A burst cartridge (or can) detection system is neces­sary to detect any defect in the cladding of a nuclear fuel element which is leaching radioactive material into the coolant gas. Radioactive isotopes in the coolant gas will be deposited over the reactor internals and the boilers and boiler voids which will give two main problems:

• Access to boilers is limited because of high radia­tion levels preventing easy access for maintenance.

• Blowdown of pressure vessels and boilers will be limited bv gas containing radioactive material.

The most serious problem associated with a burst fuel element in a magnox reactor can, is that uranium metal will quickly oxidise and there is a high risk that rapid swelling will occur which will restrict the coolant gas flow up the fuel channel. Swelling of fuei will quickly block the channel and unless action is taken immediately a channel fire could result.

With AGR fuel the principal risk arises from con­tamination of the circuit and boiler.

Burst cartridge detection equipment Burst cartridge detection equipment detects the amount of radioactivity in a sample of gas drawn from a fuel channel, a group of channels or even the general bulk coolant stream. The principle of operation is that decay products from radioactive gas in the sample are attracted to an electrostatically-charged wire which passes through a scintillation counter where the radio­activity is measured. The level of activity is counted on a ratemeter and indicated to the operator as a number. The number displayed is used relatively as each counting period ends.

The series of readings are fed to a computer where it is stored for historical display providing differential relative values against previous readings and compared with alarm settings to make the operator aware that a faulty element may exist.

Normally a number of systems exist for BCD moni­toring and each serves a different purpose. These are listed as follows:

• A system for interrogating each channel, which may sample each channel in turn or which may have to be preset to an identified channel. The routine sampling of each channel may take several hours and is therefore a slow system.

• A system for interrogating a group of between 4-16 channels which will survey the whole reactor in minutes.

• A bulk system which monitors the bulk coolant on

a continuous basis.

The slow system is primarily used for detecting very small leaks to give warning that a more serious burst may be developing, and for monitoring a single chan­nel during fuelling operations to look for damaged fuel that may be in the process of being loaded.

The fast system, which monitors groups of chan­nels, gives information to the operator of impending fast fuel cladding failure, although it will not identify the culprit channel.

The bulk gas sample gives information on fast fail­ures and any indication here must be serious because the gas sample only contains heavily diluted activity from an affected channel.