any of the components of the product gas exceeds its maximum acceptable level. Advantageously, components of the product gas maybe concentrated in one or more sample traps by adsorption of a desired component onto a solid such as activated charcoal. Alternatively, less volatile components of the product gas may be trapped using low temperature traps. Conveniently the sample traps may comprise a U-shaped tube of stainless steel packed with the desired adsorbent. Desirably, any non-radiolytic depletion of the reactant gas (eg sorption) can be detected using a third gas (an inert gas) mixed with the reactant gas and measuring the relative concentration ratio of the third gas to reactan gas. The third gas may comprise SFg. The third gas, in conjunction with the reactant gas, can be used to measure gas outleakage during the assay. Furthermore, a stable fourth or internal standard gas, substantially unaffected by the radiation to be detected, is desirably added as or is present as an impurity to the reactant gas in a known concentration of not more than a few- parts per million ft 20ppm) to enable the concentration of the product gas to be standardised. The ratio of the concentration of the product gas to that of the internal standard gas provides a normalised measure of the product gas. The detection of the amount of the reactant gas converted into the product gas may be by one of the techniques well known to those skilled in the field of analytical techniques, such as gas chromatography combined with mass spectrometry (GCMS) or by infra-red (IR) spectroscopy. The signal representative of the concentration of the product gas may be displayed on a paper printer, on a electronic video display unit or in any other suitable way. As noted above the concentration of the product gas or its components may be compared with a previously determined acceptable level and the detector and display may provide an output which indicates whether on the one hand the concentrations of all the components of the product gas are below their acceptable levels or whether on the other hand the concentration of any of the components is above the acceptable level. The signals produced as outputs by the measurement system, eg GCMS, may be further enhanced by signal processing techniques, eg band pass filtering plus signal to noise enhancement at the known peak values representing the various detected gas components. A body of material to be assayed by the method according to the first aspect of the present invention is contained in a suitable container and the reactant gas together with the leakage or non-radiolytic depletion measurement gas and the internal standard gas are desirably added to the body of material. The container does not need to be evacuated before addition of the said gases, and a suitable concentration of the added gases is greater than say 30 per cent by volume, desirably greater than 50 per cent by volume, of the total gas (including air) in the container. Where the body comprises a mass of solid waste articles contaminated with radioactive materials, the articles are desirably inspected as they are tipped into the container to search for items such as bottles or cans which are sealed and require further breakage to allow their surfaces to be contacted by the added gases. The container into which they are tipped may for example be a so-called ISO-FREIGHT container having a volume of approximately 35m 3 . After the solid material has been added to the container the added gases are introduced and the container is then sealed to the extent of sealing possible. The container is then left for a suitable period of time, eg from one day to several months, depending on the likely radioactivity levels of the material. During this period, eg every week where the container is stored for a period of several weeks, the gases in the container are re-circulated by applying a pressure pulse. When it is necessary to measure the radioactivity levels a sample of the gas in the container is extracted by a suitable gas suction pump or gas sampling equipment and the sampled gas is made the subject of an interrogation by a suitable analytical technique, eg one of the known techniques mentioned above, to determine the relative amount of reactant gas converted into product gas. The solid material is suitably consigned to the appropriate waste destination after its radioactivity has been monitored and/or permitted to fall below a border level. In the United Kingdom, for example, the border level between LLW and ILW is approximately 12 Gigabecquerels per tonne of waste for beta and gamma emissions and 4 Gigabecquerels per tonne for alpha emissions and the border level between LLW and de-minimus waste is orders of magnitude lower (with certain provisos depending on certain factors such as the manner and place of storage, the nature of the activity etc). According to a second aspect of the present invention the solid may comprise a convoluted items having sites of obscured contamination and the process of the present invention includes filling the item with a chemically stable non-volatile reactant liquid which is converted by nuclear radiation