Scholarly article on topic 'Safety assessment of molten salt reactors in comparison with light water reactors'

Safety assessment of molten salt reactors in comparison with light water reactors Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Badawy M. Elsheikh

Abstract Molten salt reactors (MSRs) have a long history with the first design studies beginning in the 1950s at the Oak Ridge National Laboratory (ORNL). Traditionally these reactors are thought of as thermal breeder reactors running on the thorium to 233U cycle and the historical competitor to fast breeder reactors. In the recent years, there has been a growing interest in molten salt reactors, which have been considered in the framework of the Generation IV International Forum, because of their several potentialities and favorable features when compared with conventional solid-fueled reactors. MSRs meet many of the future goals of nuclear energy, in particular for what concerns an improved sustainability, an inherent safety with strong negative temperature coefficient of reactivity, stable coolant, low pressure operation that don not require expensive containment, easy to control, passive decay heat cooling and unique characteristics in terms of actinide burning and waste reduction, while benefiting from the past experience acquired with the molten salt technology. As the only liquid-fueled reactor concept, the safety basis, characteristics and licensing of an MSR are different from solid-uranium fueled light water reactors. In this paper, a historical review of the major plant systems in MSR is presented. The features of different safety characteristics of MSR power plant are reviewed and assessment in comparison to other solid fueled light water reactors LWRs.

Academic research paper on topic "Safety assessment of molten salt reactors in comparison with light water reactors"

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M. V esearch & Applied Sciences

Safety assessment of molten salt reactors in comparison with light water reactors

Badawy M. Elsheikh

Egyptian Nuclear and Radiological Regulatory Authority, Egypt

ARTICLE INFO

ABSTRACT

Article history: Received 5 June 2013 Accepted 17 August 2013

Keywords: Nuclear safety LWR safety

Molten salt reactor safety Nuclear reactor accident

Molten salt reactors (MSRs) have a long history with the first design studies beginning in the 1950s at the Oak Ridge National Laboratory (ORNL). Traditionally these reactors are thought of as thermal breeder reactors running on the thorium to 233U cycle and the historical competitor to fast breeder reactors. In the recent years, there has been a growing interest in molten salt reactors, which have been considered in the framework of the Generation IV International Forum, because of their several potentialities and favorable features when compared with conventional solid-fueled reactors. MSRs meet many of the future goals of nuclear energy, in particular for what concerns an improved sustainability, an inherent safety with strong negative temperature coefficient of reactivity, stable coolant, low pressure operation that don not require expensive containment, easy to control, passive decay heat cooling and unique characteristics in terms of actinide burning and waste reduction, while benefiting from the past experience acquired with the molten salt technology. As the only liquid-fueled reactor concept, the safety basis, characteristics and licensing of an MSR are different from solid-uranium fueled light water reactors. In this paper, a historical review of the major plant systems in MSR is presented. The features of different safety characteristics of MSR power plant are reviewed and assessment in comparison to other solid fueled light water reactors LWRs.

Copyright © 2013, The Egyptian Society of Radiation Sciences and Applications. Production

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Introduction

Molten salt reactors (MSRs) are liquid-fueled reactors that can be used for burning actinides, producing electricity, producing hydrogen, and producing fissile fuels (breeding). Fissile, fertile, and fission products are dissolved in a high-temperature, molten fluoride salt with a very high boiling temperature (~1400 °C). The molten salt serves as both the reactor fuel and

the coolant as shown in (Fig. 1). Heat is generated in the reactor core and transported by the fuel salt to heat exchangers before returning to the reactor core. There exists a broad range of design choices (LeBlanc, 2010) such as whether graphite is used as moderator or not, whether fuel processing for fission product removal is employed, whether the system runs in a denatured Low-enriched uranium (LEU) state by the inclusion of 238U and also whether one operates as a Single

E-mail address: badawymel@yahoo.com. Peer review under responsibility of The Egyptian Society of Radiation Sciences and Applications

1687-8507/$ — see front matter Copyright © 2013, The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/jjrras.2013.10.008

Fig. 1 - Schematic representation of a typical MSR. Reproduced from GIF-IV (2002).

