Calandria vessel

With respect to the redundant heat removal paths, the moderator can act as an emergency heat sink even with no water in the fiiel channels. Should the moderator heat removal system subsequently fail, the large water-filled reactor vault surrounding the calandria vessel provides an additional line of defence (Fig. 5.7.2). Its primary purpose is to provide shielding of the concrete reactor vault from neutrons and gamma rays. However it can also act as a passive emergency water reservoir in case of a severe core damage accident that is, should the primary coolant... 

Gross weight (ton/kg) 711 of empty calandria vessel without 23. 41 of pressure tubes... 

In the case of a channel failure, the moderator acts as an energy-absorbing cushion, preventing failure of the calandria vessel. Even for beyond-design-basis severe core damage accidents, where a number of channels are postulated to fail, the calandria may leak (pressure relief rupture discs) but would retain its gross structural integrity. 

A unique feature of pressure tube HWRs is large volumes of heavy water or light water surrounding the fuel channels and the calandria vessel, respectively. These water volumes provide an inherent means for removal of decay heat from the core during BDBA that progresses to severe core damage. The two water sources are the heavy water moderator... 

A severe flow blockage >90% of the channel flow area is required to cause pressure tube failure due to overheating. A single channel event leading to channel failure also requires analysis of the pressure transient within the calandria, to show that the calandria vessel itself remains intact, that the SDS devices within it can still perform their fimction, and the break does not propagate by causing failures of other reactor channels. 

Luxat, D.L. and J.C. Luxat. 2007. Evaluation of failure modes of CANDU calandria vessels under severe accident conditions. In Proceedings of the 19th SMiRT Conference, August, Toronto, Canada. 

The design life of all non-replaceable stmctures (including calandria vessel, end-shields, major piping and concrete stmctures) is 100 years. All components and equipment that will have lower design life will be easily replaceable during routine shutdowns. From this consideration, the zirconium alloy pressure tubes of AHWR have been made easily replaceable. 

The Canadian SCWR core concept is illustrated in Fig. 8.5. It consists of a pressurized inlet plenum, an LP calandria vessel that contains heavy water moderator, and 336 fuel channels that are attached to a common outlet header. A counterflow fuel channel is adopted to position the inlet and outlet piping above the reactor core so that a complete break of either an inlet pipe or an oudet pipe will not result in an immediate loss of coolant at the reactor core. A nonfuel central flow channel is located at the center of the fuel channel to increase neutron moderation close to the inner fuel rings. This feature results in reasonably uniform radial power distributions across the fuel channel as well as a desirable negative coolant void reactivity throughout the bum-up cycle. 

The calandria vessel pressure transient does not cause vessel failure or loss of moderator (other than through the relief pipes), and any vessel deformation does not prevent operation of the shutdown systems. 

Since the severe core damage behaviour of PHWRs is somewhat different from that of light water reactors (LWRs), it is worth summarizing the phenomenology here. Analysis of events in category (3) initially started from heat balance calculations (to determine the times to boil off the water in the moderator, and then in the shield tank), followed by calculations of the characteristics of the debris once it collects on the bottom of the calandria vessel. 

CANDU (Canada) The exiting CANDU reactors as well as the proposed new designs rely to varying degrees on heat removal processes driven by natural circulation. Passive heat sinks based on NC, are utilized in current CANDU reactors, which are able to mitigate the accident progression for more than 24 hours. The passive heat sink consists of the heavy water moderator, which is contained within a low-pressure vessel, called a calandria. The calandria vessel is in turn contained in a calandria vault filled with light water, which provides a second... 

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