Proceedings of the 2nd International Conference on Environmental and Geological Science and Engineering

Environmental Friendly Nuclear Power Need Risk Mitigation GHEORGHE POPESCU National Institute of R&D for Cryogenics and Isotopic Technologies Rm. Valcea - code 240050, Uzinei 4, CP10, Valcea, Phone: 0040 250 736979, fax: 0040 250 732746, ROMANIA [email protected] Abstract: While the electric power industry suffered a set-back in the public acceptance of nuclear energy, the reasons for expanding construction of new nuclear power plants three decades ago have not changed and in fact have become more urgent. However environmentalists, public policy makers, and financiers like to see a risk assessment of the nuclear option compared to other possible solutions. Such stakeholders want to be convinced that an expansion of nuclear power plants can be done safely and economically before they will give it their support. Therefore in this paper we model and solve the problem of choosing a maintenance policy for a single item, from a particular class of policies, which will set the occurrence probability for worst case scenario at the best value, where damage is transformed into economies in resources. Key-Words: environment, nuclear, accident, risk, consequences, protective functions, probability, gain

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Introduction

The industrial risk management

2.1

Risk assessment

Managing operational risks is the most important part of any management strategy related to energy production and it can be summarized by the following stages:

The energy demand of the world is growing due to the increase in world population and the increasing energy (and especially electricity) use per capita. Today most of the worldwide electricity demand is satisfied by fossil fuels, basically by oil, natural gas and coal. Against this background, new energy sources are needed. Between many others, a way to obtain green energy has already been discovered, and that is the nuclear power. Nuclear power is any nuclear technology designed to extract energy from atomic nuclei using controlled nuclear reactions. But the dimension of possible accidents at the nuclear plants is mandatory to make the risk analysis involved by the operation of the designed facility. Risk can be defined as ‘the chance that something adverse will happen’ or more specifically the likelihood that a specific (undesirable) event will occur within a specified period or in specified circumstances. As risk is made up of two elements, likelihood and consequences, risk reduction can be achieved by implementing measures either to reduce the likelihood of the event or measures to reduce the consequences of the event.

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2

Identification of M possible failure scenarios, Estimating the likelihood pi and consequences Ci for each failure scenario, Prioritizing risks Ri Estimating the total risk M (1) R = ∑ p iC i i =1

The risks Pi Ci corresponding to different failure scenarios are ranked in order of greatness. After that the risk reduction efforts are concentrated on the few failure scenarios responsible for most of the total risk..

2.2

Risk abatement

Selecting appropriate risk response strategies for each determined risk consist basically in choosing between avoiding and reducing the risk. Avoiding the risk altogether is the best prevention measure because it eliminates the cause of the risk (eg. The risk of radiations is avoided if non-radioactive materials are used, and, regarding tritium, that’s why it’s best to

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detritiate the moderator and the coolant heavy water used in CANDU reactors, like the two we have at Cernavoda Nuclear Power Plant). Because nuclear equipments repairs are often very difficult or very expensive preventive measures should be used, which means reducing the likelihood of failure. At the same time, a nuclear facility is characterized by low-probability high-impact events which require also protective measures to be included in the strategy adopted for reducing the risk. So, preventive measures should be preferred to protective measures wherever possible because while protective measures mitigate the consequences from failure, preventive measures exclude failures altogether or abate the probability of their occurrence.

2.3 Reviewing and implemented measures.

maintaining

can highlight two major issues which require to be examined in turn: 1. Each disturbing event is characterized by two specific elements essential to such analysis: • the magnitude of the consequences generated in the nuclear objective when producing the analyzed disturbance, • the natural frequency of the event. 2. To counter the dangerous disturbances we can implement systems to achieve protective functions / measures in two main directions: • protective functions / measures to mitigate the consequences, which will move the risk vector to minor consequences RC,

the

Appropriate risk reduction measures are identified which will reduce the risks associated with these few failure scenarios. Next, new failure scenarios are identified, and the total risk is estimated and assessed again. This iterative process continues until the riskassessment procedure indicates that the total risk is acceptable [6].

Fig. 2 Choosing the way to risk reduction • implementation of protective functions to reduce the natural frequency of the event and to move the risk vector to low frequency RF occurrence. Operating points travel size (BB1 or CC0), respectively RF (red. frequency) and RC (red. consequences) vectors sizes, practically defines the degree of reduction of characteristic risk for the studied event and depends on the effectiveness of protective functions implemented and the complexity of these functions[1]. In nuclear installations the protective functions are performed by two systems: 1. The constraints, which intervene to limit the operational parameters. Not always leads to emergency shut-down. At the nuclear installations there are constraints on all major B

Fig. 1 Risk-assessment block diagram

3

The problem of choosing

Considering all the possible disturbances we

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Proceedings of the 2nd International Conference on Environmental and Geological Science and Engineering

equipment (isotope exchange column, the gas treatment, cryogenic distillation column, etc.). 2. Special security systems, which acts as quick stop / emergency systems, in cases when a barrier / barriers could be damaged. The special security systems, as vital protective function in a nuclear unit, once triggered by a disturbance should bring the installation into a safe state. So, no matter how little the uncertainty would be the technology requires the implementation of security equipment/systems, to reduce the probability of a fatal event. It follows that: • Risk may not be removed (a zero probability of occurrence) and this led to the adoption of the principle of ALARP(As Low As Reasonably Practicable) [9].

