A large proportion of the discussion on nuclear power is centered around the possibility of a ‘severe accident’ at a power station. More specifically, the infamous accident at Chernobyl reactor No.4 is the first thing that comes to mind when the topic is raised.

Opponents of nuclear power often highlight claims made about the projected casualties of the accident. Some proponents of nuclear power suggest that other than the immediate casualties of the blast and the clean-up there is little evidence of any further deaths caused by the accident. The claims range from 56 to 945,000 fatalities as a result of the subsequent release (The World Health Organization estimates 9,000 worldwide early deaths will occur as a result of elevated radiation levels across Europe).

In this article, I wish to discuss the likelihood (If any) of an accident similar in consequence to Chernobyl happening in a modern reactor, of a design most likely to be built here in the UK over the next 10 years.

What happens at a Nuclear Power station?

(If you feel you have a good understanding of the fission process, please skip this section).

The vast majority of nuclear fission reactors around the world use Uranium as a fuel, which is arranged as fuel rods in the reactor core.

There is a particular isotope of Uranium, within these fuel rods, which, when struck by a relatively slow moving neutron, undergoes fission producing two new elements (fission products), and a large amount of heat energy.

To carry away this heat and make something useful of it (such as electricity, via a steam turbine), a coolant must be used. In the past, many different gases, including air and Carbon Dioxide, have been used as coolant, but today the vast majority of reactors use water. Pumps must be used to ensure steady flow of this coolant into the core. In case of a problem with the coolant system, an Emergency-Core-Cooling-System (ECCS) is added as a backup.

In order to make the neutrons slow enough to enact fission, the reaction must take place in the presence of a moderator. Many different materials have been used as moderators, the most prevalent today being again, ordinary water.

Finally there are the control rods. These are placed within and around the core and are used to control the rate of the reaction by absorbing neutrons. If fully inserted into the core, these act as a kind of ‘off switch’.

What was Chernobyl?

All 4 reactors at the Chernobyl generating site where of the RBMK design (High-Power Channel Reactor). The fuel is arranged in tubes, surrounded by the graphite moderator. Water is the coolant in this design, and the tubes are designed so that water can boil in inside them and pass to the steam generator.

So what went wrong?

The bitter irony of the Chernobyl accident is that it happened as a result of an experiment to test the performance of the safety features of the plant. The test was intended to determine if the turbine would generate enough electrical energy to keep the coolant water pumped at a sufficient rate until the emergency diesel generators could come online (in the event of an emergency shutdown of the core).

To prevent disruption to the test, many safety systems were deliberately switched off, including the Emergency-Core-Cooling-System, and the automatic control rod shutdown (also known as the ‘scram’ or ‘trip’ system).

The test was due to coincide with a planned shutdown of the reactor. However, due to high electricity demand, lowering the power output below 50% was not permitted. At the time, there was a strong build up of Xenon (a fission product) in the core, which is a strong neutron absorber, and has a similar effect to that of a control rod. To compensate for this (and hence stop the reaction from slowing down completely or stopping) only 6-8 control rods were kept within the core (rather than the suggested minimum of 30). Under these conditions the core was very unstable, and so operators had to make constant adjustments.

The reduced coolant flow, due to the shutting down of the coolant pumps at the start of the test, caused the water to heavily boil in the tubes, leaving small gaps. Due to a design feature of the RBMK reactor, these ‘voids’ in the coolant led to an increase in power. The increased power led to a greater number of ‘voids’ occurring, and hence a feedback loop. It is estimated that the core reached a power level 100 times normal levels.

The situation was exacerbated when operators tried to desperately insert the control rods to get the core under control. However, due to another design feature (graphite tips on the control rods), this led to the situation only getting worse.

Ultimately, the immense heat caused the fuel rods and tubes to burst and the graphite to catch on fire. The resulting steam blasted off the shield covering the core, and allowed the release of fission products into the environment.

So could that happen here?

