2.5.1 Total level density

A revised version of the Back Shifted Fermi Gas (BSFG) model parameters was prepared by the Obninsk group to be consistent with both the recom­mended RIPL-2 neutron resonance parameters and the evaluated parameters of the recommended low-lying levels. The new BSFG systematics developed by the Brussels group is consistent with the recommended RIPL-2 neutron resonance parameters, and will be included in the RIPL-2 TECDOC. The Gilbert-Cameron (GC) and Generalized Super-fluid Model (GSM) parame­ters were revised by the Obninsk group in accordance with changes in the RIPL-2 resonance segment. The microscopic HF-BCS calculations of the nu­clear level densities are based on the realistic microscopic single-particle level scheme [11] determined within the HF-BCS mass model obtained with the MSk7 Skyrme force, and were supplied by Goriely and made available from the RIPL-2 library. Also, the single-particle schemes used in the HF-BCS calculations were provided by the Brussels group. In addition, the FRDM single-particle schemes are included as corresponding to the accepted FRDM mass table.

The advent of personal computers (leading to the emergence of the CD-ROM and the World Wide Web) has revolutionised the ability to communicate with and access large data bases rapidly and efficiently. Examples of the provision of CD-ROMs and the implementation of the Web are listed in Appendix A (ENSDF, NUBASE and the Table of Isotopes). A significant number of these data bases have been adapted so that PCs can interrogate, extract, compare and use their contents. Both the Internet and the utilisation of CD-ROMs have given users world-wide access to all of the most recently evaluated nuclear data libraries via a number of routes including the IAEA Network of Nuclear Data Centres. Communications between laboratories have improved beyond all recognition over the previous 10 years, and have revolutionised the speed with which data can be provided for various calculations (including decay — heat analyses).

Maurizio Cumo[9]

University of Rome ”La Sapienza, ”

Department of Nuclear Engineering and Energy Conversion, Rome, Italy

Lectures given at the
Workshop on Nuclear Reaction Data and
Nuclear Reactors: Physics, Design and Safety
Trieste, 25 February — 28 March 2002

LNS0520006

1. Introduction

Decommissioning of a nuclear power plant or other nuclear installations can be defined as the cessation of operations and the withdrawal of the facility from service, followed by its transformation into an out-of-service state and eventually, its complete removal, the so-called "green field" status, which, in principle, restores the site to the conditions existing before the construction of the plant. Alternative end conditions may include a situation in which the buildings, free of any radioactive contamination, are left for future conventional demolition, or situations in which these buildings, or the site itself, are used for other industrial purposes.

We will discuss the decommissioning activities starting from a situation in which spent fuel is not present any more on-site, or at least, in a completely independent storage facility. This removes from the plant more than 99,99% of the radioactivity present in the plant at the time of operation.

The main goal for the decommissioning activities is to place the facility in a condition that eliminate any risk for the health and safety of the general public and the environment, removing, in particular, from systems and structures any radioactivity that may have been accumulated during plant operations. Of course, all the decommissioning activities shall be carried out with great attention to assure the minimization of the risks to both the public and the workers involved in the process.

Decommissioning is a complex, long lasting and highly technological activity that presents smaller challenges, but similar to the plant construction activities. In some countries, in fact, it is called de-construction. Activities include use of technological tools, control of industrial safety, environmental impact minimization, licensing, safety analysis, structural analysis, etc. Other aspects very relevant are the activity planning, the calculation of related cash flow and anticipation of the funds needed to perform the activities. Aspects related to waste disposal and spent fuel strategy shall be covered as well.

Whereas the Ritz variational principle for bound states is a standard text­book example, variational principles for scattering wave functions are still under discussion, especially for composite systems, see the review by Ger — juoy [5]. I will therefore repeat the essential points and refer to [4] for some details. With respect to bound state wave functions of fragments, the

RRGM is nothing but a standard Ritz variation with an ansatz for the wave functions in terms of Gaussian functions, see eqs. (2.33 — 2.36) below. That this expansion converges pretty fast was shown in [4] and is also discussed in chapter 5.1.

There are extensive gaps in the charge distribution data for fission, and in the chain yields for the more important fission reactions; there are also significant discrepancies between chain-yield measurements. Many fission products of importance in decay-heat calculations are short-lived, and their decay characteristics are either poorly defined or entirely unknown because of the difficulties associated with their direct study. Under such circumstances, sound theoretical extrapolation procedures and modelling techniques have been adopted to generate comprehensive fission-yield data sets, while various methods have been successfully explored to derive beta-strength functions and use these approximations to estimate half-lives and mean beta and gamma energies.