to a chemically stable non-volatile product liquid. Desirably, reactant liquid has density greater than that of water whereby if any water is present on the surface to be assayed, the water will be displaced from the surface. The said item may comprise a containment, glovebox or pipework or other radioactive plant component which is being monitored to determine whether it is to be classified as intermediate level waste, low level waste or de-minimus waste. Alternatively, the item may be monitored to determine whether its containment may safely be broken. The reactant liquid may comprise a simple hydrocarbon or a halogen- substituted hydrocarbon or a mixture of such compounds. Desirably, the reactant liquid comprises a chlorofluorocarbon. Trichlorotrifluoroethane is preferred as the reactant liquid and the action of nuclear radiation on parent trichlorotrifluoroethane molecules is to produce molecules larger than the parent molecules eg by addition of CF2 and/or CFC1 units. Detection of species of larger mass than the parent species is therefore possible using a mass spectrometer. In use of the process of the second aspect of the present invention, the item, such as a glovebox is filled with the reactant liquid and left for a suitable period of time which may be several hours, but could be several days or even several weeks. Any residual radioactive material in the glovebox will cause a chemical change in the reactant liquid. Samples of liquid are taken from the glovebox and analysed by a suitable technique such as gas liquid chromatography followed by mass spectrometry. The radioactive content of the glovebox may be determined from the quantity of the various chemical compound products produced in the reactant liquid. Examples of the manner in which a reactant gas may be irradiated to form a product gas and analysis of the concentration of the product gas may be employed to indicate the level of irradiation will now be described with reference to the accompanying drawings in which: Figure 1 is a graph of ratio of product gas to internal standard gas versus dose for an irradiated gas sample; Figure 2 is a block schematic diagram of an arrangement for monitoring the radioactivity of solid waste material by a method embodying the first aspect of the present invention; A sample of each of a series of suitable reactant gases was sealed into a thin plastics gas sampling bag which was then sealed in a cardboard box. The bags were interspaced with pieces of paper. The box was then subjected to various doses of beta radiation from a commercial electron beam source of known energy level, viz 40MeV. The irradiation doses were selected to be successively 4MRad, 2MRad, IMRad and 0.5MRad. The sampled reactant gases in the bags were the specific reactant gases listed above. Each gas included the impurity CCIF3 in a concentration of parts per million. A GCMS system was programmed so that the oven temperature started at 40°C and remained at that temperature for 1 minute before rising to 100°C at a rate of 5°C per minute. The temperature then remained at 100°C for a further five minutes. The samples after irradiation were injected via a gas syringe onto the column. 10 microlitres of sample was injected. The results were printed and analysed. The results obtained using as reactant gas bromotrifluoromethane are summarised in Table 1 as follows, hi Table 1 the figures given in each of the second to fourth columns are the peak areas observed from the mass spectrometer prints at different dose levels which indicate the concentration of the relevant components. Scan 171 represents CF3Br, Scan 177 represents CF4 and Scan 230 represents CCIF3. Table 1 Peak Area Data For Selected Com ounds A graph of the ratio of the peak area figure obtained for Scan 177 to the peak area figure obtained for Scan 230 as tabulated in Table 1 was plotted against the applied dose and the result is shown in Figure 1. Reference to the linear plot shown in Figure 1 may be used therefore to provide a calibration plot to enable the dose providing a given Scan 177:Scan 230 ratio to be calculated. The dose figure is a measure of the irradiation, for a given period of exposure, provided by a source of given activity level. Figure 2 shows an arrangement for monitoring the radioactivity level of solid waste material held in a container 1 by a method embodying the present invention. Reactant gas from a source 3 is admitted into the container 1 via a valve 5. The container 1 is sealed with the gas added and left for a suitable period of time. Periodically, the added gas is recirculated within the container by the action of a pressure pulse from a pump 11 in a loop 9 containing a valve 13. Eventually, a sample of the gas from the container 1 is extracted from the loop 9 when the pump 11 is operating. The sample is diverted to a sample container 15 via a valve 17. The gas held in the sample container 15 is subsequently made the subject of analysis in an analyser 19, eg a GCMS system, the result of the analysis either as a numerical value indicating activity level or as a activity classification, ie above or below a given level, is displayed on a display 21.

