Universal Problems with Eastern Reactors
Compiled by Mike Baker: firstname.lastname@example.org
and Jim Barnes (FOEI) email@example.com
SOVIET REACTORS - INTRO
Two main reactor types were developed and built by the Former Soviet Union (FSU). The first is the RBMK which is graphite moderated and water cooled. Its fuel assemblies are in tubes inside graphite blocks. Water flows up the tubes and emerges as a mixture of steam and water. The steam is then used to drive a turbine and produce electrical power. Unlike US reactors, this reactor can be refueled while operating.
The RBMK design evolved from early plutonium production reactors, and thus was never built outside the SU. The Chernobyl reactors were of this type, as are other reactors in Russia, Lithuania, and Ukraine.
The second main reactor type is the VVER, which is conceptually like the Western countries' pressurized water reactors (PWR). It is water moderated and cooled, with three slightly different designs in use. This reactor evolved from that used for ice breakers and submarines and is currently in use in the FSU, Finland, Hungary, Bulgaria, and the Czechoslovakia region.
SOVIET REACTOR PROBLEMS - RBMK
- The RBMK reactor has a positive void coefficent. This means that if water is lost from the core, either through a coolant leak or heatup of the reactor, it will cause power to increase. The power increase causes even more water to be lost from the core. So as you can see, a vicious feedback cycle is created, making the reactor inherently unstable. (This design problem was a major contributor to the Chernobyl accident. Improvements are being made to the existing reactors to correct this problem but I was unable to find information on the status of these changes.)
- A second problem is that each control rod has a graphite tip. This causes the insertion of a control rod to initiate a power increase before it causes the power to decrease. This happened in the Chernobyl accident. (This is also being modified on operating plants, but the status is unknown.)
- The RBMK also has a slow scram system. The original method required about 20 seconds to insert rods. This is enough time for the chain reaction to increase uncontrollably. (This is also being modified on operating plants, but status is unknown.)
- Fuel channel ruptures also present a serious problem in the RBMK design. First as already explained above, when coolant is lost, power increases due to the positive void coefficient. Second, if several channels rupture, the pressure under the reactor cover can increase enough to lift it off. This would lift out all the control rods. (This is what happened at Chernobyl.)
(Steps are being taken to reduce the multiple channel rupture accident, but they cannot eliminate it.)
SOVIET REACTOR PROBLEMS - VVER-230
- The oldest VVER designs (model 230) lack some of the basic safety features found on Western reactors. The first of these is an emergency core cooling system (ECCS). This system is requied to cool the reactor core if the normal means of cooling are lost through a loss of coolant accident, such as a major pipe rupture.
- The model 230 also lacks a containment structure. The purpose of this structure is to prevent the release of large amount of radioactive substances during a severe accident and to protect the reactor from outside forces.
- Finally, there is some question as to whether or not the model 230 reactors have been designed to sufficeint siesmic standards for some of the sites currenly being used. Specifically, the sites in Armenia and Bulgaria. (The newer model VVER, model 213, includes an ECCS but still does not have a full containment system except for the reactors sold abroad to Finland and Cuba. The VVER model 1000 incorporates a full containment system and is the closest to Western standards of the Soviet designs.)
SOVIET REACTOR PROBLEMS Cont - VVER 440/213
(this section is taken from Russian Roulette (Friends of the Earth - US))
- Although an improvement over the 230, the core cooling system is still 10 to 50 times less reliable than comtemporary western systems.
- Even this, the second-generation model, does not have a containment system. Therefore, there is a high risk of radiation release.
- The pressure redirection system's reliability remains questionable. Furthermore, even if it succeeds in relieving pressure, a consequent radiation release is likely.
- The model shares the 230's problems with embrittlement, redundancy, fire protection, and substandard construction techniques and materials.
SOVIET REACTOR PROBLEMS Cont. - VVER 1000
- Although the VVER - 1000/320 reactors have a type of containment vessel, it is more vulnerable in an emergency than Western-designed containment vessels. All of these containment vessles have holes to permit various materials into the reactor area (water, fuels, etc.,) For example, the outlets for the piping system are tunnels through the concrete that extend some distance from the containment vessel about three meters above ground level, where they rest on columns. These could easily be damaged in an earthquake or other "event" and could mean disruption of the containments' integrity. In general, the containment shells are 1.2 meters thick, made of steel-reinforced concrete.
- Its pressure vessel diameter presents a special disadvantage: because of its small size, it provides an inadequate buffer of water between the neutrons emitted by the fuel and the vessel wall. As a result, reactor vessel embrittlement accelerates.
- A fundamental construction error in the reactor core causes this reactor, like the RBMK, to exhibit positive reactivity feedback at certain operating levels. To date no pactical solution has been found to this problem.
- The plant layout results in fire hazards, threatening the safety of the installation.