GB1565901A

GB1565901A - Method of detecting a change in temperature

Info

Publication number: GB1565901A
Application number: GB196977A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1977-01-18
Filing date: 1977-01-18
Publication date: 1980-04-23
Also published as: FR2377621A1; FR2377621B1; DE2801268A1

Description

(54) IMPROVEMENTS IN OR RELATING TO A METHOD OF DETECTING A CHANGE IN TEMPERATURE (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY London, a British Authority do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of detecting a change in temperature at a location and more particularly, but not exclusively, at relatively inaccessible locations, or locations in a hostile environment in which the use of existing temperature detecion devices such as thermocouples, or the maintenance thereof, may be impossible.In some applications it is only the onset of a rise in temperature of the location that is required so as to provide a warning of fault conditions occuring in a plant or apparatus.

According to the present invention, there is provided a method of detecting a change in temperature at a location, wherein at the location a source is provided adapted to release a radioactive gas at a rate which is temperature dependent, and means are provided removed from said location for detecting the rate of release of said radioactive gas to indicate change of temperature at the location.

Preferably, the location is arranged to be swept by a stream of gas to entrain the said radioactive gas therein, and the detecting means are positioned downstream of the location.

The source may comprise Ra-226 or Th228.

The invention will now be described, by way of example only, with reference to the drawings accompanying the provisonal specification in which: Figure I is a semi-diagrammatic drawing of part of a nuclear reactor plant including a trapping system; Figure la is a sectional view to an exaggerated scale of a radioactive source; Figure 2 shows in part-section an isometric view of part of a tapping circuit for the trapping system of Figure 1; Figure 3 shows a sectional view to a reduced scale of a cold trap for the trapping system of Figure 1, but with the part of Figure 2 removed; Figure 4 shows a view on the line IV-IV of Figure 3 but with the part of Figure 2 in place, and Figure 5 shows a plot of release of the daughter products of Ra-226 and Th-228 against temperature.

Referring now to Figure 1, a nuclear reactor powered plant 1 has a High Temperature Gas Cooled Reactor 2 in which the reactor coolant (helium) flows in a primary circuit 3 including the reactor 2.

The reactor 2 comprises a neutron moderator core structure 6 of graphite disposed in a pressure-tight reactor vessel 7.

Heat is transferred to a secondary circuit 5 (only part of which is shown) from the primary circuit 3 by way of a heat exchanger 8, and the secondary circuit 5 may be used as a heat source for a process plant (not shown) or a gas turbine (not shown).

At a location in the primary circuit 3 within the reactor core 2, a Ra-226 source 11 is disposed, the location being one which is relatively inaccessible during operation of the reactor 2.

The source 11 is shown in Figure la to which reference is now made, and is of circular form in plan. The source 11 is provided by a layer 12 of Ra-226 deposited by vacuum sublimation on to a circular foil 13 of platinum.

Referring again to Figure 1, a trapping system 16 is connected to the primary circuit 3 outside the reactor vessel 7, and comprises a trapping circuit 17 connected in parallel to the primary circuit 3 and to a cold trap 18 shown in greater detail in Figure 2.

Referring now to Figure 2, the portion of the trapping circuit 17 shown comprises a thick-walled glass tube coiled in a flat spiral to form a flat disc-like portion 21 having an inlet 17a and outlet 17b. The bore of the disc portion 21 is packed with activated charcoal granules 25 which are retained in position at each end of the disc portion 21 by glass wodl wadding 26. The direction of flow through the trapping circuit 17 as indicated by the arrows provides for the inlet limb 17a to enter the centre of the disc portion 21.

A vinyl liner 27 is disposed in the bore of the inlet 1 7a upstream of the wadding 26 to provide a thermal insulating layer between the inlet 17a and the gas therein so as to avoid cooling the gas before it has reached the disc portion 21.

Referring now to Figures 3 and 4, the cold trap 18 comprises a Dewar flask 30 having a cavity 31 in which a copper rod 32 is disposed axially therein. The copper rod 32 has three evenly disposed, axially aligned and radially directed fins 33 brazed to it, each fin 33 having a 450 bevel at the end corresponding with the closed end of the cavity 31, and has a copper receptacle 34 brazed to it at the open end of the cavity 31 in which the disc portion 21 of the tapping circuit 17 (not shown in Figure 3) is suspended. The receptacle 34 is filled with ISO-propyl alcohol to cover the disc portion 21 and the space between the finned rod 32 and the Dewar vessel 30 in the cavity 31 is filled either with solid CO2ISO-propyl alcohol or solid CO2/acetone.

In operation of the reactor 2, the Ra-226 source 11 releases Rn-222 which is trapped in the cold trap 18 leading to decay to Pb214 therein. Whilst the source 11 is at a constant temperature the rate of release of Rn-222 is constant, but in the event of some fault condition in the reactor 2 leading to a higher temperature where the source 11 is located, more Rn-222 is released by the source 11 leading to a greater amount trapped in the cold trap 18 in a given time and hence a higher radioactivity level. The trapping system 16 may be disconnected from the primary circuit 3 at regular intervals (e.g. hourly), and the radioactivity determined by a gamma spectrometer (not shown) to detect whether a change in temperature has occurred at the location of the source 11.

The release of Rn-222 from suitable Ra226 sources is temperature dependent as shown in Figure 5 to which reference is now made. In Figure 5 the percentage release of radioactive daughter products from vacuum sublimed sources of Ra-226 and Th-228 are shown plotted against temperature, Ra-226 releasing Rn-222 and Th-228 releasing Rn220.

The temperature where the source 11 is located may be determined from the radioactivity measured at the disc portion 21 of the trapping system 16, or an indication of change in temperature given.