Fig.3 The dependence between the investment and the occurrence probability for a specific (undesirable) event Obviously, the problem is resolvable only if (2) C ≤ S 5. Under these conditions, the event will realize losses expressed by a losses function P, with the expression: P = p ⋅D (lei) (3)

Fig. 3 The ALARP triangle • The owner of a nuclear installation has the problem of optimizing the costs of achieving the necessary security systems in order to obtain a risk value equal to or even lower than regulated by law.

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Fig. 4 The dependence of losses with the probability to have the event which can cause the maximum damage

Gain through economy

To solve this problem is necessary the analysis of the „gain function” for each case; such an analysis starts with a series of assumptions: 1. To have a safety system / subsystem which will perform a protective function against one or more harmful events, the investor is willing to spend a sum of S lei; 2. The event occurrence, in worst case, can cause a maximum damage of D Lei; 3. The event has a probability of occurrence p; 4. The designer knows that it is possible to keep the event under control through an expense of C lei, where C depends on p, so it will be C (p);

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We define the gain function E p, which is the achievable economy, and its expression is given by the relationship: (4) E p = S − [C ( p ) + p ⋅ D ] The event can take place after several possible scenarios:

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5. CONCLUSIONS

The problems developed in the present paper relates to a single-item maintenance problem. That said, this paper’s model forms the basis for higher fidelity multi-item maintenance models and in practice it can be applied to multiple items within an equipment class. This way a nuclear power facility can establish a schedule for many major equipment classes. Practically all equipment that can be safely maintained together are grouped and assigned a base calendar frequency. The engineer may at times feel pressure to reduce maintenance (to reduce short-term operating costs) but at the same time the engineer is expected to keep the equipment running well. Typically, the feedback from the effect of a change to maintenance policy is not fast enough to make a fine-tuned correction [8]. That is, at a time when cost overruns are being minimized, it is tempting to reduce maintenance on wellrunning equipment. By the time the reduction in maintenance is seen, the equipment performance may have significantly degraded and it is natural to overreact by increasing maintenance to a higher level than previous. That is why that mathematical model is so useful as a decision-support guide for engineers working on maintenance scheduling to mitigate risks and to transform the damage into economies in resources [5].

Fig. 5 Possible scenarios related to a specific event occurrence The objective is to maximize the gain function knowing that:

Max (E p ) ∈ {0; S − [C ( p ) + p ⋅ D ]}

(5) To determine the maximum we have to solve the equation:

d [S − (C ( p ) + p × D )] = 0 dp

(6) resulting that:

− C ' (p ) − D = 0

(7)

and therefore:

D =

d C (p ∗ ) d p

(8)

where p * represents the "best value" for the "fatal event" probability. The graphical solution of the problem is:

References: [1] Carabulea, A., ş.a., Reingineria industriala a riscului energetic in conceptia cercetarilor operationale, Politehnica Press, 2006. [2] Niţu, V., ş.a., Fiabilitatea în energetică, Editura Didactică şi Pedagogică, Bucureşti, 1980. [3] Gheorghiu, I. D., Reingineria Riscului, U.P.B., Bucureşti, 2007. [4] Valeca, S.C., Sisteme Energetice cu Generatoare Nucleare, Editura Academiei Romane, 2002 [5] Carabulea, A., Managementul Sistemelor Energetice, U.P.B., Bucureşti, 2006. [6] Michael, T. Todinov, Risk-Based Reliability Analysis and Generic Principles for Risk Reduction, Elsevier, 2007. [7] Christian, Kirchsteiger, Risk Assessment and Management in the Context of the Seveso II Directive, Elsevier, Industrial Safety Series, Volume 6, 1998. [8] M. Perez, F. Reventos, R. Wagner, C. Allison, Integrated Uncertainty Analysis using RELAP/SCDAPSIM/MOD4.0, NUTHOS-7: The 7th

Fig. 6 The profit “Z” for the best value probability Therefore we’ll have a real and certainly gain Z, with the value: (9) Z = S − C p* + p* ⋅ D

[ ( )

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Proceedings of the 2nd International Conference on Environmental and Geological Science and Engineering

International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety, Seoul, Korea, October 5-9, 2008, Paper 245. [9] David, J. Smith, Reliability, Maintainability and Risk, Elsevier, 2005.

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