Fluid or a Two Fluid system (a Two Fluid system has separate salts for fissile 233U and fertile Th). These choices also dictate whether a system has a Breeding Ratio > 1.0 (to produce excess fissile for future startups) or a B.R = 1.0 to break even on fissile production or if B.R < 1.0 making it a converter reactor requiring annual additions of fissile fuel of some kind. The MSR is one of six advanced reactor concepts identified by the Generation IV (GENIV) International Forum as a candidate for cooperative development.

The history of the molten salt reactor dates back to the 1950s. The design was first proposed as the propulsion system of a nuclear-powered aircraft at the Oak Ridge National Laboratory (MacPherson, 1985). After the program finished, the emphasis was put on the research of MSR running on the thorium fuel cycle. In the 1960s the project focused on the breeding possibilities, resulting in the design of the Molten Salt Breeder Reactor (MSBR) (Robertson, 1978). An experiment, called molten salt reactor experiment (MSRE) (Prince, Ball, Engel, Haubenreich, & Kerlin, 1968) has been carried out from 1965 to 1969. The reactor has been operated with 233U in early 1969. This was the first time 233U fuel was used as reactor fuel. The salt of this reactor did not contain any thorium because it was intended to simulate only the fuel stream of a two-fluid breeder reactor. The MSR program was terminated in 1976 although the results of the experiment were promising. The MSR without the complicated chemical removal processes is a converter reactor with high conversion ratio if thorium is added to the fuel

(Perry, 1975). Oak Ridge did continue a modest program until the early 1980s, with a greatly increased value placed on maximizing proliferation resistance. ORNL examined operating on denatured cycles in which all uranium stayed below the weighted average of 12% 233U and/or 20% 235U. The results were surprisingly successful, with two routes examined, both termed DMSR for Denatured Molten Salt Reactor. The first a DMSR break even design (Engel et al., 1978) with similar fission product processing to the MSBR and showed that break even breeding could be accomplished even while remaining in a denatured state using depleted 238U and Th as the fertile makeup. The second was a greatly simplified DMSR converter design called the "30 Year Once Through Design" (Engel et al., 1980) without any fuel processing beyond chemistry control for a full 30 years while still maintaining a very high conversion ratio and excellent uranium resource utilization. Both designs also featured larger, low power density cores that gave a full 30 year lifetime of the graphite.

The MSR concept deserves renewed interest and reevaluation, because it can satisfy today's priorities to (Moir et al., 2008):

- minimize weapons useable material in storage,

- minimize need for high level waste repository space,

- increase the proliferation resistance of nuclear energy

- make beneficial use of spent fuel from LWRs,

- increase resource utilization,

- greatly expand non-carbon based energy (electricity and hydrogen production) at a cost competitive with alternatives.

According to reactor safety definition, there are many possible definitions of a "safe" reactor. Where as for reactors employing solid oxide fuel, the most elementary demonstration of safety is one in which it can be proven that all (or almost all) fission products remain within the fuel sheath following all Postulated Initiating Event (PIE). Implicit in this requirement is that the fuel pellets should never reach the molten state (Meneley & Muzumdar, 2009). Where in case of MSR the safety defining approach, taking into account a fuel in a liquid form within the coolant and the safety aspects of the chemistry-controlled phenomena. This mean safety depends upon keeping actinides and fission products in solution.

2. Basic characteristics of MSR safety

The most important safety performances are coming from the

following factors (Furukawa et al., 1999):

(1) The primary and secondary systems have pressure lower than 5 bar, and do not have the danger of accidents due to high pressure such a system destruction or salt leakage.

(2) The fuel and coolant salts are chemically inert, and no firing or explosive with air or water (as occurred in the Fukushima accident).

(3) The boiling point of fuel salt is about 1670 K or more, much higher than the operation temperature 973 K. Therefore the pressure of primary system cannot increase.

(4) The fuel salt will be able to become just critical when it coexists with the graphite moderator. Therefore, leaked fuel salt will not induce any criticality accident. [EPIthermal-type MSR is not the same.]

(5) MSR has a large prompt negative temperature coefficient of fuel salt. The temperature coefficient of graphite is slightly positive, but controllable due to the slow temperature increase depending on its high heat capacity.

(6) The delayed-neutron fraction in 233U fission is smaller than that in 235U, and half of the delayed neutrons is generated outside the core. However, it is controllable owing to the longer neutron life, and large negative prompt temperature coefficient of fuel salt.