Claims: CLAIMS 1. A process for the measurement of the level of radioactivity of a solid by contacting the said solid with a chemically stable reactant fluid which is converted by nuclear radiation to a chemically stable product fluid to an extent dependentupon the radioactive content of the solid and detecting the amount of the reactant fluid converted into the product fluid. 2. A process as in Claim 1 and wherein the solid comprises a material which is porous or has re-entrant surfaces and which process includes permeating through a sample of the said material a chemically stable volatile reactant gas which is converted by nuclear radiation to a chemically stable volatile product gas. 3. A process as in Claim 2 and wherein the said material comprises bulk solid radioactive waste material which is being monitored to determine whether it is intermediate level waste, low level waste or de-minimus waste. 4. A process as in Claim 2 or Claim 3 and wherein the reactant gas is permeated through the sample using a varying pressure, to ensure that the reactant gas breathes into the re-entrant surfaces and contacts any sites of obscured contamination. 5. A process as in Claim 2, 3 or 4 and wherein the sample is contained in a totally or partially sealed container. 6. A process as claimed in any one of Claims 2 to 5 and wherein a pressure pulse from a pump is applied at intervals after the initial application of the reactant gas to permit re-circulation through the re-entrant surfaces. 1. A process as claimed in any one of Claims 2 to 6 and wherein the reactant gas comprises a simple hydrocarbon or a halogen-substituted hydrocarbon or a mixture of such compounds. 8. A process as in Claim 7 and wherein the reactant gas comprises one or more of methane, ethane, ethylene, propane, propene, butane, butane- 1, butane-2, methyl bromide, ethyl bromide, hexafluoroethane, trifluoromethane and bromotrifluoromethane. 9. A process as in any one of Claims 2 to 8 and wherein components of the product gas may be concentrated in one or more sample traps by adsorption of a desired component onto a solid adsorbent. 10. A process as in Claim 9 and wherein the solid adsorbent comprises activated charcoal. 11. A process as in Claim 9 or Claim 10 and wherein the sample trap comprises a U-shaped tube of stainless steel packed with a desired adsorbent. 12. A process as in any one of the Claims 2 to 8 and wherein components of the product gas having a relatively low volatility may be trapped using low temperature traps. 13. A process as in one of Claims 2 to 12 and wherein non-radiolytic depletion of the reactant is detected using an inert gas mixed with the reactant gas by measuring the relative concentration ratio of the inert gas to the reactant gas. 14. A process as in Claim 13 and wherein the inert gas in conjunction with the reactant gas is employed to measure gas outleakage. 15. A process as in Claim 13 or Claim 14 and wherein the inert gas is SFg. 16. A process as in any one of Claims 2 to 15 and wherein an internal standard gas unaffected by the radiation to be detected is added to or is present as an impurity in the reactant gas in a known concentration to enable the concentration of the product gas to be standardised. 17. A process as in Claim 1 and wherein the solid comprises a convoluted item and which process includes filling the item with a chemically stable non¬ volatile reactant liquid which is converted by nuclear radiation to a chemically stable non-volatile product liquid. 18. A process as in Claim 17 and wherein the said item comprises a containment, glovebox or pipework or other radioactive plant component which is being monitored to determine whether it is to be classified as intermediate level waste, low level waste or de-minimus waste. 19. A process as in Claim 17 or Claim 18 and wherein the item is being momtored to determine whether its containment may safely be broken. 20. A process as in Claim 17, 18 or 19 and wherein the reactant liquid comprises a simple hydrocarbon or a halogen-substituted hydrocarbon or a mixture of such compounds. 21. A process as in Claim 20 and wherein the reactant liquid has a density greater than that of water. 22. A process as in Claim 20 and wherein the reactant liquid comprises a chlorofluorocarbon.

Inventors: HOLMES ROBERT GEORGE GODFREY, HOLMES, ROBERT, GEORGE, GODFREY
Applicants: BRITISH NUCLEAR FUELS PLC, HOLMES ROBERT GEORGE GODFREY
Application year: 1993
Main classification: G01T1
IPC codes: G01T1