Inherently safe reactors

These arguments are often put forward to promote the use ofNuclear Energy. However not all is well with the Nuclear Energy. There are the questions of the waste problem so far unsolved, safety ofNuclear Reactors is not guaranteed to the extent that they are inherently safe. If we aim to construct inherently safe reactors, then the economics of a Nuclear Reactor makes it unacceptable. 

Taylor, J.J. Improved and Safer Nuclear Power, Science, 318 (April 21, 1989). Weinberg, A M. and I. Spiewak Inherently Safe Reactors and a Second Nuclear Era, Science, 224, 1398-1402 (1984). 

This concept means that the risk of core meltdown in case the heat from fission processes cannot be led away must be absent. Two examples of proposed inherently safe reactor designs are ... 

Dr Cope, a member of the Chernobyl Working Group and Director of UK CEED, welcomed Dr Hayns and said that it seemed to him that in the past there had been a lack of discussion on inherently safe reactors in the UK compared with other countries. He was therefore encouraged by the research undertaken by the UKAEA. 

A module has a power rating of 170 MW(th) at 950°C helium outlet temperature for process heat applications, while for electricity production or coproduction of electricity and process steam or district heating a rating of 200 MW(th) at an exit temperature of 700 °C is achieved. For reasons of economics and demand, plant sizes of 8 units appear to be an upper limit. Additionally the modular HTR as a small and inherent safe reactor system fullfil the criterion for the design of an autarc barge-mounted energy station in an optimum way. 

Inherently safe reactor because of the large coolant body surrounding the fuel. 

Reactors are designed to be inherently safe based on physical principles, supplemented by redundant equipment and special procedures. Nuclear power benefits from the appHcation of the concept of defense in depth, ie, by using fuel form, reactor vessel, building containment, and emergency backup procedures to ensure safety. 

GP 11] [R 19] An impressive example of the impact of miniaturization on the explosion limit is given in [9], For a conventional reactor of 1 m diameter, explosive behavior sets in at 420 °C at ambient pressure (10 Pa). In turn, an explosion occurs at about 750 °C when the reactor diameter is decreased to about 1 mm. A further reduction to 100 pm shifts the explosive regime further to higher pressures and temperatures. Even the first explosion limit is above ambient pressure. Now, explosive behavior can be excluded and so the reaction becomes inherently safe. 

Energy development is international by its very nature. International cooperation in the sphere of energy will ensure faster progress and will make possible the realization of projects which no country could cope with on its own. Large-scale joint efforts between different nations, in particular, between East and West in the field of energy should be initiated. These efforts could include joint research on controlledfusion, and research and development on more inherently safe, as well as more reliable, fission reactors. 

The procedure to determine both the basis of safety for the reactor (see Annex 1) and the worst case scenario for that basis of safety is iterative. The same screening tests which help determine the worst case for pressure relief sizing may lead to the conclusion that pressure relief is not the best basis of safety. The results of screening tests may also indicate that it is worthwhile to seek a more inherently safe solution by designing out the possibility of certain maloperations or system failures (for example, if the screening indicates that a very large relief system would be required). 

The cooling failure is not considered here, as the adiabatic reactor is designed to work without cooling. If the conditions listed above are fulfilled, the adiabatic batch reactor is inherently safe as far as the charge is guaranteed. The reaction course is not affected by any eventual cooling failure or breakdown of utilities. The batch reactor can be made safe only if it is designed for adiabatic conditions. 

The reactor is proposed to be modular, but with module sizes of 500 MW imits. This is chosen for convenience in sizing Brayton turbines, but is not small enough to constitute an inherently safe design as defined below. 

There are two proposed technologies that use accelerators. One is called accelerator-breeding, as it aims at converting fertile material (e.g., Th-232) into fissile fuel to be used elsewhere in a (conventional) reactor (Lecocq and Furukawa, 1994). This does not in itself reduce accident probabilities, and to achieve this, the reactors should be of the above-mentioned inherently safe type. 

A reduction in size means also a better controlled process and the managing of lower amounts of chemicals per reactor. In addition, microreactors often offer intrinsically safer operations [3]. In fact, with respect to conventional reactors, they offer a much better control of heat transfer (e.g., reduced possibility of runaway reactions) and a higher wall to volume ratio, which minimize the possibility of side radical reactions (e.g., of explosion). PI concepts are thus essential to develop inherently safe processes as well as energy efficient operations. 

In addition to the inherently safe features of microchannel reactors, they allow rapid catalyst testing and have the option of the same catalyst type use, for example, washcoated, throughout the whole development cycle, up to pilot and production. Furthermore, mass transfer is improved through the increase in gas-liquid interfaces, and thermal control is also better due to larger exchange surfaces. 

Fusion power generators are inherently safe. The magnetic confinement of the plasma must be carefully controlled and balanced to sustain the nuclear reaction. Any disturbance of the operating conditions will result in termination of the reaction. No combination of system failure, operator error, natural disaster or sabotage can cause the fusion reaction to run away. A nuclear explosion, melt down or similar catastrophic accident is not possible. A violent event, one of sufficient magnitude to disrupt the total reactor, could cause a chemical or electrical fire similar to any industrial fire. 

Because of the small inventories of reactants and products, MSR operation may be inherently safe even when the reactor is operated in the explosive regime [17-19]. Furthermore, small reactor dimensions facilitate distributed production at the location where the product is needed or the reagents are available, and the transport and storage of dangerous materials can be minimized. 

Research reactors are used in nuclear physics, in nuclear, analytical and structural chemistry, in radiobiology, in medicine, etc. They are usually easy to operate, inherently safe, and of moderate cost. Many of them are of the pool type the reactor core is located in the center of a stainless steel clad concrete vessel, 6—8 m deep and 2—5 m wide, containing purified water. The water provides the main radiation shielding, moderation, and cooling. The concrete walls are about 2 m thick wh a the reactor is located above groimd otherwise much thinner walls can be used. The main radiation protection demand comes from the reaction... 

FIG. 19.14. Working principle of ABB-Atom SECURE inherently safe district heating reactor. 

This procedure, which for the example considered results in a solution, that the reactor responds inherently safely to a cooling failure for almost all process temperatures, can be performed in a complete analogical way for any other kinetic rate law. 

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