Uri Gat and the Ultimately Safe Reactor

Uri Gat, a now retired ORNL Engineer, wrote one of the most important papers on Reactor Safety. Yet Gats work on Nuclear safety is almost completely unknown. The Reason why Gat's papers and their conclusions are so little known, Gats analysis leads to conclusions about nuclear safety that are are unacceptable to the nuclear industry, the United States Department of Energy and to the Nuclear Regulatory Commission as well as almost every university Department of nuclear studies, and several national Laboratories. Gat was actually the Lead Author in two significant papers on Nuclear safety. The first was Molten-Salt Reactors–Safety Options Galorere. That paper was reproduced in Energy from Thorium and Nuclear Green. The second paper, THE ULTIMATE SAFE (U.S.) REACTOR - A CONCEPT FOR THE THIRD MILLENNIUM Covers some of the concepts introduced by Galore.

THE ULTIMATE SAFE (U.S.) REACTOR - A CONCEPT FOR THE THIRD MILLENIUM Ur1 Gat

Oak Ridge National Laboratory (ORNL)* OakRidge,TN 37831

ABSTRACT

The Ultimate Safe (U.S.) Reactor is based on a novel safety concept. Fis- sion products in the reactor are allowed to accumulate only to a level at which they would constitute a harmless source term. Removal of fission products also removes the decay heat — the driving force for the source term. The reactor has no excess criticality and is controlled by the reactivity temperature coefficient. Safety 1s inherent and passive. Waste is removed from the site promptly.h

1. ULTIMATE SAFETY
Ultimate safety for a nuclear reactor is considered here to mean that a reactor event (accident), however remote, leading to a significant health hazard to the public cannot occur . Ultimate safety also requires passive and inherent safety . That means active intervention by an operator or one that requires a power source or mechanical activation is not required, nor can the safety features be overridden by external means since they are inherent to the system and rely on the physical properties of the system . Such an extreme safety usually implies that no significant damage to the plant will occur and no extraordinary economic penalty will result even if there is an unusual occurrence. The Nuclear Power Options Viability Study determined that a degree of passive safety and low economic risk are prerequisites for the success of an advanced concept.1]

There are two major possible events that can lead to a dispersal of radio-activity from a nuclear reactor and become a health hazard . The first is an uncontrolled reactivity increase that will yield a power burst which would damage the reactor and disburse the radioactive fission products inventory . The other event could be failure to remove the decay heat of the fission pro- ducts resulting in overheating and dispersal of inventory. The so-called source term - the likelihood for a quantified release of radioactivity - is the product of the inventory of fission products and the driving force or the energy to disperse this inventory.
The U.S. Reactor described here retains the inventory of fission products at such a low level that even if dispersed the safety hazard is within acceptable limits . But, furthermore, the driving forces - the decay heat and any excess criticality - are kept so low that there is insufficient energy to bring about such a dispersal.

2. SOURCE TERM CONTROL

The source term in the U.S. Reactor is controlled by continuous removal of fission products at the rate they are produced. Fission products are allowed to accumulate only to a level of 1 to 6 hours of full power operation equivalent . That is an equilibrium level as if the reactor had operated for the equivalent time without any removal of fission products ; then any additional fission products are removed as they are produced. This retains the accumulated activity at a low level that reduces a hypothetical maximum accident to a manageable level . The exact equivalent build-up time is determined by the degree of desired hypothetical safety and the economics of the chemical processing plant removing these fission products. The need ease, and efficiency of removal of the various chemical elements may result in different equivalent build-up times for different elements.

It has been determined2] that at the 1 to 6 hours equivalent build-up time the fused fuel salt of The U.S. Reactor will not reach boiling due to after-heat even without any heat removal. Thus decay heat cannot provide sufficient energy to disperse the fission products, and there is no source term associated with the decay heat.
The reactor is designed such that any excess buildup of fission products will not allow it to become critical due to neutron poisoning. There is no excess criticality to override this limitation. This safety feature is thus inherent, passive, and can- not be overridden.

3. CRITICALITY CONTROL

The U.S. Reactor is operated at no excess criticality as none is necessary for burnup or poisoning compensation. The fuel, based on the thorium-uranium cycle, is internally replaced by an exact breeding ratio of 1.0. The fuel composition, the fertile material contents, and the processing are coordinated such that the breeding ratio cannot exceed 1.0 and no excess fuel can be produced . The core also has a negative temperature coefficient . An additional benefit of this arrange- ment is that if an attempt is made to divert fuel from the reactor, the reactor becomes
subcritical and shuts itself down.

The criticality control is inherent, passive and cannot be overridden.

4. REACTOR CONTROL
The U.S. Reactor is designed with no mechanical controls or externally operated controls . Load following is accomplished by a negative temperature coefficient . The reactor operates at a constant core exit temperature which is determined by the core dimensions and the invariable fuel composition. This arrangement enhances the thermodynamic efficiency at partial loads and improves the economy of the reactor . This re- actor control is inherent, passive and can- not be overridden.

Reactor shutdown, or scram, is accomplished by dumping the fuel into dump tanks which guarantee subcriticality. The dump valve can be designed as a freeze valve which will open by removal of cooling or by overheating of the fuel - a safety feature
which cannot be overridden.

Reactor shutdown, or scram, is accom- plished by dumping the fuel into dump tanks which guarantee subcriticality. The dump valve can be designed as a freeze valve which will open by removal of cooling or by overheating of the fuel - a safety feature
which cannot be overridden.

