Nils Göran was born in Eksjö on 28 March 1925.
He showed a great interest in physics already at young age. His father was a
high-school teacher who lectured in mathematics, physics and chemistry, and his
mother was member of parliament between 1949-56.
Nils Göran had enrolled to the newly started Engineering
Physics program of the Royal Institute of Technology (KTH) in Stockholm and he
got his diploma in 1949. He continued with graduate studies and his choice fell
on a subject representing one of the most promising and exciting new branches
of physics of the time – nuclear engineering, more concretely reactor physics.
This was a very exciting time worldwide and in Sweden, and Nils Göran became
one of the true pioneers of this field. This was the time of the start of the
“Atomic age”, with Eisenhower’s talk “Atoms for Peace” in the UN in 1953, which
opened up the scene for peaceful applications of nuclear energy with, among
others, the historic Geneva conferences with start 1955. Due to her neutrality,
Sweden had a good position to have access to technical and scientific know-how,
as well as to raw materials such as Norwegian heavy water. Together with rich access to low-grade
uranium ores in Sweden, there was a promise of developing a domestic nuclear
energy program and to become self-supporting in electricity production.
It was in such a setting that Nils Göran
started his PhD at KTH, while having an assistant researcher status at the
newly founded AB Atomenergi, the governmental research institute for the
development of nuclear power in Sweden. His advisor was Sigvard Eklund, head of
the research group, who later became General Director of the IAEA. During this
time Nils Göran took part in the construction and operation of the first
research reactor in Sweden, the R1, which was built at the campus of KTH in an
underground location. He took part in the start-up of R1 which started
operation in 1954. During his PhD studies N. G. Sjöstrand was also visiting
scientist at Brookhaven National Laboratories in 1952-53 and in AERE Harwell in
The R1 reactor was used for research in reactor
and neutron physics, mostly pulsed neutron experiments. The goal of these
measurements was to determine the properties and dynamic response of the
reactor in the critical state. However, for obvious reasons, such pulsed
experiments can only be performed in the subcritical state of the reactor. To
extrapolate the results to the critical state was extremely complicated,
which started to become a hinder for
large-scale development of nuclear technology. During his PhD time, Nils Göran
Sjöstrand found an elegant, exact solution to the problem, by developing a
methodology of measuring reactivity in subcritical systems, the so-called “area
ratio method”, also called the “Sjöstrand method”. It was published in the
journal of the Swedish Physical Society, Arkiv för Fysik, in 1956, and it is
one of the most cited Swedish articles of all categories. Nils Göran’s PhD
thesis had the title ”A theoretical and experimental investigation of the
pulsed source method as a means for neutron diffusion studies”. He got his PHD degree at KTH in 1959.
The cover of article in Arkiv för Fysik (left) and the schematics of the area ratio method (right) from the article
The area ratio method earned Nils Göran
international fame. The theory, performance and capabilities of the method were
studied intensively around the world, among others at the Department of Reactor
Physics in the Central Research Institute for Physics in Budapest, where undersigned
was PhD student in the early 1970’s. The method plays a continued important
role today in the recent development of the so-called Accelerator Driven
Systems (ADS) for transmutation of nuclear waste, and the area ratio method is
cited just as frequently today.
The area ratio method in the book Bell and Glasstone: Nuclear Reactor Theory
Research and education in reactor physics was
started in Sweden in the early 1960’s, where departments for reactor physics
were established in Chalmers and KTH. Chalmers founded the Department of
Reactor Physics in 1960. Nils Göran became the first professor in reactor
physics in Sweden when he was appointed first as acting, then in 1961 as a full
professor in Chalmers and moved to Göteborg. Since the research at the “syster
department” in Stockholm was focused on neutron scattering and diffraction, for
a long time Chalmers had the only department with research and education in
reactor physics in Sweden.
Since no education in nuclear engineering
subjects was in existence in Sweden at
the time of Nils Göran’s appointment, he had to build up education in the Swedish
language at the undergraduate and graduate level from scratch. The lecture
notes he wrote were the only teaching material in Swedish, and were used at
other Swedish universities as well.
