GB2274339A

GB2274339A - Leak testing.

Info

Publication number: GB2274339A
Application number: GB9300930A
Authority: GB
Inventors: Adrian John Sandbach; David Ian Barlow; Leslie Grantham James
Original assignee: NUCLEAR TRANSPORT Ltd
Current assignee: NUCLEAR TRANSPORT Ltd
Priority date: 1993-01-19
Filing date: 1993-01-19
Publication date: 1994-07-20
Anticipated expiration: 2013-01-19
Also published as: GB2274339B; GB9300930D0

Abstract

A system 10 for performing leak tightness measurements on a nuclear fuel transport flask 13 of the type comprising a lid 12 having at least two radially spaced ring seals 15 defining an annular interspace 17 therebetween. The system 10 comprises means for providing a reference volume 48 in intimate thermal contact with the flask lid 12, the reference volume 48 being substantially equal to the volume of the interspace 17, means for supplying a gas for pressurising the reference volume 48 and interspace 17 simultaneously to the same extent, a means 34 for isolating the reference volume 48 from the interspace 17, and a means 14 for measuring the difference in pressure between the reference volume 48 and the interspace 17. <IMAGE>

Description

LEAK TESTING This invention relates to a system for performing leak tightness measurements on nuclear fuel transport flasks.

Nuclear fuel transport flasks are used to carry irradiated nuclear fuel from a nuclear reactor site to a nuclear fuel reprocessing facility. After fuel has been irradiated in a reactor it is removed from the reactor and stored in waterfilled storage ponds at the reactor site. The irradiated fuel is stored for periods of up to several years in order to allow the fuel to reduce its radioactivity and dissipate its high initial heat production.

When it is desired to transfer the irradiated fuel to the reprocessing facility, the fuel is loaded into a transport flask under water and the flask lid is bolted in place. The lid has at least two radially-spaced 0-ring seals which provide leak-tightness by sealing against the flask body. After the flask is removed from the storage pond, its outer surface is decontaminated and a check must be made to ensure that the flask lid is leak tight prior to transport to the reprocessing facility.

In a known method of testing the leak tightness of a transport flask, the annular interspace between the 0-ring seals is first pressurised to a pressure of 7 bar (gauge) (8.1 x 105 Pa). This pressure is monitored over a period of time and any pressure drop is compared to the maximum permissible leak rate for the design of flask in question (for example 5 psi (34,400 Pa) in 4 hours). However, during the time that pressure measurements are being made, the temperature of the flask increases due to the heat generated by its contents, typically 30 kW. A disadvantage of the above prior art method is that this temperature rise results in an increase in the pressure of the gas inside the flask lid seal interspace and thereby masks the pressure drop being measured.

An object of the present invention is to provide a system and method for performing leak tightness measurements on nuclear fuel transport flasks and which compensates for the aforementioned problems associated with the changes in temperature of the flask following loading with irradiated nuclear fuel.

Similar systems, which seek to compensate for the temperature-induced pressure increase in the flask lid seal interspace, have been tried by others. One such system attempts to measure the temperature variation of the flask arising from the heating by its contents and uses a computer program to compensate for the associated pressure increase. A disadvantage of such a system is the requirement to measure very small temperature changes, typically of the order of 0.0010C and, as a consequence, such systems, and their associated spare parts, tend to be expensive.

According to the present invention, in a first aspect, there is provided a system for performing leak tightness measurements on a nuclear fuel transport flask of the type comprising a lid having at least two radially spaced ring seals defining an annular interspace therebetween, the system comprising a reference volume in intimate thermal contact with the flask lid, the reference volume being substantially equal to the volume of the interspace, means for supplying a gas for pressurising the reference volume and interspace simultaneously to the same extent, a means of isolating the reference volume from the interspace, and a means for measuring the difference in pressure between the reference volume and the interspace.

Preferably, the reference volume may comprise a cavity in a hollow bolt inserted into a bolt hole in the flask lid. Alternatively, the reference volume may comprise a chamber integral with the flask lid.

Conveniently, the pressurising gas may be compressed air or nitrogen.

Advantageously, the system may include a means for performing a preliminary leak test on the system as a whole, and the said means may be a Bourdon gauge. However, a pressure transducer may be used as an alternative means.

Preferably, the means for measuring the difference in pressure between the reference volume and the interspace may be a water manometer. As an alternative, pressure transducers may be used.

