Calt Leak Testing CALT 9 Service
CALT is the Croft Associates Leak Tester — equipment built to measure the leak rate of the closure seals on packages used to transport radioactive material. We carry out leakage testing to BS EN ISO 12807:2021, and our technicians travel across the UK and Europe doing this work for nuclear clients including LLW Repository and Sellafield Ltd.
If you have arrived here searching for CALT, it is worth saying plainly what it is not: this is not a general-purpose leak test for pipework or vessels. It is a specific method for a specific problem — proving the integrity of a double O-ring closure on a transport package.
The problem it solves
A transport package for radioactive material is only as good as its lid seal. The containment case rests on that closure holding, in normal transport and in the accident conditions the package design has to withstand.
Most such packages close with two concentric O-rings rather than one. That is not simply redundancy. The gap between the two seals — the interspace — is what makes the closure testable. With a sealed volume trapped between an inner and an outer O-ring, you have somewhere to apply a pressure or a vacuum and watch what happens to it. A single seal gives you nowhere to measure from without opening the package.
So the two rings serve two purposes: the inner one contains, and the pair together allow the containment to be demonstrated rather than assumed.
How the test works
The interspace between the O-rings is accessed through a test port in the closure. That volume is evacuated or pressurised, isolated, and the pressure then monitored over a measured period.
A perfect seal holds. A leaking seal does not, and the rate at which the pressure changes — combined with the interspace volume and the test conditions — gives a quantified leak rate rather than a pass-or-fail impression. That number is the deliverable. It can be compared against the rate the package's design safety case permits, and compared against the same package's result from last time.
The CALT unit records the test parameters alongside the result and produces a printed record of both, which matters as much as the measurement itself: on nuclear transport work the evidence has to stand up long after the technician has left site.
Why it is measured rather than judged
Plenty of leak testing is qualitative. Bubbles or no bubbles. Pressure held or it did not. That is perfectly adequate for a great many applications and completely inadequate here.
A transport package approval specifies a maximum permissible leakage rate. Demonstrating compliance therefore requires a rate, in the right units, obtained by a method capable of resolving it — not an opinion that the lid seemed tight. BS EN ISO 12807:2021 is the standard governing leakage testing on packages for the safe transport of radioactive material, and it is what our testing is carried out to.
A quantified result also lets you see a seal degrading across successive tests while it is still within limits, which is the difference between planned maintenance and a package failing when you need it.
When it is carried out
Typically at the points where the closure's condition has to be known: after a package is loaded and closed, before consignment; after maintenance or a seal change; periodically through a package's service life as required by its approval; and after any event that might have disturbed the closure.
Seals are consumables. Elastomer O-rings take a compression set, harden with age and temperature, and pick up damage from handling and from debris on the sealing face. A seal that passed a year ago is not evidence about the seal today.
What we need to know from you
Before we mobilise it helps to have the package type and its closure arrangement, the permissible leakage rate from the design safety case or approval certificate, the test class or method required by BS EN ISO 12807, the test port arrangement, where the work is happening and what site access involves, and whether the package will be loaded at the time. That last point drives the radiological arrangements rather than the test method.
If you are not sure of some of that, tell us what you do know. Working out what a package needs is part of the job, and we would rather establish it in a phone call than on the day.
Working on nuclear sites
This work takes place almost entirely on licensed nuclear sites and at facilities handling radioactive material, which means the testing is the straightforward part — the arrangements around it are not. Our technicians hold current site inductions and security clearances for the sites we work on regularly, and we are used to working to permit, to a defined method statement, and inside someone else's safety case.
We travel across the UK and Europe for this work. If your package is somewhere awkward, that is normal.
Talk to us
If you have transport packages needing closure leak testing, or you have inherited a fleet and are not certain what its approval requires, tell us what you have and we will tell you what the testing should look like. Where a package needs more than closure testing we can bring the rest of the group's NDT capability to the same visit rather than sending separate crews.