(7) As the fuel composition can be made up anytime if necessary, the excess reactivity and required control rod reactivity are sufficiently small, and the reactivity shift by control rods is small.

(8) Gaseous fission products such as Kr, Xe and T are continuously removed from fuel salt, minimizing their leakage in accidents and in the chemical processing.

2.1. MSRs safety concept

In general the safety of nuclear reactors consists of:

- Inherent safety (self regulation system) features: Negative temperature coefficient. Negative void coefficient. 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.

- Engineered safety (defense in depth).

2.1.1. Inherent safety features

It has always been the dream of reactor designers to produce plants with inherent safety—reactor assembly, fuel and power-generation components engineered in such a way that the reactor will, without human intervention, remain stable or shut itself down in response to any accident, electrical outage, abnormal change in load or other mishap. All reactors should have inherent safety, which is achieved by suppression of power change by designing the reactor with a negative power coefficient. Because the temperature coefficient of fuel salt is prompt negative and large, this condition is satisfied in MSR in addition to others inherent features:

- Firstly negative temperature coefficient of reactivity. MSRs passively regulate their own temperature. If the reactor overheats, then reactivity in the core automatically slows down. That is, strong negative temperature coefficient of reactivity. The temperature dependence comes from 3 sources. The first is that thorium absorbs more neutrons if it overheats, the so-called Doppler effect (Mathieu et al., 2006). This leaves fewer neutrons to continue the chain reaction, reducing power. The second effect it has to do with thermal expansion of the fuel (Mathieu et al., 2006). If the fuel overheats, it expands considerably, which, due to the liquid nature of the fuel, will push fuel out of the active core region. In a small or well moderated core this reduces the reactivity. However in a large under-moderated core less fuel salt means better moderation and thus more reactivity (the size does not have a significant impact on the feedback coefficients because the neutron spectrum changes very little with the size. The slight evolution of the coefficient is due to the difference in neutron escapes, which are more likely in smaller reactors). This response permits the desirable property of load following—under conditions of changing electricity demand (load), the reactor requires no intervention to respond with automatic increases or decreases in power production. The third part is the graphite moderator, that usually causes a positive contribution to the temperature coefficient (Mathieu et al., 2006).

- Secondly, MSRs operate at atmospheric pressure and use no water, thus eliminating the risk of a steam or hydrogen explosion. The MSRs design appears, in its present state of research and design, to possess an extremely high degree of inherent safety. The single most volatile aspect of current nuclear reactors is the pressurized water. In boiling light-water, pressurized light-water, and heavy water reactors (accounting for nearly all of the 441 reactors worldwide), water serves as the coolant and neutron moderator. The heat of fission causes water to boil, either directly in the core or in a steam generator, producing steam that drives a turbine. The water is maintained at high pressure to raise its boiling temperature. The explosive pressures involved are contained by a system of highly engineered, highly

Table 1 - Comparison of reactor shutdown functions.

Demand function LWR MSR Merit on MSR

High speed shutdown system (scram) Control rod Control rod Enough with small numbers

Second shutdown system Boric acid injection system Fuel-salt drain system No re-criticality in drain tank

Third shutdown system Fuel-salt composition Also used for makeup of thorium

adjusting system component

expensive piping and pressure vessels (called the "pressure boundary"), and the ultimate line of defense is the massive, expensive containment building surrounding the reactor, designed to withstand any explosive calamity and prevent the release of radioactive materials propelled by pressurized steam. A signature safety feature of the LFTR design is that the coolant—liquid fluoride salt—is not under pressure. The fluoride salt does not boil below 1400 °C. Neutral pressure reduces the cost and the scale of LFTR plant construction by reducing the scale of the containment requirements, because it obviates the need to contain a pressure explosion. Disruption in a transport line would result in a leak, not an explosion, which would be captured in a noncritical configuration in a catch basin, where it would passively cool and harden (Hargraves & Moir, 2010). - Also the fuel in an MSR is already in liquid form, it cannot melt down and in an emergency situation it can be quickly drained out of the reactor into a passively cooled dump tank. MSRs designs have a freeze plug at the bottom of the core—a plug of salt, cooled by a fan to keep it at a temperature below the freezing point of the salt. If temperature rises beyond a critical point, the plug melts, and the liquid fuel in the core is immediately evacuated, pouring into a sub-critical geometry in a catch basin. This formidable safety tactic is only possible if the fuel is a liquid.