Desirably, the manometer, where employed, has two limbs and is fitted with two valves. The first limb of the manometer is connected to the interspace and the second limb is connected to the reference volume. The first valve is used to charge and vent the pressurising gas to and from the system, and the second valve is used to isolate the limbs and, thereby, the interspace from the reference volume so as to allow the measurement of any pressure difference between the interspace and the reference volume.

Advantageously, a tubular T-piece may be used to connect the preliminary leak testing means and the pressure difference measuring means to the flask lid seal interspace.

According to another aspect of the present invention there is provided a method for performing leak tightness measurements on a nuclear fuel transport flask, by means of the system according to the first aspect, the method comprising: - a first stage in which a preliminary leak test is carried out upon the system as a whole, the first stage comprising: (i) pressurising the system as a whole, and (ii) monitoring pressure changes in the system using the preliminary leak testing means, and - a second stage in which the pressure difference between the reference volume and the interspace is measured, the second stage comprising: (i) isolating the reference volume from the interspace, and (ii) monitoring the pressure difference between the reference volume and the interspace.

Conveniently, the system is pressurised to a pressure of 7 bar (gauge) when performing leak tightness measurements.

An advantage of the system of the present invention is its ease of use, giving a direct indication of the leak tightness of the flask without the need for sensitive temperature sensors or for computers to calculate pressure variations. This leads to a further advantage of the system in that it is inexpensive in comparison to such other systems currently in use and means that spare parts may be carried at low cost.

Embodiments of the present invention will now be described, by way of example only, with reference to the single Figure of the accompanying drawing which shows a part-sectional view of a leak testing system for a nuclear fuel transport flask.

Referring now to the Figure, a leak testing system 10 is shown connected to a lid 12 of a fuel transport flask 13. The lid 12 has two O-ring seals 15 located in grooves 19, which seals 15, upon bolting of the lid 12 to the flask 13, act so as to seal the lid 12 to the flask 13 and thereby define an annular flask lid seal interspace 17.

The system 10 comprises a water manometer 14 having a range of +25cm and having two limbs 16, 18. The limb 16 is connected by a tube 20 and a quick release coupling 22 to a tubular T-piece 24. The limb 18 is connected by a tube 26 to a hollow test bolt 28 by means of a quick release coupling 30. The manometer 14 is fitted with two valves 32, 34, the valve 32 being used to charge and vent compressed air, from a compressed air supply (not shown), to and from the system 10 through a tube 36 which connects with the limb 16 of the manometer 14. The valve 34 is located on a tube 38 which connects the limbs 16, 18 of the manometer 14, the valve 34 being operable to isolate or connect the limbs 16, 18.

The tubular T-piece 24 is further connected to a Bourdon pressure gauge 40 and, by means of a threaded connection 42, to a duct 44 in the flask lid 12 which connects to the flask lid seal interspace 17.

The hollow test bolt 28 is located in a jacking bolt hole 46 in the flask lid 12 and contains a reference volume 48 which has been machined to a volume approximately equal to the volume of the lid seal interspace 17.

In operation of the system 10, the leak rate measurement is performed in two stages. In the first stage, a coarse test is performed, by means of the Bourdon gauge 40, to determine gross leaks from the system 10. In the second stage, a more sensitive test is performed, by means of the manometer 14, to ensure compliance with the relevant permitted leak rates for the design of transport flask in question.

In carrying out the first stage, the valves 32, 34 are opened and the compressed air supply is connected to the charge/vent valve 32. The system 10 is pressurised to approximately 7 bar, using the Bourdon pressure gauge 40 to measure the system pressure. When the required pressure is reached, the valve 32 is closed. The Bourdon gauge 40 is then used to monitor pressure changes in the system 10.

Initially, a pressure drop will occur as the O-ring seals 17 settle into their respective grooves 19 in the flask lid 12. The period required for settling to occur is dependent on the flask design. Following this settling period the pressure observed by the Bourdon gauge should stabilise.

If any further pressure drops are observed these would indicate a gross leak from the system 10 which would require to be identified and remedied. The above procedure is repeated until no pressure drops are observed on the Bourdon gauge over a period of 10 minutes. The operate of the system 10 then proceeds to the second stage.

