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Hilda Murrell was abducted on the 21st of March 1984, as she prepared to testify at the Sizewell B Nuclear Reactor Inquiry. She was found dead on the 24th of March. https://miningawareness.wordpress.com/2016/03/23/the-murder-of-hilda-murrell-an-abiding-mystery/

According to her nephew, Robert Green, “Hilda had conferred with Don Arnott who, uniquely among anti-nuclear scientists, had discovered a serious flaw in the control rods of the US nuclear power plant design which had failed at Three Mile Island, but which Thatcher was determined to build. His concerns were later endorsed by Dr Edward Radford, chair of the TMI Scientific Advisory Board. At the time key leaders of the nuclear industry supporting Thatcher’s plans took a close interest in what Don knew; he was tipped off that he was under surveillance, and some documents went missing. Before he could testify at the Sizewell Inquiry, he nearly died of a suspicious heart attack. In June 1985 he found his motorbike had been sabotaged. His death from natural causes in 2000 had nothing to do with his heart.https://wikispooks.com/wiki/Hilda_Murrell/Outstanding_questions (Emphasis added)

[Important aside: Don Arnott, “kept the phial he had used at Amersham for mixing radium. Over the years, as a result, it had developed fine hairline cracks. He realised that incorporating high-level waste into glass before deep dumping would not bind it securely for all time, as the industry claimed.http://www.theguardian.com/news/2000/jan/13/guardianobituaries ]

Zion Rd. Three Mile Island Nuclear Power Station
Road ironically named Zion near Three Mile Island Nuclear Power Station in Pennsylvania. Apparently using Zion as metaphor for dying and going to heaven, Pennsylvania born Robert Lowery wrote the refrain and music in 1867 “We’re marching to Zion, Beautiful, beautiful Zion; We’re marching upward to Zion, The beautiful city of God.” to a 1707 work by Englishman Isaac Watts: http://www.cyberhymnal.org/htm/m/a/marching.htm

Dr. Don Arnott discussed the problems with the PWR found at Three Mile Island, Sizewell B, and many other sites https://en.wikipedia.org/wiki/List_of_nuclear_reactors in “UNDER PRESSURE: THE DANGERS OF THE PWR“. Westinghouse is now owned by Toshiba, and proposes new PWR, AP 1000 reactors, for Moorside in Cumbria, UK.

Excerpted from Dr. Arnott’s “Under Pressure“, written prior to construction of Sizewell B:
At Sizewell, in Suffolk, the CEGB wants to build a Pressurised Water Reactor(PWR) – the same type of nuclear reactor as went seriously wrong at Three Mile Island on March 28, 1979.
[…]
the Pressurised Water Reactor(PWR) is the most dangerous because it contains a whole complex of hazards which are in addition to those found in other types of nuclear power reactor.
[…]
PWRs originated in the USA where Westinghouse, though not the only manufacturers, are the best known. Their model, chosen for Sizewell, has been extensively modified. In respect of the reactor-steam generator assembly, it is important, first of all, to understand the exact status of those modifications.

In 1973, five US manufacturing companies entered into an agreement to build nuclear power plants to a common design. This is called SNUPPS (Standard Nuclear Unit Power Plant System). Five such plants were ordered but only two are actually being built and they will not operate for another two years.

In Britain, a Task Force, headed by Dr -now Sir – Walter Marshall, was set up to introduce further modifications to the SNUPPS design, with the object of reducing cost and increasing safety — a combination guaranteed to raise eyebrows. The important fact to realise, however, is that the SNUPPS design has not been operationally tested anywhere in the world. So the Marshall re-design is a further modification of an already modified and untested version!

The fuel is enriched Uranium oxide clad in a Zirconium alloy called Zircaloy. The moderator/coolant is water under pressure and in that form does not react significantly with Zircaloy.

Turning once more to pressurisation, we find a situation very different from that described for the gas-cooled reactors. The reactor core is enclosed as before in a pressure vessel; but the purpose of the presure is to raise the boiling point of the water to above 300 degrees Centigrade – three times normal – whilst preventing it from boiling by the application of pressure. This is essential, if electrical generation is to be efficient. The pressure required is around 2300 psi.

