Pressurized heavy water-moderated reactor

The pressurized heavy water-moderated reactor is also included in this category. This plant can use natural uranium as fuel. One early plant of this t5qje was built and operated in Parr, South Carolina. It operated at 17 MWe from 1963 to 1967. This is the type of reactor used in Canada. 

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). 

On-power refuelling provides the principal means for controlling reactivity in the CANDU 6. Additional reactivity control, independent of the safety shutdown systems, is achieved through use of reactivity control mechanisms. These include light-water zone compartments, absorber rods, and adjuster rods all are located between fiiel channels within the low pressure heavy water moderator and do not penetrate the heat transport system pressure boundary. The reactor is controlled by the dual redundant computer control system. The overall station control system is described in Section 5.7.2.3. 

Inherent in the concept of heavy water moderated reactors is the separation between heat transport coolant and moderator coolant. This has significant safety advantages for low probability events such as postulated dual failures when normal heat transport systems becomes ineffective, e.g. loss of coolant accident coupled with loss of emergency core cooling system. For this postulated event, the pressure tubes balloon, contact the calandria tubes and thereby provide a heat transfer path to the moderator. Design improvements to the moderator heat sink will be made for these low probability events. 

In an Oak Ridge study [61] various types of fluidized-bed reactors were compared. Systems investigated were (a) a sodium-cooled fast reactor, (b) a gas-cooled system, (c) an organic-moderated reactor, (d) a heavy-water-moderated reactor, and (e) a light-water system. A detailed study of this latter system was carried out to compare its characteristics and performance with solid-fuel heterogeneous pressurized-water reactors. The results indicated that both the light-water-moderated and organicmoderated fluidized reactors showed promise, while the gas-cooled, the D20-cooled, and the fast (unmoderated) reactors were found to be less satisfactory for application of the fluidized-bed technique. 

There are various types of nuclear power reactors, including boiling water reactors (BWR) and pressurized water reactors (PLWR or LWR), which are both light-water reactor (LWR) designs and are cooled and moderated by water. There also are pressurized heavy-water reactor (PHWR or HWR) designs. 

Hatcher, S.R., Banerjee, S, Lane, A.D, Tamm, H., Veeder,J.I. "Thorium Cycle in Heavy Water Moderated Pressure Tube (CANDU) Reactors" American Nuclear Society Meeting, San Francisco AECL-5398, 1975... 

One of the technical problems of ranoval of tritium is its low concentration in technological and waste streams. In heavy water reactor moderator (pressurized heavy water reactor [PHWR]), the ratio of DTO to D2O is 10 , and in water... 

Reactor. Experimental heavy water moderated, carbon dioxide cooled pressure tube reactor. 

The Group were interested in visiting SGHWR because it is a direct cycle pressure tube reactor, with some superficially similar features to the Chernobyl reactor. The moderator is, however, heavy water rather than graphite as at Chernobyl. Control is by pumping boric acid solution into liquid shutdown tubes and dumping heavy water moderator. 

Water Reactors. After pressure to design Hanford eased, Wigner and his group turned to examination of other combinations of moderators, coolants, and fuel. The most thoroughly studied were the light-water cooled and moderated, and the heavy-water moderated systems. 

The reactor core consists of 380 fuel channels, held in a square lattice array by circular end-shields, and contained within a cylindrical low-pressure tank called the calandria (Figure 5.7.2.). The calandria contains the heavy water moderator at low temperature and at near-atmospheric pressure. The calandria is positioned within a low-pressure steel lined concrete vault, filled with ordinary water. This tank provides biological shielding from neutron and gamma radiation from the reactor core (Figure 5.7.2.). 

The Indian PHWR is a pressure tube type reactor using heavy water moderator, heavy water coolant (in a separate high pressure high temperature system) and natural uranium oxide fuel. 

The CANDU series of reactors is designed to use natural uranium, but it can also use SEU or a variety of fuels. Typically, the reactor core is contained in a cylindrical austenitic stainless steel tank (calandria) that holds the heavy water moderator at low temperatures (<80°C) and low pressure ( 0.1 MPa). The ends of the cylinder are closed with two parallel end shields that are perforated with holes for the fuel channels, the holes being arranged in a square lattice pattern. Thin-walled Zircaloy 2 tubes are fastened to each inner tube sheet and act as stays for the end shields to form a leak tight tank. The holes in each end shield are connected with stainless steel tubes (lattice tubes) (Figure 4.1). Each fuel channel consists of a Zr-2.5%Nb pressure tube joined to martensitic stainless steel end fittings... 

Four gas-cooled pressure tube reactors of relatively small size were built in the 1960s with the object of exploring fhe use of CO2 as a heaf fransport fluid in combination with heavy water moderation instead of graphite. The reactors had innovative fuel designs and most had the pressure tubes vertically oriented, although the most successful unit, the EL4 plant in France had the pressure tubes horizontal. 

The EL4 reactor (70 MWe) was constructed at the Mont dAree site near Brennilis, France. The heavy water moderator is contained in a horizontal cylinder 4.6-m long and 4.8 m in diameter. The 216 fuel channels, arranged on a square pitch of 234 mm, are contained in Zircaloy tubes. The Zircaloy pressure tubes (107-mm inside diameter and 3.2-mm wall thickness) can operate at a low temperature by virtue of their being thermally isolated from fhe hot CO2 gas by a stainless steel guide tube and by thermal insulation between the guide tube and the pressure tube. 

In the previous sections, several reactor designs using heavy water moderation are briefly described. Based on the pressure tube boundary conditions, the fuel channel designs can be divided into three types ... 

Light water cooled heavy water moderated pressure tube reactor AHWR (BARC, India) ... 

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