One of the current requirements of the USA Nuclear Regulatory Commission (NRC) for certification of a new nuclear plant design is that in the event of a complete electricity outage, the reactor remains at least stable for several days if it is not automatically deactivated. As it happens, the freeze-plug safety feature is as old as Alvin Weinberg's 1965 Molten Salt Reactor Experiment design, yet it meets the NRC's requirement; at ORNL, the "old nukes" would routinely shut down the reactor by simply cutting the power to the freeze-plug cooling system. This setup is the ultimate in safe power outage response. Power isn't needed to shut down the reactor, for example by manipulating control elements. Instead power is needed to prevent the shutdown of the reactor (Hargraves & Moir, 2010).

So, from "inherently safe" point of view MSRs comes closer to this ideal case than does any water-cooled reactors. This is inherently much safer, eliminating almost all the (water-based) risks of current reactors.

2.1.2. Engineering safety features

Engineering safety features are based on the defense in depth (DID) philosophy. Which adapted to assure higher safety of the nuclear facility, taking in the following three different levels:

Level 1: Prevention of abnormal operation and failures: by provisions in design, manufacturing, construction, operating and maintenance.

Level 2: Control of abnormal operation and detection of failures by: abnormality detection in an early stage, plant Inherent features and the reactor shutdown system. Level 3: Prevention of the large release of radioactive materials : by setting up containment and Emergency Core Cooling System ECCS. The multiple defense concept in the MSR should be the same as LWR but it must be obvious that defense in depth for a MSR must operate in a quite different way than for a LWR.

In LWR, the defense in depth approach to the safety design followed intuitively from the configuration of a LWR, which provides 3 important physical barriers to the release of the fission products to the environment viz, the clad on the fuel element, where the fission products are generated, the reactor vessel, which contains all the fuel elements forming a reactor core and the leak-tight containment, which is supposed to keep any fission products inside the containment from escaping to the environment. Assuring the integrity of each of these physical barriers in any accident scenario becomes the defense in depth approach against the release of radioactivity to the public environment (Sehgal, 2006).

In case of MSR many of the defense features of the LWR are not required. For example, the massive steel pressure vessel can be dispensed with since MSRs operate under atmospheric pressure. If the pressure vessel is removed from design in case of MSR, we loose one of the physical barriers present in the LWR defense in depth system. Also, LWRs coolant failure is a hazard because it leads to core meltdown. We have seen that much of the LWR's defense in depth system is devoted to prevention of core melt down, a molten core would represent a partial failure of the reactor defense system. Since the core of the MSR is already molten, from the viewpoint of the NRC the MSR violates profoundly important safety rules.

So, It must be obvious then that defense in depth for a MSR must operate in a quite different fashion than for a LWR. Reactor defenses are the most reliable if they depend on the automatic operation of laws of nature, rather than human intervention (Moir & Teller, 2008). Defense in depth in necessitated with undesirable events an unlikely but not impossible. Defense in depth thus is about defense against the unlikely. The purpose of defense in depth is to make the unlikely even more unlikely, if not impossible. More defense in depths are not needed if undesirable consequences cease to be matters of practical concern, or when they stop being theoretically impossible.

MSR Defense in Depth: According to Moir-Teller View (Moir & Teller, 2008), the levels of MSR defenses are:

Table 2 - Comparison of cooling functions of core in emergency.

Demand function

Remark on MSR

Cooling water

makeup Heat removal

Decay-heat removal system

Unnecessary

Decay-heat removal system

Unnecessary (drain system can be used as backup) For severe accident countermeasure

Table 3 - Comparison of radioactive materials confinement functions.

