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Uprating nuclear reactors reduces safety

Nuclear Monitor Issue: 

The process of increasing the licensed power level of a commercial nuclear power plant is called a “power uprate.” Power uprates are generally categorized based on the magnitude of the power increase and the methods used to achieve the increase. Currently a significant number of the nuclear power plants have plans for power uprate by larger or smaller amounts.

The increase in the electricity produced in a nuclear power plant can be achieved in two ways. One way of increasing the thermal output from a reactor is to increase the amount of fissile material in use. The amount of fissile material is increased either by increasing the degree of enrichment, or the density of the fuel. In boiling water reactors, the increased core power is achieved by increasing the core feed water flows and steam flows. In pressurized water reactors, the increased power outputs call for an increase either in the core coolant flows or in the main coolant temperature rise across the cores, or both.

In Japan, in February 2009 a working group on uprating was established within the Nuclear and Industrial Safety Subcommittee of the Advisory Committee for Natural Resources and Energy. The working group met on six occasions and released a report on March 2 this year.

The first reactor slated for uprating is Tokai No. 2 (BWR, 1100MW), owned by Japan Atomic Power Company (JAPCO). The company is likely to apply in 2011. According to JAPCO's management policy for the 2010 fiscal year, the plant will be uprated during a periodic inspection in the latter half of 2012. However, the other nuclear power companies do not appear to be very enthusiastic. Plans were supposed to be released during the 2009 fiscal year, but they have not appeared yet.

Method of uprating
Both the thermal and electrical output of Tokai No. 2 will be uprated by 5%. When completed the plant will have an electrical output of 1150MWe.

A 5% increase in electrical output will be produced by a 5% increase in the flow of steam to the turbines. The rate of revolution of the high-pressure turbine will be increased by replacing the stationary blades with blades with a wider flow-path surface area. It is said that this is the only change required.

To increase the flow of steam to the turbines by 5% it is necessary to raise the flow of water to the reactor core by 5%. To produce extra steam it is also necessary to increase the thermal output of the core. So as to avoid the need to make adjustments to the core, more new fuel assemblies will be loaded during periodic inspections. The average uranium-235 enrichment of the fuel assemblies is 3.7%. Although the output of individual fuel assemblies will not change, the total amount of fissile material in the core will increase, thus increasing thermal output overall.

It is said that this approach will raise output with the minimum of changes. There will be no need to make major modifications, or to increase the uranium enrichment. Nevertheless, many safety issues arise as a result of the increased supply of feedwater and steam generation.

Problems arising as a result of uprating
Safety-related problems include the following:

* The increased number of fission reactions will produce more radiation within the reactor building. Embrittlement of the pressure vessel due to neutron irradiation will proceed at a faster rate. This will reduce safety, especially if nuclear power plants are to be operated for 50 or 60 years.

* Replacing fuel at a faster rate will increase the amount of spent fuel. This will put extra stress on the cooling equipment of the spent fuel pools and will affect future treatment and disposal.

* Increased fission reactions will reduce the effectiveness of the control rods and reduce their life. They will have to be replaced more frequently. This will increase the volume of waste produced.

* The increased flow of steam will cause more wear and tear and hence exacerbate wall thinning of the steam tubes. There will also be more wear and tear on the turbine blades.

* The increased feedwater flow will place extra stress on the feedwater pump.

Another problem relates to cost. Although JAPCO has not said anything so far, it can be expected that costs will rise as a result of uprating. In the first place, a 7% increase in the rate of replacement of fuel assemblies results in only a 5% increase in electrical output. Add to this the increased rate of replacement of control rods and the increased wear and tear on pipes and turbine blades and one would expect costs to rise.

The Nuclear and Industrial Safety Subcommittee's report claims that there are "basically no safety problems", but it can be seen from the problems listed above that uprating reduces the safety margin. The chair of the working group tried to defend the uprating program on the grounds of "the needs of the people".

Uprating is one of many fronts on which Japan's nuclear safety is being whittled away. Others include extended operation cycles, life extensions for aging reactors and the use of MOX fuel in light water reactors. There is little sign so far that the Democratic Party led government will fulfil the pledge in its 2009 election Manifesto to place safety first in Japan's nuclear administration.

Source: Nuke Info Tokyo, May/June 2010 / IAEA;
Contact: CNIC, (Citizens' Nuclear Information Center), Akebonobashi Co-op 2F-B, 8-5 Sumiyoshi-cho, Shinjuku-ku, Tokyo, 162-0065, Japan.
Tel:  + 81-3-3357-3800