Isomeric states are normally low-lying metastable states (< 1 MeV) that occur when the angular momentum differences between this nuclear level and all lower levels are large. Electromagnetic transition probabilities are significantly reduced under such circumstances, and the lifetimes of the states are long (i. e., metastable). The half­lives of both the ground and metastable states can span many orders of magnitude (~1015 sec) due to variations in the form of P" decay (end-point energies and beta — strength functions). Over 150 of the fission products formed in the thermal-neutron fission of U, U and Pu have known isomeric states with half-lives > 0.1 sec.

These isomeric states play an important role in decay-energy release, since this time — dependent phenomenon depends on the relative populations between the ground and metastable states. Madland and England (1977) have developed a simple model to calculate the independent yield branching ratios between the ground and metastable states, and this approach has been extended by Rudstam et al (IAEA-CRP, 2000).

1.2 National Nuclear Data Centre (NNDC), Brookhaven National Laboratory (BNL), USA:

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Enter the code when prompted for assigned authorization code. New users may adopt the user name GUEST for a time-limited trial, and can apply for registration by using an electronic form which appears on exiting from the system.

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The region of light nuclei houses some of the neutron standards [36]. Starting from proton-neutron scattering below 20 MeV up to carbon-neutron scat­tering, all cross sections can be calculated by the methods described above, at least in principle, in case suitable nucleon-nucleon potentials are given. In the form of the RGM n-p scattering is just potential scattering, hence, any calculation just reflects how well the extracted potential was fitted to the original data.

The next simplest reaction is 3He (n, p)3H, which has 4He as compound nucleus. For this A = 4 system realistic nucleon-nucleon forces are feasi­ble, whereas for all heavier systems one has to rely on simplified, so-called effective forces. To demonstrate the differences, advantages and disadvan­tages I discuss in some detail in the following the 4He-system and the 7Li — system, which contains another standard reaction 6Li (n, t)4He. The reaction 10B(n, a) closes this section.

Nuclides with high N/Z ratios are neutron rich, and undergo radioactive decay to reduce this value by the emission of an electron (representing the conversion of a neutron to a proton within the nucleus). Conversely, nuclides with low N/Z ratios will decay by the emission of a positron (representing the conversion of a proton to a neutron); electron capture decay is an alternative process to positron emission, in which the unstable nucleus captures an orbiting electron to produce the same daughter nuclide.

Alpha decay becomes a dominant process above Z = 80, with the emission of an alpha particle (helium nucleus). Other relatively common modes of decay include isomeric transition (gamma-ray decay from a well-defined energy state of a radionuclide to a lower energy state in the same nuclide), spontaneous fission and delayed-neutron emission.

(a) Beta Decay

The mass number remains unchanged, and the atomic number Z increases by one unit when a radionuclide undergoes P- decay. An electron and an antineutrino are emitted, as a neutron in the nucleus is transformed into a proton:

n 4 p + в + V

Ax ч z+Y+P~+V

The maximum P-energy is represented by the equation:

Eemax = Q"- E

where Q — is the overall disintegration energy, equal to the difference in atomic masses between the ground states of the parent and daughter, and El is the energy level to which the decay occurs. Similarly, P+ decay is described by:

p 4 n + в+ +V

Ax ч z-y + в+ +v

and Eв+ = Q + — 2m0c2 — Et

where V is a neutrino, Q + is the overall disintegration energy, and

2m0c2 = 1.021998 MeV in which m0 is the mass of an electron at rest.

P+ emission occurs when

Q +- Et > 2m0c2

The P transition energy is shared between the electron (or positron) and antineutrino (or neutrino), as a continuous distribution for the two particles extending from 0 up

to E

(b) Alpha decay

A nucleus of atomic number Z and mass number A disintegrates by the emission of an a particle to give a daughter nucleus with atomic number Z — 2 and mass number A — 4:

Ax ч A-4y + Ahp

Z^ N Z-21 N-2 2^C2

The a disintegration energy can be represented by the equation:

where Ea is the energy of the emitted a particle, Et is the nuclear-level energy of

the daughter nuclide, and Er is the recoil energy:

in which MN is the mass of the recoiling daughter nucleus, and Ma is the mass of the a particle. The a particle is held within the nucleus by the Coulomb potential barrier, and escapes from the nucleus by means of a tunnelling mechanism.

(c) Gamma transitions

A gamma transition occurs when a nucleus in an excited state de-excites to a lower energy level, leading to the emission of a у ray and conversion electron (and an electron-positron pair when energy conditions permit). The gamma transition probability is defined as:

PTP — PY + Pce + Pe±

where PY, Pce and Pe± are the у -ray, conversion-electron and electron-positron pair emission probabilities, respectively.