Wall number LWR MSR

Remark on MSR

Pellet Cladding

Pressure vessel, pipes Containment

Reactor building

None (liquid fuel)

None (liquid fuel) Reactor vessel, pipes High temperature confinement Reactor building

No LOCA, gaseous fission products are removed always Same as above Very low pressure No steam generation, no flammable gas generation Same as LWR

- the negative coefficient of reactivity - increased temperature slows down and eventually stops the nuclear reaction;

- the low fuel burn up margin and fast burnup rate - failure to add new fuel slows and then stops the chain reaction process;

- the continuous removal of radioactive gasses;

- the addition primary core containment structure, piping, drain tanks and other fuel holding and processing structures;

- the reactor system chamber;

- an outer containment vessel;

- an underground location requiring escaping radioactive materials to counteract the forces of gravity before any above surface excursion.

Other potential barriers exist. In two fluid MSRs, the blanket containment structure constitutes another safety barrier.

Core salts breaching core containment must mix with blanket

salts and then breach the blanket.

3. Key aspect of nuclear safety (safety functions)

For the confinement of radioactivity all reactor should have the following three safety functions:

a Reactor shutdown function: in case of trouble occurs, quickly insert all control rods into reactor to stop (shut down) the fission and to terminate the energy generation.

b Cooling function of the reactor: in case reactor water level usually decreases, inject water into reactor to keep the integrity of the fuel, and to prevent the release of radioactivity.

c Confinement function of radioactive materials: in case of big accidents and radioactive material is released to primary containment vessel, prevent a leakage from primary containment vessel (PCV) to outside environment.

The above three safety function in MSR will be explained as follows (Furukawa et al., 1999):

a Reactor shutdown function:

This condition is satisfied in MSR through different systems in addition to inherent self stabilization as in all reactors by designing the reactor with a negative power coefficient. As shown in Table 1, Control rods are used for a rapid shutdown, and the fuel-salt drain system is also able to be used as another reactor shutdown function. Because the excess reactivity is small, the number of control-rods is few and the diameter is large. The reliability will be high. The drain system is always necessary and effective on the pipe rupture accident. Since the fuel salt falls to the drain tank by gravity through the freeze valve with a simple mechanism, its reliability is high. Although the freeze-valve operation may be slow, rapid response needs not due to no re-criticality.

As a third measure, the adjustment of fuel composition using fuel-salt controlling system is possible to shutdown the reactor. One approach will be the Th addition, which is necessary to make up fuel salt in any MSR, and again a slow action of this system does not cause any problem. b Cooling function of the reactor (Table 2): In MSR, the possibility of piping rupture is very low due to the low pressure, and the ECCS will not need the same as Fast breeder Reactor FBR (Monju FBR in Japan). It is possible to deal with the drain system, even if a piping rupture causes the fuel salt loss. Of course the decay heat removal system is necessary for the drain system. The MSR may have a capability of natural circulation when all pumps stop, because the pressure loss in the core is small. When natural circulation cannot be expected, or when a turbine system is isolated and the cooling by the secondary loop is impossible, the decay-heat removal system is necessary. As a final heat sink, the decay heat removal system by a static air cooler as in FBR is preferable to endure a long term severe accident, such as all AC power supply loss ("station black out") accident. c Confinement function of radioactive materials at accident (Table 3):

For this purpose, five barriers are applied in LWR. The first two barriers do not exist in MSR because MSR uses fluid

fuel. The chance of radiation exposure by gaseous fission products (FP) is smaller due to their continuous removal from fuel salt, and the danger of piping rupture is also very low. Therefore it is thought that the MSR safety is better than LWR.

The primary system of MSR is enclosed in a "high temperature confinement" and the entire reactor system is covered in the "containment" which is a reactor building itself. These arrangements are basically equal to the LWR. Since there is no water and no flammable gas generation, the MSR safety is excellent due to very few events which can threaten the integrity of containment.

4. Accidents control

4.1. Design basis accident (DBAs)

4.1.1. Power increase accident or RIA (reactivity initiated accident) (Yoshioka, Shimazu, & Mitachi, 2012)

Control rod withdrawal/ejection accident: This is a most typical reactivity initiated accident (RIA). If we adopt a control rod made of neutron absorbing material, and when this control rod is inserted in operation and is withdrawn or ejected by some equipment failure or operator error, then RIA occurs. If we adopt a control rod made of graphite, which was proposed for MSBR, insertion of graphite control rod increases more neutron moderation and it may cause RIA. Owing to large negative reactivity coefficient of fuel-salt temperature, power excursion terminates, even if control rod scram function fails. Meanwhile, reactivity coefficient of graphite temperature is slightly positive, but this does not cause any problem, because heat transfer to graphite is slow. On the other hand, MSR has a longer prompt neutron lifetime than LWR, and this fact mitigates the maximum neutron flux.