The experimental circumstances were very
rudimentary and restrictive at the beginning. The Department rented two floors
in a block of flats in central Göteborg, where also a low-yield neutron
generator was also installed. This naturally led to some complications. During
the turbulent times around the national vote on nuclear energy in 1980, some
journalists believed that the neutron generator was a nuclear reactor, which of
course gave headlines in the newspapers.
After some time the Department could move to
the Chalmers campus and have access to a good laboratory with international
standards. The Department got access to an own building, which was originally
built as a transformer station for the trams in Göteborg. A new powerful
neutron generator was installed, which was suitable for research and PhD work.
During his time as professor, about 20 PhD students graduated at the Department,
and a large number of master theses and licentiate exams were taken. His
students have taken many leading positions in academia and industry both in
Sweden and world-wide.
During his time as professor at Chalmers, Nils
Göran shifted his research to semi-analytical and numerical solution methods of
the transport equation, in particular high accuracy calculation of its higher
order eigenfunctions and eigenvalues. He has been pursuing this activity with
large productivity under a number of years, partly in collaboration with D. C.
Sahni from BARC, India. He initiated in 1983 the biannual series “Reactor
Physics Research in the Nordic countries”. His reputation in the field was an
important factor that in 1997 Chalmers hosted the 15th International
Conference in Transport Theory.
Nils Göran continued with this research long
after his retirement in 1991. He also used to have some hours in the courses in
reactor and neutron physics when he told many interesting stories and anecdotes
from the pioneering years, which were a highlight of the courses. One of these was
the legendary story when he escorted a highly secret transport of heavy water
from Rjukan in Norway to Stockholm, for the construction of the R1 reactor.
About this, and about the history of the Department from the beginnings to his
retirement in 1991, he wrote two very entertaining internal reports.
Nils Göran Sjöstrand around 1995
Nils Göran was a member of the Editorial Board
of “Nuclear Instruments and Methods A” under 28 years, and reviewed about 600
manuscripts submitted for publication. He was the Vice Dean of the School of
Physics in Chalmers for 15 years. He was also a member of the Committee for the
Safety of Nuclear Installations at the Swedish Nuclear Power Inspectorate.
Nils Göran’s publications are known not only
for their technical content, but also for their literary qualities. He was an
archetype of a writer of high quality articles, both scientific and literary
ones. His help as a language reader for younger colleagues was very much
appreciated. He was not only interested in language and terminology, but he
played an active role in elaborating and
maintaining the terminology of reactor physics and nuclear power both in
Swedish and English.
The lifetime achievement of Nils Göran
Sjöstrand was acknowledged internationally in many ways. The international
journal Progress of Nuclear Energy published a “Festschrift”, a special issue,
to honour him on his 70th birthday in 1995. In late 2006, a
mini-symposium was held in Chalmes, to commemorate the 50th
anniversary of the publication of his seminal paper in Arkiv för Fysik. He is
also recipient of the prestigious Eugene P. Wigner Reactor Physicist Award of the
American Nuclear Society, which he received as late as 2011. Since by that time
Nils Göran could not travel, Dimitrios Cokinos, the Head of the Honours and
Awards committee of the ANS Reactor Physics Division, came to Sweden to present
the Award to Nils Göran at a ceremony in the Chalmerska Huset in Göteborg.
The Wigner Award ceremony in the Chalmerska Huset in 2011
Nils Göran was not only a prominent scientist,
but also very interested and knowledgeable in arts and humanities. He had a
large collection of the books on the history of nuclear energy, which he donated
to the Department in 2006. During 44 years he was a Fellow of the Royal Society
of Arts and Sciences in Göteborg, and in 1996 he was its president. His humble
and helpful attitude together with his huge knowledge and insight earned him
much respect and many friends. As a learned friend and colleague put it, “he
was a fine man and a scholar of distinction”. The reactor physics community
lost one of its most outstanding founding members, but he will always be
remembered for his achievements, wide knowledge, and kind personality.