Reactor Safety Systems & Why Is A Reactor Safe. Twilight Style.

From the creator of Nuclear Talk Review. Harry Potter Style comes the anticipated ... Oh just get on with it!

I watch as Edward pulls out a pocket mirror and starts putting make-ups on. A little bit of eyeliner here, a pat of mascara there and a dap of Narcissist-Vampire lipstick for the final touch. He’s perfect and he was mine. Me! An average girl who’s annoyingly modest, that people just want to shove some faeces down my throat and slap me silly. Life was good but then again, I just couldn’t get this disturbing matter off my mind. Not even if Edward decides to go heavy on the cosmetics to make himself more beautiful.

Edward: What’s wrong Bella? You look troubled. Is my lipstick the wrong shade?

Bella aka Me: I’m alright, it’s nothing. Your lipstick’s fine.

Edward: Come on now. You know you’ve always been a bad liar. (While putting away the sissy-man beautification kit) Tell me.

Me: It’s just... I’m worried about...

Edward: About what? Auww I mean Argghhh! It’s frustrating that yours is the only mind I can’t read!

Me: Oh well, it’s the nuclear reactors...

Edward: What about it?  

   

Me: I’m worried about its safety features. What if things go wrong and it explodes and starts leaking out radioactivity all over the place? I’m scared Edward, terrified...

Edward: Oh Bella my dear, don’t worry about that. It’s all going to be fine.

Me: How do you know that? You’re always busy putting make-ups on! How could you possibly know anything about nuclear reactors?

Edward: Because Bella, this make-up infatuation is only a recent hobby. I didn’t live for almost a 100 years (yet still looks young and great mind you) for nothing you know. I’m quite familiar with this nuclear thing.

Me: Really? You mean nuclear reactor’s a safe thing?

Edward: Of course! You see Bella; it’s true that there has been accidents involving nuclear reactors in the past but if you think about it, in over 14,000 cumulative reactor-years of commercial operation in 32 countries, there have been only two major accidents involving nuclear power plants - Three Mile Island and Chernobyl.

Me: Still there were accidents! OMG! OMG! In Chernobyl, (Ukraine 1986) the destruction of the reactor by steam explosion and fire killed 56 people and had significant health and environmental consequences. The death toll has since increased to about 70. In Three Mile Island (USA 1979), the reactor was severely damaged...though I have to say that the radiation was contained and there were no adverse health or environmental consequences.

Edward: What are you? A walking encyclopaedia? See, even you admitted that the latter accident was not so bad. True, lives were lost but if you compared to accidents at coal, gas and hydro plants, the casualties are much higher. Nearing 12 000 combined from 1970 - 1992! Whilst nuclear fatalities are only just around 50-70. The blames however lies in humans, not the technology. Remember that.

Me: 50 thou... oh okay. But still if ever the reactor were to malfunction, wouldn’t it be disastrous?

Edward: No, not really. The disaster at the Chernobyl nuclear power plant was due to:

·        major design deficiencies in the RBMK type of reactor

·        the violation of operating procedures and

·        the absence of a safety culture 

A weird thing about the RBMK design was that, the engineers there seem to think that coolant failure could lead to a strong increase in power output from the fission process. I really don’t know what they were thinking. The Chernobyl accident was a unique event and the only time in the history of commercial nuclear power that radiation-related fatalities occurred.

An OECD expert report on it concluded that “the Chernobyl accident has not brought to light any new, previously unknown phenomena or safety issues that are not resolved or otherwise covered by current reactor safety programs for commercial power reactors in OECD Member countries.  In other words, the concept of 'defence in depth' was conspicuous by its absence, and tragically shown to be vitally important.”

Me: So ... you’re saying that, only that one type of reactor is defective ... but what do you mean by defence-in-depth concept?

Edward: You see my Bella, nuclear plants all over the world in achieving optimum safety, operates using a 'defence-in-depth' approach with multiple safety systems supplementing the natural features of the reactor core. Key aspects of the approach are:

• high-quality design & construction,
• equipment which prevents operational disturbances or human failures and errors turning into problems,
• comprehensive monitoring and regular testing to detect equipment or operator failures,
• redundant and diverse systems to control damage to the fuel and prevent significant radioactive releases,
• provision to confine the effects of severe fuel damage (or any other problem) to the plant itself.

Or in other words: Prevention, Monitoring, and Action (to mitigate consequences of failures).

The safety provisions include: 

·        Between the radioactive reactor core and the environment, there will be a series of physical barriers

·        The provision of multiple safety systems, each with backup and designed to accommodate human error. 

Which is why about one quarter of the capital cost of such reactors goes to the safety systems.

The barriers in a typical plant are: 

·        The fuel is in the form of solid ceramic (UO2) pellets and while it’s burned, the radioactive fission products remain largely bound inside these pellets.