4.1.2. Flow decrease accident (pump trip accident)

If all fuel-salt pumps trip (stop), heat removal function is lost. (In LWR licensing, one pump trip is categorized to AOT, and all pumps trip is categorized to DBA.) Then, fuel-salt temperature increases. Also, delayed neutrons increase in the core when salt circulation stops, and it causes the same effect as positive reactivity insertion. This is because normally some of delayed neutrons are lost out of the core. However, owing to the negative reactivity coefficient, its consequence is not as severe as pump seizure accident. In this pump trip accident case, control rods are inserted and nuclear fissions stop (Yoshioka et al., 2012). In MSR, there is judgment that "the reactor is safe for the stop of all primary pumps, if an appropriate scram system is designed" (Furukawa et al., 1999).

4.1.3. Fuel-salt ¡eak accident (primary ¡oop break accident) (Yoshioka et al., 2012)

If rupture or break of vessel, pipes, pumps, heat exchangers, and other small pipes occurs by some reasons other than pressure/heat, then the integrity of primary loop is lost, and fuel salt will leak out. Of course, leaked salt is caught by a catch-pan, and collected in a drain tank or an emergency drain tank. Regarding the rupture of heat exchanger, mixing of fuel salt and secondary-salt must be evaluated. The causes of

these accidents may be manufacturing flaw, excessive wall temperatures and stresses, corrosion, thermal stress cycling, and so on. In this accident scenarios as shown, fuel salt must be transferred to a drain tank, and this system assures high safety of MSR.

4.2. Severe accidents

For nuclear reactors it is common to consider three types of severe accidents: criticality accident, failure to remove after heat and a melt down. The melt down is not an accident by itself but rather a description of a consequence of an accident.

4.2.1. The source term

The source term is the measure of the radiation which needs to be contained from reaching any sensitive location or target. The energy contained in the source term also provides the driving force for dispersion of the source term as it also a measure of the after heat, or the energy, to damage a reactor in the event of heat-removal failure or loss of coolant accident (LOCA). For an MSR, as for any fluid fuel reactor, on-line fuel processing can be applied. The on-line processing, at the least, removes the gaseous and volatile part of the source term (Gat & Dodds, 2008). Fission products (with the exception of xenon and krypton) and nuclear materials are highly soluble in the salt and will remain in the salt under operating and expected accident conditions. The fission products that are not soluble (e.g. xenon and krypton) are continuously removed from the molten fuel salt, solidified, packaged, and stored in passively cooled storage vaults (C.W. Forsberg, 2004). The MSRs processing can be adjusted to have a small source term. The safety advantages of this small source term are many fold (Gat & Dodds, 2008): The driving force for dispersion is reduced and there are no major stored energy sources within containment such as high pressure fluids [helium and water] or reactive fluids [sodium]. This reduce requirements for the containment (C.W. Forsberg, 2004); also the gaseous and volatile components, which are the most likely to disperse, are essentially all but eliminated (Gat & Dodds, 2008); the long half-life isotopes (elements) are reduced such that the long-term effect of even the most unlikely accident is not severe; and, the short-lived isotopes require a proportionately short-term protection time till they decay (Gat & Dodds, 2008).

4.2.2. Criticality accident

In MSRs with processing, the criticality accident is essentially eliminated. There are two factors that make an excess reactivity incident unlikely, temperature control and optimized geometry. The MSR can be temperature controlled. The large negative temperature coefficient allows for control without control rods or other mechanically operated control mechanism (some designs included low worth rods for minor temperature control). The control rods can be used for temperature regulation. Continuous fuel processing, with the ability to externally add fissile material when needed, reduces the need for excess reactivity inventory (Gat & Dodds, 2008). The MSR can be designed so that bred fuel, at a breeding ratio of 1.0, keeps the reactor at equilibrium with fertile-material feed and with no need to add fissile material. Since the fuel is also the coolant, the reactor is largely temperature controlled regardless of the power.

