Reactor fuel channel

Systems That Shut Down the Reactor in the Case of Accidents The emergency core cooling system (ECCS) fulfils this purpose. It is a system that refills the reactor fuel channels with light water to remove residual or decay heat from the fuel. The fuel requires heavy water for the reactor to go critical and the light water of the ECCS suppresses criticality. There is no need to add boron to the ECCS water. 

Figure A1.25 3-D image of CANDU-reactor fuel channel with bundle. Based on data from AECL.

A small LOCA is a break in any pipe, with an area up to the size of twice the cross-section of the largest feeder pipe. Spurious opening of a liquid relief valve (LRV) is included in this category. Events affecting a single reactor fuel channel or one or more steam generator tubes also involve small breaks, but because of their specific phenomenology are covered separately. 

Single channel events are a particular subset of small LOCAs affecting only one reactor fuel channel. They consist of ... 

The potential for propagation of the break to other reactor fuel channels. Calandria overpressure due to the discharge of high enthalpy fluid into the moderator. 

The failure does not propagate to other reactor fuel channels. 

The reactor core was made up of stacks of hexagonal graphite blocks. Each fuel element block had 210 axial fuel holes and 108 axial coolant holes (Section 5, Fig. 14). The fuel particles were formed into a fuel compact (Section 5.3) and sealed into the fuel channels. 

Pressure-tubes allow the separate, low-pressure, heavy-water moderator to act as a backup hesit sink even if there is no water in the fuel channels. Should this fail, the calandria shell ilsdf can contain the debris, with the decay heat being transferred to the water-filled shield tank around the core. Should the severe core damage sequence progress further, the shield tank and the concrete reactor vault significantly delay the challenge to containment. Furthermore, should core melt lead to containment overpressure, the concrete containment wall will leak and reduce the possibility of catastrophic structural failure (Snell, 1990). 

A fuel channel in a natural uranium reactor is 5 m long and has a heat release of 0.25 MW. If the thermal conductivity of the uranium is 33 W/mK, what is the temperature difference between the surface and the centre of the uranium element, assuming that the heat release is uniform along the rod ... 

Overall, however, the release of refractory fission products from Windscale was less than the release of volatile elements by two or three orders of magnitude, relative to the inventories in the reactor fuel (Table 2.4). Alpha activity on the stack filters and environmental filters was mainly 210Po, derived from the bismuth irradiated in the isotope channels (Crouch Swainbank, 1958 Crooks et al., 1959). The 210Po/137Cs ratio on the environmental filters was about 0.2, with no significant change with distance, suggesting that both activities were carried on the same fume particles. 

Barium hollandite, Bai.4(Al,Ti)2.28Ti60i6, is an important component of Synroc, a synthetic material developed for the immobilisation of high-level waste from nuclear reactor fuel. The hollandite component of Synroc takes up alkali metal ions such as radioactive Cs by substitution for Ba in the structural channels. This uptake has been studied by Cs MAS NMR which shows a single resonance at 211 ppm from Cs in the channel sites in the absence of paramagnetic ions (Hartman et al. 1998). Replacement of Al by Ti in the channel walls causes the Cs NMR peak to broaden and shift to 640 ppm, and also provides a sensitive means of monitoring the formation of water-soluble CsAlTi04 which, if present, would compromise the aqueous durability of Synroc. 

The reactor core is composed of fuel rods with square cross-section. At the comer there are fins that are spiral with respect to longitudinal axis of the fuel rod. Fuel composition is uranium-zirconium alloy with - 20 % enrichment by U235. Fuel cladding is made of zirconium alloy. Fuel rods are grouped in fuel assemblies (FA). Burnable rods placed in FAs and absorber rods moving outside fuel channels are used to compensate for reactivity change in the core. 

In a reactor which is air cooled and where the air is filtered and then released to the atmosphere, this was serious. After carbon dioxide was tried without success, the fire was extinguished by water jets. The immediate concern was the release of relatively short lived radioactive iodine and caesium. It was later learnt that polonium had been released. Effects were detectable as far away as France. One hundred and fifty of the fuel channels were affected, with considerable radioactive release from the stack, although filters retained most of the particulate material. District and... 

The entire active zone is divided arbitrarily into groups of cells containing 12 fuel channels with fuel elements, two channels with auxiliary absorber rods and two channels with control and safety rods. This grouping is varied at the reactor periphery by reducing the number of control and safety rods to flatten the radial power distribution. 

The reactor was considerably over-moderated to allow the use of low enrichment levels for the fuel elements. The water in the fuel channels... 

There are 308 fuel channels and 81 control rod channels per reactor. 

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