The energy of the emitted y-ray can be represented by the equation:

EY — (i — Ef ) — Er

where Ei — Ef is the energy difference between the initial and final levels of the Ytransition, and Er is the recoil energy of the nucleus in the final state:

E —feL

r 2 M N c2

where MN is the mass of the recoiling daughter nucleus. The recoil energy is negligible, except for high Y energies and nuclei with low atomic number.

Gamma transitions can be classified in terms of multipole order, which is a function of the orbital angular momentum and quantum number L carried by the photon: L = 0, monopole; L = 1, dipole; L = 2, quadrupole, etc… If J and Jf are the total angular momenta quantum numbers of the initial and final levels connected by the Y transition, the vectorial relationship between the angular momenta is given by the formulation:

Ji J f — L — Ji +Jf I

Moreover, the angular momentum carried off by the photon cannot be zero, and consequently a 0 4 0 transition cannot occur except by internal conversion or internal-pair creation.

Gamma transitions are divided into electric and magnetic radiations:

electric radiation (emitted by the oscillation of electrical charges), with a parity change of (-1)L;

magnetic radiation (caused by the magnetic moment of the nucleus), with a parity change of (-1)^+4

A gamma transition can be a mixture of two (or sometimes three) multipole transitions. Two transitions in competition will have multipole order 2L and 2L+1, one electric and the other magnetic (e. g., mixture of M1 + E2).

The de-excitation energy of the nucleus can also be transferred directly to an electron (K, L, M..) which is ejected from the atom in preference to a gamma-ray emission:

where Ex is the binding energy of the electron in the X shell.

The internal conversion coefficient of the electron in the K shell is defined as:

Pce

ceK

PY where P and Pf are the K conversion-electron and y-ray emission probabilities, respectively; similar terms are also defined for the L, M, N… shells.

The total conversion coefficient is:

«total — OK + aL + 06m + ••• — — pe

PY

where Pce is the total conversion-electron emission probability of the related transition.

Although a considerable progress has been made in the evolutionary designs of LWRs, these are large reactors and many believe25 that development and demonstration of new, smaller, innovative designs with short construction and start­up times and low capital costs are necessary to usher a new era of nuclear power. Since the early 1990s, the interest of developing countries, mainly in Asia, has resulted in increased efforts on the design of small and medium sized power reactors. This is because in the next 50 years, electric demand is expected to be tripled, most of which will come from developing countries with small grid capacities. Also, in industrialized countries, electricity market deregulation is calling for power generation flexibility that smaller reactors may offer. Small and medium reactors (SMRs) are also of particular interest for non-electric applications such as seawater desalination and district heating, fuel synthesis, and, in the future, hydrogen production.

Small and medium sized reactors are, however, not new. We have currently 150 SMRs operational in the world, 41 of these with power levels less than 300 MWe and 109 having power levels between 300 and 700 MWe. The detailed breakdown show 32 gas cooled reactors in UK (AGR and GCR), 32 PWR, 24 BWR, 29 WWER and 27 HWRs.

Recent major drive for innovation in light water reactors has been toward integral reactors, where the core, pumps, pressurizers, and steam generators are contained inside a single reactor pressure vessel (RPV). They are of enhanced safety because there is no large break LOCA; they also endure less fluence on the reactor pressure vessel and employ passive safety systems. Three primary examples of these reactors are CAREM (Argentina), IRIS (USA), and SMART (Republic of Korea). Being small, they allow more shop-fabrication and hence improved quality. These are being designed primarily for sizes up to 700 MWe due to easy constructability of Reactor Pressure Vessels and to better match smaller electric grids.

SMR designs are also attempting to increase the fuel core life to enhance proliferation-resistant features and also to reduce the O&M costs. Eight to even 20 years of single core life has been envisioned. Another idea in this regard is to have refueling services provided by a central refueling organization, with crew dedicated to refueling, visiting each site as required. This would also improve efficiency. Similarly, barge mounted reactors could be returned to a central location for refueling.

Some designs have proposed to make extensive use of modularization, in which a significant portion of the plant is built as modules, which are fabricated outside of the principal buildings of the nuclear power plant. In some cases, the modules are fabricated off-site, to take advantage of existing fabrication facilities. Modularization serves to transfer a significant portion of the construction labor from the nuclear power plant to more easily controlled manufacturing environment. This reduces the site construction infrastructure and shortens the construction schedule, and hence the capital cost.

In order to improve economics, small reactor designs strive to minimize the manpower costs associated with the operation of the reactors. The inherent reactor shutdown and passive decay heat removal capability of some designs, in combination with modern advanced communication systems, may even facilitate remote operation with fewer operators, or even unattended, for some applications.