The adequately designed MSR has an optimum geometrical design for criticality in the core. The externally cooled reactor has neither coolant nor structural materials in the core that may require design compromises and thus can truly be optimized for safety. This core optimization also assures that no criticality, or re-criticality, outside the core can occur (Gat & Dodds, 2008).

4.2.3. Decay heat accident

Molten salt reactors use passive emergency core cooling systems that are radically different from those used in solid-fuel reactors. The fluid nature of the fuel means that the reactor core meltdown is an irrelevant term. The liquid state of the core also enables in most emergencies a passive, thermally triggered fuel salt draining into bunkered, and geometrically sub-critical, multiple dump tanks, which are provided with passive decay heat cooling systems (see Fig. 1). Actually, at the bottom of the core, MSR designs have a freeze plug (a plug of salt, actively cooled by a fan to keep it at a temperature below the freezing point of the salt). If the fuel salt overheats and its temperature rises beyond a critical point, the freeze plug melts, and the liquid fuel overflows by gravity and is immediately evacuated from the core, pouring into the emergency dump tanks. This formidable safety tactic is only possible if the fuel is a liquid. Power is not needed to shutdown the reactor, for example by manipulating control elements, but it is needed to prevent the shutdown of the reactor (Luzzi, Aufiero, Cammi, & Fiorina, 2012).

5. Licensing

Although two experimental MSRs have been built and operated in the United States under government ownership, none has ever been subjected to formal licensing or even detailed review by the NRC (Engel et al., 1980). Accident analysis for molten salt reactors MSR has been investigated and several calculations were made for experimental reactors: MSRE. However, it was 50 years ago, and it may not be applicable from the standpoint of recent licensing approach for light water reactor (LWR). Since then, no guidelines or safety criteria have been defined for MSR accident analysis. Regarding the safety criteria, and as shown in Refs. Yoshioka et al. (2012) and Shimazu and Yoshioka (2010) authors showed one proposal based on a temperature limitation of the component: Hastelloy N, in the previous London Conference.

The licensing-related design and safety features combine those of a reactor and a chemical processing plant. Because prescriptive safety regulations were developed for solid-fuel reactors, many of the prescriptive safety regulations are not applicable to an MSR. Further, the licensing experience of solid-fueled reactors can be used as only a general guide because of significant fundamental differences between those systems and MSRs. Liquid fueled reactors use different approaches to reactor safety than solid fueled reactors. These include (https://inlportal.inl.gov/portal/ server): (1) draining the fuel into critically safe, passively cooled tanks if off-normal conditions occur, (2) limiting excess reactivity by on-line fuel processing and/or

continuous fueling, and (3) limiting fission product source terms by on-line processing. The current regulatory structure was developed with the concept of solid-fuel reactors. The comparable regulatory requirements for this system must be defined. Using current tools, appropriate safety analysis is required followed by appropriate research on the key safety issues.

The MSR requires that the safety basis be defined in terms of performance goals with a rethinking of how those goals are met. While a probabilistic safety analysis has not been done on an MSR, the available evidence suggests significantly different concerns. The characteristics of the MSR suggests that the probability and consequences of a large accident to be much smaller than most solid-fuel reactors.

At the same time, the processing and other operations indicate greater concerns associated with smaller accidents. The MSR incorporates most of the fuel cycle with the reactor; thus, risk comparisons with other reactors must consider the entire fuel cycle (C. Forsberg, 2004).

6. Conclusions

Safety of MSRs are reviewed and assessment compared to conventional solid fueled LWRs. MSRs are safer and more stable since they don't reach high enough temperatures for meltdown (since the fuel is in a molten state) and the primary system is at a low operating pressure even at high temperature, due to the high boiling point (~ 1400 °C at atmospheric pressure) and therefore do not require expensive containment or highly pressurized hot water. The MSR is not subject to safety concerns from chemical or mechanical violent reactions or explosions. The basic features of MSRs give the solutions for many problems for others solid fueled light water reactors, and eliminate the reasons for serious last accidents like TMI, Chernobyl and Fukush-ima and more of basis and severe accidents will be decreased and limited. The MSR comparable regulatory requirements must be defined and probabilistic safety analysis is required and quantitative evaluation for several accidents are needed.

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