Reactor startup is accomplished by pumping the fuel into the core . The pump is sized such that at maximum capacity the design startup parameters cannot be ex- ceeded. As the fuel reaches cold criti- cality level, it becomes critical and heats to the temperature corresponding to that level . As the level rises it continues to heat until it reaches the design temperature at full core, which it then maintains by the temperature coefficient control .

5. WASTE CONTROL
The radioactive fission products are processed into the desired and acceptable chemical composition and concentration . This composition will be determined by regu- lations and governing authorities and will affect only the design details of the pro- cessing plant tail end. The fission pro- ducts waste contains significant amounts of radioactivity and decay heat. The safety of this waste is provided by small container units and adequate geometry for cooling . The waste will be contained in very small containers, perhaps as small as 100 ml or less, in narrow cylinders of 10 mm diameter or less . The small quantity per unit reduces the hazard associated with each to tolerable levels even for a major hypothetical accident . The narrow cylinders.

make it easy to design the cylinders such that they will retain their integrity under all circumstances . The narrow cylinders also assure that they will not overheat without requiring any active cooling. The cylinders can be designed so that conduction to the shielding container, or natural convection, as augmented by radiative heat loss, will assure the temperature can not rise above the design level . This safety is inherent and passive.

The small waste containers are shipped frequently, daily or several times a day, to the waste repository site. There is no accumulation of radioactivity anywhere on the site to feed a source term.

6. FUEL CONTROL
The U.S. Reactor is designed to be exactly self-sufficient in fuel . No fuel shipments are necessary . Overproduction of fuel cannot be done since the neutron
economy of the system will not allow it. An attempt to remove fuel will shut the reactor
down as it will become subcritical .

7. FISSION PRODUCT REMOVAL
To facilitate the on-line continuous fission product removal, a liquid fuel is used . Fluoride molten salt was chosen to take advantage of the already developed technology .31 Specifically the processing has been investigated in great detail .4] In addition the thorium-uranium fuel cycle was chosen. This eliminates the plutonium as a fuel component and all but eliminates the actinides from the waste products .5]
8. REACTOR DESIGN.

Though many design options are open for The U.S. Reactor, a single fluid, epithermal, externally cooled concept is chosen as the first concept.6] The single fluid sim- plifies the design . The epithermal neutron spectrum also simplifies the design, eliminates the need for a moderator, and provides for the breeding ratio of 1.0 with relative simplicity . The external cooling makes the design of the reactor into a core and a primary heat exchanger connected by pipes, with the fuel circulated by a mp. Connections are needed to and from the processing and into the dump tanks.

9. ECONOMIC OUTLOOK
It is anticipated that the ultimate safety features of The U.S. Reactor will allow the elimination of most safety systems, required on reactors. There is no safety need for a containment . The extreme simplicity of the design should result in a very economic, low in capital cost, reac- tor. The inherent and passive safety features do not require mechanical precision or extreme reliability of any operating parts. The construction cost and cost associated with quality assurance can be relatively low . There is no expectation of regulatory delays or back fitting, as safety is not dependent on construction or design details. This reduces the risk of unplanned or unanticipated cost. The small dimensions of the components and the absence of sophisticated or complicated systems allow for off-site fabrication and quick and simple on-site assembly, yielding further potential for an economic system. The simplicity of the system also allows for size flexibility or modular units which are economic .
The relative simplicity of the design and absence of mechanically operating parts, except the pump, are expected to make maintenance simple and availability high; also there is no downtime for fuel exchange.

The onsite processing facility is an added capital cost which comes in lieu of most of the fuel cycle cost.

10. CONCLUSION
The U.S. Reactor is designed to achieve ultimate safety . The source term is reduced by reducing the radioactive inventory to such low levels that they cannot constitute a major hazard and so that they lack the power or energy to cause damage to the reac- tor or provide the driving force for the source term.

The reactor has constant reactivity at exact criticality . Excess criticality is neither needed to compensate for burnup or poison buildup nor available; thus a criticality accident during operation is not possible. Power control is provided by the negative temperature coefficient . All these safety features are inherent and passive and
cannot be overridden resulting in the ulti- mate safe features. The extreme safety sim- plifies the reactor design and can be anticipated to yield good economy .

The waste is packaged in dedicated small containers of size and shape that prvide assurance against exceeding a set temperature. The size is small enough so that the maximum credible accident does not exceed acceptable level . The waste is shipped frequently from the site so there is no site hazard associated with it.

The benefits and advantages of a liquid fuel-molten salt system are also utilized .

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Concluding comment

In addition to the safety features Uri Gat points out, I should add the safety and economic advantages of underground reactor housing. An underground reactor cannot be damaged by an aircraft or a truck bomb. The construction cost 0f an underground reactor housing may varie, by circumstances, for example siting MSRs in salt mines, may make underground reactor housing a very low cost proposition indeed. Thus underground housing can significantly lower reactor cost, while enhancing reactr safety.

I cannot argue that there will never be an accident with Uri Gat's Ultimately dafe molten Salt React0r, ;yomatre;y dsfe reactor, but that there will never be a fatality or injury as a consequences of Gat's ultimately safe reactor, and such safety will probably be possible while costing less than the safety investments necessitated by current nuclear technology. - CB