As compared with a gas, water is almost incompressible and relatively little extra has to be forced in to reach the pressure required. In consequence, even a relatively small coolant leak (known in the trade as a LOCA, or Loss of Coolant Accident) can lead to rapid and serious depressurisation. What happens then is best illustrated by reference to a device which most people know- the familiar pressure-cooker.

The pressure-cooker cooks vegetables quickly by heating them hotter than the normal boiling point of water, which is 100 degrees Centigrade at atmospheric pressure. At that temperature, there is a phase-change: the water boils and turns to steam.

And no amount of extra heat will raise the temperature higher; the additional heat merely produces steam more quickly. If now the weighted knob, typically weighing 1/4 lb, is placed in position on the pressure-cooker vent, steam can no longer escape; therefore, it is no longer generated; and the extra heat supplied raises the temperature of the water above its normal boiling point.

Most people know what then happens if the knob of the pressure-cooker is removed too trustfully. The increase of pressure has been suddenly removed; the water inside instantly reverts to atmospheric pressure – but at a temperature several degrees above its correct boiling point. It therefore boils with explosive violence and a jet of steam escapes through the vent.

When the extra heat accumulated under pressure has been got rid of in this way, things quieten down. We can now easily understand what happens to a PWR core when there is a leak, or LOCA, somewhere in the pressurised coolant system. The pressure inside the reactor core falls rapidly and the water contained can flash into steam. And steam reacts with the Zircaloy fuel-cladding forming Hydrogen. Moreover, this reaction itself guarantees heat, thus worsening the situation. Furthermore, steam is a bad conductor of heat. The partial destruction of the fuel cladding leads inevitably to fission-product release; this was a prominent feature on the TMI-2 accident.

A direct LOCA is not the only, nor indeed the most common source of trouble. When a reactor gets too hot, it is not usually because the core has suddenly started producing more heat, but because the coolant, through some interruption in flow, is no longer removing the heat generated. In the PWR; since water absorbs neutrons, matters are arranged so that as little as possible is in the reactor at any given moment. In order to get the heat out, the coolant must be pumped through at very great speed. In fact, very nearly 19 tonnes of pressurised water per second must be pumped through the pressure vessel – which is about the volume of a medium-sized bedroom. If now a pump fails, the coolant no longer flows, and both temperature and pressure start to rise rapidly within the reactor. At this point, a pressure-operated release valve (called a PORV by the trade) opens to restore conditions to normal. Its function is strictly parallel to that of the knob on our pressure-cooker; should it fail to close, the consequences will be as described in the previous paragraph and no further description is necessary. A stuck PORV was the major contributor to the TMI 2 accident.

It is not the case that the risks of depressurisation occur only in the reactor pressure vessel. For the coolant is removed from the reactor to a heat-exchanger (in the course of generating steam for the turbines) and thence returned to the reactor to extract more heat; and all pipes, pumps and valves associated with this circuit must necessarily operate under nearly the same conditions of extremely high pressure which are found in the reactor itself. Opportunities for leaks or other malfunctions are greater here than in the reactor itself and even transient changes can produce mechanical shock to the system. Another example from everyday life can illustrate this.

Gardeners who live in tower blocks are often aware of how an open-ended hosepipe behaves when connected to a high head of water; it writhes like a living snake. This is known as pipe-whip. And , inside the tower block, the sudden turning off of a tap can produce a shock to the pipe which feeds it, suggesting that somebody has hit it pretty hard with a mallet. This is water-hammer.

Similar phenomena can occur in the system just described in the event of malfunction. Actual physical disintegration would not be slow but violent. There is a section in the CEGBs Preconstruction Safety Reportwhich is headed missiles. This is not concerned, as might be supposed, with anything lobbed our way by the USSR or casually dropped by the US military aircraft which ceaselessly patrol the East Anglian skies. It refers to flying debris, mostly metallic, produced from any disintegration in the heat extraction circuits and to the problems of containing the damage it would do.

As may be imagined, a significant part of the whole design consists of systems, back-up systems and alternative systems whose function is to contain these and other hazards. There are too many of them to be described here. But in number and complexity they exceed those to be found in any other type of commercial power reactor now operating.