Of course the level of worker qualification and regulation is much higher in the UK and most other nuclear generating nations than it was in the former soviet union. In the UK, the Nuclear Installations Inspectorate (a branch of the Health and safety Executive) monitors any tests at nuclear facilities to a high level of scrutiny. It is likely that few, or perhaps no tests will need to be carried out at nuclear power stations following any future build. This is due to the fact that systems will be, or have already been tested in specially designed prototypes.

However, for many (including myself), this level of reassurance is not enough. What physical barriers exist to prevent an accident of that magnitude?

To answer the above question, I’ve decided to look in depth at some of the safety features of the two types of reactor which would be constructed in the UK as part of a new build. They are the Westinghouse AP-1000 and the Frematome European-Pressurized-Reactor (EPR).

Both of these designs are referred to as Light-Water-Reactors (LWR) meaning that they use ordinary water as a coolant and moderator. One interesting feature of this arrangement is that if, for whatever reason, the coolant fails to flow at the optimal rate, then the power level also falls, as it is required to sustain the reaction (as its also the moderator). If the core gets hotter than it should, then the water expands and becomes less dense, less dense means less moderation and therefore, less power – It’s self regulating.

Many point to the fact that western reactors are built with ‘Containment domes’ over the reactor core. This structure is designed to prevent the release of large amounts of radioactive material following a severe accident. The containment dome around the Three-Mile-Island Unit-2 reactor prevented the release of a harmful level of radioactive material, following an accident in 1979.

But perhaps the biggest barrier to an event similar to either Chernobyl or TMI is knowledge – operators and plant designers know what happened at those events, and act accordingly. The creation of the World Associated of Nuclear Operators (WANO) was one positive outcome of the Chernobyl disaster, and it enables sharing of operating experience among nuclear workers on a world scale.

Designers also build on ‘lessons learned’ over the past 50 years from reports from performance of older plant’ hence the term ‘Evolutionary Reactors’. Lets take a closer look.

The Westinghouse AP1000

This reactor is designed in a way that safety systems are ‘Passive’ – they come on automatically when there’s a problem.

In previous designs of Nuclear plant, the Emergency-Core-Cooling-System is powered by diesel generators. However, a concern is that these backup systems may fail, or that an operator may not respond in time and not press the relevant button to switch them on. The design of the AP1000 averts these concerns in two ways.

The first is that the safety systems rely on natural forces, such as gravity or gas pressure. Water can be automatically injected from tanks by means of compressed air. Water can also loop between a large tank within the containment and the core via natural circulation and gravity.

Secondly, the valves that activate the ECCS are designed in a ‘fail safe’ manner. Under normal operation of the plant, the electrical output holds the valve in a ‘closed’ position against springs. If the plant is tripped (unexpectedly shut down) then lack of power causes the valve to spring open.

These systems therefore do not rely heavily on operator abilities or reliability of safety systems and they can not be switched off. In the case of an emergency, the reactor is designed to essentially solve the problem itself (although they can also be manually operated).

Frematome European-Pressurized-Reactor

This design has received a lot of attention in the press and from certain environmentalist groups for its supposed lack of safety capability. After studying the plant designs for some time, I really don’t think this could be further from the truth. In fact, I think it’s fair to say that the EPR is one of the safest nuclear power stations designs yet conceived.

The reactor design features a containment system. However, unlike previous nuclear power station designs, the EPR has a double layer containment – an inner steel shell and an outer high strength concrete dome. This extra layer of protection makes it wildly improbable that harmful radioactive material would ever be released even under serious accident conditions, and protects the plant from external hazards (an aeroplane crash for example).

As usual, there is also a ECCS. The EPR design has what’s known as a four-fold redundancy in terms of safety systems – What this means is that there are back-ups to the back-up systems. And there are back ups to these back-ups, and finally one more set of back-ups. If for some reason 3 quarters of the emergency systems failed, the remaining systems can still keep the core intact.

Conclusions

Of course the question I’ve set out to answer is much, much more complicated than the issues that have been touched on here. I at least hope however, that I have at least highlighted some of the major differences between the RBMK design and a modern plant. I will eventually get around to writing a follow up blog to this one, discussing the issues regarding the relative safety of Nuclear Power compared with other sources of Energy.

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