The gamma spectrometer might be incorporated in the cold trap 18 to provide a continuous readout of the radioactivity of the gas passing through the trapping system 16, and thereby provide a relatively fast response to fault conditions in the reactor 2.

Although the invention has been described in relation to an application in a nuclear reactor, it may be applied in nonnuclear installations although naturally care must be taken to avoid discharging radioactive gas into the surrounding environment.

The rate of release of the radioactive gas might be arranged for a particular temperature range, for example, by adjustment of the grain size of the parent active source, or the nature of the substrate or the incorporation of the source in some ceramic or similar material.

The use of a material such as glass having a low thermal conductivity to provide the disc portion 21 has advantages in assisting to confine the cooling effect of the refrigerated ISO-propyl alcohol to the vicinity of the disc portion 21 to avoid premature cooling of the gas, but other materials preferably also having low gamma absorption properties may be used.

WHAT WE CLAIM IS: 1. A method of detecting a change in temperature at a location, wherein at the location a source is provided adapted to release a radioactive gas at a rate which is temperature dependent, and means are provided removed from said location for detecting the rate of release of the said radioactive gas to indicate change of temperature at the location.

2. A method as claimed in Claim 1, wherein the location is arranged to be swept by a stream of gas to entrain the said radioactive gas therein, and the detecting means are positioned downstream of the location.

3. A method as claimed in Claim 2, wherein the stream of gas comprises a coolant gas in a cooling circuit of a gascooled nuclear reactor, which circuit includes the core of the reactor.

4. A method as claimed in Claim 2 or Claim 3, wherein the detecting means includes means for trapping and condensing the said radioactive gas.

5. A method as claimed in Claim 4, wherein the condensed said radioactive gas emits gamma radiation, and the detecting means includes a gamma spectrometer associated with the trapping means to detect the rate of release of said radioactive gas from the rate of increase in gamma

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. thick-walled glass tube coiled in a flat spiral to form a flat disc-like portion 21 having an inlet 17a and outlet 17b. The bore of the disc portion 21 is packed with activated charcoal granules 25 which are retained in position at each end of the disc portion 21 by glass wodl wadding 26. The direction of flow through the trapping circuit 17 as indicated by the arrows provides for the inlet limb 17a to enter the centre of the disc portion 21. A vinyl liner 27 is disposed in the bore of the inlet 1 7a upstream of the wadding 26 to provide a thermal insulating layer between the inlet 17a and the gas therein so as to avoid cooling the gas before it has reached the disc portion 21. Referring now to Figures 3 and 4, the cold trap 18 comprises a Dewar flask 30 having a cavity 31 in which a copper rod 32 is disposed axially therein. The copper rod 32 has three evenly disposed, axially aligned and radially directed fins 33 brazed to it, each fin 33 having a 450 bevel at the end corresponding with the closed end of the cavity 31, and has a copper receptacle 34 brazed to it at the open end of the cavity 31 in which the disc portion 21 of the tapping circuit 17 (not shown in Figure 3) is suspended. The receptacle 34 is filled with ISO-propyl alcohol to cover the disc portion 21 and the space between the finned rod 32 and the Dewar vessel 30 in the cavity 31 is filled either with solid CO2ISO-propyl alcohol or solid CO2/acetone. In operation of the reactor 2, the Ra-226 source 11 releases Rn-222 which is trapped in the cold trap 18 leading to decay to Pb214 therein. Whilst the source 11 is at a constant temperature the rate of release of Rn-222 is constant, but in the event of some fault condition in the reactor 2 leading to a higher temperature where the source 11 is located, more Rn-222 is released by the source 11 leading to a greater amount trapped in the cold trap 18 in a given time and hence a higher radioactivity level. The trapping system 16 may be disconnected from the primary circuit 3 at regular intervals (e.g. hourly), and the radioactivity determined by a gamma spectrometer (not shown) to detect whether a change in temperature has occurred at the location of the source 11. The release of Rn-222 from suitable Ra226 sources is temperature dependent as shown in Figure 5 to which reference is now made. In Figure 5 the percentage release of radioactive daughter products from vacuum sublimed sources of Ra-226 and Th-228 are shown plotted against temperature, Ra-226 releasing Rn-222 and Th-228 releasing Rn220. The temperature where the source 11 is located may be determined from the radioactivity measured at the disc portion 21 of the trapping system 16, or an indication of change in temperature given. The gamma spectrometer might be incorporated in the cold trap 18 to provide a continuous readout of the radioactivity of the gas passing through the trapping system 16, and thereby provide a relatively fast response to fault conditions in the reactor 2. Although the invention has been described in relation to an application in a nuclear reactor, it may be applied in nonnuclear installations although naturally care must be taken to avoid discharging radioactive gas into the surrounding environment. The rate of release of the radioactive gas might be arranged for a particular temperature range, for example, by adjustment of the grain size of the parent active source, or the nature of the substrate or the incorporation of the source in some ceramic or similar material. The use of a material such as glass having a low thermal conductivity to provide the disc portion 21 has advantages in assisting to confine the cooling effect of the refrigerated ISO-propyl alcohol to the vicinity of the disc portion 21 to avoid premature cooling of the gas, but other materials preferably also having low gamma absorption properties may be used. WHAT WE CLAIM IS:

1. A method of detecting a change in temperature at a location, wherein at the location a source is provided adapted to release a radioactive gas at a rate which is temperature dependent, and means are provided removed from said location for detecting the rate of release of the said radioactive gas to indicate change of temperature at the location.

radiation from the condensed said radioactive gas.

6. A method as claimed in any of the respective preceding Claims, wherein the source comprises Ra-226, or Th-228.

7. A method of detecting a change in temperature at a location substantially as hereinbefore described with reference to Figures 1 to 4 of the drawings filed with the provisional specification.