In fact, the whole operation is an exercise in technological brinkmanship. One example of how perverse this can become must be given. In order that the fuel rods (over 50,000 of them) can withstand, without collapsing inwards, the enormous water pressure within the reactor core they too must be pressurised. This is done by pumping Helium gas into them at the time of manufacture; which is fine unless the coolant depressurises. In that case the cladding, which is little more than 0.5mm thick, may swell, thus distorting the fuel rod assembly. This danger is graphically known as clad-ballooning and at the present time the problem is not solved.

In the gas-cooled nuclear stations, the reactor itself is isolated from virtually all of the steam-generating and other equipment by a concrete vessel called the biological shield. All equipment can be serviced with the reactor on load; and, in the later Magnoxes, refuelling can also take place under load (theoretically the AGR scan also be refuelled under load, but there have been a few technical problems about that).

By contrast, in the PWR, the whole of the steam-generating equipment, the reloading equipment and much else has to be housed close-in to the reactor, the whole of it being mounted in a very large building called the containment. The name itself explains its purpose. If there is a severe accident, the whole of this equipment is liable to become severely contaminated; the containment is there in order to ensure that none of this radioactivity reaches the environment.

Therefore, as the National Nuclear Corporation Observed in its evidence to the House of Commons Select Committee on Energy, the basic construction and mode of operation of SNUPPS, no matter how modified, is bound to lead to increased radiation-exposure for reactors. In fact, it is expected that this will be 2-4 times higher than for the gas-cooled reactors. There will also be increased production of radioactive waste, arising from servicing and refuelling, and itself contributing to increased operator exposure.

Mention must finally be made of the emergency procedure which will come into play should things go seriously wrong. This has been almost entirely automated with little room left for human intervention. With the example of Three Mile Island in mind, this might seem to be no bad thing-until one knows that the system to be adopted is new, untested, and has not been licensed for any PWR in the world. In fact, should it ever be brought into action, what we shall be witnessing is not an established emergency procedure but a scientific experiment to determine whether the system works or not.

So how did we get stuck with it?

Like much else in the nuclear field, the origin of the PWR is military. If, for the sake of argument, you assume that a submarine that can remain submerged without refuelling for long periods is a good thing to have, there is only one way of propelling it; that is by nuclear power. And, space being at a premium in a submarine, a small unit of high power output is needed; a small PWR provides this. And from this has grown the notion that the best power station reactor is the one which produces the most output from the smallest volume – even where space is not at a premium. The Sizewell PWR is basically a scaled-up submarine reactor. It produces enormous output in small volume (the pressure vessel containing the reactor is cylindrical and only 13 metres X 4) and not a few of its instability problems arise from that fact.

It is a commonplace in technology- and not only in nuclear engineering- that scaling-up produces problems of its own.

What about the Nuclear Installations Inspectorate?

The NII is the watchdog which oversees the operation of a nuclear power station from start-up to decommissioning; and if an entirely new type of reactor is proposed for Britain, then it must also approve the design. In theory, the NII is independent; but, in reality, it is staffed by members of the nuclear constellation. Recently, concern has been expressed about the NII being under-staffed and its employees being under-paid. It is interesting to reflect on the degree to which it is likely to carry out these tasks- or indeed is able to.

II there is an accident leading to shutdown – or even a lesser incident – at a functioning nuclear station, NII approval is necessary before the station can be restarted. But a most peculiar position arises with new introductions, such as the PWR. For, once the NII has decided that the design is generically acceptable on safety grounds, there is thereafter no way off the hook. For the emphasis now shifts from scientific principles to design details. And inevitably, for such a huge project, approval is given in stages with many amendments, themselves requiring approval. The advisor to Suffolk County Council, Professor Leslie, graphically describes the NII as shooting at a moving target. It is in theory possible that some insurmountable snag late-on in the development process might arise which would cause the NII to reject the whole proposal. In practice, by that time tens of millions of pounds will have been spent on development work and the pressure to solve outstanding problems at any price will have grown; there may even be the dangerous assumption that all problems are soluble. It is therefore realistic to assume that approval of fundamental design will be followed by approval of a finished power-station: this is no more than an assertion of the fact that all of us are subject to pressures from other persons and other interests.