·        The pellets are packed inside sealed zirconium alloy tubes to form fuel rods. These are confined inside a large steel pressure vessel with walls up to 30 cm thick

·        The associated primary water cooling pipe work is also substantial.

·        All this in turn, is enclosed inside a robust reinforced concrete containment structure with walls at least one metre thick.

 

This amount to three significant barriers around the fuel, which in itself is stable.

Continuous monitoring is also applied to these barriers:

·        The fuel cladding is monitored by measuring the amount of radioactivity in the cooling water.

·        The high pressure cooling system is monitored by the leak rate of water and

·        The containment structure is monitored by periodically measuring the leak rate of air at about five times atmospheric pressure. 

To control reactivity, to cool the fuel and to contain radioactive substances are the three basic safety functions in a nuclear reactor.

The main safety features of most reactors are inherent:

·        Negative temperature coefficient which means that beyond an optimal level, as the temperature increases, the efficiency of the reaction decreases (this in fact is used to control power levels in some new designs).  

·        Negative void coefficient which means that if any steam has formed in the cooling water, there is a decrease in moderating effect so that fewer neutrons are able to cause fission and the reaction slows down automatically.

Beyond the control rods which are inserted to absorb neutrons and regulate the fission process, the main engineered safety provisions are the back-up emergency core cooling system (ECCS) to remove excess heat (though it is more to prevent damage to the plant than for public safety) and the containment.

Traditional reactor safety systems are 'active' in the sense that they involve electrical or mechanical operation on command. Some engineered systems operate passively like pressure relief valves. Both require parallel redundant systems. Inherent or full passive safety design depends only on physical phenomena such as convection, gravity or resistance to high temperatures, not on functioning of engineered components. All reactors have some elements of inherent safety as mentioned, but in some recent designs the passive or inherent features substitute for active systems in cooling etc.

Nuclear power plants are designed with sensors to shut them down automatically in an earthquake, and this is a vital consideration in many parts of the world.

Me: Oh Edward, I see that now. But then again, what about the workers in nuclear power plants? Surely they’ll be exposed to radioactivity?! Think Edward! Those poor people...

Edward: Oh Bella, you have such beautiful heart. (kiss2) But you don’t have to worry about that. Operational safety is a prime concern for those working in nuclear plants. Radiation doses are controlled by the use of remote handling equipment for many operations in the core of the reactor. Other controls include physical shielding and limiting the time workers spend in areas with significant radiation levels. These are supported by continuous monitoring of individual doses and of the work environment to ensure very low radiation exposure compared with other industries. I mean come on, an x-ray technician who works with x-ray machines every day is exposed to about 500 milirem of radiation each year whilst nuclear power plant workers are only exposed to 180 milirem per year.

Me: Really? Now that’s a mind opener. Oh Edward I forgot! Terrorism! What if terrorists decide to hijack a plane and crash a nuclear power plant like what happened with the WTC? It’ll be the end of the world! Apocalypse! Apocalypse! Apocalypse! 

Edward: Calm down Bella! (Slapping me fervently) People have been aware of this for years so of course preventive measures have been taken.  Since the World Trade Centre attacks in New York in 2001 there has been concern about the consequences of a large aircraft being used to attack a nuclear facility with the purpose of releasing radioactive materials. Various studies have looked at similar attacks on nuclear power plants. They show that nuclear reactors would be more resistant to such attacks than virtually any other civil installations. A thorough study was undertaken by the US Electric Power Research Institute (EPRI) using specialist consultants and paid for by the US Dept. of Energy. It concludes that US reactor structures "are robust and (would) protect the fuel from impacts of large commercial aircraft".

The analyses used a fully-fuelled Boeing 767-400 of over 200 tonnes as the basis, at 560 km/h - the maximum speed for precision flying near the ground. The wingspan is greater than the diameter of reactor containment buildings and the 4.3 tonne engines are 15 metres apart. Hence analyses focused on single engine direct impact on the centreline - since this would be the most penetrating missile - and on the impact of the entire aircraft if the fuselage hit the centreline (in which case the engines would ricochet off the sides). In each case no part of the aircraft or its fuel would penetrate the containment. Other studies have confirmed these findings.

Penetrating (even relatively weak) reinforced concrete requires multiple hits by high speed artillery shells or specially-designed "bunker busting" ordnance - both of which are well beyond what terrorists are likely to deploy. Thin-walled, slow-moving, hollow aluminium aircraft, hitting containment-grade heavily-reinforced concrete disintegrate, with negligible penetration. But further realistic assessments from decades of analyses, lab work and testing, find that the consequence of even the worst realistic scenarios - core melting and containment failure - can cause few if any deaths to the public, regardless of the scenario that led to the core melt and containment failure. 

