Squib valves

A typical high speed deluge system uses an electrically actuated (solenoid or squib) valve to initiate the flow of water from the nozzles. See Figure 9. The valve is positioned as close to the nozzles as possible. The piping between the valve and nozzles is fully primed and contains few if any air bubbles. 

The GDCS is composed of three identical divisions completely independent of each other both electrically and mechanically. A confirmed RPV level 1 signal will actuate the ADS to reduce RPV pressure. Simultaneously, 150-second short-term system timers, and 30-minute longterm system timers in the GDCS logic are started, which after time-out, actuate squib valves providing an open flow path from the respective water sources to the vessel. 

Systems and components must prevent an explosion that would preclude the APIOOO Ifom maintaining its KSFs. This would include the accumulation of explosive gases, inadvertent initiation of squib valve propellant and failure of gas pressurised equipment. 

Failures of in-containment refuelling water storage tank gravity injection line check valves and squib valves. 

Failures of fourth-stage automatic depressurisation system squib valves. 

The leading risk decrease common cause failures of hardware are associated with ADS fourth stage squib valves, gravity injection and recirculation line components, and I C components and sensors. 

The APIOOO Level 1 shutdown PRA has estimated the CDF to be 1.23x10 events/year, see Table 5. 4.3-1. This CDF is conservative because credit is not taken forthe design enhancement in using diverse squib valves in the recirculation lines. If credit is taken forthe diverse squib valves in the recirculation lines of the APIOOO plant, the estimated CDF of the Level 1 shutdown PRA would be the 1.04x 10 per year. 

Diversity is provided in the actuation by using diverse squib valves. The motor-operated valve is designed so that it remains open in case of failure. Recirculation flow is provided through parallel check valve/squib valve flow paths the check valves open once a sufficient gravity injection head is established. The success criterion forthe containment sump recirculation is defined as injection into the reactor coolant system from the containment sump through one out of four recirculation lines and the corresponding gravity injection lines. 

Common cause failures of the automatic depressurization system Stage-4 squib valves contribute approximately 18 percent to the total shutdown core damage frequency. This indicates that maintaining the reliability of the automatic depressurization system is important. 

Selection of Squib Valves in Preference to Conventional Valve Types... 

Squib valves are self-contained valves that are actuated by an explosive charge on receipt of a firing signal. They are widely used on Boiling Water Reactors and in the aerospace industry for one-off emergency operation. Squib valves are used for the following three applications within the safety systems of the APIOOO ... 

The squib valves are the only active components within these otherwise passive safety systems. Their operation thus needs to be very reliable. Experience has shown that squib valves are more reliable than other types of valve, because of the reliability of the actuating propellants, and also because of the simplicity of the mechanical design of a squib valve. For comparison, the... 

The nature of a squib valve body design makes it virtually leak free that is, the valve is not subj ect to internal leakage, as with standard valve designs such as globe, butterfly, gate, and nonreturn valves. This is a very important safety function, because any such leakage would be the equivalent of a small LOCA, given the location of the squib valves in the primary circuit. 

The following advantages result from choosing squib valves over other valve types ... 

The squib valve body is virtually leak free. 

The explosive charge and igniter used within a squib valve have a limited shelf life and service life however, the required maintenance and testing can easily be accommodated within the APlOOO s refuelling outages. 

Using squib valves within the design of the three chosen safety systems provides a simple, reliable and leak free solution for the one active element within otherwise passive safety systems. 

Common mode failure within the three systems is minimised by using a different valve design for each of the three applications one for the Stage 4 ADS squib valves a second high-pressure valve design for the IRWST injection line and check valve recirculation line squib valves and a third low-pressure recirculation line squib valve. 

In summary, design variations and design tolerances between the various designs provide adequate design diversity to protect against squib valve common failure modes. 

Having a diverse valve design for each of the three squib valve applications on the APIOOO minimises the simultaneous common mode failure of the three applications. The only disadvantages are that more spares have to be held for maintenance on three valve designs, although this is minimal given the difference in valve size and pressure rating between the three applications. 

This option improves risk at negligible cost over the option of having the same squib valve design throughout. It is thus the ALARP option. 

However, the cost estimate for this potential design improvement assumes that a second squib valve vendor does exist and that this vendor provides only the two diverse IRWST squib valves. The cost impact does not include the additional first time engineering and qualification testing that would be incurred by the second vendor. Such costs would be expected to be more than a million dollars. 

Design for rapid depressurization to allow water injection to be assisted by the use of relief or squib valves and multiple loops. 

---