New research is underway to utilize the unique thermo-physical properties of supercritical water to enhance nuclear plant thermal efficiency to 40 — 45% from the current 33 -34%. This will also lead to considerable plant simplification. Because there will be no change of phase in the core, the need for steam separators and dryers as well as for BWR-type recirculation pumps is eliminated, which will lead to smaller reactor vessels. In a direct cycle steam generators are not needed. However, to make this possible, advances are required in high temperature materials to improve corrosion, stress corrosion cracking, and wear resistance.

Major innovative reactors in the world26 are tabulated in Table VII. Key features of SMRs include simplification and streamlining of designs as well as emphasis placed on safety features avoiding off-site impacts in case of accident. Such characteristics should facilitate their acceptability by local communities. However, none of these reactors have been built; only recently announcements have been made for beginning the preparatory phase for construction of KLT-40 in Severodvinsk in Russia and of a 65 MWt pilot version of SMART in KAERI, Republic of Korea. Two KLT-40 nuclear submarine reactors will be built on a floating barge with a displacement capacity of 20,000 tonnes. It is expected that the floating nuclear plant in Russia will produce power in 2006 and the pilot plant in Korea in 2008.

TABLE VII. MAJOR INNOVATIVE REACTOR DESIGNS UNDER DEVELOPMENT AROUND THE WORLD

5. UTILIZATION OF THORIUM FUEL

There has been a recent renewed interest in thorium fuel cycles. The reasons for this are to (1) burn excess weapons Pu without creating more, (2) generate less long-lived radioactive waste, (3) design reactors to operate in a safer mode, (4) reduce U-235 enrichment, (5) go to higher temperatures, and finally having large thorium deposits.

Thorium-232 is three times more abundant than uranium and available in India, Brazil, USA, Turkey and China. It is not a fissile material but it can produce U-233 in a reactor, which, from a neutronic standpoint, is an excellent nuclear fuel among the three nuclear fuels — U-235, Pu-239 and U-233. It also produces much less minor actinides from fission. Thorium dioxide is the only stable oxide of thorium, which accounts for its greater stability compared to uranium dioxide. It is also much more resistant to chemical interactions and has a high thermal conductivity. The melting point of thorium dioxide is 3050 degree centigrade. Thorium contains naturally up to about 100 ppm of Th-230; this and other neutron reactions of Th-232 and U-233 produces U-232, which decays with emission of hard gamma rays. Thorium fuel fabrication is similar to U-fuel but it requires remote operation because of the gamma emission from U-232 decay chains. In addition high chemical inertness of thorium dioxide makes it very difficult to be dissolved and reprocessed. Because of these drawbacks the thorium fuel cycle is considered a more proliferation-resistant fuel.

Thorium fuel cycles have been studied in the past in several countries on a smaller scale but its importance has increased in recent years as a non-proliferating fuel and also for reducing the inventory of Pu. Germany had used Thorium fuels for several years on the AVR, a pebble-bed high temperature research reactor, and on the THTR, Thorium High Temerature Reactor. Both in Germany and the US the fuel fabrication technology has been developed under high temperature reactor programs to a well proven, industrial process. The coated fuel particles for the HTGRs have shown excellent performance under irradiation and reactor operation. In Russia also tests of thorium-based fuels for WWER and LMFBRs have shown an excellent irradiation behavior.

The US has shown new interest in thorium fuel and has initiated four projects under the Nuclear Energy Research Initiative. Their primary motive is to develop an advanced proliferation-resistant, low cost uranium-thorium dioxide fuel. The Radkowski Thorium Reactor (RTR), being investigated in the US, Russia and Israel, revives the seed-blanket concept of the US Light Water Breeder Reactor design that operated in Shippingport in the late 50s. The concept assumes a once-through fuel cycle with no reprocessing; U-233 is bred and mostly burned in the reactor.

Most prominently, India has been pursuing a strong program on thorium fuel cycle activities. India has a closed fuel cycle strategy, which calls for using U-Pu fuel cycle for fast breeder reactors and a closed Th-U-233 fuel cycle in the next stage with advanced heavy water reactors. The Advanced Heavy Water Reactor (AHWR), currently under design, plans to use thorium for 75% of the power. Utilization of thorium is their focal point for development. All aspects of the fuel cycle including the back end are being studied in India. Activities for Thorium fuel development in India include studying: (1) dissolution of irradiated thorium fuel, (2) effective utilization of recovered fissile and fertile material, and (3) thorium fuel fabrication.

A critical review was undertaken of the methods for calculating partial level densities to be used in pre-equilibrium model calculations. A code for a com­binatorial calculation of particle-hole state densities, based on a convolution of shell-model single particle-states with BCS pairing, is included in RIPL-2 along with the corresponding tools for retrieving single-particle levels from Segment I. The most useful analytical approaches in the frame of the equidis­tant single-particle model are implemented in the revised AVRIGEANU code [12]. The finite hole-depth and binding energy restrictions are taken into account in these calculations.