In 1988 Sandia National Laboratories in USA demonstrated the unequal distribution of energy absorption that occurs when an aircraft impacts a massive, hardened target. The test involved a rocket-propelled F4 Phantom jet (about 27 tonnes, with both engines close together in the fuselage) hitting a 3.7m thick slab of concrete at 765 km/h. This was to see whether a proposed Japanese nuclear power plant could withstand the impact of a heavy aircraft. It showed how most of the collision energy goes into the destruction of the aircraft itself - about 96% of the aircraft's kinetic energy went into the ‘itself’ destruction and some penetration of the concrete, while the remaining 4% was dissipated in accelerating the 700-tonne slab. The maximum penetration of the concrete in this experiment was 60 mm, but comparison with fixed reactor containment needs to take account of the 4% of energy transmitted to the slab. The study of a 1970s US power plant in a highly-populated area is assessing the possible effects of a successful terrorist attack which causes both meltdown of the core and a large breach in the containment structure - both extremely unlikely. It shows that a large fraction of the most hazardous radioactive isotopes, like those of iodine and tellurium, would never leave the site. 

Much of the radioactive material would stick to surfaces inside the containment or becomes soluble salts that remain in the damaged containment building. Some radioactive material would nonetheless enter the environment some hours after the attack in this extreme scenario and affect areas up to several kilometres away. The extent and timing of this means that with walking-pace evacuation inside this radius, it would not be a major health risk. However it could leave areas contaminated and hence displace people in the same way as a natural disaster, giving rise to economic rather than health consequences. 

Looking at spent fuel storage pools, similar analyses showed no breach. Dry storage and transport casks retained their integrity. "There would be no release of radionuclides to the environment".

Likewise, the massive structures mean that any terrorist attack even inside a plant (which are well defended) and causing loss of cooling, core melting and breach of containment would not result in any significant radioactive releases.

Switzerland's Nuclear Safety Inspectorate studied a similar scenario and reported in 2003 that the danger of any radiation release from such a crash would be low for the older plants and extremely low for the newer ones.

The conservative design criteria which caused most power reactors to be shrouded by massive containment structures with biological shield has provided peace of mind in a suicide terrorist context. Ironically and as noted earlier, with better understanding of what happens in a core melt accident inside, they are now seen to be not nearly as necessary in that accident mitigation role as was originally assumed.

I watched enthralled as Edward speaks though I didn’t really understand a thing. Just as I was about to lean over and kiss the hell out of him, I heard footsteps approaching us from behind. I turned and saw Jacob half-way between his wolf-to-human transformation.

Jacob: Aooo woof aumm. I mean, go team Jacob! Bella? You okay? You look troubled. Bloodsucker! What did you do!

Edward: Relax mongrel! She was just upset about the safety of nuclear reactors that’s all.

Jacob: OIC. Silly Bella, there really is nothing to worry about. The designs for nuclear plants being developed for implementation in coming decades contain numerous safety improvements based on operational experience. The first two of the advanced reactor began operating in Japan in 1996.

One major feature they have in common (beyond safety engineering already standard in Western reactors) is passive safety systems, requiring no operator intervention in the event of a major malfunction.

It was not until the late 1970s that detailed analyses and large-scale testing, followed by the 1979 meltdown of the Three Mile Island reactor, began to make clear that even the worst possible accident in a conventional western nuclear power plant or its fuel could not cause dramatic public harm. The industry still works hard to minimize the probability of a meltdown accident, but it is now clear that no-one need fear a potential public health catastrophe.

The decades-long test and analysis program showed that less radioactivity escapes from molten fuel than initially assumed, and that this radioactive material is not readily mobilized beyond the immediate internal structure. Thus, even if the containment structure that surrounds all modern nuclear plants were ruptured, it would still be highly effective in preventing escape of radioactivity.

It is the laws of physics and the properties of materials that preclude disaster, not the required actions by safety equipment or personnel. In fact, licensing approval now requires that the effects of any core-melt accident must be confined to the plant itself, without the need to evacuate nearby residents.

The main metric used to assess reactor safety is the likelihood of the core melting due to loss of coolant. These new designs are one or two orders of magnitude less likely than older ones to suffer a core melt accident, but the significance of that is more for the owner and operator than the neighbours, who - as Three Mile Island showed - are entirely safe also with older types.

Edward: Hey! I was about to tell her that! Back off dog!

Jacob: Yeah? You wanna piece of me?

Laurel: No, I want a piece of Bella. So mouth-watering!

Edward, Jacob & Me: Laurel! 

Dush! Pow! Poof! Some werewolf and vampire fighting action commence.

Me: Non-human guys! Stop it! I’ve had enough of this! Mr. Bean, swap with me! 

Edward & Jacob: Bella? No!!!

Laurel: Ouch and euww! 

 

The End

nuclear reactor safety paper