life of nuclear power plants

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life of nuclear power plants

Postby goddyffa » 07 Apr 2013, 08:01

What determines the life of nuclear power plants? I researched that on average, nuclear power plants are designed to last 30-40 years, and in some cases 60 years. What determines the life of the plant? Why does the plant have to shut down after a certain amount of time?
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Last edited by goddyffa on 09 Apr 2013, 12:11, edited 1 time in total.
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Re: life of nuclear power plants

Postby Matte » 08 Apr 2013, 09:35

The ageing of nuclear power plants and components are estimates based no how the plant is operated (how often it is shut down, scramed, how the coolant chemistry performs and the neutron flux in the core, mechnical wear of components etc.). This leads to a number of factors being affected:
1. mechanical wear and corrosion
2. embrittlement of materials through temperature cycling and neutron radiation
3. changes in conductivity of electrical cables and isolator materials
4. crack initiation and crack propagation through mechanical wear, chemical influences and temperature variance/cycling

Experience and development of how the nuclear plant is operated (mainly chemistry, materials and core loading pattern) and better knowledge of what materials are best suited for nuclear applications have proved the initial estimates from the early 1970's on plants technical lifetime to be underestimated.

Today it is possible to replace most components in a nuclear powerplant, the only components that have not been replaced is reactor pressure vessels (vessel head and reactor internals do get replaced for various reasons!) and primary circuit pipework.

From a technical viewpoint the only component limiting the life time of a nuclear plant is the pressure vessel. This is a large and heavy component containing the reactor core and fuel. During operation the pressure vessel is bombarded by neutrons from the fuel which changes the mechanical properties of the vessel materials slowly over time. At a point the material has changed its' mechanical properties so much that it is not suitable for further use. This is monitored in several ways, there are flux meters monitoring the amount of neutrons from the core, material sampling pods located near the pressure vessel wall, non destructive testing of the vessel etc.

Usually the pressure vessel is the limiting factor in determening the technical lifetime of a nuclear power plant. A few times the pressure vessel have been ok to operate but as the sampling pods have run out (they contain materials from the same batch as the vessel in various forms suitable for destructive testing) at which point the operator can't proove to the regulator that their pressure vessel is safe to operate and have to shut down.

More common is the economic reasons for shutting down. The plant has not been maintained in a suitable manner to conform to new regulations and require more money for upgrades than the owners/investors feel is "sustainable" (this is quite common I belive), there is a plethora of financial reasons why nuclear powerplants shut down but that is not my expertiese...

There is no reason for a Gen II plant that have been suitably operated and been diligently maintaned to operate way past its' initially estimated 30-40 year technical life span. New loading patterns minimizes the neutron flux att the pressure vessel wall and most other components can be replaced as needed. As long as you have material sampling pods in the reactor and can fulfill new regulator requirements you can keep operating until you hit the neutron dose rate limit in the reactor wall or the maintenance costs exceed your revenues.

Suitable answer to your question?

EDIT2: I have been put to right about SCRAMS and transients. It is really called Design Transient Specifications (DTS) which deterimnes if you can continue running a plant or not. There are other events that counts towards your DTS than just the SCRAMS. It is possible to analyse a facility and recalculate the DTS to show that it can handle more events than initially specified for. I know very little about this topic so I can't elaborate further (big thank you to a collegue for putting me straight.).
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