design and safety of nuclear reactor core melting

Design and Safety of nuclear reactor core melting

In 1950 attention turned to harness power of atom in a controlled way, as demonstrated at Chicago in 1942 and subsequently for military research, and applying the steady heat yield to generate electricity. This naturally gave rise to concern about accidents and their possible effects. In particular the scenario of loss of cooling which resulted in melting of nuclear reactor core motivated studies on both physical and chemical possibilities and biological effects of any dispersed radio activity.

Those responsible for nuclear power technology in the west devoted extra -ordinary effort to ensuring that melt down of reactor core would not takes place, since it was assumed that melt down of core would create major public hazard, and if contained, a tragic accident with likely fat abilities.

In avoiding such accidents the industry has been outstanding successful. In over 14,000 cumulative reactor -years of commercial operation in 32 countries, there have been only two major accidents to nuclear power plants  - three Mile Island and Chernobyl , latter being of little relevance outside the old Soviet bloc .

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

It is laws of physics and properties of materials that preclude disaster, not required actions by safety equipment or personnel. In fact licensing approval now requires that effects of any core melt accident must be confined to plant it self, without need to evacuate nearby residents . It should be emphasized that commercial type power reactor  simple cannot under any circumstances explode like a nuclear bomb.

The international Atomic Energy Agency (IAEA) was set up by United Nations in 1957. One of its function was to act as a auditor of world nuclear safety. It prescribes safety procedures and reporting of even minor accidents and its role strengthened in 1996. Every country which operates nuclear power plants has a nuclear safety inspectorate  and all these work closely  with IAEA.

While nuclear power plants are designed to be safe in the event of any mal function or accident , no industrial activity can be represented as entire risk free. However a nuclear accident a western type reactor is now understood to have severe financial consequences for the owner but will give rise to minimal off site consequences.

Achieving safety

Operational safety is prime concern for those working in nuclear power plants. Radiation doses are controlled by the use of remote handling equipment for many operations in the core of reactor. Other controls include physical shielding and limiting the time of workers spend in the 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 to other industries.

Concerning possible accidents, up to early 1970, some extreme assumptions were made about the possible chain of consequences. They gave rise to genre of dramatic fiction (e.g. China Syndrome) in public domain and also some solid engineering including containment structures (at least western reactor design ) in the industry itself. Licensing regulations were framed accordingly.

One mandated safety indicator is calculated probable frequency of degraded core or core melt accidents. The U.S. Nuclear regularity commission (NRC) specifies that reactor designs must meet a 1 in 10,000 year core damage frequency, but modern designs exceed this. U.S. utility requirements are 1 in 100,000 year the best currently operating plants are about 1 in million and those likely to built in next decade are almost 1in 10million.
Even months after  the three Mile Island accident in 1979 it was assumed that there had been no core melt because there were no indications of severe radioactive release even inside the containment . It turned out to be fact about half of core melted . This remains the only core melt in a reactor confirming to NRC safe criteria ,and the effects were contained designed without radiological harm to any one.

However apart from this accident and Chernobyl disaster they have been about 10 core melt accidents-mostly in military or experimental reactors. None resulted in any hazard due to burning fuel in hot graphite ( similar to Chernobyl but small scale.

Regularity requirements today are that the effects of any core melt must to be confined to plant itself, without need to evacuate nearby residents.

The main safety concern has always been the possibility of an uncontrolled release of radio active materials leading to  contamination and consequent radiation exposure off site. Earlier assumption were this would be likely in the event  of major loss of cooling accident (LOCA) which resulted in a core melt. Experience has proved otherwise in any circumstances relevant to western reactor designs. In the light of better understanding of physics and chemistry of material in a reactor core under extreme conditions it became evident that even a severe core melt coupled with breach of containment could not in fact create a major radiological disaster from any western reactor design studies of post accident situation at three Mile Island ( where there was no breach of containment supported this.

An OECD / NEA report in 2010 pointed out that the  theoretically -calculated frequency for a larger release of radioactivity from severe nuclear power plant accident has reduced by a factor of 1600 between the early generation 1 reactors as originally built, and generation 111/ 111+ plants being built today. Earlier designs however have been progressively up graded through their operating lives.

It has been asserted that nuclear reactor accidents are epitome of low -probability but high consequences risks. Understandingly , with  this in mind some people were  declined to accept the risk however low probability  . However physics and chemistry of reactor core, coupled with but not wholly depending on the engineering, mean that the consequences of an accident are likely in fact be much less severe than those from other industrial and energy sources. Experience bear this out.

The use of nuclear energy for electricity generation can be considered extremely safe. Every year severe thousand people die in coal mines to provide this widely used fuel for  electricity . There  are also significant health and environmental effects arising from fossil fuel use.

Achieving optimum nuclear safety

To achieve optimum safety, nuclear power plants in western world operate using a defensive-in depth approach with multiple safety systems supplementing  the natural features of reactor core. Key aspects of approach are

. High quality  design and construction
. Equipment which prevents operational disturbances or human failures, errors developing into problem.
. comprehensive monitoring and regular testing to detect equipment or operator failures.
. Redundant and diverse systems to control damage to the fuel and prevent significance radioactive releases.
. Provision to confine the effects of severe fuel damage (or any other problem ) to plant it self.

These can be summoned by prevention, monitoring and action to mitigate consequences of failures.