In carrying out the second stage, valve 34 is closed in order to isolate the limbs 16, 18 of the manometer 14. The water levels in the limbs 16, 18 will be identical since the pressure in the lid seal interspace 17 will initially be the same as the pressure in the reference volume 48 of the test bolt 28. Since the seal interspace 17 and the reference volume 48 are subjected to the same increase in temperature, the resulting pressure change (by virtue of the Universal Gas Law) is the same for both and so the water levels in the limbs 16, 18 of the manometer 14 remain the same. Therefore, any difference in the pressures measured between the two limbs 16, 18 of the manometer 14 must be due to leakage. A leak at the O-ring seals will be manifest by an increase in the water level in limb 16 of the manometer 14. The pressure drop on the manometer 14 is monitored over a 20 minute period. If the measured pressure drop is outside the specified leak rate for the design of flask being tested, the leak must be identified and repaired. The whole operation of the system 10 (including the first stage) is then repeated until the measured leak is within the specified limits.

If or when the leak rate complies with the specified limits, the valve 34 is opened, the compressed air supply is disconnected from the charge/vent valve 32, and valve 32 is opened slowly to depressurise the system 10. The manometer 14 is disconnected from the T-piece 24 and the test bolt 28. The T-piece 24 and the testbolt 28 are then removed from the flask 13. The flask 13 is now ready to be transported to its desired destination.

Claims

1. A system for performing leak tightness measurements on a nuclear fuel transport flask of the type comprising a lid having at least two radially spaced ring seals defining an annular interspace therebetween, the system comprising means for providing a reference volume in intimate thermal contact with the flask lid, the reference volume being substantially equal to the volume of the interspace, means for supplying a gas for pressurising the reference volume and interspace simultaneously to the same extent, a means of isolating the reference volume from the interspace, and a means for measuring the difference in pressure between the reference volume and the interspace.

2. A system as claimed in Claim 1, wherein the reference volume comprises a cavity in a hollow bolt inserted into a bolt hole in the flask lid.

3. A system as claimed in Claim 1, wherein the means for providing a reference volume comprises a chamber integral with the flask lid.

4. A system as claimed in any one of Claims 1 to 3, wherein the pressurising gas is compressed air.

5. A system as claimed in any one of Claims 1 to 3, wherein the pressurising gas is nitrogen.

6. A system as claimed in any one of the preceding Claims, wherein the system includes a means for performing a preliminary leak test on the system as a whole.

7. A system as claimed in Claim 6, and wherein the said means for performing a preliminary leak test is a Bourdon gauge.

8. A system as claimed in Claim 6 and wherein the preliminary leak testing means comprises pressure transducers.

9. A system as claimed in any one of the preceding Claims, wherein the means for measuring the difference in pressure between the reference volume and the interspace is a water manometer.

10. A system as claimed in any one of Claims 1 to 8, wherein the pressure difference measuring means comprise pressure transducers.

11. A system as claimed in Claim 9, and wherein the manometer comprises two limbs and is fitted with two valves, the first limb being connected to the interspace and the second limb being connected to the reference volume, the first valve being used to charge and vent the pressurising gas to and from the system, and the second valve being used to isolate the limbs, thereby isolating the interspace from the reference volume so as to allow the measurement of any pressure difference therebetween.

12. A system as claimed in any one of the preceding Claims, and further comprising a tubular T-piece to connect the preliminary leak testing means and the pressure difference measuring means to the flask lid seal interspace.

13. A method for performing leak tightness measurements on a nuclear fuel transport flask, by means of the system as claimed in any one of Claims 1 to 12-, the method comprising: - a first stage in which a preliminary leak test is carried out upon the system as a whole, the first stage comprising: (i) pressurising the system as a whole, and (ii) monitoring pressure changes in the system using the preliminary leak testing means, and - a second stage in which the pressure difference between the reference volume and the interspace is measured, the second stage comprising: (i) isolating the reference volume from the interspace, and (ii) monitoring the pressure difference between the reference volume and the interspace.

14. A method as claimed in Claim 13, wherein the system is pressurised to a pressure of at least 7 bar (gauge) (8.1 x 105 Pa) when performing leak tightness measurements.

15. A system for performing leak tightness measurements on a nuclear fuel transport flask, the system being substantially as hereinbefore described with reference to the single Figure of the accompanying drawing.

16. A method for performing leak tightness measurements on a nuclear fuel transport flask, the method being substantially as hereinbefore described with reference to the single Figure of the accompanying drawing.