On the PWR, the NII’s position is particularly shaky. It has had the safety factors under review for several years; and, shortly before the Three Mile Island accident, it had decided that the generic risks of PWRs, properly contained, were acceptable. Also, in its evaluation at that time, it concluded that the likelihood of a Loss of Coolant Accident (LOCA), such as subsequently happened at TMI-2, was small. There need be no implication of slipshod work in this – but it is a sharp statement about human fallibility! LOCAs, we now know, have been relatively common in the American PWR experience. To decide that British modifications can make this unstable reactor safe, to give it in effect a new lease of life when cancellations are the order of the day, is an awesome responsibility for the NII. And this responsibility must be set against what is the most disquieting issue of all. This is the sudden change in British policy.“(pp. 12-18). Emphasis added. The original document is found here: https://ia802605.us.archive.org/30/items/UnderPressure_201410/Under%20Pressure_text.pdf

From the President’s Commission on Three Mile Island Nuclear Disaster:
We estimate that there were failures in the cladding around 90 percent of the fuel rods. The interaction of the very hot cladding with water generated somewhere between 1,000 and 1,300 pounds of hydrogen gas and converted 44 to 63 percent of the zirconium to relatively weak zirconium oxide. As a result of oxidation and embrittlement of the fuel rod cladding, several feet of the upper part of the core fell into the gaps between the fuel rods, causing partial blocking of the flow of steam or water that could remove heat from the damaged fuel.

Fuel temperatures may have exceeded 4,000°F in the upper 30 to 40 percent of the core (approximately 30 to 40 tons of fuel). Temperatures in parts of the damaged fuel that were not effectively cooled by steam may have reached the melting point of the uranium oxide fuel, about 5,200°F.

An NRC study suggests that some of the fuel may have become liquid at temperatures above 3,500°F by dissolving in a zirconium-zirconium oxide mixture. The study estimates that the amount of fuel that may have melted by this process is from zero to a few tons” (pp. 30-31) “The President’s Commission On The Accident At Three Mile Islandhttp://www.threemileisland.org/downloads/188.pdf

From Wikipedia: “The Kemeny Commission noted that Babcock & Wilcox’s pilot-operated relief valve had previously failed on 11 occasions, nine of them in the open position, allowing coolant to escape. More disturbing, however, was the fact that the initial causal sequence of events at TMI had been duplicated 18 months earlier at another Babcock & Wilcox reactor, the Davis-Besse Nuclear Power Station owned at that time by Toledo Edison. The only difference was that the operators at Davis-Besse identified the valve failure after 20 minutes, where at TMI it took 80 minutes, and the Davis-Besse facility was operating at 9% power, against TMI’s 97%. Although Babcock engineers recognized the problem, the company failed to clearly notify its customers of the valve issue.[68]https://en.wikipedia.org/wiki/Three_Mile_Island_accident
[…]
About 130 minutes after the first malfunction, the top of the reactor core was exposed and the intense heat caused a reaction to occur between the steam forming in the reactor core and the Zircaloy nuclear fuel rod cladding, yielding zirconium dioxide, hydrogen, and additional heat. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes to the reactor coolant, and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon
In a 2009 article, Gilinsky wrote that it took five weeks to learn that “the reactor operators had measured fuel temperatures near the melting point”.[37] He further wrote: “We didn’t learn for years – until the reactor vessel was physically opened – that by the time the plant operator called the NRC at about 8:00 a.m., roughly half of the uranium fuel had already melted.”[37]

[…]
… was later found that about half the core had melted, and the cladding around 90% of the fuel rods had failed,[15][48] with 5 ft (1.5 m) of the core gone, and around 20 short tons (18 t) of uranium flowing to the bottom head of the pressure vessel, forming a mass of corium.[49]https://en.wikipedia.org/wiki/Three_Mile_Island_accident

http://www.nytimes.com/2001/10/22/world/edward-radford-79-scholar-of-the-risks-from-radiation.html